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 Lenovo N200 Battery 5200mAh .
Through the phenomenon of secondary emissionthis plate multiplies the number of electrons reaching the phosphor screen, giving a significant improvement in writing rate (brightness), and improved sensitivity and spot size as well Lenovo T410 Battery 5200mAh .
Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements. 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 Lenovo T510 Battery 5200mAh .
An internal graticule provides an advantage in that it eliminates parallax error. 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 Lenovo T400S Battery 5200mAh .
Convergence in color CRTs
The three beams in color CRTs would not strike the screen at the same point without convergence calibration. Instead, the set would need to be manually adjusted to converge the three color beams together to maintain color accuracy Lenovo Y430 Battery 5200mAh ; .
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. This degaussing field is strong enough to remove most cases of shadow mask magnetization Ibm ThinkPad G40 Battery 5200mAh .
Vector monitors were used in early computer aided design systems and in some late-1970s to mid-1980s arcade games such as Asteroids. They draw graphics point-to-point, rather than scanning a raster Ibm ThinkPad X200T Battery 5200mAh .
Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs. 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 Apple A1280 battery 5200mAh .
Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture grille CRTs require one or two crosswise (horizontal) support strips Apple A1281 battery 5200mAh .
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) Samsung R522 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 P467 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 Hp Pavilion dm3 Battery 5400mAh .
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 dm4 Battery 5200mAh
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 cq62 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 cq42 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 cq72 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 cq20 Battery 2600mAh .
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 2230 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 Dell Inspiron Mini 12 battery 7800mAh .
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 XPS M1730 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 M2010 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 Studio XPS 1340 battery 5200mAh/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 Inspiron 9100 battery 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 N4010 battery 5200mAh .
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 N3610 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 N5010 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 N7010 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 1464 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 1564 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 1764 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 1320 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. 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 Dell Inspiron 1320n battery 5200mAh-.
CRTs are useful for displaying photos with high pixels per unit area and correct color balance. 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 Dell Vostro 1710 battery 5200mAh .
CRTs are still popular in the printing and broadcasting industries as well as in the professional video, photography, and graphics fields due to their greater color fidelity, contrast, and better viewing from off-axis (wider viewing angle) Dell Vostro 1720 battery 5200mAh.
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.
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 Dell Studio XPS 1640 battery 5200mAh.
The amount of radiation escaping the front of the monitor is widely considered unharmful. 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 Dell Vostro A840 battery 5200mAh ;
since 2007, most CRTs have emissions that fall well below this limit.
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.
Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two metal electrodes to which a voltage is applied Ibm THINKPAD X200 Battery 8800mAh ,
a cathode or positive electrode and an anode or negative electrode. They were first observed in 1869 by German physicist Johann Hittorf, and were named in 1876 by Eugen Goldstein kathodenstrahlen, or cathode rays. Electrons were first discovered as the constituents of cathode rays Ibm THINKPAD X200 Battery 8800mAh .
In 1897 British physicist J. J. Thomson showed the rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode ray tubes (CRTs) create the image in a classic television set.
Cathode rays are so named because they are emitted by the positive electrode, or cathode, in a vacuum tube Ibm THINKPAD X200 Battery 8800mAh .
To release electrons into the tube, they first must be detached from the atoms of the cathode. In the early cold cathode vacuum tubes, called Crookes tubes, this was done by using a high electrical potential between the anode and the cathode to ionize the residual gas in the tube SONY VAIO VGN-FW21E Battery ;
the ions were accelerated by the electric field and released electrons when they collided with the cathode. Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filamentwhich is heated by a separate electric current passing through it Asus M52N Battery .
The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament, into the evacuated space of the tube.
Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode Dell Latitude E6400 ATG Battery .
They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube SONY VAIO VGN-FW21J Battery ,
exciting the atoms of the glass and causing them to emit light, a glow called fluorescence. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be travelling in straight lines from the cathode SONY VAIO VGN-FW21L Battery .
After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube SONY VAIO VGN-FW21M Battery .
The current in a beam of cathode rays through a tube can be controlled by passing it through a metal screen of wires (a grid) to which a small voltage is applied. The electric field of the wires deflects some of the electrons, preventing them from reaching the anode. Thus a small voltage can be made to control a much larger voltage SONY VAIO VGN-FW21Z Battery .
This is the principle used in vacuum tubes to amplify electrical signals. High speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets) SONY VAIO VGN-FW25T/B Battery .
These are used in cathode ray tubes, found in televisions and computer monitors, and in electron microscopes.
After the 1690 invention of the vacuum pump by Otto von Guericke, physicists began to experiment with passing high voltage electricity through rarefied air SONY VAIO VGN-FW26T/B Battery .
In 1705, it was noted that electrostatic generator sparks travel a longer distance through low pressure air than through atmospheric pressure air.
Gas discharge tubes
In 1838, Michael Faraday passed a current through a rarefied air filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode) SONY VAIO VGN-FW27/B Battery.
In 1857, German physicist and glassblower Heinrich Geissler sucked even more air out with an improved pump, to a pressure of around 10?3 atm and found that, instead of an arc, the glow filled the tube. The voltage applied between the two electrodes of the tubes, generated by an induction coil, was anywhere between a few kilovolts and 100 kV SONY VAIO VGN-FW27/W Battery .
These were called Geissler tubes, similar to today's neon signs.
The explanation of these effects was that the high voltage accelerated electrically charged atoms (ions) naturally present in the air of the tube SONY VAIO VGN-FW31ZJ Battery .
At low pressure, there was enough space between the gas atoms that the ions could accelerate to high enough speeds that when they struck another atom they knocked electrons off of it, creating more positive ions and free electrons in a chain reaction. The positive ions were all attracted to the cathode SONY VAIO VGN-FW27T/H Battery .
