1. Radiation Detection Instrumentation Fundamentals

2. DETECTORS

3. General Principles of Radiation Detection
In order for the detector to respond at all, the radiation must undergo interaction through one interaction mechanisms. The interaction or stopping time is very small (typically a few nanoseconds in gases or a few picoseconds in solids). In most practical situations, these times are so short that the deposition of the radiation energy can be considered instantaneous

4. General Principles of Radiation Detection
The net result of the radiation interaction in a wide category of detectors is the appearance of a given amount of electric charge within the detector active volume. Typically, collection of the charge is accomplished through the imposition of an electric field within the detector, which causes the positive and negative charges created by the radiation to flow in opposite directions. The time required to fully collect the charge varies greatly from one detector to another. For example, in ion chambers the collection time can be as long as a few milliseconds, whereas in semiconductor diode detectors the time is a few nanoseconds. These times reflect both the mobility of the charge carriers within the detector active volume and the average distance that must be traveled before arrival at the collection electrodes.

5. General Principles of Radiation Detection
We can now introduce a fundamental distinction between three general modes of operation of radiation detectors. The three modes are called pulse mode, current mode, and mean square voltage mode. Pulse mode is easily the most commonly applied of these, but current mode also finds many applications. MSV mode is limited to some specialized applications that make use of its unique characteristics. Although the three modes are operationally distinct, they are interrelated through their common dependence on the sequence of current pulses that are the output of our simplified detector model

6. General Principles of Radiation Detection
In pulse mode operation, the measurement instrumentation is designed to record each individual quantum of radiation that interacts in the detector. In most common applications, the time integral of each burst of current, or the total charge Q, is recorded since the energy deposited in the detector is directly related to Q. All detectors used to measure the energy of individual radiation quanta must be operated in pulse mode. Such applications are categorized as radiation spectroscopy and are subject of this lecture.

7. General Principles of Radiation Detection
Characteristics of Pulse Mod Measurements
Puls Height Spectra
Energy resolution
Detection Efficiency
Dead Time

8. General Principles of Radiation Detection
Gas-Filled Detectors
Scintillation Detectors
Solid State Detectors 8

9. Gas-Filled Detectors - Components
Variable voltage source
Gas-filled counting chamber
Two coaxial electrodes well insulated from each other 9

10. Gas-Filled Detectors Basic principle
Electron-ions pairs
produced by radiation in fill gas
move under influence of electric field
produce measurable current on electrodes, or
transformed into pulse

11. Gas- Filled Detectors - one example
wall
fill gas
Output
Anode (+)
End window
Or wall A
Cathode (-) or R 11

12. Indirect Ionization Process
wall
e -
e -
e -
e -
e -
e -
e -
e -
Incident gamma photon 12

13. Direct Ionization Process
wall
beta (β-)
e -
e -
e -
e -
e -
e -
e -
e -
Incident
charged
particle 13

14. Competing Processes - recombination+
e -
Output +
e - R 14

15. Operating Regions of Gas-Filled Detectors
Proportional Region
Pulse Height
Ionization Region
Recombination Region
Continuous Discharge Region
Limited Proportional
Region
Geiger-Mueller Region   
Voltage 15

16. Ionization Chambers Voltage versus Ions Collected
Recom-
bination
region
Ionization region
Number
of Ion
Pairs
collected
Saturation Voltage
100 % of initial
ions are collected
Voltage 16

17. Saturation Current
The point at which 100% of ions begin to be collected
All ion chambers operate at a voltage that produces a saturation current
The region over which the saturation current is produced is called the ionization region
It levels the voltage range because all charges are already collected and rate of formation is constant 17

18. Observed Output: Pulse Height
Ions collected
Number of ionizations relate to specific ionization value of radiation
Gas filled detectors operate in either
current mode
Output is an average value resulting from detection of many values
pulse mode
One pulse per particle 18

19. Pulse Height Variation
Alpha
Particles
Pulse
Height
Beta
Particles
Gamma Photons
Detector Voltage 19

20. Ionization Region Recap
Pulse size depends on number of ions produced in detector.
No multiplication of ions due to secondary ionization (gas amplification is unity)
Voltage produced (V) = Q/C
Where
Q is total charge collected
C is capacitance of the ion chanber

21. Ionization Chambers, continued
Chamber’s construction determines is operating characteristics
Physical size, geometry, and materials define its ability to maintain a charge
Operates at a specific voltage
When operating, the charge collected due to ionizing events is
Q = CΔV

22. Ionization Chambers, continued
The number of ions (N) collected can be obtained once the charge is determined:
N = Q / k
Where k is a conversion factor
(1.6 x 10-19C/e)

