Nuclear PhysicsNuclear PhysicsNuclear Reaction Nuclear Reaction

Dr. Nidal Dr. Nidal Dwaikat Dwaikat

An- Najah Nation An- Najah Nation University University December, December,

20092009

Nuclear Reaction In order nuclear reaction to occur two nuclei or particle And nucleus must approach each other.

Two types of nuclear reactions :

1- fission reaction

2- fusion reaction

aNuclear Reaction is denoted By X (Pi, Pf)Y

Example : 14N ( , p ) 17O

fPYYX 21iPIncident Particle

Target Recoiling Nucleus

Scattered Particle

Recoiling Nucleus

Fission Reaction :

using fissionable material such as Uranium(235U) , 239Pu and 2333U.

Thermal neutron : < 1 eV

The process of fission The process of fission

1- A neutron collides with a uranium- 1- A neutron collides with a uranium- 235 atom and creates a highly unstable 235 atom and creates a highly unstable

uranium-236 atom. uranium-236 atom. 2- As soon as the uranium-236 atom is 2- As soon as the uranium-236 atom is created, it splinters into an isotope of created, it splinters into an isotope of barium (barium (144144BA), an isotope of krypton BA), an isotope of krypton

((8989Kr), and three free neutrons are Kr), and three free neutrons are produced as well. This process can be produced as well. This process can be

summarized using the notation that was summarized using the notation that was introduced earlier (the n in the equation introduced earlier (the n in the equation

stands for the word “neutron”):stands for the word “neutron”):

The excitation energy added to the The excitation energy added to the compound nucleus is equal to the sum compound nucleus is equal to the sum of the binding energy of the incident of the binding energy of the incident

neutron and its kinetic energy. neutron and its kinetic energy.

The criteria for choosing Nuclear fuel is :

The criteria for choosing Fissionable material is :

1- Fissionable

2- Cross section for capturing neutron

3- Number of neutron emitted must be larger than one

In order to Chain reaction continue

Thermal neutron at room temperature, E = KT= 300 K= Thermal neutron at room temperature, E = KT= 300 K= 1/40 eV1/40 eV

Low energy neutrons, electrically neutral, cant get much Low energy neutrons, electrically neutral, cant get much closer to the nuclei and interact with them closer to the nuclei and interact with them

- Capture of low energy was used to produce nuclei of Capture of low energy was used to produce nuclei of higher A. It is, experimentally, observed thathigher A. It is, experimentally, observed that

- The interaction of low energy neutron with odd-A nuclei The interaction of low energy neutron with odd-A nuclei such as such as 235235U did not produce heavier nuclei. In stead the U did not produce heavier nuclei. In stead the parent nucleus fragmented into two nuclei of smaller parent nucleus fragmented into two nuclei of smaller size. size.

- The interaction of thermal neutrons with even-A such as - The interaction of thermal neutrons with even-A such as 238238U nuclei such as U238 does not produce U nuclei such as U238 does not produce fragmentation. fragmentation. Fission can take place in such nuclei when Fission can take place in such nuclei when the neutrons have kinetic energies of the order of 2MeV. the neutrons have kinetic energies of the order of 2MeV.

-The percentage of -The percentage of 235235U is 0.7% of U is 0.7% of natural Uranium natural Uranium

- - 238238U is around 98.8% of natural U is around 98.8% of natural Uranium.Uranium.

- - 235235U is fissionable U is fissionable - - 238 238 U is non fissionable U is non fissionable Uranium enrichment Uranium enrichment

Is the process by which the Is the process by which the concentration of concentration of 235235U is increased U is increased

over its natural valueover its natural value 4% for power generation4% for power generation

20% for Medical Application 20% for Medical Application >90% for nuclear weapons >90% for nuclear weapons

diagram represents the steps of diagram represents the steps of Uranium enrichment by Gaseous Uranium enrichment by Gaseous

diffusion diffusion

Other separation methods are also used Other separation methods are also used

2- Electromagnetic method 2- Electromagnetic method The earliest successful methods were The earliest successful methods were

electromagnetic isotope separation electromagnetic isotope separation (EMIS), in which large magnets are used (EMIS), in which large magnets are used to separate ions of the two isotopes, and to separate ions of the two isotopes, and gaseous diffusion, in which the gas gaseous diffusion, in which the gas uranium hexafluoride (UF6 ) is passed uranium hexafluoride (UF6 ) is passed through a porous barrier material; the through a porous barrier material; the lighter molecules containing lighter molecules containing 235235U U penetrate the barrier slightly more penetrate the barrier slightly more rapidly, and with enough stages rapidly, and with enough stages significant separation can be significant separation can be accomplished. Both gaseous diffusion accomplished. Both gaseous diffusion and EMIS require enormous amounts of and EMIS require enormous amounts of electricity. More efficient methods have electricity. More efficient methods have been developed.been developed.

