The Cathode Ray Tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen Lenovo S10-2 Battery 5200mAh .
The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.
The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile Lenovo Y730 Battery 5200mAh .
The experimentation of cathode rays is largely accredited to J.J. Thomson, an English physicist who, in his three famous experiments, was able to deflect cathode rays, a fundamental function of the modern CRT Lenovo Y710 Battery 5200mAh .
The earliest version of the CRT was invented by the German physicist Ferdinand Braunin 1897 and is also known as the Braun tube. It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen Lenovo Y530 Battery 5200mAh .
In 1907, Russian scientist Boris Rosing used a CRT in the receiving end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television Lenovo Y510 Battery 5200mAh .
The first cathode ray tube to use a hot cathode was developed by John B. Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922 Lenovo S10-3 Battery 5200mAh .
A cathode ray tube is a vacuum tube which consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target. In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called araster Lenovo G450 Battery 5200mAh .
An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference. In all modern CRT monitors and televisions, the beams are bent by magnetic deflection Lenovo E30 Battery 5200mAh ,
a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of diagnostic instrument Lenovo E40 Battery 5200mAh .
In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below Lenovo E50 Battery 5200mAh .
Oscilloscopes use electrostatic rather than magnetic deflection because the inductive reactance of the magnetic coils would limit the frequency response of the instrument.
Various phosphors are available depending upon the needs of the measurement or display application Lenovo G460 Battery 5200mAh .
The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds. For visual observation of brief transient events, a long persistence phosphor may be desirable Lenovo Y460 Battery 5200mAh .
For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.
When displaying fast one-shot events the electron beam must deflect very quickly, with few electrons impinging on the screenLenovo Y450 Battery 5200mAh ;
leading to a faint or invisible image on the display. Oscilloscope CRTs designed for very fast signals can give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emissionthis plate multiplies the number of electrons reaching the phosphor screen Lenovo N200 Battery 5200mAh ,
giving a significant improvement in writing rate (brightness), and improved sensitivity and spot size as well.
Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements Lenovo T410 Battery 5200mAh .
The graticule may be permanently marked inside the face of the CRT, or it may be a transparent external plate. External graticules are typically made of glass or acrylic plastic. An internal graticule provides an advantage in that it eliminates parallax error Lenovo T510 Battery 5200mAh .
Unlike an external graticule, an internal graticule can not be changed to accommodate different types of measurements.Oscilloscopes commonly provide a means for the graticule to be side-illuminated, which improves its visibility when used in a darkened room or when shaded by a camera hood Lenovo SL410 Battery 5200mAh .
Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs) Lenovo SL510 Battery 5200mAh .
Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor Lenovo C100 Battery 5200mAh .
A shadow mask tube uses a metal plate with tiny holes, placed so that the electron beam only illuminates the correct phosphors on the face of the tube. Another type of color CRT uses an aperture grille to achieve the same result.
Convergence in color CRTs
The three beams in color CRTs would not strike the screen at the same point without convergence calibration Lenovo T400S Battery 5200mAh .
Instead, the set would need to be manually adjusted to converge the three color beams together to maintain color accuracy.
Most CRT television sets and computer monitors have a built-in degaussing (demagnetizing) coil, which upon power-up creates a brief, alternating magnetic field which decays in strength over the course of a few seconds Lenovo Y430 Battery 5200mAh .
This degaussing field is strong enough to remove most cases of shadow mask magnetization.
Vector monitors were used in early computer aided design systems and in some late-1970s to mid-1980s arcade games such as Asteroids Ibm ThinkPad G40 Battery 5200mAh .
They draw graphics point-to-point, rather than scanning a raster.
Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs Ibm ThinkPad X200T Battery 5200mAh .
In these, as the scanned resolution approaches the dot pitch resolution, moiré appears, as the detail being displayed is finer than what the shadow mask can render. Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail Apple A1280 battery 5200mAh .
In smaller CRTs, these strips maintain position by themselves, but larger aperture grille CRTs require one or two crosswise (horizontal) support strips.
CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship in an electron gun between applied video voltage and light intensity) Apple A1281 battery 5200mAh .
In better quality tube radio sets a tuning guide consisting of a phosphor tube was used to aid the tuning adjustment. This was also known as a "Magic Eye" or "Tuning Eye". Tuning would be adjusted until the width of a radial shadow was minimized Samsung R522 battery 5200mAh .
This was used instead of a more expensive electromechanical meter, which later came to be used on higher-end tuners when transistor sets lacked the high voltage required to drive the device. The same type of device was used with tape recorders as a recording level meter Samsung P467 battery 5200mAh .
Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask (stencil), which shapes a wide electron beam to form a character on the screen Hp Pavilion dm3 Battery 5400mAh .
The system selects a character on the mask using one set of deflection circuits, but that causes the extruded beam to be aimed off-axis, so a second set of deflection plates has to re-aim the beam so it is headed toward the center of the screen. A third set of plates places the character wherever required Hp Pavilion dm4 Battery 5200mAh .
The beam is unblanked (turned on) briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center Hp Pavilion cq62 Battery 5200mAh ;
this effectively disabled the character mask, and the system reverted to regular vector behavior. Charactrons had exceptionally-long necks, because of the need for three deflection systems.
Nimo was the trademark of a family of small specialised CRTs manufactured by Industrial Electronics Engineers Hp Pavilion cq42 Battery 5200mAh .
These had 10 electron guns which produced electron beams in the form of digits in a manner similar to that of the charactron. The tubes were either simple single-digit displays or more complex 4- or 6- digit displays produced by means of a suitable magnetic deflection system Hp Pavilion cq72 Battery 5200mAh .
Having little of the complexities of a standard CRT, the tube required a relatively simple driving circuit, and as the image was projected on the glass face, it provided a much wider viewing angle than competitive types (e.g. nixie tubes) Hp cq20 Battery 2600mAh .
Zeus Thin CRT Displays
In the late 1990s and early 2000s Philips Research Laboratories experimented with a type of thin CRT known as the Zeus display which contained CRT-like functionality in a flat panel. The devices were demonstrated but never marketed Hp 2230 Battery 2600mAh .
Although a mainstay of display technology for decades, the demand for CRT screens has dropped precipitously since 2000, and this falloff has been accelerating in the latter half of that decade Dell Inspiron Mini 12 battery 7800mAh .
The rapid advances and falling prices of LCD flat panel technology, first for computer monitors and then for televisions, has been the key factor in the demise of competing display technologies such as CRT, rear-projection, and plasma display Dell XPS M1730 battery 7800mAh .
The end of most high-end CRT production by around 2010 (including high-end Sony and Mitsubishi product lines) means an erosion of the CRT's capability. In Canada and the United States, the sale and production of high-end CRT TVs (30-inch screens) in these markets has all but ended by 2007 Dell XPS M2010 battery 7800mAh ;
just a couple years later inexpensive combo CRT TVs (20-inch screens with an integrated VHS or DVD player) have disappeared from discount stores. It has been common to replace CRT-based televisions and monitors in as little as 5–6 years, although they generally are capable of satisfactory performance for a much longer time Dell Studio XPS 1340 battery 5200mAh/7800mAh .
Companies are responding to this trend. Electronics retailers such as Best Buy have been steadily reducing store spaces for CRTs. In 2005, Sony announced that they would stop the production of CRT computer displays Dell Inspiron 9100 battery 7800mAh .