23. Current vs. Voltage for Fill Gases in a Cylindrical Ion Chamber
Air at high pressure
100
Helium at high pressure
Relative
Current
(%) 10
Air at low pressure
1.0
Helium at low pressure
0.1
Applied Voltage (volts) 23

24. Examples
CanberraL The portable Ground Monitor System comprises a beta/gamma detector assembly consisting of five pancake GM tubes mounted in a plastic housing attached to a lightweight adjustable carrying handle.
Output of the detector assembly is connected to a battery operated ratemeter carried by a strap round the user's neck.
A headphone connected to the ratemeter provides an audible output of the detector assembly.
LND - examples of types of detector tubes - end window, pancake, etc. 24

25. Proportional Counters
Operates at higher voltage than ionization chamber
Initial electrons produced by ionization
are accelerated with enough speed to cause additional ionizations
cause additional free electrons
produces more electrons than initial event
Process is termed: gas amplification 25

26. Pulse-Height Versus Voltage
Ionization
Region
Proportional Region
Recombination Region
Pulse Height  
Voltage 26

27. Selection of the Operating Voltage
For charged radiations such as alpha or beta particles, a signal pulse will be produced for every particle that deposits a significant amount of energy in the fill gas.
The proportional counters are seldom operated in a mode that is sensitive to single avalanches in order to prevent nonlinerities for larger pulses due to space charge efects.

28. Fill gas selected to enhance gas multiplication
no appreciable electron attachment
most common is P-10 (90% Argon and 10% methane) 28

29. Distinguishing Alpha & Beta
Proportional counters
can distinguish between different radiation types
specifically alpha and beta-gamma
Differential detection capability
due to size of pulses produced by initial ionizing events
requires voltage setting in range of 900 to 1,300 volts
alpha pulses above discriminator
beta/gamma pulses too small 29

30. Alpha Counting
For alpha particle sources, proportional counters can record easily each particle that enters the active volume with virtually 100% efficiency.
Absolute measurements of alpha source activity are therefore relatively straightforward and involve evaluation of the thje effective solid angle subtended by the counter active volume.
Windowless

31. Beta Counting
For beta particles of typical energies, the particle range greatly exceeds the chamber dimensions.
The numbers of ion pairs formed in the gas is then proportional to only that small fraction of the particcle energy lost in the gas before reaching the opposite wall

32. Gas-Flow Proportional Counter

33. Gas-Flow Proportional Counter
Fill gas
outlet
Fill gas
inlet
anode
(window-
optional)
Detector
O-ring
sample
Sample planchet 33

35. Variants of the Proportional Counter Design Position-Sensitive Proportional Counters

36. Multiwire Proportional Counters
In some situations, it is advantageous to provide more than one anode wire within a proportional counter.
Detectors with very large surface area can be constructed by placing a grid of anode wires between two large flat planes that serves as cathodes on either side of the counter

37. Multiwire Proportional Counters

38. Microstrip Gas Chamber
On insulating substrate , metallic electrodes are formed by etching techniques.
The anode structures are kept quite narrow (typically 10 microns) so that the same type of concentration of the electric field that occurs around the wire is realized near the surface of the anode strip.
Thus avalanches will be formed as electrons are drawn in to the anode strip surface

39. Microstrip Gas Chamber

40. Geiger Mueller Detectors
Operate at voltages above proportional detectors
Each primary ionization
produces a complete avalanche of ions throughout the detector volume
called a Townsend Avalanche
continues until maximum number of ion pairs are produced
avalanche may be propagated by photoelectrons
quenching is used to prevent process 40

41. Geiger Mueller Detectors
A typical pulse from Geiger tube represents a large amount of collected charge, about ion pairs being formed in the discharge.
No proportional relationship between energy of incident radiation and number of ionizations detected
A level pulse height occurs throughout the entire voltage range 41

42. Geiger Mueller Detectors Fill Gases
Noble gases with helium and argon the most popular choices.
Even trace amounts of gases that forms negative ions (such as oxygen) must be avoided
Operate at atmospheric pressure or tenths of an atmospheric presurre.
Operating voltages up to 2000 V.

43. Geiger Mueller Detectors Quenching
The problem of multiple pulsing is severe in Geiger tubes.
Internal quenching is accomplished by adding a second component called the quenching gas to the primary fill gas, in proportion of 5-10%.
Large organic molecules like ethyl alcohol and ethyl formade prevent the reemission of free electrons from the cathod.