When they struck it they knocked many electrons out of the metal. The free electrons were all attracted to the anode.
In the Geissler tubes, there was so much air that the electrons could only travel a tiny distance before colliding with an atom SONY VAIO VGN-FW29/B Battery .
The electrons in these tubes moved in a slow diffusion process, never gaining much speed, so these tubes didn't produce cathode rays. The glow in the gas was caused when the electrons or ions struck gas atoms, exciting their orbital electrons to higher energy levels SONY VAIO VGN-FW290JRB Battery .
The electrons released this energy as light. This process is called fluorescence.
By the 1870s, British physicist William Crookes and others were able to evacuate tubes to a lower pressure, below 10?6 atm SONY VAIO VGN-FW290JTB Battery .
These were called Crookes tubes. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space" SONY VAIO VGN-FW290JTH Battery .
Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow SONY VAIO VGN-FW33G/E1 Battery .
What was happening was that as more air was pumped from the tubes, the electrons could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision SONY VAIO VGN-FW33G/W Battery .
With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays.
When they reached the anode end of the tube, they were travelling so fast that, although they were attracted to it SONY VAIO VGN-FW33GW Battery ,
they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher energy levels, causing them to fluoresce. Later researchers painted the inside back wall with fluorescent chemicals such as zinc sulfide, to make the glow more visible Dell XPS M1210 Battery) http://www.hdd-shop.co.uk .
Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist Johann Hittorf was first to realize that something must be travelling in straight lines from the cathode to cast the shadows Dell KM958 battery .
Eugen Goldstein named them cathode rays.
Discovery of the electron
At this time, atoms were the smallest particles known, and were believed to be indivisible Dell Studio 1555 battery .
What carried electric currents was a mystery. During the last quarter of the 19th century many experiments were done to determine what cathode rays were. There were two theories. Crookes and Artur Shuster believed they were particles of "radiant matter", that is, electrically charged atoms Sony VGP-BPS13 battery .
German scientists Eilhard Wiedemann, Heinrich Hertz and Goldstein believed they were "aether waves", some new form of electromagnetic radiation, and were separate from what carried the electric current through the tube Sony VGP-BPS13/B battery .
The debate was resolved in 1897 when J. J. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen. Therefore they were not atoms, but a new particle which he originally called "corpuscle" but was later named electron Sony VGP-BPS13/S battery .
He also showed they were identical with particles given off by photoelectric and radioactive materials. It was quickly recognised that they are the particles that carry electric currents in metal wires, and carry the negative electric charge of the atom Sony VGP-BPS13A/B battery .
Thomson was given the 1906 Nobel prize for physics for this work. Philipp Lenard also contributed a great deal to cathode ray theory, winning the Nobel prize for physics in 1905 for his research on cathode rays and their properties Sony VGP-BPS13B/B battery .
The gas ionization (or cold cathode) method of producing cathode rays used in Crookes tubes was unreliable, because it depended on the pressure of the residual air in the tube. Over time, the air was absorbed by the walls of the tube, and it stopped working Sony VGP-BPL9 battery .
A more reliable and controllable method of producing cathode rays was investigated by Hittorf and Goldstein, and rediscovered by Thomas Edison in 1880. A cathode made of a wire filament heated red hot by a separate current passing through it would release electrons into the tube by a process called thermionic emission Sony VGP-BPS13B/B battery .
The first true electronic vacuum tubes, invented around 1906, used this hot cathode technique, and they superseded Crookes tubes. These tubes didn't need gas in them to work, so they were evacuated to a lower pressure, around 10?9 atm (10?4 P) Sony VGP-BPL15 battery .
The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as krytrons.
Cathode rays are now usually called electron beams Dell Inspiron E1505 battery .
The technology of manipulating electron beams pioneered in these early tubes was applied practically in the design of vacuum tubes, particularly in the invention of the cathode ray tube by Ferdinand Braun in 1897. and is today employed in sophisticated devices such as electron microscopes, electron beam lithography, and particle accelerators HP Pavilion dv6000 Battery .
Particles of Cathode Rays
Like a wave:
- they travelled in straight lines
- Produced a shadow when obstructed by objects HP Pavilion dv8000 battery
- could pass through thin metal foils without disturbing them (Tested by New Zealander Ernest Rutherford using gold foil.)
These conflicting properties caused disruptions when trying to classify it as a wave or particle. Crookes insisted it was a particle, whilst Hertz maintained it was a wave HP PAVILION DV6000 Battery, 8800mAh .
The debate was resolved when an electric field was used to deflect the rays by J. J. Thomson. This evidence that the beams were composed of particles was strong because scientists knew it was impossible to deflect electromagnetic waves with an electric field HP PAVILION DV8000 Battery, 8800mAh .
Louis de Broglie later (1924) showed in his doctoral dissertation that electrons are in fact much like photons in the respect that they act both as waves and as particles in a dual manner as Einstein had shown earlier for light. The wave-like behaviour of Cathode Ray particles was later directly demonstrated using a crystal lattice by Davisson and Germer in 1927 SONY VAIO VGN-FZ Battery .
Alpha particles (named after and denoted by the first letter in the Greek alphabet, ?) consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is produced in the process of alpha decay HP Pavilion DV6000 battery - 8800mAh .
The alpha particle can be written as He2+, 4
2He2+ or 4
2He (as it is possible that the ion gains electrons from the environment; also, electrons are not important in nuclear chemistry) HP Pavilion DV8000 battery - 7800mAh .