Samsung did not introduce any CRT models for the 2008 model year at the 2008 Consumer Electronics Show and on February 4, 2008 Samsung removed their 30" wide screen CRTs from their North American website and has not replaced them with new modelsDell Inspiron N4010 battery 5200mAh .
The demise of CRT, however, has been happening more slowly in the developing world. According to iSupply, production in units of CRTs was not surpassed by LCDs production until 4Q 2007, owing largely to CRT production at factories in China Dell Inspiron N3610 battery 5200mAh .
In the United Kingdom, DSG (Dixons), the largest retailer of domestic electronic equipment, reported that CRT models made up 80–90% of the volume of televisions sold at Christmas 2004 and 15–20% a year later, and that they were expected to be less than 5% at the end of 2006. Dixons ceased selling CRT televisions in 2007 Dell Inspiron N5010 battery 5200mAh .
CRTs, despite recent advances, have remained relatively heavy and bulky and take up a lot of space in comparison to other display technologies. CRT screens have much deeper cabinets compared to flat panels and rear-projection displays for a given screen size, and so it becomes impractical to have CRTs larger than 40 inches (102 cm) Dell Inspiron N7010 battery 5200mAh .
The CRT disadvantages became especially significant in light of rapid technological advancements in LCD and plasma flat-panels which allow them to easily surpass 40 inches (102 cm) as well as being thin and wall-mountable, two key features that were increasingly being demanded by consumers Dell Inspiron 1464 battery 5200mAh .
By 2006, although the price points of CRTs are generally much lower than LCD and plasma flat panels, large screen CRTs (30-inches or more) are as expensive as a similar-sized LCD. While LCDs are generally the most expensive TV display technology, major innovations have caused prices to drop significantly Dell Inspiron 1564 battery 5200mAh .
Monochrome CRTs are even more efficient than color CRTs. This is because up to 2/3rds of the backlight power of LCD and rear-projection displays are lost to the RGB stripe filter. Most LCDs also have poorer color rendition and can change color with viewing angle, though modern PVA and IPS LCDs have greatly attenuated these problems Dell Inspiron 1764 battery 5200mAh .
Resurgence in specialized markets
In the first quarter of 2008, CRTs retook the #2 technology position in North America from plasma, due to the decline and consolidation of plasma display manufacturers Dell Inspiron 1320 battery 5200mAh .
DisplaySearch has reported that although in the 4Q of 2007 LCDs surpassed CRTs in worldwide sales, CRTs then outsold LCDs in the 1Q of 2008.
CRTs are useful for displaying photos with high pixels per unit area and correct color balance Dell Inspiron 1320n battery 5200mAh.
LCDs, as currently the most common flatscreen technology, have generally inferior color rendition (despite having greater overall brightness) due to the fluorescent lights commonly used as a backlight.
CRTs are still popular in the printing and broadcasting industries as well as in the professional video Dell Vostro 1710 battery 5200mAh ,
photography, and graphics fields due to their greater color fidelity, contrast, and better viewing from off-axis (wider viewing angle). CRTs also still find adherents in video gaming because of their higher resolution per initial cost, lowest possible input lag, fast response time, and multiple native resolutions Dell Vostro 1720 battery 5200mAh .
CRTs can emit a small amount of X-ray radiation as a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors. The amount of radiation escaping the front of the monitor is widely considered unharmful Dell Studio XPS 1640 battery 5200mAh .
The Food and Drug Administration regulations in 21 C.F.R. 1020.10 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour (mR/h) (0.13 µC/(kg·h) or 36 pA/kg) at a distance of 5 cm (2 in) from any external surface; since 2007, most CRTs have emissions that fall well below this limit Dell Vostro A840 battery 5200mAh .
Color and monochrome CRTs may contain toxic substances, such as cadmium, in the phosphors. The rear glass tube of modern CRTs may be made from leaded glass, which represent an environmental hazard if disposed of improperly Dell Vostro A860 battery 5200mAh .
By the time personal computers were produced, glass in the front panel (the viewable portion of the CRT) used barium rather than lead, though the rear of the CRT was still produced from leaded glass. Monochrome CRTs typically do not contain enough leaded glass to fail EPA tests Dell Studio 1535 battery 5200mAh/7800mAh .
In October 2001, the United States Environmental Protection Agency created rules stating that CRTs must be brought to special recycling facilities. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration Dell Studio 1735 battery 5200mAh/7800mAh .
Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment.
In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive Dell Vostro 1310 battery 5200mAh .
At low refresh rates (below 50 Hz), the periodic scanning of the display may produce an irritating flicker that some people perceive more easily than others, especially when viewed with peripheral vision. A high refresh rate (above 72 Hz) reduces the effect Dell Latitude E6400 battery,10400mAh/7800mAh .
Computer displays and televisions with CRTs driven by digital electronics often use refresh rates of 100 Hz or more to largely eliminate any perception of flicker.Non-computer CRTs or CRT for sonar or radar may have long persistence phosphor and are thus flicker free Dell Latitude E6400 Battery,10400mAh/7800mAh .
If the persistence is too long on a video display, moving images will be blurred.
CRTs used for television operate with horizontal scanning frequencies of 15,734 Hz (for NTSC systems) or 15,625 Hz (for PAL systems) Dell Latitude E6400 Battery,10400mAh/7800mAh .
These frequencies are at the upper range of human hearingand are inaudible to many people; some people will perceive a high-pitched tone near an operating television CRT. The sound is due to magnetostriction in the magnetic core of the flyback transformerDell Latitude E6500 battery,10400mAh/7800mAh .
A high vacuum exists within all cathode ray tubes, putting the envelope under relatively high stress. If the outer glass envelope is damaged, the glass will break and pieces will fly out at high speed Dell Latitude E6500 battery,10400mAh/7800mAh .
While modern Cathode Ray Tubes used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs removed from equipment must be handled carefully to avoid personal injury Dell Latitude E6500 battery,10400mAh/7800mAh .
Under some circumstances, the signal radiated from the electron guns, scanning circuitry, and associated wiring of a CRT can be captured and used to remotely reconstruct what is shown on the CRT, using a process called Van Eck phreaking Dell Latitude E6500 battery,10400mAh/7800mAh .
Special TEMPEST shielding can mitigate this effect. Such radiation of a potentially exploitable signal however occurs also with LCDs and with all electronics in general.
As electronic waste, CRTs are considered one of the hardest types to recycle HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .
CRTs have relatively high concentration of lead and phosphorus, both of which are necessary for the display. There are several companies in the United States that charge a small fee to collect CRTs, then subsidize their labor by selling the harvested copper, wire, and printed circuit boards HP Pavilion DV7 battery - 7200mAh .
Leaded CRT glass is sold to get remelted into other CRTs, or even broken down and used in road construction.
The Williams tube or the Williams-Kilburn tube (after inventors Freddie Williams and Tom Kilburn), developed in about 1946 or 1947, was a cathode ray tube used to electronically store binary data Ibm THINKPAD X200 Battery 8800mAh .
It was the first random-access digital storage device,and was used successfully in several early computers.
The Williams tube depends on an effect called secondary emission Ibm THINKPAD X200 Battery 8800mAh .
When a dot is drawn on a cathode ray tube, the area of the dot becomes slightly positively charged and the area immediately around it becomes slightly negatively charged, creating a charge well. The charge well remains on the surface of the tube for a fraction of a second, allowing the device to act as a computer memory Ibm THINKPAD X200 Battery 8800mAh .
The lifetime of the charge well depends on the electrical resistance of the inside of the tube.