46. Advantages/Disadvantages of Gas-Filled Detectors
Ion Chamber: simple, accurate, wide range, sensitivity is function of chamber size, no dead time
Proportional Counter: discriminate hi/lo Linear Energy Transfer, higher sensitivity than ion chamber
GM Tube: cheap, little/no amplification, thin window for low energy; limited life 46

47. Issues with Gas Filled Detectors: Dead Time
Minimum time at which detector recovers enough to start another avalanche (pulse)
The dead time may be set by:
limiting processes in the detector, or
associated electronics
“Dead time losses”
can become severe in high counting rates
corrections must be made to measurements 47

48. Issues with Gas Filled Detectors: Recovery Time
Time interval between dead time and full recovery
Recovery Time = Resolving time- dead time 48

49. Issues with Gas Filled Detectors: Resolving Time
Minimum time interval that must elapse after detection of an ionizing particle before a second particle can be detected. 49

50. Relationship among dead time, recovery time, and resolving time
100
200
300
400
500
Pulse
Height
Dead
Time
Recovery time
Resolving time
Time, microseconds 50

51. SCINTILLATION DETECTORS
CANBERRA; Scintillation Detectors
A gamma ray interacting with a scintillator produces a pulse of light, which is
converted to an electric pulse by a photomultiplier tube. The photomultiplier
consists of a photocathode, a focusing electrode and 10 or more dynodes
that multiply the number of electrons striking them several times each. The
anode and dynodes are biased by a chain of resistors typically located in a
plug-on tube base assembly. Complete assemblies including scintillator and
photomultiplier tube are commercially availabl.
The properties of scintillation material required for good detectors are
transparency, availability in large size, and large light output proportional to
gamma ray energy. Relatively few materials have good properties for
detectors. Thallium activated NaI and CsI crystals are commonly used, as
well as a wide variety of plastics. NaI is the dominant material for gamma
detection because it provides good gamma ray resolution and is economical.
However, plastics have much faster pulse light decay and find use in timing
applications, even though they often offer little or no energy resolution. 51

52. SCINTILLATION DETECTORS

53. Scintillators Emit light when irradiated Can be promptly (<10-8s)
fluorescence
delayed (>10-8s)
phosphorescence
Can be
liquid
solid
gas
organic
inorganic 53

54. Scintillators
The fluorescence process in organics arises from transitions in the energy level structure of a single molecule and therefore can be observed from a given molecular species independent of its physical state
In contrast, crystalline inorganic scintillators require a regular crystalline lattice as basic for the scintillation process.

55. Basis of Scintillation - Energy Structure in a Molecule
Excited state A1
EA1
Ground state
EB1 B1
EB0 Bo
EA0 Ao
Interatomic distance

56. Basis of Scintillation - Energy Structure in an Atom
Excited state
Ground state, last filled (outer) orbital

57. Organic Scintillators
Examples
Differences
Energy
CH3
Excited state A1
EA1
Toluene
Ground state
EB1
The materials that are efficient organic scintillators belong to the class of aromatic compounds. They consist of planar molecules made of of benzene rings. Two examples are toluene and anthracene, having the structure shown above.
Organic scintillators are mixtures of compounds that consist of a solvent and solute(s). Examples of solvents (the substance with the highest concentration) include benzene, toluene, and p-Xylene. There are many others.
Examples of solutes include p-Terphenyl, PPO (2,5-diphenalyoxazole), and PDB ((2-Pheny l, 5-(4-biphenyl)-1,2,4-oxadiazole). There are primary and secondary solutes and solvents.
Light production in organic scintillators is the result of molecular transitions. The potential energy of a molecule (which comes from electronic, vibrational and translational energies) changes with interatomic distance. The ground state is at point A0. Ionizing radiation passing through the scintillator can raise the molecule to an excited state (A1). The molecule wants to return to the ground state, and loses energy through vibrations (heat) to move to point B1. This is still not at the lowest energy state, and so the molecule will emit energy (a photon) to move it to point B0. This transition is rapid (10-8 s), and results in a measurable photon of energy different than the incident one. B1
EB0 Bo
EA0 Ao
Anthracene
Interatomic distance 57

58. Inorganic (Crystal) Scintillators
Most are crystals of alkali metals (iodides)
NaI(Tl)
CsI(Tl)
CaI(Na)
LiI(Eu)
CaF2(Eu)
Impurity in trace amounts
“activator” causes luminescence
e.g., (Eu) is 10-3 of crystal
The high Z of iodine in NaI gives good efficiency for gamma ray detection. A
small amount of Tl is added in order to activate the crystal, so that the
designation is usually NaI(Tl) for the crystal. The best resolution achievable
ranges from 7.5%-8.5% for the 662 keV gamma ray from 137Cs for 3 in.
diameter by 3 in. long crystal, and is slightly worse for smaller and larger
sizes. Figure 1.6 shows, respectively, the absorption efficiencies of various
thicknesses of NaI crystals, and the transmission coefficient through the most
commonly used entrance windows. 58

59. Energy band structure of an activated cystalline scintillator

60. Scintillator efficiency
One electron-pair creation requires in, NaI, aprox. 20 eV charged particle energy lost (three times the gap band).
For 1 MeV energy deposited in scintillator aprox 5 x 10 4 electron-holes are created.
For NaI(Tl) the absolute scintillation efficiency is about 12% which results in 1.2 x 105 eV in light energy or 4 x 104 photons with average energy of 3 eV.