Not all high-velocity helium nuclei are considered by all authors as alpha particles. As with beta and gamma rays/particles, the name used for the particle carries some mild connotations about its production process and energy HP Pavilion DV6000 battery - 8800mAh .
Some science authors may use doubly-ionized helium nuclei (He2+) and alpha particles as interchangeable terms. Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions (for example the alpha processes), and even when they occur as components of cosmic rays HP Pavilion DV8000 battery - 6600mAh .
However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles" because the high energies produced by these sources highlights the striking difference in behavior of their particles from the classical alpha particles produced (and originally defined) by the process of radioactive alpha decay SONY VAIO VGN-FZ18 Battery .
Alpha particles, like helium nuclei, have a net spin of zero and (due to the mechanism of their production in nuclear decay), in the classical sense, a total energy of about 5 MeV. They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) have low penetration depth SONY VAIO VGN-FZ21E Battery.
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, and are thus capable of being highly penetrating and able to traverse the human body and also many meters of dense solid shielding, depending on their energy.
When alpha particle emitting isotopes are ingested SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh ,
they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage, after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes (such as transuranics or actinides) are an average of about 20 times more dangerous SONY VAIO VGN-FW21E Battery ,
and in some experiments up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes.
Because of the short range of absorption, alphas are not, in general, dangerous to life unless the source is ingested or inhaled SONY VAIO VGN-NR11S/S Battery ,
in which case they become extremely dangerous. Because of this high mass and strong absorption, if alpha-emitting radionuclides do enter the body (upon being inhaled, ingested, or injected, as with the use of Thorotrast for high-quality X-ray images prior to the 1950s), alpha radiation is the most destructive form of ionizing radiation SONY VAIO VGN-NR11M/S Battery .
It is the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning. It is estimated that chromosome damage from alpha particles is anywhere from 10 to 1000 times greater than that caused by an equivalent amount of gamma or beta radiation, with the average being set at 20 times SONY VAIO VGN-NR11Z/S Battery .
The powerful alpha emitter polonium-210 (a milligram of 210Po emits as many alpha particles per second as 4.215 grams of 226Ra) is suspected of playing a role in lung cancer and bladder cancerrelated to tobacco smoking. 210Po was used to kill Russian dissident and ex-FSB officer Alexander V. Litvinenko in 2006 SONY VAIO VGN-NR11Z/T Battery .
Not only do alphas themselves cause damage, but approximately equal ionization is caused by the recoiling nucleus after alpha emission, and this energy may in turn be especially damaging to genetic material, since the positive cations of many soluble transuranic elements that emit alphas Sony VAIO VGN-FZ21E Battery ,
are chemically attracted to the net negative charge of DNA, causing the recoiling atomic nucleus to be in close proximation to the DNA.
History of discovery and use
In the years 1899 and 1900, physicists Ernest Rutherford and Paul Villard separated radiation into three types Sony VAIO VGN-FW21E Battery :
alpha, beta, and gamma, based on penetration of objects and ability to cause ionization. Alpha rays were defined by Rutherford as those having the lowest penetration of ordinary objects.
Rutherford's work also included measurements of the ratio of an alpha particle's mass to its charge Sony VAIO VGN-NR11S/S Battery,
which led him to the hypothesis that alpha particles were doubly charged helium ions (later shown to be bare helium nuclei).In 1907, Ernest Rutherford and Thomas Royds finally proved that alpha particles were indeed helium ions. To do this they allowed alpha particles to penetrate a very thin glass wall of an evacuated tube Sony VAIO VGN-NR11Z/S Battery ,
thus capturing a large number of the hypothesized helium ions inside the tube. They then caused an electric spark inside the tube, which provided a shower of electrons that were taken up by the ions to form neutral atoms of a gas. Subsequent study of the spectra of the resulting gas showed that it was helium and that the alpha particles were indeed the hypothesized helium ions Sony VAIO VGN-NR11M/S Battery .
Because alpha particles occur naturally, but can have energy high enough to participate in a nuclear reaction, study of them led to much early knowledge ofnuclear physics. Rutherford used alpha particles emitted by radium bromide to infer that J. J Sony VAIO VGN-NR11Z/T Battery .
Thomson's Plum pudding model of the atom was fundamentally flawed. In Rutherford's gold foil experiment conducted by his students Hans Geiger and Ernest Marsden, a narrow beam of alpha particles was established, passing through very thin (a few hundred atoms thick) gold foil SONY VAIO VGN-FZ180E Battery .
The alpha particles were detected by a zinc sulfide screen, which emits a flash of light upon an alpha particle collision. Rutherford hypothesized that, assuming the "plum pudding" model of the atom was correct, the positively charged alpha particles would be only slightly deflected, if at all, by the dispersed positive charge predicted SONY VAIO VGN-FZ220E Battery .
It was found that some of the alpha particles were deflected at much larger angles than expected (at a suggestion by Rutherford to check it) and some even bounced almost directly back. Although most of the alpha particles went straight through as expected SONY VAIO VGN-FZ340E Battery ,
Rutherford commented that the few particles that were deflected was akin to shooting a fifteen-inch shell at tissue paper only to have it bounce off, again assuming the "plum pudding" theory was correct. It was determined that the atom's positive charge was concentrated in a small area in its center SONY VAIO VGN-FZ430E Battery ,
making the positive charge dense enough to deflect any positively charged alpha particles that came close to what was later termed the nucleus.
Note: Prior to this discovery, it was not known that alpha particles are themselves atomic nuclei SONY VAIO VGN-FZ460E Battery,
nor was the existence of protons or neutrons known. After this discovery J.J. Thomson's "plum pudding" model was abandoned, and Rutherford's experiment led to the Bohr model (named for Niels Bohr) and later the modern wave-mechanical model of the atomSONY VAIO VGN-FZ480E Battery .