The dot can be erased by drawing a second dot immediately next to the first one, thus filling the charge well SONY VAIO VGN-FW21E Battery .
Most systems did this by drawing a short dash starting at the dot position, so that the extension of the dash erased the charge initially stored at the starting point.
The computer reads information from the tube by means of a metal pickup-plate that covers the front of the tube Asus M52N Battery.
Each time a dot is created or erased, the change in electrical charge induces a voltage pulse in the pickup-plate. Since the computer knows which location on the screen is being targeted at that instant, it can use the voltage pulse from the plate to read the data stored on the screen Dell Latitude E6400 ATG Battery .
Reading a memory location creates a new charge well, destroying the original contents of that location, and so any read has to be followed by a write to reinstate the original data. Since the charge gradually leaked away, it was necessary to scan the tube periodically and rewrite every dot (similar to the memory refresh cycles of DRAMin modern systems) SONY VAIO VGN-FW21J Battery .
Some Williams tubes were made from radar-type cathode ray tubes with a phosphor coating that made the data visible to the eye, while other tubes were purpose-built without such a coating SONY VAIO VGN-FW21L Battery .
The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators since the face of the tube was covered by the pickup-plate. If a visible output was needed, a second tube with a phosphor coating was used as a display device SONY VAIO VGN-FW21M Battery .
Each Williams tube could store roughly 512 – 1024 bits of data.
Developed at the University of Manchester in England, it provided the medium on which the first electronically stored-memory program was stored in the Manchester Small-Scale Experimental Machine computer SONY VAIO VGN-FW21Z Battery .
Tom Kilburn wrote a 17-line program to calculate the highest factor of 218. Tradition at Manchester University has it that this was the only program Kilburn ever wrote.
The Williams tube tended to become unreliable with age, and most working installations had to be "tuned" by hand SONY VAIO VGN-FW25T/B Battery .
By contrast, mercury delay line memory was slower and also needed hand tuning, but it did not age as badly and enjoyed some success in early digital electronic computing despite its data rate, weight, cost, thermal and toxicity problems SONY VAIO VGN-FW26T/B Battery .
However, the Manchester Mark 1 was successfully commercialised as the Ferranti Mark 1. Some early computers in the USA also used the Williams tube, including the IAS machine, originally designed for Selectron tube memory, the UNIVAC 1103, IBM 701, IBM 702 and the Standards Western Automatic Computer(SWAC) SONY VAIO VGN-FW27/B Battery .
Williams tubes were also used in the Soviet computer, Strela-1.
Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically phototubes SONY VAIO VGN-FW27/W Battery ,
are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is very low SONY VAIO VGN-FW31ZJ Battery.
The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear andparticle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners SONY VAIO VGN-FW27T/H Battery .
Semiconductor devices, particularly avalanche photodiodes, are alternatives to photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated SONY VAIO VGN-FW29/B Battery .
While photomultipliers are extraordinarily sensitive and moderately efficient, research is still underway to create a photon-counting light detection device that is much more than 99% efficient. Such a detector is of interest for applications related to quantum information and quantum cryptography SONY VAIO VGN-FW290JRB Battery .
Elements of photomultiplier technology, when integrated differently, are the basis of night vision devices.
Combining two scientific discoveries
The invention of the photomultiplier is predicated upon two prior achievements, firstly discovering the photoelectric effect and secondly discovering secondary emission SONY VAIO VGN-FW290JTB Battery
(i.e., the ability of electrons in a vacuum tube to cause the emission of additional electrons by striking an electrode).
The first demonstration of the photoelectric effect was carried out in 1887 by Heinrich Hertz who demonstrated it using ultraviolet light SONY VAIO VGN-FW290JTH Battery .
Significant for practical applications, Elster and Geitel two years later demonstrated the same effect using visible light striking alkali metals (potassium and sodium). The addition of caesium, another alkali metal, has permitted the range of sensitive wavelengths to be extended towards longer wavelengths in the red portion of the visible spectrum SONY VAIO VGN-FW33G/E1 Battery .
Historically, the photoelectric effect is associated with Albert Einstein, who relied upon the phenomenon to establish the fundamental principle of quantum mechanics, in 1905,an accomplishment for which Einstein received the 1921 Nobel Prize SONY VAIO VGN-FW33G/W Battery .
It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized that the kinetic energy of the emitted electrons is proportional to the frequency but independent of the optical intensity. This fact implied a discrete nature of light, i.e. the existence of quanta, for the first time SONY VAIO VGN-FW33GW Battery .
The phenomenon of secondary emission was first limited to purely electronic inventions (i.e., those lacking photosensitivity). In 1902, Austin and Starke reported that the metal surfaces impacted by electron beams emitted a larger number of electrons than were incident Dell XPS M1210 Battery) http://www.hdd-shop.co.uk .
The application of the newly discovered secondary emission to the amplification of signals was only proposed after World War I by Westinghouse scientist Joseph Slepian in a 1919 patent.
The race towards a practical electronic television camera
The ingredients for inventing the photomultiplier were coming together during the 1920s as the pace of vacuum tube technologies accelerated Dell KM958 battery .
The primary goal for many, if not most, workers was the need for a practical television camera technology. Television had been pursued with primitive prototypes for decades prior to the 1934 introduction of the first practical camera (the iconoscope). Early prototype television cameras lacked sensitivity Dell Studio 1555 battery .
Photomultiplier technology was pursued to enable television camera tubes, such as the iconoscope and (later) the orthicon, to be sensitive enough to be practical. So the stage was set to combine the dual phenomena of photoemission (i.e., the photoelectric effect) with secondary emission Sony VGP-BPS13 battery ,
both of which had already been studied and adequately understood, to create a practical photomultiplier.
First photomultiplier, single-stage (early 1934)
The first documented photomultiplier demonstration dates to the early 1934 accomplishments of an RCA group based in HarrisonSony VGP-BPS13/B battery ,
NJ. Harley Iams and Bernard Salzberg were the first to integrate a photoelectric-effect cathode and single secondary emission amplification stage in a single vacuum envelope and the first to characterize its performance as a photomultiplier with electron amplification gain Sony VGP-BPS13/S battery .
These accomplishments were finalized prior to June 1934 as detailed in the manuscript submitted to Proceedings of the Institute of Radio Engineers (Proc. IRE).The device consisted of a semi-cylindrical photocathode, a secondary emitter mounted on the axis, and a collector grid surrounding the secondary emitter Sony VGP-BPS13A/B battery .
The tube had a gain of about eight and operated at frequencies well above 10 kHz.
Magnetic photomultipliers (mid 1934–1937)
Higher gains were sought than those available from the early single-stage photomultipliers. However, it is an empirical fact that the yield of secondary electrons is limited in any given secondary emission process, regardless of acceleration voltage Sony VGP-BPS13B/B battery .
Thus, any single-stage photomultiplier is limited in gain. At the time the maximum first-stage gain that could be achieved was approximately 10 (very significant developments in the 1960s permitted gains above 25 to be reached using negative electron affinity dynodes) Sony VGP-BPL9 battery .
For this reason, multiple-stage photomultipliers, in which the photoelectron yield could be multiplied successively in several stages, were an important goal. The challenge was to cause the photoelectrons to impinge on successively higher-voltage electrodes rather than to travel directly to the highest voltage electrode Sony VGP-BPS13B/B battery .