61. Selecting Scintillators - Density and Atomic number
Efficient detection of gamma-rays requires material with a high density and high Z
Inorganic scintillation crystals meet the requirements of stopping power and optical transparency,
Densities range from roughly 3 to 9 g/cm3
Very suitable to absorb gamma rays.
Materials with high Z-values are used for spectroscopy at high energies (>1 MeV).

62. Commonly Used Scintillators
Material
Density [g/cm3]
Emission Max [nm]
Decay Constant (1)
Refractive Index (2)
Conversion Efficiency (3)
Hygro-scopic
NaI(Tl)
3.67
415
0.23 ms
1.85
100
yes
CsI(Tl)
4.51
550
0.6/3.4 ms
1.79 45 no
CsI(Na)
420
0.63 ms
1.84 85
slightly
CsI
undoped
315
16 ns
1.95
4 - 6
CaF2 (Eu)
3.18
435
0.84 ms
1.47 50
6LiI (Eu)
4.08
470
1.4 ms
1.96 35
6Li - glass
2.6
60 ns
1.56
CsF
4.64
390
3 - 5 ns
1.48
5 - 7
(1) Effective average decay time For g-rays. (2) At the wavelength of the emission maximum. (3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode.

63. Conversion Efficiency (3)
Commonly Used Scintillators
Material
Density [g/cm3]
Emission Maximum [nm]
Decay Constant (1)
Refractive Index (2)
Conversion Efficiency (3)
Hygroscopic
BaF2
4.88
315
220
0.63 ms
0.8 ns
1.50
1.54 16 5 no
YAP (Ce)
5.55
350
27 ns
1.94
GSO (Ce)
6.71
440 ns
1.85
BGO
7.13
480
0.3 ms
2.15
CdWO4
7.90
470 / 540
20 / 5 ms
2.3
Plastics
1.03
1 - 3 ms
1.58
(1) Effective agerage decay time For g-rays. (2) At the wavelength of the emission maximum. (3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode.

64. Mechanical and Optical Properties
NaI(Tl) is one of the most important scintillants.
Hygroscopic
Can only be used in hermetically sealed metal containers
Some scintillation crystals may easily crack or cleave under mechanical pressure
CsI is “plastic” and will deform.
Important aspects of commonly used scintillation materials are listed on the next 2 slides.
The list is not exhaustive, and each scintillation crystal has its own specific application.
For high resolution spectroscopy, NaI(Tl), or CsI(Na) (high light output) are normally used.
For high energy physics applications, the use of bismuth germanate Bi4Ge3O12 (BGO) crystals (high density and Z) improves the lateral confinement of the shower.
For the detection of beta-particles, CaF2(Eu) can be used instead of plastic scintillators (higher density).

65. Decay time
Scintillation light pulses (flashes) are usually characterized by a fast increase of the intensity in time (pulse rise time) followed by an exponential decrease.
Decay time of a scintillator is defined by the time after which the intensity of the light pulse has returned to 1/e of its maximum value.
Most scintillators are characterized by more than one decay time and usually, the effective average decay time is given
The decay time is of importance for fast counting and/or timing applications

66. Afterglow
Defined as the fraction of scintillation light still present for a certain time after the X-ray excitation stops.
Originates from the presence of millisecond to even hour long decay time components.
Can be as high as a few % after 3 ms in most halide scintillation crystals .
CsI(Tl) long duration afterglow can be a problem for many applications.
Afterglow in halides is believed to be intrinsic and correlated to certain lattice defects.
BGO and Cadmium Tungstate (CdWO4) crystals are examples of low afterglow scintillation materials

67. Emission Spectra of Scintillation Crystals
Each scintillation material has characteristic emission spectrum.
Spectrum shape is sometimes dependent on the type of excitation (photons / particles).
Emission spectrum is important when choosing the optimum readout device (PMT /photodiode) and the required window material.
Emission spectrum of some common scintillation materials shown in next two slides.