3- centrifuge3- centrifuge The third method in widespread use is the The third method in widespread use is the

gas centrifuge [Urenco (Netherlands, gas centrifuge [Urenco (Netherlands, Germany, UK), Russia, Japan] in which UFGermany, UK), Russia, Japan] in which UF6 6

gas is whirled inside complex rotor gas is whirled inside complex rotor assemblies and centrifugal force pushes assemblies and centrifugal force pushes molecules containing the heavier isotope to molecules containing the heavier isotope to the outside. Again, many stages are the outside. Again, many stages are needed to produce the highly enriched needed to produce the highly enriched uranium needed for a weapon, but uranium needed for a weapon, but centrifuge enrichment requires much less centrifuge enrichment requires much less electricity than either of the older electricity than either of the older technologies.technologies.

4- Laser Isotopes 4- Laser Isotopes Separation Separation

Atomic and molecular laser isotope Atomic and molecular laser isotope separation (LIS) techniques use lasers to separation (LIS) techniques use lasers to selectively excite atoms or molecules selectively excite atoms or molecules containing one isotope of uranium so containing one isotope of uranium so that they can be preferentially extracted. that they can be preferentially extracted. Although LIS appears promising, the Although LIS appears promising, the technology has proven to be extremely technology has proven to be extremely difficult to master and may be beyond difficult to master and may be beyond the reach of even technically advanced the reach of even technically advanced states.states.

5- Thermal diffusion5- Thermal diffusion

Thermal diffusion utilizes the transfer of Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to heat across a thin liquid or gas to accomplish isotope separation. By cooling accomplish isotope separation. By cooling a vertical film on one side and heating it on a vertical film on one side and heating it on the other side, the resultant convection the other side, the resultant convection currents will produce an upward flow along currents will produce an upward flow along the hot surface and a downward flow along the hot surface and a downward flow along the cold surface. Under these conditions, the cold surface. Under these conditions, the lighter the lighter 235235U gas molecules will diffuse U gas molecules will diffuse toward the hot surface, and the heavier toward the hot surface, and the heavier 238238U U molecules will diffuse toward the cold molecules will diffuse toward the cold surface.surface.

Fissile Isotopes Fissile Isotopes Production Production

It is possible to produce fissile It is possible to produce fissile isotopes from abundant non isotopes from abundant non fissionable material, a process fissionable material, a process known as conversion known as conversion

The two most important fissile The two most important fissile isotopes that can be produced by isotopes that can be produced by conversion are conversion are 233233U and U and 239239PuPu

238238U+n U+n 239239U Np PuU Np Pu

Chain Reaction Chain Reaction

Neutron emitted by fission nuclei Neutron emitted by fission nuclei induce fissions in other fissionable induce fissions in other fissionable nuclei ; the neutrons from these nuclei ; the neutrons from these fissions induce fissions in still of the fissions induce fissions in still of the fissionable nuclei, and so on. fissionable nuclei, and so on.

Chain reaction

Multiplication factor (K)Multiplication factor (K)

generationprecedinginfissionsofNumber

generationoneinfissionsofNumberK

K=1 , critical (steady state ), chain K=1 , critical (steady state ), chain reaction proceeds at constant rate .reaction proceeds at constant rate .K > 1 ,supercritical , the number of K > 1 ,supercritical , the number of fissions increase with time, therefore fissions increase with time, therefore

the energy released the energy released by the chain reaction increases with by the chain reaction increases with

time.time.K<1 , subcritical, the number of K<1 , subcritical, the number of

fission decreases with time fission decreases with time

Energy Released in Energy Released in Fission (E)Fission (E)

n+ X Yn+ X Y1 1 + Y+ Y22 + 3n + 3n

E = [ ME = [ MXX+M+Mnn - (My - (My11 + My + My22 + N M + N Mnn)]c)]c22

Where MWhere MXX is the mass of the fissile is the mass of the fissile nucleus before fission, Mynucleus before fission, My11, My, My22 and M and Mnn are Mass of fission fragments and fission are Mass of fission fragments and fission neutron after fission.neutron after fission.