- Most smoke detectors contain a small amount of the alpha emitter americium-241. The alpha particles ionize air between a small gap. A small current is passed through that ionized air SONY VAIO VGN-FZ4000 Battery .
- Smoke particles from fire that enter the air gap reduce the current flow, sounding the alarm. The isotope is extremely dangerous if inhaled or ingested, but the danger is minimal if the source is kept sealed. Many municipalities have established programs to collect and dispose of old smoke detectors, to keep them out of the general waste stream SONY VAIO VGN-FZ31E Battery .
- Alpha decay can provide a safe power source for radioisotope thermoelectric generators used for space probes and artificial heart pacemakers. Alpha decay is much more easily shielded against than other forms of radioactive decay SONY VAIO VGN-FZ31B Battery
- Plutonium-238, a source of alpha particles, requires only 2.5 mm oflead shielding to protect against unwanted radiation.
- Static eliminators typically use polonium-210, an alpha emitter, to ionize air, allowing the 'static cling' to more rapidly dissipate SONY VAIO VGN-FZ31J Battery .
- Researchers are currently trying to use the damaging nature of alpha emitting radionuclides inside the body by directing small amounts towards a tumor. The alphas damage the tumor and stop its growth while their small penetration depth prevents radiation damage of the surrounding healthy tissue SONY VAIO VGN-FZ31M Battery .
- This type of cancer therapy is called unsealed source radiotherapy.
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40 SONY VAIO VGN-FZ31Z Battery .
The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (?). There are two forms of beta decay, ?? and ?+, which respectively give rise to the electron and thepositron SONY VAIO VGN-FZ38M Battery .
An unstable atomic nucleus with an excess of neutrons may undergo ?? decay, where a neutron is converted into a proton, an electron and anelectron-type antineutrino (the antiparticle of the neutrino):
This process is mediated by the weak interaction SONY VGP-BPS8 Battery .
The neutron turns into a proton through the emission of a virtual W? boson. At the quark level, W? emission turns a down-type quark into an up-type quark, turning a neutron (one up quark and two down quarks) into a proton (two up quarks and one down quark). The virtual W? boson then decays into an electron and an antineutrino SONY VGP-BPS13/S Battery .
Beta decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons also decay via this process. This is the source of the copious amount of electron antineutrinos produced by fission reactors SONY VGP-BPS13A/B Battery .
?+ decay (positron emission)
Unstable atomic nuclei with an excess of protons may undergo ?+ decay, also called positron decay, where a proton is converted into a neutron, a positronand an electron-type neutrino SONY VGP-BPS13B/B Battery :
- p ? n + e+
Beta plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is higher than that of the mother nucleus SONY VGP-BPS13A/S Battery .
Interaction with other matter
Of the three common types of radiation given off by radioactive materials, alpha, beta and gamma, beta has the medium penetrating power and the medium ionising power. Although the beta particles given off by different radioactive materials vary in energy, most beta particles can be stopped by a few millimeters of aluminum SONY VGP-BPS13AS Battery .
Being composed of charged particles, beta radiation is more strongly ionising than gamma radiation. When passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off bremsstrahlung x-rays Dell Inspiron 1320n Battery .
Beta particles can be used to treat health conditions such as eye and bone cancer, and are also used as tracers. Strontium-90 is the material most commonly used to produce beta particles. Beta particles are also used in quality control to test the thickness of an item, such as paper, coming through a system of rollers Dell Studio 1450 Battery .
Some of the beta radiation is absorbed while passing through the product. If the product is made too thick or thin, a correspondingly different amount of radiation will be absorbed. A computer program monitoring the quality of the manufactured paper will then move the rollers to change the thickness of the final product Dell Studio 1457 Battery .
The well-known 'betalight' contains tritium and a phosphor.
Beta plus(or positron) decay of a radioactive tracer isotope is the source of the positrons used in positron emission tomography (PET scan) Dell Latitude D610 Battery .
Henri Becquerel, while experimenting with fluorescence, accidentally found out that Uranium exposed a black paper wrapped photographic plate with some unknown radiation that could not be turned off like X-rays. Ernest Rutherford continued these experiments and discovered two different kinds of radiation racks Toshiba NB100 Battery :
- alpha particles that did not show up on the Becquerel plates because they were easily absorbed by the black wrapping paper
- beta particles which are 100 times more penetrating than alpha particles.
He published his results in 1897 Toshiba Satellite M65 battery .
Beta particles are able to penetrate living matter to a certain extent and can change the structure of struck molecules. In most cases s
Beta sources can be used in radiation therapy to kill cancer cells.
One can envisage betavoltaic cells to supply power to consumer electronic devices such as laptops and mobile phones without recharging for the expected useful life of the product Dell Latitude D830 Battery .
The power output would be limited by the half-life of the radioactive source(s). For example tritium has a half-life of approximately 12 years; Strontium 90 has a half-life of about 29 years.
Gamma radiation, also known as gamma rays (denoted as ?), is electromagnetic radiation of high frequency (very short wavelength) Dell Latitude D620 Battery .
They are produced bysub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, fusion, fission or inverse Compton scattering in astrophysical processes. A classical gamma ray source is a type of radioactive decay called gamma decay where an excited nucleus emits a gamma ray almost immediately on formation Dell Inspiron Mini 10 Battery .
However, gamma decay may also describe isomeric transition which involves an inhibited gamma decay with a relatively much longer half life.
Gamma rays have frequencies above 10 exahertz (1019 Hz), and therefore have energies above 100 keV and wavelength less than 10 picometers, less than the diameter of an atom Sony VGN-FW11S Battery .
Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. Energies from astronomical sources can be much higher, ranging over 10 TeV (this is far too large to result from radioactive decay) Sony VGN-FW11M Battery .
There is effectively no lower limit to gamma energy (though they are sometimes classed as X-rays if their frequencies are lower than 1019 Hz). Because gamma rays are a form of ionizing radiation, they pose a health hazard.
Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radiumSony VGN-FW139E/H battery .
Alpha and beta "rays" had already been separated and named by the work of Ernest Rutherford in 1899, and in 1903 Rutherford named Villard's distinct new radiation "gamma rays."
The distinction between X-rays and gamma rays has changed in recent decades Dell Latitude E5400 Battery .
Originally, the electromagnetic radiation emitted by X-ray tubes had a longerwavelength than the radiation emitted by radioactive nuclei (gamma rays). Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10?11 m, defined as gamma rays Dell Latitude E4200 Battery .
However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by thenucleus Dell Inspiron 300M Battery.
Naming conventions and overlap in terminology
In the past, the distinction between X-rays and gamma rays was based on energy (or equivalently frequency or wavelength), with gamma rays being considered a higher-energy version of X-rays Dell Studio 1737 battery .
However, modern high-energy (megavoltage) X-rays produced by linear accelerators ("linacs") for megavoltagetreatment, in cancer radiotherapy usually have higher energy (typically 4 to 25 MeV) than classical gamma rays produced by radioactive gamma decay Dell Inspiron E1505 battery .
Conversely, one of the most common gamma-ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of about the same energy (140 keV) as produced by a diagnostic X-ray machine, and significantly lower energy than therapeutic photons from linacs Dell RM791 battery .
Because of this broad overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce Bremsstrahlung-type radiation) Toshiba Satellite P10 Battery ,
while gamma rays are emitted by the nucleus or from other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet and even lower energy photons produced by these processes would also be defined as "gamma rays" Acer Aspire One battery 10400mAh .
In certain fields such as astronomy, higher energy gamma and X-rays are still sometimes defined by energy, since the processes which produce them may be uncertain. Occasionally, high energy photons in nature which are known not to be produced by nuclear decay, are nevertheless referred to as gamma radiation SONY VGN-FZ210CE Battery .
An example is "gamma rays" from lightning discharges at 10 to 20 MeV, which are known to be produced as Bremsstrahlung.
Units of measure and exposure
The measure of gamma rays' ionizing ability is called the exposure Dell Precision M70 Battery :
- The coulomb per kilogram (C/kg) is the SI unit of ionizing radiation exposure, and is the amount of radiation required to create 1 coulomb of charge of each polarity in 1 kilogram of matter.
- The röntgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create 1 esu of charge of each polarity in 1 cubic centimeter of dry air Toshiba Satellite L305 Battery .
- 1 röntgen = 2.58×10?4 C/kg
However, the effect of gamma and other ionizing radiation on living tissue is more closely related to the amount of energy deposited rather than the charge. This is called the absorbed dose Toshiba Satellite T4900 Battery :
- The gray (Gy), which has units of (J/kg), is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.
- The rad is the (obsolete) corresponding traditional unit, equal to 0.01 J deposited per kg. 100 rad = 1 Gy Toshiba PA3399U-2BRS battery .
The equivalent dose is the measure of the biological effect of radiation on human tissue. For gamma rays it is equal to the absorbed dose.
- The sievert (Sv) is the SI unit of equivalent dose, which for gamma rays is numerically equal to the gray (Gy) Sony VGP-BPS10 battery 5200mAh/7800mAh/10400mAh .
- The rem is the traditional unit of equivalent dose. For gamma rays it is equal to the rad or 0.01 J of energy deposited per kg. 1 Sv = 100 rem.
Shielding from gamma rays requires large amounts of mass Sony VGP-BPS9 battery 5200mAh/7800mAh/10400mAh .
They are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray. For this reason, a lead shield is only modestly better (20–30%) as a gamma shield than an equal mass of another shielding material such as aluminium Sony VGP-BPS11 battery 5200mAh/7800mAh/10400mAh ,
concrete, or soil; the lead's major advantage is its density.
The higher the energy of the gamma rays, the thicker the shielding required Sony VGP-BPS22 battery 5200mAh/7800mAh/10400mAh .
Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example gamma rays that require 1 cm (0.4?) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 Sony VGP-BPL8 battery 5200mAh/7800mAh/10400mAh
, 6 cm (2½?) of concrete, or 9 cm (3½?) of packed soil. However, the mass of this much concrete or soil is only 20–30% larger than that of lead with the same absorption capability. Depleted uranium is used for shielding in portable gamma ray sources, but again the savings in weight over lead is modest, and the main effect is to reduce shielding bulk Sony VGP-BPS14 battery 5200mAh/7800mAh/10400mAh .
When a gamma ray passes through matter, the probability for absorption in a thin layer is proportional to the thickness of that layer. This leads to an exponential decrease of intensity with thickness Sony VGP-BPS21 battery 5200mAh/7800mAh/10400mAh .
The exponential absorption holds only for a narrow beam of gamma rays. If a wide beam of gamma rays passes through a thick slab of concrete the scattering from the sides reduces the absorption to where ? = n? is the absorption coefficient, measured in cm?1, n the number of atoms per cm3 in the material Toshiba Satellite A200 Battery .
? the absorption cross section in cm2 and d the thickness of material in cm.
In passing through matter, gamma radiation ionizes via three main processes: the photoelectric effect, Compton scattering, and pair production Toshiba Satellite 1200 Battery .
- Photoelectric effect: This describes the case in which a gamma photon interacts with and transfers its energy to an atomic electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron Toshiba Satellite M300 Battery .