68. Emission Spectra of Scintillators

69. Emission Spectra of Scintillators A typical human eye will respond to wavelengths from about 390 to 700 nm.

70. Temperature Influence on the Scintillation Response
Light output (photons per MeV gamma) of most scintillators is a function of temperature.
Radiative transitions, responsible for the production of scintillation light compete with non-radiative transitions (no light production).
In most light output is quenched (decreased) at higher temperatures.
An exception is the fast component of BaF2 where intensity is essentially temperature independent.

71. Temperature Influence on the Scintillation Response

72. Scintillators - Neutron Detection
Neutrons do not produce ionization directly in scintillation crystals
Can be detected through their interaction with the nuclei of a suitable element.
6LiI(Eu) crystal -neutrons interact with 6Li nuclei to produce an alpha particle and 3H which both produce scintillation light that can be detected.
Enriched 6Li containing glasses doped with Ce as activator can also be used.

73. Neutron Detection

74. Neutron Detection
Conventional neutron meters surround a thermal neutron detector with a large and heavy (20 lb) polyethylene neutron moderator.
Other meters utilizes multiple windows formed of a fast neutron scintillator (ZnS in an epoxy matrix), with both a thermal neutron detector and a photomultiplier tube.

75. Radiation Damage in Scintillators
Radiation damage results in change in scintillation characteristics caused by prolonged exposure to intense radiation.
Manifests as decrease of optical transmission of a crystal
decreased pulse height
deterioration of the energy resolution
Radiation damage other than activation may be partially reversible; i.e. the absorption bands disappear slowly in time.

76. Radiation Damage in Scintillators
Doped alkali halide scintillators such as NaI(Tl) and CsI(Tl) are rather susceptible to radiation damage.
All known scintillation materials show more or less damage when exposed to large radiation doses.
Effects usually observed in thick (> 5 cm) crystals.
A material is usually called radiation hard if no measurable effects occur at a dose of 10,000 Gray. Examples of radiation hard materials are CdWO4 and GSO.

77. Detecting Scintillator Output:- PhotoCathode & Photomultiplier Tubes
Radiation interaction in scintillator produces light (may be in visible range)
Quantification of output requires light amplification and detection device(s)
This is accomplished with the:
Photocathode
Photomultiplier tube
Both components are
placed together as one unit
optically coupled to the scintillator 77

78. Cutaway diagram of solid-fluorescent scintillation detector
Photocathode
Scintillation
event
Photomultiplier tube
Gamma ray
Dynodes
Photoelectrons
Fluor crystal NaI (Tl)
Reflector housing 78

79. Photomultiplier

80. Photomultiplier tubes
PMTs perform two functions:
Conversion of ultraviolet and visible light photons into an electrical signal
Signal amplification, on the order of millions to billions
Consists of an evacuated glass tube containing a photocathode, typically 10 to 12 electrodes called dynodes, and an anode

82. Dynodes
Electrons emitted by the photocathode are attracted to the first dynode and are accelerated to kinetic energies equal to the potential difference between the photocathode and the first dynode
When these electrons strike the first dynode, about 5 electrons are ejected from the dynode for each electron hitting it
These electrons are attracted to the second dynode, and so on, finally reaching the anode

83. PMT amplification
Total amplification of the PMT is the product of the individual amplifications at each dynode
If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately 10,000,000
Amplification can be adjusted by changing the voltage applied to the PMT

84. Position-Sensing Photomultipliers Tubes
Low noise
(1 p.e. level detectable)
Position sensitive PMT
Large effective surface
> 80 % physical area
Compact
26 x 26 x 27 mm3
Operate at low voltage
~ 900 V, gain ~ 2 ×106
R M16MOD-UBA
We have made two additional improvements
Ultra bi-alkali photo-cathode Q.E. > 40 %

85. Multichannel Plates

86. Field Applications for Liquid and Solid Scintillation Counters
Solid Scintillators
in-situ measurement of low to high energy gammas
laboratory systems
spectroscopy
SCA or MCA mode
Liquid Scintillators
wipe tests
contaminants in solids (concrete)
contaminants in aqueous/organic liquids 86

87. Light Collection and Scintillator Mounting
Light Pipes

88. Fiber Scintillators
Some scintillation materials can be fabricated as small diametr fibers in which a fraction of the scintillation light is conducted over substantial distance by the total internal reflection

89. 43-93 Alpha/Beta Scintillator
The Model is a 100 cm² dual phosphor alpha/beta scintillator that is designed to be used for simultaneously counting alpha and beta contamination

90. 44-2 Gamma Scintillator
The Model 44-2 is a 1" X 1" NaI(Tl) Gamma Scintillator that can be used with several different instruments including survey meters, scalers, ratemeters, and alarm ratemeters

91. Scintillation Detectors Conclusions
Best:
Measure low gamma dose rates
Also:
Measure beta dose rates (with corrections)
However:
Somewhat fragile and expensive
CANNOT:
Not intended for detecting contamination, only radiation fields

92. Semiconductor Detectors92

93. Semiconductor Detectors
For the measurements of high energy electrons or gamma rays, solid detectors can be kept much smaller than the equivalent gas-filled detectors (solid densities 1000 times greater than for a gas).
Scintillators have a poor energy resolution; the chain of events that must take place in converting the incident radiation energy to light and then to electrical signal involves many inefficient steps.