Initially this challenge was overcome by using strong magnetic fields to bend the electrons' trajectories. Such a scheme had earlier been conceived by inventor J. Slepian by 1919 (see above) Sony VGP-BPL15 battery .
Accordingly, leading international research organizations turned their attention towards improving photomultiplers to achieve higher gain with multiple stages. This work proceeded against a background of economic boom and bust, tyrannical dictatorship, and cataclysmic war clouds collecting on the horizon Dell Inspiron E1505 battery .
In the USSR, RCA-manufactured radio equipment was introduced on a large scale by Joseph Stalin to construct broadcast networks, and the newly formed All-Union Scientific Research Institute for Television was gearing up a research program in vacuum tubes that was advanced for its time and place HP Pavilion dv6000 Battery .
Numerous visits were made by RCA scientific personnel to the USSR in the 1930s, prior to theCold War, to instruct the Soviet customers on the capabilities of RCA equipment and to investigate customer needs. During one of these visits, in September 1934, RCA's Vladimir Zworykin was shown the first multiple-dynode photomultiplier, or photoelectron multiplier HP Pavilion dv8000 battery .
This pioneering device of 28-year-old Leonid A. Kubetsky achieved gains of 1000x or more when demonstrated in June 1934. The work was submitted for print publication only two years later, in July 1936 as emphasized in a recent 2006 publication of the Russian Academy of Sciences (RAS) HP PAVILION DV6000 Battery, 8800mAh .
which terms it "Kubetsky's Tube." The Soviet device used a magnetic field to confine the secondary electrons and relied on the Ag-O-Cs photocathode which had been demonstrated by General Electric in the 1920s.
By October 1935, Vladimir Zworykin, George Ashmun Morton, and Louis Malter of RCA in Camden HP PAVILION DV8000 Battery, 8800mAh ,
NJ submitted their manuscript describing the first comprehensive experimental and theoretical analysis of a multiple dynode tube — the device later called a photomultiplier — to Proc. IRE. The RCA prototype photomultipliers also used a Ag-O-Cs (silver oxide-caesium) photocathode. They exhibited a peak quantum efficiency of 0.4% at 800 nm SONY VAIO VGN-FZ Battery .
Electrostatic photomultipliers (1937–present)
Whereas these early photomultipliers used the magnetic field principle, electrostatic photomultipliers (with no magnetic field) were demonstrated by Jan Rajchman of RCA Laboratories in Princeton HP Pavilion DV6000 battery - 8800mAh ,
NJ in the late 1930s and became the standard for all future commercial photomultipliers. The first mass-produced photomultiplier, the Type 931, was of this design and is still commercially produced today.
Also in 1936, a much improved photocathode, Cs3Sb (caesium-antimony), was reported by P. Gorlich HP Pavilion DV8000 battery - 7800mAh .
The caesium-antimony photocathode had a dramatically improved quantum efficiency of 12% at 400 nm, and was used in the first commercially successful photomultipliers manufactured by RCA (i.e., the 931-type) both as a photocathode and as a secondary-emitting material for the dynodes HP Pavilion DV6000 battery - 8800mAh .
Different photocathodes provided differing spectral responses.
Spectral response of photocathodes
In the early 1940s the JEDEC (Joint Electron Devices Engineering Council), an industry committee on standardization, developed a system of designating spectral responses HP Pavilion DV8000 battery - 6600mAh .
The philosophy included the idea that the product's user need only be concerned about the response of the device rather than how the device may be fabricated. Various combinations of photocathode and window materials were assigned "S-numbers" (spectral numbers) ranging from S-1 through S-40, which are still in use today SONY VAIO VGN-FZ18 Battery .
For example, S-11 uses the caesium-antimony photocathode with a lime glass window, S-13 uses the same photocathode with a fused silica window, and S-25 uses a so-called "multialkali" photocathode (Na-K-Sb-Cs, or sodium-potassium-antimony-caesium) that provides extended response in the red portion of the visible light spectrum SONY VAIO VGN-FZ21E Battery .
No suitable photoemissive surfaces have yet been reported to detect wavelengths longer than approximately 1700 nanometers, which can be approached by a special (InP/InGaAs(Cs)) photocathode.
Role of RCA
For decades, RCA was responsible for performing the most important work in developing and refining photomultipliers SONY VAIO VGN-FW31J Battery 10400mAh .
RCA was also largely responsible for the commercialization of photomultiplers. The company compiled and published an authoritative and very-widely used Photomultiplier Handbook. RCA made printed copies available for free upon request SONY VAIO VGN-FW31J Battery 10400mAh .
The handbook, which continues to be made available online at no cost by the successors to RCA, is considered to be an essential reference.
Following a corporate break-up in the late 1980s involving the acquisition of RCA by General Electric and disposition of the divisions of RCA to numerous third-parties, RCA's photomultiplier business became an independent company SONY VAIO VGN-FW21E Battery .
Lancaster, Pennsylvania facility
The Lancaster, Pennsylvania facility was opened by the U.S. Navy in 1942 and operated by RCA for the manufacture of radio and microwave tubes. Following the Allied victory in World War II, the naval facility was acquired by RCA SONY VAIO VGN-NR11S/S Battery .
RCA Lancaster, as it became known, was the base for development and production of commercial television products. In subsequent years other products were added, such as cathode ray tubes, photomultiplier tubes, motion-sensing light control switches, and closed-circuit television systems SONY VAIO VGN-NR11M/S Battery .
Burle Industries, as a successor to the RCA Corporation, carried the RCA photomultiplier business forward after 1986, based in the Lancaster, Pennsylvania facility SONY VAIO VGN-NR11Z/S Battery .
The 1986 acquisition of RCA byGeneral Electric resulted in the divestiture of the RCA Lancaster New Products Division. Hence, 45 years after being founded by the U.S. Navy, its management team, led by Erich Burlefinger, purchased the division and in 1987 founded Burle Industries SONY VAIO VGN-NR11Z/T Battery .
In 2005, after eighteen years as an independent enterprise, Burle Industries and a key subsidiary were acquired by Photonis, a European holding company Photonis Group. Following the acquisition, Photonis was composed of Photonis Netherlands, Photonis France, Photonis USA, and Burle Industries Sony VAIO VGN-FZ21E Battery .
Photonis USA operates the former Galileo Corporation Scientific Detector Products Group (Sturbridge, Massachusetts), which had been purchased by Burle Industries in 1999. The Group is known for microchannel plate detector (MCP) electron multipliers—an integrated micro-vacuum tube version of photomultipliers Sony VAIO VGN-FW21E Battery .
MCPs are used for imaging and scientific applications, including night vision devices.
On 9 March 2009 Photonis announced that it would cease all production of photomultipliers at both the Lancaster, Pennsylvania and the Brive, France plants Sony VAIO VGN-NR11S/S Battery .
The Japan-based company Hamamatsu Photonics (also known as Hamamatsu) has emerged since the 1950s as a leader in the photomultiplier industry Sony VAIO VGN-NR11Z/S Battery .
Hamamatsu, in the tradition of RCA, has published its own handbook, which is available without cost on the company's website. Hamamatsu uses different designations for particular photocathode formulations and introduces modifications to these designations based on Hamamatsu's proprietary research and development Sony VAIO VGN-NR11M/S Battery .
Structure and operating principles
Photomultipliers are constructed from a glass envelope with a high vacuum inside, which houses a photocathode, severaldynodes, and an anode Sony VAIO VGN-NR11Z/T Battery .