On average, some 200 MeV is released On average, some 200 MeV is released per thermal fission. This energy is per thermal fission. This energy is distributed as shown distributed as shown

Nuclear reactor :Nuclear reactor :is a device inside it the fission is a device inside it the fission

chain reaction can proceed in a chain reaction can proceed in a controlled manner controlled manner

- The control is accomplished by varying the The control is accomplished by varying the value of K, which can be done by the person value of K, which can be done by the person operating the systemoperating the system

- Neutron can disappear in two ways: Neutron can disappear in two ways: 1- absorption in some type of nuclear reaction 1- absorption in some type of nuclear reaction 2- by escaping from the surface of reactor 2- by escaping from the surface of reactor

(leakage) (leakage) In case of critical state In case of critical state Neutron production rate = absorption rate + Neutron production rate = absorption rate +

leakage rate leakage rate

Moderation is a substance whose Moderation is a substance whose nuclei absorb energy from nuclei absorb energy from

incident fast neutrons that collide incident fast neutrons that collide with them without an excessive with them without an excessive

tendency to capture the neutrons tendency to capture the neutrons - to accomplish the slow down of - to accomplish the slow down of

fission neutrons, the uranium in a fission neutrons, the uranium in a reactor dispersed in a matrix of reactor dispersed in a matrix of

moderator moderator - the energy transfer between - the energy transfer between

collide objects is a maximum when collide objects is a maximum when the mass of these objects are the mass of these objects are

equal equal

Type of moderators

Three moderators in common use

1- light water (normal water)

2- heavy water (deuterium atom instead of ordinary hydrogen

3- graphite, a form of pure carbon.

Control of nuclear reaction rate

-The rate of nuclear chain reaction can be controlled by the control rods. The rods made of a material such as cadmium or boron that absorbs slow neutrons.

- As the control rods inserted further and further into the reactor , the reaction rate is progressively damped.

Type of nuclear reactor

All nuclear reactors have the same principal components ,nuclear fuel, control rods, moderator and cooling system, but they differ in cooling system .

1- Pressurized- Water Reactor (PWR) use light water reactor as a coolant. Most of commercial reactor use light water both as a moderator and as a coolant. The water that circulates past the core is kept at a sufficiently high pressure, about 150 atom to prevent boiling

The water enters the pressure vessel at a bout 280 co and leaves at about 320 c0, passing through a heat exchanger which produce steam that drives turbine.

2- Boiling – Water reactor (BWR)

Low pressure, about 68 atom, inside the vessel allowed the steam to form and this steam is separated directly and sent to the turbine.

The BWR is simpler than PWR-Light water tend to capture thermal neutron and form deuterium 1H(n, )2H. Therefore, natural uranium cannot be used as a nuclear fuel in light water reactor. Instead, enrichment uranium use-The fuel is in the form of UO2 sealed in long, thin zirconium-alloy tubes that are assembled together with movable control rods into a core that is enclosed a steel pressure vessel. - Heavy water Reactor (2H2O) is a better moderator than light water . Natural Uranium can be used as a nuclear fuel-High Temperature Gas -cooled reactor (HTGR). Graphite use her as a moderator. -Efficiency of HTGR (39%) is higher than PWR and BWR (32 to 33 %.-Graphite has a small cross section for neutron capture, so it is a good moderator.

Nuclear fuel

3.08.2007

Monju nuclear power plant

Japan

A breeder reactor Contains fissionable and non fissionable material one that can be made fissionable by absorbing a neutron. For example235U and 238U suppose 3 neutrons per fission one is needed to induce A fission in another fuel atom and keep the chain reaction going. If the other two neutrons can be used to convert two non fisionable atoms (238U) into fissionable (239Pu), the two fuel atoms are produced where one is consumed .In the example , neutrons from 235U may be used to convert non fissionable 238U to fissionable 239Pu.238U+ n 239U+ 239Np 239Pu

A prerequisite to breeding is that, the number of neutrons produced per neutron absorbed in the fuel, should be larger than 2 (>2). In the example this achieved by the used of fast neutron and so no moderator is needed.

)(

)(ReRe

JoulefissionperEnergy

wattpoweractoractorinfissionsofNumber

Example: Estimate the number of fission per second in a 100 MW reactor.

Solution:

Each fission of uranium releases about 200 MeV= 320x10-13 J.