94. Idealized Gamma-Ray Spectrum in NaI
theoretical
Counts
per
Energy
Interval
Actual
Energy Eo 94

95. Components of Spectrum Semiconductor detector
Compton edge
Backscatter
Peak
Photopeak
Counts
per
Energy
Interval
X-ray
Peak
Annihilation
Peak
Energy Eo 95

96. NaI(Tl) vs. HPGE 96

97. NaI(Tl) vs. HPGE 97

98. Semiconductor Detectors
Solids have
lattice structure (molecular level)
quantized energy levels
valence bands
conduction bands
Semiconductors have lattice structure
similar to inorganic scintillators
composed of Group IVB elements
ability to easily share electrons with adjoining atoms
Solids have allowed and forbidden bands of energy values which the most loosely bound (valence) electrons can possess. The origin of the band of energy levels is a consequence of the splitting of discrete atomic energy levels when multiple atoms are brought together into a regular array. When the number of atoms is very large, a continuous energy band results, and you have the formation of the conduction and valence bands for the solid.
The valence band can be considered the outermost electron level around the atom which is still held to the atom. The conduction band loosely encompasses all the atoms in the solid. If an electron is promoted to the conduction band, then it can freely migrate across the solid. An electron promoted from the valence band leaves behind a positively charged “hole”. This hole can similarly migrate across all the atoms in the solid.
The gap between the valence and conduction band is called the “forbidden gap”. Electrons will have a continua of energy values in the valence and conduction band. No states with energies that fall within the forbidden band will exist.
If the valence band is full, and the forbidden gap is large (> 5 eV), then the material will be an insulator. 98

99. Semiconductor Detectors Band Structure

100. Semiconductor Diode Detectors Crystal LatticeGe
As+
Addition of a small quantity of pentavalent As to Ge crystal lattice produces very loosely bound “extra” electrons that have a high probability of being thermally excited into the conduction band at room temperatures. Arsenic is called a donor impurity because it has five valence electrons whereas germanium only has four. There is one electron left over after the As “sits” in the lattice and all the covalent bonds have been formed. The loop respresents the orbit of the extra electron. Because it is not tightly bound to the As ion it can be excited into the conduction band. The conductivity of the doped Ge is therefore much higher than the pure material. Arsenic is the “dopant” in this lattice and donates an extra electron. As a result, this semiconductor is called an n-type (for negative charge).
Shared
electron pair
100

101. Basic Nature of Semiconductors
Schematic view of lattice of Group IVB element Si
Dots represent electron pair bonds between the Si atoms Si

102. Basic Nature, cont’d
Schematic diagram of energy levels of crystalline Si.
Pure Si is a poor conductor of electricity
Conduction Band
1.08 eV
Energy
Forbidden Gap
Valence Band

103. Basic Nature, cont’d
Schematic view of lattice of Group IV element Si, doped with P (Group VB) as an impurity – note extra electron Si P

104. Basic Nature, cont’d Conduction Band Energy Valence Band 0.05 eV
Schematic diagram of disturbed energy levels of crystalline Si.
Si with Group V impurities like P is said to be an n-type silicon because of the negative charge carriers (the electrons)
Conduction Band
0.05 eV
Donor level
Energy
Valence Band

105. Basic Nature, cont’d
Schematic view of lattice of Group IV element Si, doped with B (Group IIIB) as an impurity – note hole in electron orbital Si Si Si B Si Si

106. Basic Nature, cont’d
Schematic diagram of disturbed energy levels of crystalline Si with B impurity.
Si with Group III impurities is said to be a p-type silicon because of the positive charge carriers (the holes)
Conduction Band
Energy
Acceptor level
0.08 eV
Valence Band