Incident photons strike the photocathode material, which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission SONY VAIO VGN-FZ180E Battery .
The electron multiplier consists of a number of electrodes called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon (minus the work function of the photocathode) SONY VAIO VGN-FZ220E Battery .
As the electrons move toward the first dynode, they are accelerated by the electric field and arrive with much greater energy. Upon striking the first dynode, more low energy electrons are emitted, and these electrons in turn are accelerated toward the second dynode SONY VAIO VGN-FZ340E Battery .
The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally, the electrons reach the anode, where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode SONY VAIO VGN-FZ430E Battery .
There are two common photomultiplier orientations, the head-on or end-on (transmission mode) design, as shown above, where light enters the flat, circular top of the tube and passes the photocathode, and the side-on design (reflection mode), where light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode SONY VAIO VGN-FZ460E Battery .
Besides the different photocathode materials, performance is also affected by the transmission of the window material that the light passes through, and by the arrangement of the dynodes. A large number of photomultiplier models are available having various combinations of these, and other, design variables SONY VAIO VGN-FZ480E Battery .
Either of the manuals mentioned will provide the information needed to choose an appropriate design for a particular application.
The photocathodes can be made of a variety of materials, with different properties SONY VAIO VGN-FZ4000 Battery .
Typically the materials have low work function and are therefore prone to thermionic emission, causing noise and dark current, especially the materials sensitive in infrared; cooling the photocathode lowers this thermal noise. The most common photocathode materials are SONY VAIO VGN-FZ31E Battery :
- Ag-O-Cs: also called S1. Transission-mode, sensitive from 300–1200 nm. High dark current; used mainly in near-infrared, with the photocathode cooled.
- GaAs:Cs: caesium-activated gallium arsenide SONY VAIO VGN-FZ31B Battery .
- Flat response from 300 to 850 nm, fading towards ultraviolet and to 930 nm.
- InGaAs:Cs: caesium-activated indium gallium arsenide. Higher infrared sensitivity than GaAs:Cs. Between 900–1000 nm much higher signal-to-noise ratio than Ag-O-Cs SONY VAIO VGN-FZ31J Battery .
- Sb-Cs: caesium-activated antimony. Used for reflective mode photocathodes. Response range from ultraviolet to visible. Widely used.
- Bialkali (Sb-K-Cs, Sb-Rb-Cs): caesium-activated antimony-rubidium or antimony-potassium alloy SONY VAIO VGN-FZ31M Battery .
- Similar to Sb:Cs, with higher sensitivity and lower noise. Can be used for transmission-mode; favorable response to a NaI:Tl scintillator flashes makes them widely used in gamma spectroscopy and radiation detection SONY VAIO VGN-FZ31Z Battery .
- High-temperature bialkali (Na-K-Sb): can operate up to 175 °C, used in well logging. Low dark current at room temperature.
- Multialkali (Na-K-Sb-Cs): wide spectral response from ultraviolet to near-infrared; special cathode processing can extend range to 930 nm. Used in broadband spectrophotometers SONY VAIO VGN-FZ38M Battery .
- Solar-blind (Cs-Te, Cs-I): sensitive to vacuum-UV and ultraviolet. Insensitive to visible light and infrared (CsTe has cutoff at 320 nm, CsI at 200 nm) SONY VGP-BPS8 Battery .
The windows of the photomultipliers act as wavelength filters; this may be irrelevant if the cutoff wavelengths are outside of the application range or outside of the photocathode sensitivity range, but special care has to be taken for uncommon wavelengths SONY VGP-BPS13/S Battery .
- Borosilicate glass is commonly used for near-infrared to about 300 nm. Glass with very low content of potassium can be used with bialkali photocathodes to lower the background radiation from thepotassium-40 isotope.
- Ultraviolet glass transmits visible and ultraviolet down to 185 nm. Used in spectroscopy SONY VGP-BPS13A/B Battery .
- Synthetic silica transmits down to 160 nm, absorbs less UV than fused silica. Different thermal expansion than kovar (and than borosilicate glass that's expansion-matched to kovar), a graded seal needed between the window and the rest of the tube. The seal is vulnerable to mechanical shocks SONY VGP-BPS13B/B Battery .
- Magnesium fluoride transmits ultraviolet down to 115 nm. Hygroscopic, though less than other alkali halides usable for UV windows.
Photomultiplier tubes typically utilize 1000 to 2000 volts to accelerate electrons within the chain of dynodes SONY VGP-BPS13A/S Battery .
The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. Negative high-voltage supplies (with the positive terminal grounded) are preferred, because this configuration enables the photocurrent to be measured at the low voltage side of the circuit for amplification by subsequent electronic circuits operating at low voltage Dell Inspiron 1320n Battery .
Voltages are distributed to the dynodes by a resistive voltage divider, although variations such as active designs (with transistors or diodes) are possible. The divider design, which influences frequency response or rise time, can be selected to suit varying applications Dell Studio 1450 Battery .
Some instruments that use photomultipliers have provisions to vary the anode voltage to control the gain of the system.
While powered (energized), photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation Dell Studio 1457 Battery .
If used in a location with strong magnetic fields, which can curve electron paths, steer the electrons away from the dynodes and cause loss of gain, photomultipliers are usually shielded by a layer of mu-metal. This magnetic shield is often maintained at cathode potential Dell Latitude D610 Battery .
When this is the case, the external shield must also be electrically insulated because of the high voltage on it. Photomultipliers with large distances between the photocathode and the first dynode are especially sensitive to magnetic fields Toshiba NB100 Battery .
- Photomultipliers were the first electric eye devices, being used to measure interruptions in beams of light.
- Photomultipliers are used in conjunction with scintillators to detect nuclear and particle radiation in physics experimentsToshiba Satellite M65 battery .
High sensitivity applications
After fifty years, during which solid-state electronic components have largely displaced the vacuum tube, the photomultiplier remains a unique and important optoelectronic component Dell Latitude D620 Battery .
xtracting the tiny currents associated with weak light signals. There is no Johnson noise associated with photomultiplier signal currents even though they are greatly amplified, e.g., by 100 thousand times (i.e., 100 dB) or more. The photocurrent still contains shot noise Dell Inspiron Mini 10 Battery .
Photomultiplier-amplified photocurrents can be electronically amplified by a high-input-impedance electronic amplifier (in the signal path, subsequent to the photomultiplier), thus producing appreciable voltages even for nearly infinitesimally small photon fluxes Sony VGN-FW11S Battery .
Photomultipliers offer the best possible opportunity to exceed the Johnson noise for many configurations. The aforementioned refers to measurement of light fluxes that, while small, nonetheless amount to a continuous stream of multiple photons Sony VGN-FW11M Battery .
For smaller photon fluxes, the photomultiplier can be operated in photon counting or Geiger mode (see also: single-photon avalanche diode). In Geiger mode the photomultiplier gain is set so high (using high voltage) that a single photo-electron resulting from a single photon incident on the primary surface generates a very large current at the output circuit Sony VGN-FW139E/H battery .
However, owing to the avalanche of current, a reset of the photomultiplier is required. In either case, the photomultiplier can detect individual photons. The drawback, however, is that not every photon incident on the primary surface is counted either because of less-than-perfect efficiency of the photomultiplier Dell Latitude E5400 Battery ,
or because a second photon can arrive at the photomultiplier during the "dead time" associated with a first photon and never be noticed.