- The photoelectric effect is the dominant energy transfer mechanism for X-ray and gamma ray photons with energies below 50 keV (thousand electron volts), but it is much less important at higher energies SONY VGP-BPS13A/Q Battery .
- Compton scattering: This is an interaction in which an incident gamma photon loses enough energy to an atomic electron to cause its ejection, with the remainder of the original photon's energy being emitted as a new, lower energy gamma photon with an emission direction different from that of the incident gamma photon SONY VGP-BPS13A/Q Battery .
- The probability of Compton scatter decreases with increasing photon energy. Compton scattering is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV SONY VGP-BPS13B/Q Battery .
- Compton scattering is relatively independent of the atomic number of the absorbing material, which is why very dense metals like lead are only modestly better shields, on a per weight basis, than are less dense materials.
- Pair production: This becomes possible with gamma energies exceeding 1.02 MeV SONY VGP-BPS13B/Q Battery ;
- color:black;mso-ansi-language:EN-US;mso-fareast-language:ZH-CN;mso-bidi-language: AR-SA'> , and becomes important as an absorption mechanism at energies over about 5 MeV (see illustration at right, for lead) SONY VGP-BPS13/Q Battery .
- By interaction with the electric field of a nucleus, the energy of the incident photon is converted into the mass of an electron-positron pair. Any gamma energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus SONY VGP-BPS13/Q Battery .
- At the end of the positron's range, it combines with a free electron. The entire mass of these two particles is then converted into two gamma photons of at least 0.51 MeV energy each (or higher according to the kinetic energy of the annihilated particles) SONY VGP-BPS21A/B Battery .
The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionizationthemselves SONY VGP-BPS21/S Battery .
High-energy (from 80 to 500 GeV) gamma rays arriving from far far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus their density may be estimated by analyzing the incoming gamma-ray spectrums SONY VGP-BPS21B Battery .
Gamma ray production
Gamma rays from radioactive gamma decay are produced alongside other forms of radiation such as alpha or beta, and are produced after the other types of decay occur. The mechanism is that when a nucleus emits an ? or ? particle, the daughter nucleus is usually left in an excited state SONY VGP-BPS21A Battery .
It can then move to a lower energy state by emitting a gamma ray, in much the same way that an atomic electron can jump to a lower energy state by emitting infrared, visible, or ultraviolet light. Emission of a gamma ray from an excited nuclear state typically requires only 10-12 seconds SONY VGP-BPS21 Battery ,
and is thus nearly instantaneous, following types of radioactive decay that produce other radioactive particles. Gamma decay from excited states may also happen rapidly following nuclear reactions such as nuclear fission or nuclear fusion Sony VGP-BPS21A/B Battery .
In certain cases, the excited nuclear state following the emission of a beta particle may be more stable than average, and is termed a metastable excited state, if its decay is 100 to 1000 times longer than the average 10-12 seconds. Such nuclei have half lives that are easily measurable, and are termed nuclear isomers Sony VGP-BPS21/S Battery .
Some (metastable) nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. Isomeric transition is the name given to a gamma decay from such a state. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves metastable excited states of the nuclei Sony VGP-BPS21A Battery .
An emitted gamma ray from any type of excited state may transfer its energy directly to one of the most tightly bound electrons causing it to be ejected from the atom, a process termed thephotoelectric effect (it should not be confused with the internal conversion process, in which no real gamma ray photon is produced as an intermediate particle) Sony VGP-BPS21 Battery .
Gamma rays, X-rays, visible light, and radio waves are all forms of electromagnetic radiation. The only difference is the frequency and hence theenergy of the photons. Gamma rays are generally the most energetic of these, although broad overlap with X-ray energies occurs. An example of gamma ray production follows Sony Vaio PCG-5G2L Battery :
Co decays to excited 60
Ni by beta decay. Then the 60
Ni drops down to the ground state (see nuclear shell model) by emitting two gamma rays in succession (1.17 MeV then 1.33 MeV)Sony Vaio PCG-5G3L Battery :
Another example is the alpha decay of 241
Am to form 237
Np; this alpha decay is accompanied by gamma emission. In some cases, the gamma emission spectrum for a nucleus (daughter nucleus) is quite simple, (e.g. 60
Co/60 Sony Vaio PCG-5J1L Battery
Ni) while in other cases, such as with (241
Ir/192 Sony Vaio PCG-5K2L Battery
Pt), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels can exist. The fact that an alpha spectrum can have a series of different peaks with different energies reinforces the idea that several nuclear energy levels are possible Sony Vaio PCG-5J2L Battery .
Because a beta decay is accompanied by the emission of a neutrino which also carries energy away, the beta spectrum does not have sharp lines, but instead is a broad peak. Hence from beta decay alone it is not possible to probe the different energy levels found in the nucleus Sony Vaio PCG-5K1L Battery .
In optical spectroscopy, it is well known that an entity which emits light can also absorb light at the same wavelength (photon energy). For instance, a sodium flame can emit yellow light as well as absorb the yellow light from a sodium vapor lamp Sony Vaio PCG-6S2L Battery .
In the case of gamma rays, this can be seen in Mössbauer spectroscopy. Here, a correction for the energy lost by the recoil of the nucleus is made and the exact conditions for gamma ray absorption through resonance can be attained.
This is similar to the Franck Condon effects seen in optical spectroscopy Sony Vaio PCG-6S3L Battery .
All ionizing radiation causes similar damage at a cellular level, but because rays of alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage, e.g. radiation burns to the skin Sony Vaio PCG-6V1L Battery .
Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body (e.g. radiation sickness), increasing incidence of cancer rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances Sony Vaio PCG-6W1L Battery ,
which, depending on the substance's chemical nature, can produce both diffuse and localized internal damage. The most biological damaging forms of gamma radiation occur in the gamma ray window, between 3 and 10 MeV, with higher energy gamma rays being less harmful because the body is relatively transparent to them. See cobalt-60 Sony Vaio PCG-6W2L Battery .
Gamma rays travel to Earth across vast distances of the universe, only to be absorbed by Earth's atmosphere. Different wavelengths of light penetrate Earth's atmosphere to different depths Sony Vaio PCG-6W3L Battery .
Instruments aboard high-altitude balloons and such satellites as the Compton Observatory provide our only view of the gamma-spectrum sky.
Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz Sony Vaio PCG-7111L Battery .
In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These US$5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to screen merchant ship containers before they enter US ports Sony Vaio PCG-7112L Battery .
Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor Sony Vaio PCG-7113L Battery .
Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer, since the rays kill cancer cells also. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells Sony Vaio PCG-7133L Battery .
The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.
Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques Sony Vaio PCG-7Z2L Battery .
A number of different gamma-emitting radioisotopes are used. For example, in a PET scan a radiolabled sugar called fludeoxyglucose emits positrons that are converted to pairs of gamma rays that localize cancer (which often takes up more sugar than other surrounding tissues) Sony Vaio PCG-8Y1L Battery .
The most common gamma emitter used in medical applications is the nuclear isomer technetium-99m which emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, a gamma camera can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted Sony Vaio PCG-8Y2L Battery .
Depending on what molecule has been labeled with the tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones in a bone scan) Sony Vaio PCG-8Z1L Battery .
When gamma radiation breaks DNA molecules, a cell may be able to repair the damaged genetic material, within limits. However, a study of Rothkamm and Lobrich has shown that this repair process works well after high-dose exposure but is much slower in the case of a low-dose exposure Sony Vaio PCG-8Z2L Battery .
The natural outdoor exposure in Great Britain ranges from 2 × 10?7 to 4 × 10?7 cSv/h (centisieverts per hour). Natural exposure to gamma rays is about 0.1 to 0.2 cSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 0.36 cSv SONY VAIO VGN-FW31J Battery .
There is a small increase in the dose, due to naturally occurring gamma-radiation, around small particles of high atomic number materials in the human body caused by the photoelectric effect.
By comparison, the radiation dose from chest radiography is a fraction of the annual naturally occurring background radiation dose, and the dose from fluoroscopy of the stomach is, at most, 5 cSv on the skin of the back SONY VAIO VGN-FW31E Battery .
For acute full-body equivalent dose, 100 cSv causes slight blood changes, but 200–350 cSv causes very severe syndrome of nausea, hair loss, and hemorrhaging, and will cause death in a sizable number of cases— about 10% to 35% without medical treatment SONY VAIO VGN-FW31B Battery .
A dose of 500 cSv is considered approximately the LD50 (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. A dose higher than 500 cSv brings an increasing chance of death above 50% SONY VAIO VGN-FW31M Battery .
Above 750–1000 cSv, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see Radiation poisoning).
For low dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 1.9 cSv SONY VAIO VGN-FW31Z Battery ,
the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 10 cSv, that risk increase is at 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the atomic bombing of Hiroshima and Nagasaki SONY VAIO VGN-FW38M Battery .
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from both stable molecules andisotopes, and these free neutrons react with nuclei of other stable molecules to form new isotopes of previously non-isotopic molecules SONY VAIO VGN-AW41JF Battery ,
which in turn produce radiation. This will result in a chain reaction of nuclear radiation, which makes radiation dangerous and harmful over great areas of space.
Neutrons may be emitted from nuclear fusion or nuclear fission SONY VAIO VGN-AW41JF/H Battery ,
or from any number of different nuclear reactions such as from radioactive decay or reactions from particle interactions (such as from cosmic rays or particle accelerators). Large neutron sources are rare, and are usually limited to large-sized devices like nuclear reactors or particle accelerators (such as the Spallation Neutron Source) SONY VAIO VGN-AW41MF Battery .
Neutron radiation was discovered as a result of observing a beryllium nucleus reacting with an alpha particle thus transforming into a carbon nucleus and emitting a neutron, Be(?, n)C. The combination of an alpha particle emitter and an isotope with a large (?, n) nuclear reaction probability is still a common neutron source SONY VAIO VGN-AW41MF/H Battery .
Cold, thermal and hot neutron radiation is most commonly used for scattering and diffraction experiments in order to assess the properties and the structure of materials in crystallography, condensed matter physics, biology SONY VAIO VGN-FW30B Battery ,
solid state chemistry, materials science, geology, mineralogy and related sciences. Neutron radiation is also used in select facilities to treat cancerous tumors due to its highly penetrating and damaging nature to cellular structure. Neutrons can also be used for imaging of industrial parts termed neutron radiography when using film SONY VAIO VGN-FW50B Battery ,
neutron radioscopy when taking a digital image, such as through image plates, and neutron tomography for three dimensional images. Neutron imaging is commonly used in the nuclear industry, the space and aerospace industry, as well as the high reliability explosives industry SONY VAIO VGN-FW51B/W Battery .
Ionization mechanisms and properties
Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the same way that charged particles such as protons and electrons do (exciting an electron), because neutrons have no charge SONY VAIO VGN-FW51MF Battery .
However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the gamma ray (photon) subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms SONY VAIO VGN-FW51MF/H Battery .
Because neutrons are uncharged, they are more penetrating thanalpha radiation or beta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high atomic number. In hydrogen, a low energy neutron may not be as penetrating as a high energy gamma ray SONY VAIO VGN-FW51ZF Battery .