107. Occupation of energy states for n and p-type semiconductors
As donor
impurity
levels
Conduction Band
0.013 eV
0.67 eV
0.011eV
The energy level diagram for a p-type Ge semiconductor with Ga acceptor atoms added, and for an n-type semiconductor with As donor atoms added
When the crystal is doped with an electron donating material, the electrons can enter the conduction band, greatly increasing the conductivity of the material. When the crystal is doped with a hole donating material (such as Ga), the dope crystal contains positively charged holes which can accept electrons. Holes in the valence band move like positive charges as electrons from neighboring atoms fill them. This also results in enhanced conductivity of the crystal..
Ga acceptor
impurity
levels
Valence Band
After Turner
107

108. Operating Principles of Semiconductor Diode detectors
Si semiconductor is a layer of p-type Si in contact with n-type Si.
What happens when this junction is created?
Electrons from n-type migrate across junction to fill holes in p-type
Creates an area around the p-n junction with no excess of holes or electrons
Called a “depletion region”
Apply (+) voltage to n-type and (-) to p-type:
Depletion region made thicker
Called a “reverse bias”

109. Energy-level diagram for n-p junction
Conduction
Band
n-type
Semiconductors are useful as radiation measuring devices because of special properties that occur at a junction where n- and p-type semiconductors are brought together. If they are placed in good contact they form a single system. This system has its own Fermi energy. Because it is just below the conduction band in the n-region, and just above the valence band in the p-region, the bands must be deformed over the region of the junction.
When the two materials are initially placed in contact, electrons flow from the donor levels to the acceptor side. As a result, negative charge accumulates on the p side and positive charges are left on the n side. There is a net separation of charges (indicated by + and - in the figure). This region is called the depletion region, because, initially, there are no mobile charges.
As a result of depletion in this region, the detector behaves much like a parallel plate ionization chamber. When ionizing radiation creates ion pairs in this region of the crystal, they will migrate according to sign, and give rise to an electrical signal. A bias voltage can also be placed on the system to prevent recombination and to limit noise.
Junction
region
Valence
Band
p-type
109

110. Detector specifics
Depletion region acts as sensitive volume of the detector
Passage of ionizing radiation through the region
Creates holes in valence band
Electrons in conduction band
Electrons migrate to positive charge on n side
Holes migrate to negative voltage on p side
Creates electrical output
Requires about 3.6 eV to create an electron hole pair in Si

111. Detector Specifics, cont’d
Reverse bias n-p junction is good detector
Depletion region
Has high resistivity
Can be varied by changing bias voltage
Ions produced can be quickly collected
Number of ion pairs collected is proportional to energy deposited in detector
Junction can be used as a spectrometer
Types of detectors:
HPGe
GeLi (lithium drifted detectors)
Surface barrier detectors
Electronic dosimeters

112. Germanium Gamma-Ray Detectors
The simple junction and surface barrier detectors are not so suitable for more penetrating radiation than apha particles. The major limitation is the maximum depletion depth or active volume; maximum depletion is about 2 mm in normal purity semiconductors.

113. Germanium Gamma-Ray Detectors
Germanium detectors produced by lithium drifting process, Ge(Li), have a thickness up to 2 cm. After the growth of the germanium crystal, interstitial lithium donors atoms are added in order to compensate the residual acceptor impurities.
High Purity Germanium detectors with a impurity concentration of 1010 atoms/cm3 have a depletion depth of several cm.

114. 114

115. Cooled Semiconductor Detector

116. PRACTICAL SOLID STATE DETECTORS
The main applications for semiconductor detectors are in the laboratory for the spectrometry of both heavy charged (alpha) particle and gamma radiations.
However, energy compensated PIN diodes and special photodiodes are used as pocket electronic (active) dosimeters.
PIN diode: Acronym for positive-intrinsic-negative diode.
A photodiode with a large, neutrally doped intrinsic region sandwiched between p-doped and n-doped semiconducting regions.
A PIN diode exhibits an increase in its electrical conductivity as a function of the intensity, wavelength, and modulation rate of the incident radiation. Synonym PIN photodiode.