So the number of fissions per second in a 100-MW reactor is

N=( 100x106)/(320x10-13) = 3x1018.

Fusion Reaction Fusion Reaction -Two nuclei, below about A=56, combine to form a heavier nucleus

-The energy released in fusion per unit mass of material is comparable with that released in fission( ≈ 1MeV).

The advantages of Nuclear fusion

_ Light nuclei are more plentiful than fissile nuclei

-Waste from fusion is less radioactive than waste from fission. Also the half-life of radionuclide from fusion is short and would decay away relatively rapidly. Therefore no need to store the waste for geological periods of time

--

- To fuse two nuclei wee need energy to overcome coulomb barrier. In fusion reactor, it is intended to generate this energy by heating the reactants.

-When fusion is driven by heat energy , the process is called thermo nuclear fusion. It requires a very high temperature. However, thermonuclear fusion has been achieved in laboratory.

-- when fusion driven by laser is called inerial confinement

Thermonuclear Reaction in Thermonuclear Reaction in the sun pp Chain the sun pp Chain

P + P D + eP + P D + e++ + + eeD + P D + P 33He + He + 33He + He + 33He He 44He + 2PHe + 2PThe resulting reaction being4P+ 2D + 2P +23He 2D+ 2e+ +

2e+23He + 4He + 2P Or 4P 4He + 2e+ + 2eQ = [ 4 M (1H) – M (4He) ] c2= 26.9 MeV

The fusion reactions that appear most promising for a terrestrial fusion power reactor involve deuterium 2H1 and tritium

3H2

Neutrons resulting from the reactions can be used to induce fission in a fission- fusion reaction or to take part in reactions like

Li + n 4He2 + 3H2 (T)

To release more energy.

Magnetic field Magnetic field confinement confinement

Inerial confinement Inerial confinement

Fast ingnitor conception in inertial confinement

Fusion by laser

Hiroshima before destructed by American Atomic Bomb

Hiroshima After destructed by American Atomic Bomb

Survivors

Nuclear Reactor Design

Fuel rods

Control rods

General types of proportional General types of proportional counter:counter:

Gas flow proportional counter – with Gas flow proportional counter – with window ( alpha-beta) or windowless window ( alpha-beta) or windowless (tritium measurements)(tritium measurements)

Air proportional counter (Alpha counting Air proportional counter (Alpha counting only)only)

Sealed proportional counter (e.g. BF3, Sealed proportional counter (e.g. BF3, He-3 neutron detectors).He-3 neutron detectors).

Cylindrical proportional Cylindrical proportional countercounter

Filling gasFilling gas

The fill gas in the proportional counter The fill gas in the proportional counter (and a GM detector) is usually a noble (and a GM detector) is usually a noble gases because:gases because:

noble gases are not electronegative noble gases are not electronegative don not react chemically with the don not react chemically with the

detector components detector components

Argon is the most widely used because of its low costArgon is the most widely used because of its low cost

Krypton and Xenon might be used if increased sensitivity to x- rays or Krypton and Xenon might be used if increased sensitivity to x- rays or gamma rays is requiredgamma rays is required

Air ( alpha radiation )Air ( alpha radiation ) Hydrocarbon gases (e.g., methane, propane and ethylene) can also Hydrocarbon gases (e.g., methane, propane and ethylene) can also

serve as a fill gas, but they have the disadvantage of being flammableserve as a fill gas, but they have the disadvantage of being flammable He-3 and BF3 are the most commonly employed gases in neutron He-3 and BF3 are the most commonly employed gases in neutron

detectorsdetectors

              n   +   He-3    n   +   He-3    ےے        H-3   +   pH-3   +   p++

                                n   +   B-10    n   +   B-10    ےے        Li-7   +   αLi-7   +   α++

Tissue equivalent gas mixture such as : 64.4% Tissue equivalent gas mixture such as : 64.4% methane, 32.4% carbon dioxide and 3.2% nitrogen methane, 32.4% carbon dioxide and 3.2% nitrogen might be used for certain application of dosimetry might be used for certain application of dosimetry

In general, the proportional gas should In general, the proportional gas should not contain electronegative components not contain electronegative components such as (O2,H2O,CO2,CCl4,SF6)) such as (O2,H2O,CO2,CCl4,SF6)) electron heading towards the anode will electron heading towards the anode will combine with electronegative gas. combine with electronegative gas. Happens, a negative ion goes to the Happens, a negative ion goes to the anode rather than an electron, and anode rather than an electron, and unlike the electron negative ion will fail unlike the electron negative ion will fail to produce and avalanche to produce and avalanche