A photomultiplier will produce a small current even without incident photons; this is called the dark current. Photon counting applications generally demand photomultipliers designed for low dark current Dell Latitude E4200 Battery .
Nonetheless, the ability to detect single photons striking the primary photosensitive surface itself reveals the quantization principle that Einstein put forth. Photon-counting (as it is called) reveals that light, not only being a wave, consists of discrete particles (i.e., photons) Dell Inspiron 300M Battery .
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic (usually containing anthracene), or organic liquid (seeliquid scintillation counting) that fluoresces when struck by ionizing radiation Dell Studio 1737 battery .
A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an electronic amplifierand other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier Dell Inspiron E1505 battery .
The scintillation counter was invented in 1944 by Sir Samuel Curran whilst he was working on the Manhattan Project at the University of California at Berkeley, and it is based on the earlier work ofAntoine Henri Becquerel, who is generally credited with discovering radioactivity Dell RM791 battery ,
whilst working on the phosphorescence of certain uranium salts (in 1896). Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency. The quantum efficiency of a gamma-ray detector (per unit volume) depends upon the density of electrons in the detector Toshiba Satellite P10 Battery ,
and certain scintillating materials, such as sodium iodide and bismuth germanate, achieve high electron densities as a result of the high atomic numbers of some of the elements of which they are composed. However, detectors based on semiconductors, notably hyperpure germanium Acer Aspire One battery 10400mAh ,
have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-rayspectrometry. In the case of neutron detectors, high efficiency is gained through the use of scintillating materials rich in hydrogen that scatter neutrons efficiently SONY VGN-FZ210CE Battery .
Liquid scintillation counters are an efficient and practical means of quantifying beta radiation.
Scintillation counter apparatus
When a charged particle strikes the scintillator, a flash of light is produced, which may or may not be in the visible region of the spectrum Dell Precision M70 Battery .
Each charged particle produces a flash. If a flash is produced in a visible region, it can be observed through a microscope and counted - an impractical method. The association of a scintillator and photomultiplier with the counter circuits forms the basis of the scintillation counter apparatus Toshiba Satellite L305 Battery .
When a charged particle passes through the phosphor, some of the phosphor's atoms get excited and emit photons. The intensity of the light flash depends on the energy of the charged particles. Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles Toshiba Satellite T4900 Battery ;
sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves.
The scintillation counter has a layer of phosphor cemented in one of the ends of the photomultiplier. Its' inner surface is coated with a photo-emitter with less work potential Toshiba PA3399U-2BRS battery .
This photoelectric emitter is called as photocathode and is connected to the negative terminal of a high tension battery. A number of electrodes called dynodes are arranged in the tube at increasing positive potential. When a charged particle strikes the phosphor, a photon is emitted Sony VGP-BPS10 battery 10400mAh .
This photon strikes the photocathode in the photomultipier, releasing an electron. This electron accelerates towards the first dynode and hits it. Multiple secondary electrons are emitted, which accelerate towards the second dynode. More electrons are emitted and the chain continues, multiplying the effect of the first charged particle Sony VGP-BPS9 battery 10400mAh .
By the time the electrons reach the last dynode, enough have been released to send a voltage pulse across the external resistors. This voltage pulse is amplified and recorded by the electronic counter.
Applications for scintillation counters
Scintillation counters can be used in a variety of applications Sony VGP-BPS11 battery 10400mAh .
- Medical imaging
- National and homeland security
- Border security
- Nuclear safety used for measurement of radiation Sony VGP-BPS22 battery 10400mAh
Several products have been introduced in the market utilising scintillation counters for detection of potentially dangerous gamma-emitting materials during transport. These include scintillation counters designed for freight terminals, border security, ports, weigh bridge applications, scrap metal yards and contamination monitoring of nuclear waste Sony VGP-BPL8 battery 10400mAh .
There are variants of scintillation counters mounted on pick-up trucks and helicopters for rapid response in case of a security situation due to dirty bombs or radioactive waste , Hand-held units are also commonly used Sony VGP-BPS14 battery 10400mAh.
Scintillation counter as a spectrometer
Scintillators often convert a single photon of high energy radiation into high number of lower-energy photons, where the number of photons per megaelectronvolt of input energy is fairly constant Sony VGP-BPS21 battery 10400mAh .
By measuring the intensity of the flash (the number of the photons produced by the x-ray or gamma photon) it is therefore possible to discern the original photon's energy.
The spectrometer consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier Toshiba Satellite A200 Battery .
The pulses are counted and sorted by their height, producing a x-y plot of scintillator flash brightness vs number of the flashes, which approximates the energy spectrum of the incident radiation, with some additional artifacts. A monochromatic gamma radiation produces a photopeak at its energy Toshiba Satellite 1200 Battery .
The detector also shows response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak Toshiba Satellite M300 Battery .
Higher energies can be measured when two or more photons strike the detector almost simultaneously (pile-up, within the time resolution of the data acquisition chain), appearing as sum peaks with energies up to the value of two or more photopeaks added. See.it is used for measurment of radiation SONY VGP-BPS13A/Q Battery .
A micro-channel plate (MCP) is a planar component used for detection of particles (electrons or ions) and impinging radiation (ultravioletradiation and X-rays). It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electronsvia secondary emission SONY VGP-BPS13A/Q Battery .
However, because a microchannel plate detector has many separate channels, it can additionally provide spatial resolution.
A micro-channel plate is a slab made from highly resistive material of typically 2 mm thickness with a regular array of tiny tubes or slots (microchannels) leading from one face to the opposite, densely distributed over the whole surface SONY VGP-BPS13B/Q Battery.
The microchannels are typically approximately 10 micrometers in diameter (6 micrometer in high resolution MCPs) and spaced apart by approximately 15 micrometers; they are parallel to each other and often enter the plate at a small angle to the surface (~8° from normal) SONY VGP-BPS13B/Q Battery .
Each microchannel is a continuous-dynode electron multiplier, in which the multiplication takes place under the presence of a strong electric field. A particle or photon that enters one of the channels through a small orifice is guaranteed to hit the wall of the channel due to the channel being at an angle to the plate and thus the angle of impact SONY VGP-BPS13/Q Battery .
The impact starts a cascade of electrons that propagates through the channel, which amplifies the original signal by several orders of magnitude depending on the electric field strength and the geometry of the micro-channel plate. After the cascade, the microchannel takes time to recover (or recharge) before it can detect another signal SONY VGP-BPS13/Q Battery .
The electrons exit the channels on the opposite side where they are themselves detected by additional means, often simply a single metal anode measuring total current. In some applications each channel is monitored independently to produce an image. Phosphors in combination with photomultiplier tubes have also been used SONY VGP-BPS21A/B Battery .
Most modern MCP detectors consist of two microchannel plates with angled channels rotated 90° from each other producing a chevron (v-like) shape. The angle between the channels reduces ion feedback in the device SONY VGP-BPS21/S Battery .
In a chevron MCP the electrons that exit the first plate start the cascade in the next plate. The advantage of the chevron MCP over the straight channel MCP is significantly more gain at a given voltage. The two MCPs can either be pressed together or have a small gap between them to spread the charge across multiple channels SONY VGP-BPS21B Battery .
Z stack MCP
This is an assembly of three microchannel plates with channels aligned in a Z shape. Single MCPs can have gain up to 10000 but this system can provide gain more than 10 million SONY VGP-BPS21A Battery .