Health hazards and protection
In health physics neutron radiation is considered a fourth radiation hazard alongside the other types of radiation. Another, sometimes more severe hazard of neutron radiation, is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of the workers themselves SONY VAIO VGN-FW51ZF/H Battery .
This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon SONY VAIO VGN-FW52JB Battery .
It is also a problem innuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.
Neutron radiation protection relies on radiation shielding SONY VAIO VGN-FW54FB Battery .
In comparison with conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei, so a large mass of hydrogen-rich material is needed. Neutrons readily pass through most material, but interact enough to cause biological damage SONY VAIO VGN-FW70DB Battery .
Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe and dangerous radiation available. The most effective materials are e.g. water, polyethylene, paraffin wax, or concrete, where a considerable amount of water molecules are chemically bound to the cement SONY VAIO VGN-FW71DB/W Battery .
The light atoms serve to slow down the neutrons by elastic scattering, so they can then be absorbed by nuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding has to be provided to absorb it SONY VAIO VGN-FW72JGB Battery .
Because the neutrons that strike the hydrogen nucleus (proton, or deuteron) impart energy to that nucleus, they in turn will break from their chemical bonds and travel a short distance, before stopping. Those protons and deuterons are high linear energy transfer particles, and are in turn stopped by ionization of the material through which they travel SONY VAIO VGN-FW73JGB Battery .
Consequently, in living tissue, neutrons have a relatively high relative biological effectiveness, and are roughly ten times more effective at causing cancers or LD-50s compared to photon or beta radiation of equivalent radiation exposure SONY VAIO VGN-FW74FB Battery .
Effects on materials
Neutrons also degrade materials; bombardment of materials with neutrons creates collision cascades that can produce point defects and dislocations in the materials. At high neutron fluences this can lead to embrittlement of metals and other materials, and to swelling of some of them SONY VAIO VGN-FW81HS Battery .
This poses a problem for nuclear reactor vessels, and significantly limits their lifetime (which can be somewhat prolonged by controlled annealing of the vessel, reducing the number of the built-up dislocations). Graphite moderator blocks are especially susceptible to this effect, known as Wigner effect, and have to be annealed periodically SONY VAIO VGN-FW81NS Battery ;
the well-known Windscale fire was caused by a mishap during such an annealing operation.
Neutron radiation and nuclear fission
The neutrons in reactors are generally categorized as slow (thermal) neutrons or fast neutrons depending on their energy SONY VAIO VGN-FW81S Battery .
Thermal neutrons are similar to a gas in thermodynamic equilibrium but are easily captured by atomic nuclei and are the primary means by which elements undergo atomic transmutation.
In order to achieve an effective fission chain reaction SONY VAIO VGN-FW82DS Battery ,
the neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the nuclear fuel is not sufficiently refined to be able to absorb enough fast neutrons to carry on the fission chain reaction, due to the lower cross section for higher-energy neutrons SONY VAIO VGN-FW82JS Battery ,
so aneutron moderator must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include graphite, light water and heavy water. A few reactors (fast neutron reactors) and all nuclear weapons rely on fast neutrons SONY VAIO VGN-FW82XS Battery .
This requires certain changes in the design and in the required nuclear fuel. The element beryllium is particularly useful due to its ability to act as a neutron reflector or lens. This allows smaller quantities of fissile material to be used and is a primary technical development that led to the creation of neutron bombs SONY VAIO VGN-FW83DS Battery .
Cosmogenic neutrons, neutrons produced from cosmic radiation in the Earth's atmosphere or surface, and those produced in particle accelerators can be significantly higher energy than those encountered in reactors SONY VAIO VGN-FW83JS Battery .
Most of them activate a nucleus before reaching the ground; a few react with nuclei in the air. The reactions with nitrogen 14 lead to the formation of carbon 14, widely used inradiocarbon dating.
A delta ray is sometimes used to describe any recoil particle caused by secondary ionization SONY VAIO VGN-FW83XS Battery.
The term was coined by J.J. Thomson. It is entirely unrelated to the family of subatomic particles named delta baryon.
A delta ray is characterized by very fast electrons produced in quantity by alpha particles or other fast energetic charged particles knocking orbiting electrons out of atoms SONY VAIO VGN-FW90NS Battery .
Collectively, these electrons are defined as delta radiation when they have sufficient energy to ionize further atoms through subsequent interactions on their own. Delta rays appear as branches in the main track of a cloud chamber SONY VAIO VGN-FW37J/B Battery .
These branches will appear nearer the start of the track of a heavy charged particle, where more energy is imparted to the ionized electrons.
Delta rays in particle accelerators
Otherwise called a knock-on electron, the term "delta ray" is also used in high energy physics to describe single electrons in particle accelerators that are exhibiting characteristic deceleration SONY VAIO VGN-FW37J Battery .
In abubble chamber, electrons will lose their energy more quickly than other particles through Bremsstrahlung and will create a spiral track due to their small mass and the magnetic field. The Bremsstrahlung rate is proportional to the square of the acceleration of the electron SONY VAIO VGN-FW378J/B Battery .
References in popular culture
In science fiction, "delta ray" is often used to describe an undefined fictional type of ray or radiation, sometimes as a hazard, sometimes used in energy weapons. For example, in the two-part Star Trek episode "The Menagerie", Fleet Captain Christopher Pike is said to have been exposed to delta radiation SONY VAIO VGN-FW378DH Battery .
Epsilon radiation, coined by J. J. Thomson, is tertiary radiation caused by secondary radiation (e.g., delta radiation). Epsilon rays are a form of particle radiation and are composed of electrons. The term is very rarely used today SONY VAIO VGN-FW378J/B Battery .