During the formation of an avalanche, some During the formation of an avalanche, some gas molecules/atoms are excited rather gas molecules/atoms are excited rather than ionized. When the electrons dethan ionized. When the electrons de excite and return to their original energy excite and return to their original energy levels, they emit photons of visible light or levels, they emit photons of visible light or UV and the problem in these photons is:UV and the problem in these photons is:

can interact with the proportional gas and can interact with the proportional gas and cause the avalanche to spread along the cause the avalanche to spread along the anodeanode

can interact with the cathode wall can interact with the cathode wall

spurious pulses spurious pulses

The solution is to add a small amount of The solution is to add a small amount of a polyatomic quench gas such as a polyatomic quench gas such as methane. The quench gas preferentially methane. The quench gas preferentially absorbs the photons, but unlike the fill absorbs the photons, but unlike the fill gas (e.g., argon), it does so without gas (e.g., argon), it does so without becoming ionized becoming ionized

Operation mechanism

The Size of the Pulse The Size of the Pulse 

The size of the pulse in a proportional counter The size of the pulse in a proportional counter depends on  two things:depends on  two things:

1. Operating Voltage. The higher the operating 1. Operating Voltage. The higher the operating voltage, the larger each avalanche becomes and voltage, the larger each avalanche becomes and the larger the pulse. The applied voltage can be the larger the pulse. The applied voltage can be increased until the amount of recombination is increased until the amount of recombination is very low. However, further increase do not very low. However, further increase do not appreciably increase the number of electrons appreciably increase the number of electrons collectedcollected

2. Energy Deposited in Detector Gas.  The greater 2. Energy Deposited in Detector Gas.  The greater the energy deposited in the detector gas by an the energy deposited in the detector gas by an incident particle of radiation, the larger the incident particle of radiation, the larger the number of primary ion pairs, the larger the number number of primary ion pairs, the larger the number of avalanches, and the larger the pulse of avalanches, and the larger the pulse

Ion Pairs Collected -vs- Applied Ion Pairs Collected -vs- Applied

VoltageVoltage

Recombination Recombination The  voltage  is  such  a  low  value  that  recombination  takes The  voltage  is  such  a  low  value  that  recombination  takes

 place  before  most  of  the negative ions are collected by the  place  before  most  of  the negative ions are collected by the electrode electrode

Ionization Region Ionization Region The voltage is sufficient to ensure all ion pairs produced by the The voltage is sufficient to ensure all ion pairs produced by the

incident radiation are collected. incident radiation are collected. No gas amplification takes place. No gas amplification takes place. Proportional Region Proportional Region The voltage is sufficient to ensure all ion pairs produced by the The voltage is sufficient to ensure all ion pairs produced by the

incident radiation are collected. incident radiation are collected. Amount of gas amplification is proportional to the applied Amount of gas amplification is proportional to the applied

voltage. voltage. Limited Proportional RegionLimited Proportional Region As voltage increases, additional processes occur leading to As voltage increases, additional processes occur leading to

increased ionizations. increased ionizations. Since positive ions remain near their point of origin, further Since positive ions remain near their point of origin, further

avalanches are impossible. avalanches are impossible.

Geiger-Müller Region Geiger-Müller Region The ion pair production is independent of The ion pair production is independent of

the radiation, causing the initial the radiation, causing the initial ionization. ionization.

The field strength is so great that the The field strength is so great that the discharge continues to spread until discharge continues to spread until amplification cannot occur, due to a amplification cannot occur, due to a dense positive ion sheath surrounding dense positive ion sheath surrounding the central wire. the central wire.

Continuous Discharge Region Continuous Discharge Region The  applied voltage is  so high  that, The  applied voltage is  so high  that,

once ionization  takes place,  there is  a once ionization  takes place,  there is  a continuous discharge of electricity. continuous discharge of electricity.

Proportional counter SummaryProportional counter Summary

When radiation enters a proportional counter, the When radiation enters a proportional counter, the detector gas, at the point of the incident radiation, detector gas, at the point of the incident radiation, becomes ionizedbecomes ionized

The detector voltage is set so that the electrons The detector voltage is set so that the electrons cause secondary ionizations as they accelerate cause secondary ionizations as they accelerate toward the electrodetoward the electrode

The electrons produced from the secondary The electrons produced from the secondary ionization cause additional ionizationsionization cause additional ionizations

This multiplication of electrons is called gas This multiplication of electrons is called gas amplification.amplification.