An external voltage divider is used to apply 100 volts to the acceleration optics (for electron detection), each MCP, the gap between the MCPs, and the backside of the last MCP and the collector (anode). The last voltage dictates the time of flight of the electrons and in this way the pulse-width SONY VGP-BPS21 Battery .
The anode is a 0.4 mm thick plate with an edge of 0.2 mm radius to avoid high field strengths. It is just large enough to cover the active area of the MCP, because the backside of the last MCP and the anode act as a capacitor with 2 mm separation and large capacitance slows down the signal Sony VGP-BPS21A/B Battery .
The positive charge in the MCPinfluences positive charge in the backside metalization. A hollow torus conducts this around the edge of the anode plate. A torus is the optimum compromise between low capacitance and short path and for similar reasons usually no dielectric (Markor) is placed into this region Sony VGP-BPS21/S Battery .
After a 90° turn of the torus it is possible to attach a large coaxial waveguide. A taper permits minimizing the radius so that an SMA connector can be used. To save space and make the impedance match less critical, the taper is often reduced to a small 45° cone on the backside of the anode plate Sony VGP-BPS21A Battery .
The typical 500 volts between the backside of the last MCP and the anode cannot be fed into the preamplifier. Therefore the inner or the outer conductor needs a DC-block, that is, a capacitor. Often it is chosen to only have 10-fold capacitance compared to the MCP-anode capacitance and is implemented as a plate capacitor Sony VGP-BPS21 Battery .
Rounded, electro-polished metal plates and the ultra high vacuum allow very high field strengths and high capacitance without a dielectric. The bias for the center conductor is applied via resistors hanging through the waveguide (see bias tee). If the DC block is used in the outer conductor, it is in parallel with the larger capacitor in the power supply Sony Vaio PCG-5G2L Battery .
Assuming good screening, the only noise is due to current noise from the linear power regulator. Because the current is low in this application and space for large capacitors is available, and because the DC-block capacitor is fast, it is possible to have very low voltage noise, so that even weak MCP signals can be detected Sony Vaio PCG-5G3L Battery .
Sometimes the preamplifier is on a potential (off ground) and gets its power through a low-power isolationtransformer and outputs its signal optically.
The gain of a MCP is very noisy, especially for single particles Sony Vaio PCG-5J1L Battery .
With two thick MCPs (>1 mm) and small channels (< 10 µm), saturation occurs, especially at the ends of the channels after many electron multiplications have taken place. The last stages of the following semiconductor amplifier chain also go into saturation Sony Vaio PCG-5K2L Battery .
A pulse of varying length, but stable height and a low jitter leading edge is sent to the time to digital converter. The jitter can be further reduced by means of a constant fraction discriminator. That means that MCP and the preamplifier are used in the linear region Sony Vaio PCG-5J2L Battery
(space charge negligible) and the pulse shape is assumed to be due to an impulse response with variable height but fixed shape from a single particle.
Because MCPs have a fixed charge that they can amplify in their life, especially the second MCP has a lifetime problem Sony Vaio PCG-5K1L Battery .
It is important to use thin MCPs, low voltage and instead more sensitive and fast semiconductor amplifiers after the anode.
With high count rates or slow detectors (MCPs with phosphor screen or discrete photomultipliers) pulses overlap. In this case a high impedance (slow, but less noisy) amplifier and an ADC is used Sony Vaio PCG-6S2L Battery .
Since the output signal from the MCP is generally small, the presence of the thermal noise limits the measurement of the time structure of MCP signal. However with the fast amplification schemes, is possible to have valuable information on the signal amplitude, even at very low signal values Sony Vaio PCG-6S3L Battery .
But yet, not successful on the time structure information of the wideband signals.
Delay line detector
In a delay line detector the electrons are accelerated to 500 eV between the back of the last MCP and a grid. Then they fly for 5 mm and are dispersed over an area of 2 mm. A grid follows Sony Vaio PCG-6V1L Battery .
Each element has a diameter of 1 mm and consists of an electrostatic lens focusing arriving electrons through a 30 µm hole of a grounded sheet of aluminium. Behind that a cylinder of the same size follows. The electron cloud induces a 300 ps negative pulse when entering the cylinder and a positive when leaving Sony Vaio PCG-6W1L Battery .
After that another sheet, a second cylinder follows, and a last sheet follows. Effectively the cylinders are fused into the center-conductor of a stripline. The sheets minimize cross talk between the layers and adjacent lines in the same layer, which would lead to signal dispersionand ringing Sony Vaio PCG-6W2L Battery .
These striplines meander across the anode to connect all cylinders, to offer each cylinder 50 ohm impedance, and to generate a position dependent delay. Because the turns in the stripline adversely affect the signal quality their number is limited and for higher resolutions multiple independent striplines are needed Sony Vaio PCG-6W3L Battery .
At both ends the meanders are connected to detector electronics. These electronics convert the measured delays into X- (first layer) and Y-coordinates (second layer). Sometimes a hexagonal grid and 3 coordinates are used. This redundancy reduces the dead space-time by reducing the maximum travel distance and thus the maximum delay, allowing for faster measurements Sony Vaio PCG-7111L Battery .
Examples of use
- A 1-GHz real-time display CRT for an analog oscilloscope (the Tektronix 7104) used a microchannel plate placed behind the phosphor screen to intensify the image. Without the plate, the image would be excessively dim, because of the electron-optical design Sony Vaio PCG-7112L Battery .
- MCP detectors are often employed in instrumentation for physical research, and they can be found in devices such as electron and mass spectrometers Sony Vaio PCG-7113L Battery .
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode Sony Vaio PCG-7133L Battery .
Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction Sony Vaio PCG-7Z2L Battery .
Principle of operation
A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron and a (positively charged electron hole). This mechanism is also known as the photoelectric effect Sony Vaio PCG-8Y1L Battery .
If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced Sony Vaio PCG-8Y2L Battery .
This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimised to enhance the sensitivity of the device.
When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up Sony Vaio PCG-8Z1L Battery .
This mode exploits the photovoltaic effect, which is the basis for solar cells– in fact, a traditional solar cell is just a large area photodiode.
In this mode the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise Sony Vaio PCG-8Z2L Battery .
This increases the width of the depletion layer, which decreases the junction'scapacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same SONY VAIO VGN-FW31J Battery .
For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).
Although this mode is faster, the photoconductive mode tends to exhibit more electronic noise SONY VAIO VGN-FW31E Battery .
The leakage current of a good PIN diode is so low (< 1nA) that the Johnson–Nyquist noiseof the load resistance in a typical circuit often dominates.
Other modes of operation
Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse biasSONY VAIO VGN-FW31B Battery .
This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.
Phototransistors also consist of a photodiode with internal gain SONY VAIO VGN-FW31Z Battery .
A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach thebase-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain ? (or hfe) SONY VAIO VGN-FW31M Battery .
Note that while phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also have significantly longer response times SONY VAIO VGN-FW38M Battery .
The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents SONY VAIO VGN-AW41JF Battery .
Materials commonly used to produce photodiodes include:
Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm SONY VAIO VGN-AW41JF/H Battery .
Since transistors and ICs are made of semiconductors, and contain P-N junctions, almost every active component is potentially a photodiode SONY VAIO VGN-AW41MF Battery .
Many components, especially those sensitive to small currents, will not work correctly if illuminated, due to the induced photocurrents. In most components this is not desired, so they are placed in an opaque housing. Since housings are not completely opaque to X-rays or other high energy radiation SONY VAIO VGN-AW41MF/H Battery ,
these can still cause many ICs to malfunction due to induced photo-currents.