Varying the detector voltage within the Varying the detector voltage within the proportional region increases or decreases the proportional region increases or decreases the gas amplification factorgas amplification factor

A quenching gas is added to give up electrons to A quenching gas is added to give up electrons to the chamber gas so that inaccuracies are not the chamber gas so that inaccuracies are not introduced due to ionizations caused by the introduced due to ionizations caused by the positive ion positive ion

Scintillation detector Scintillation detector

The second most common type of radiation The second most common type of radiation detecting instrument is the scintillation detecting instrument is the scintillation detector. The basic principle behind this detector. The basic principle behind this instrument is the use of a special material instrument is the use of a special material which glows or “scintillates” when radiation which glows or “scintillates” when radiation interacts with it. The most common type of interacts with it. The most common type of material is a type of salt called sodium-material is a type of salt called sodium-iodide. The light produced from the iodide. The light produced from the scintillation process is reflected through a scintillation process is reflected through a clear window where it interacts with device clear window where it interacts with device called a photomultiplier tube. called a photomultiplier tube.

The first part of the photomultiplier tube is made of another special

material called a photocathode. The photocathode has the unique

characteristic of producing electrons when light strikes its surface.

These electrons are then pulled towards a series of plates called

dynodes through the application of a positive high voltage. When

electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument. Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments.

Thermoluminescence Dosimeters(TLDs)Thermoluminescence Dosimeters(TLDs)

Exposure of a TLD to radiation causes electrons to be Exposure of a TLD to radiation causes electrons to be excited to the conduction band from which they can fall excited to the conduction band from which they can fall into one of the isolated levels provided by impurities in into one of the isolated levels provided by impurities in the crystal and be “trapped”; they will remain so until the crystal and be “trapped”; they will remain so until energy is supplied (usually by heat) to free it. Heating energy is supplied (usually by heat) to free it. Heating the crystal elevates the trapped electrons back to the the crystal elevates the trapped electrons back to the conduction band, and when they return to a valence conduction band, and when they return to a valence “hole,” a photon of visible light is emitted. The total “hole,” a photon of visible light is emitted. The total light emitted is a measure of the number of trapped light emitted is a measure of the number of trapped electrons and therefore of the total absorbed radiation electrons and therefore of the total absorbed radiation even after months of storage. TLDs are aptly named even after months of storage. TLDs are aptly named because thermal heating (thermo-) produces because thermal heating (thermo-) produces luminescence emitted by the crystal. TLDs are generally luminescence emitted by the crystal. TLDs are generally more accurate than film badges, are less subject to more accurate than film badges, are less subject to fading, are usable over a much larger range of radiation fading, are usable over a much larger range of radiation levels, can be used over and over, and processing does levels, can be used over and over, and processing does not require film development but can be done by a one-not require film development but can be done by a one-step electronic reader.step electronic reader.

Energy level diagram of a TLD crystal. Absorption of radiation excites valence electrons up to electron “traps” thus storing information on the amount of radiation absorbed; heating the TLD de-excites the trapped electrons with the emission of light photons with intensity that is proportional to the amount of radiation absorbed.

Semiconductor detectors Semiconductor detectors are solid-are solid-state devices that operate essentially like state devices that operate essentially like ionization chambers. The charge carriers ionization chambers. The charge carriers in semiconductors are not electrons and in semiconductors are not electrons and ions, as in the gas counters, but electrons ions, as in the gas counters, but electrons and hole. At present, the most successful and hole. At present, the most successful semiconductor detectors are made of semiconductor detectors are made of silicon and germanium. Other materials silicon and germanium. Other materials have been tried, however, with some have been tried, however, with some success, e.g., CdTe and HgIsuccess, e.g., CdTe and HgI22..

SEMICONDUCTOR DETECTORS

The most important advantage of the semiconductor detectors, compared to other types of radiation counters, is their superior energy resolution: the ability to resolve the energy of particles out of a poly energetic energy spectrum. Other advantages are 1. Linear response (pulse height versus particle energy) over a wide energy range 2. Higher efficiency for a given size, because of the high density of a solid relative to that of a gas 3. Possibility for special geometric configurations 4. Fast pulse rise time (relative to gas counters) 5. Ability to operate in vacuum 6. Insensitivity to magnetic fields