Critical performance parameters of a photodiode include SONY VAIO VGN-FW30B Battery :
- The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as a Quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity SONY VAIO VGN-FW50B Battery .
- Dark current
- The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction SONY VAIO VGN-FW51B/W Battery .
- Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system SONY VAIO VGN-FW51MF Battery .
- Noise-equivalent power
- (NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. The related characteristicdetectivity (D) is the inverse of NEP, 1/NEP; and the specific detectivity () is the detectivity normalized to the area (A) of the photodetector, SONY VAIO VGN-FW51MF/H Battery .
- The NEP is roughly the minimum detectable input power of a photodiode.
When a photodiode is used in an optical communication system, these parameters contribute to the sensitivity of the optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error ratio SONY VAIO VGN-FW51ZF Battery .
P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes SONY VAIO VGN-FW51ZF/H Battery .
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions SONY VAIO VGN-FW52JB Battery .
In other consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used SONY VAIO VGN-FW54FB Battery .
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators) or instruments to analyze samples (immunoassay) SONY VAIO VGN-FW70DB Battery .
They are also used in pulse oximeters.
PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for optical communications and in lighting regulation SONY VAIO VGN-FW71DB/W Battery .
P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding SONY VAIO VGN-FW72JGB Battery .
Comparison with photomultipliers
Advantages compared to photomultipliers:
- Excellent linearity of output current as a function of incident light
- Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
- Low noise SONY VAIO VGN-FW73JGB Battery
- Ruggedized to mechanical stress
- Low cost
- Compact and light weight
- Long lifetime SONY VAIO VGN-FW74FB Battery
- High quantum efficiency, typically 80%
- No high voltage required
Disadvantages compared to photomultipliers:
- Small area SONY VAIO VGN-FW81HS Battery
- No internal gain (except avalanche photodiodes, but their gain is typically 102–103 compared to up to 108 for the photomultiplier)
- Much lower overall sensitivity SONY VAIO VGN-FW81NS Battery
- Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
- Response time for many designs is slower
A one-dimensional array of hundreds or thousands of photodiodes can be used as a position sensor, for example as part of an angle sensor SONY VAIO VGN-FW81S Battery .
One advantage of photodiode arrays (PDAs) is that they allow for high speed parallel read out since the driving electronics may not be built in like a traditional CMOS or CCD sensor.
A traveling-wave tube (TWT) is an electronic device used to amplify radio frequency signals to high power, usually in an electronic assembly known as a traveling-wave tube amplifier (TWTA) SONY VAIO VGN-FW82DS Battery .
The bandwidth of a broadband TWT can be as high as one octave, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 70 decibels SONY VAIO VGN-FW82JS Battery .
The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end SONY VAIO VGN-FW82XS Battery .
A directional coupler, which can be either awaveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix.
The helix acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam SONY VAIO VGN-FW83DS Battery .
The electromagnetic field due to the RF signal in the helix interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the helix (i.e. the current builds up and thus is amplified as it passes down) SONY VAIO VGN-FW83JS Battery .
A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the helix. An attenuator placed on the helix, usually between the input and output helices, prevents reflected wave from traveling back to the cathode SONY VAIO VGN-FW83XS Battery .
Higher powered TWT’s usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT because of its special electrical, mechanical, and thermal properties SONY VAIO VGN-FW90NS Battery .
Invention, development and early use
The invention of the TWT is widely attributed to Rudolf Kompfner in 1942–1943, though Nils Lindenblad did patent a device in May 1940 which was remarkably similar to Kompfner's TWT :2 Kompfner invented the TWT in a British Admiralty radar lab during World War II SONY VAIO VGN-FW37J/B Battery .
His first sketch of a TWT is dated November 12, 1942, and he built the first TWT in early 1943. The TWT was refined by Kompfner and John Pierce at Bell Labs.
By the sixties TWTs were produced by such companies as the English Electric Valve Company, followed by Ferranti in the seventies SONY VAIO VGN-FW37J Battery .
On July 10, 1962, the first communications satellite, Telstar 1, was launched with a 2 W, 4 GHz RCA-designed TWT transponder used for transmitting RF signals back to the earth. Syncom 2, the first synchronous satellite (Syncom 1 did not reach its final orbit), launched on July 26, 1963 with two 2 W, 1850 MHz Hughes-designed TWT transponders (one active and one spare) SONY VAIO VGN-FW378J/B Battery .
Helix TWTs are limited in peak RF power by the current handling (and therefore thickness) of the helix wire. As power level increases, the wire can overheat and cause the helix geometry to warp. Wire thickness can be increased to improve matters, but if the wire is too thick it becomes impossible to obtain the required helix pitch for proper operation SONY VAIO VGN-FW378DH Battery .
Typically helix TWTs achieve less than 2.5 kW output power.
The coupled-cavity TWT overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. Conceptually, this structure provides a helical waveguide and hence amplification can occur via velocity modulation SONY VAIO VGN-FW378J/B Battery .
Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). A coupled-cavity TWT can achieve 60 kW output power SONY VAIO VGN-FW37J Battery .
Operation is similar to that of a klystron, except that coupled-cavity TWTs are designed with attenuation between the slow-wave structure instead of a drift tube. The slow-wave structure gives the TWT its wide bandwidth. A free electron laser allows higher frequencies SONY VAIO VGN-FW41E/H Battery .
Traveling-wave tube amplifier
A TWT integrated with a regulated power supply and protection circuits is referred to as a traveling-wave tube amplifier (abbreviated TWTA and often pronounced "TWEET-uh"). It is used to produce high-power radio frequency signals SONY VAIO VGN-FW41J/H Battery .
The bandwidth of a broadband TWTA can be as high as one octave, although tuned (narrowband) versions exist; operating frequencies range from 300 MHz to 50 GHz.
A TWTA consists of a traveling-wave tube coupled with its protection circuits (as in klystron) and regulated power supply Electronic Power Conditioner (EPC), which may be supplied and integrated by a different manufacturer SONY VAIO VGN-FW41M/H Battery .
The main difference between most power supplies and those for vacuum tubes is that efficient vacuum tubes have depressed collectors to recycle kinetic energy of the electrons and therefore the secondary winding of the power supply needs up to 6 taps of which the helix voltage needs precise regulation SONY VAIO VGN-FWZJ/H Battery .
The subsequent addition of a linearizer (as for inductive output tube) can, by complementary compensation, improve the gain compression and other characteristics of the TWTA; this combination is called a linearized TWTA (LTWTA, "EL-tweet-uh").
Broadband TWTAs generally use a helix TWT, and achieve less than 2.5 kW output power SONY VAIO VGN-FW45GJB Battery .
TWTAs using a coupled cavity TWT can achieve 15 kW output power, but at the expense of bandwidth.
TWTAs are commonly used as amplifiers in satellite transponders, where the input signal is very weak and the output needs to be high power SONY VAIO VGN-FWZJ/H Battery .
A TWTA whose output drives an antenna is a type of transmitter. TWTA transmitters are used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems. In these types of applications, a control grid is typically introduced between the TWT's electron gun and slow-wave structure to allow pulsed operation SONY VAIO VGN-FW45GJB Battery .
The circuit that drives the control grid is usually referred to as a grid modulator.
Another major use of TWTAs is for the electromagnetic compatibility (EMC) testing industry for immunity testing of electronic devices SONY VAIO VGN-FW45TJB Battery .