117. Semioconductor Detectors for Alpha Spectroscopy

118. PIN Diodes
Ordinary Silicon PIN photodiodes can serve as detectors for X-ray and gamma ray photons. The detection efficiency is a function of the thickness of the silicon wafer. For a wafer thickness of 300 microns (ignoring attenuation in the diode window and/or package) the detection efficiency is close to 100% at 10 KeV, falling to approximately 1% at 150 KeV(3).
For energies above approximately 60 KeV, photons interact almost entirely through Compton scattering. Moreover, the active region of the diode is in electronic equilibrium with the surrounding medium--the diode package, substrate, window and outer coating, etc., so that Compton recoil electrons which are produced near--and close enough to penetrate--the active volume of the diode, are also detected.
For this reason the overall detection efficiency at 150 KeV and above is maintained fairly constant (approximately 1%) over a wide range of photon energies.
Thus, a silicon PIN diode can be thought of as a solid-state equivalent to an ionization-chamber radiation detector.
Ordinary Silicon PIN photodiodes can serve as detectors for X-ray and gamma ray photons. The detection efficiency is a function of the thickness of the silicon wafer. For a wafer thickness of 300 microns (ignoring attenuation in the diode window and/or package) the detection efficiency is close to 100% at 10 KeV, falling to approximately 1% at 150 KeV(3).
For energies above approximately 60 KeV, photons interact almost entirely through Compton scattering. Moreover, the active region of the diode is in electronic equilibrium with the surrounding medium--the diode package, substrate, window and outer coating, etc., so that Compton recoil electrons which are produced near--and close enough to penetrate--the active volume of the diode, are also detected. For this reason the overall detection efficiency at 150 KeV and above is maintained fairly constant (approximately 1%) over a wide range of photon energies. Thus, a silicon PIN diode can be thought of as a solid-state equivalent to an ionization-chamber radiation detector.
DC Current-Mode Operation
The DC-current response to gamma radiation incident on a PIN diode detector can be estimated as follows:
Let A = area of the diode in cm2
N = flux of incident gamma rays (gamma's / second-cm2)
r = detection efficiency (assume constant = .01)
Ê = average energy of recoil electrons in detector, expressed in eV
s = ionization constant in silicon = 3.6 eV / electron-hole pair(1)
e = electronic charge = 1.6 x coulombs
Then the average current produced in the diode is given by:
I = N A r Ê e / s
Example--Consider a 10 mCi point-source of ß+- emitting activity (gamma-ray energy = 511 KeV annihilation radiation) at a distance of 10 cm from a type PDC 24S PIN diode detector (Detection Technology, Inc., Micropolis, Finland). The diode area is .057 cm2. The flux at the detector, N = (2 x 10 x 3.7 x 107) / (4 pi x 102 ) = 588,870 gamma photons / cm2-second. This is equivalent to an exposure doserate at the face of the detector of 0.54 roentgens / hour.
An exact expression for the average energy of a Compton recoil electron may be found in (6). An approximate formula -- accurate enough to be useful over a wide range of gamma ray energies* -- is given by Ê = 1/2 Emax, where Emax is derived from Compton's formula for the energy of the 180o (backscattered) photon:(4)
Emax = hv {2 hv / (m0c2 + 2 hv)}
hv = the energy of the incident photon (expressed in eV)
m0c2 = 511,000 eV
In this example hv = 511,000 eV, so that Emax = 340,000 eV, and Ê = 170,000 eV.
The radiation-induced current in the diode is therefore 588,870 x .057 x .01 x 170,000 x 1.6 x / 3.6 = 2.53 x amperes. Thus the scale factor relating current to exposure doserate in this example is (2.53 picoamperes) / (0.54 roentgens per hour), or 1.68 x 10-8 coulomb / roentgen.
One roentgen produces, by definition, 3.33 x coulombs in 1 cm3 of standard air(5). Thus, our .057 cm2 diode has the same radiation-induced-current sensitivity as a (1.68 x 10-8) / (3.33 x ) = 50 cm3 standard air-ionization chamber.
*The estimate, Ê, is ~ 2% high at 80 KeV, dropping to ~3% low at 511 KeV, dropping further to ~10% low at 1000 KeV.
Pulse-Mode Operation--Radiation Survey and Monitoring Applications
Stable, reliable operation at low-to-medium exposure doserates in general radiation survey and monitoring applications is enhanced by operating the PIN diode detector in AC-coupled pulse-mode. This essentially eliminates drift and instability due to changes in system parameters, such as diode leakage current, with time and temperature.
In this mode of operation the diode is closely coupled to a charge-integrating preamplifier of our own design (USA patents 5,990,745 and 6,054,705), so that individual x-ray or gamma-ray photon interactions are detected as discrete pulses of current. The preamplifier gain, expressed in units of "volts per unit charge" is 1 / Cint, where Cint is the value of the integrating capacitor which, in this implementation, is of the order of 2 x farads.
Example: Assume that an incident 511 KeV photon produces a 340 Kev recoil electron in the diode. This, in turn, produces a charge of 340,000 x 1.6 x / 3.6 = 1.51 x coulombs deposited in 2 picofarads, or a voltage pulse whose amplitude = 7.55 millivolts. Individual voltage pulses are then further amplified, thresholded, and integrated.
We eliminate system noise by introducing a low-energy threshold before the input to a bipolar junction transistor - charge-pump. This, in turn, is followed by an RC-integrating filter with a time-constant nominally = 1 second. The overall system gain beyond the preamp is set so that a 1.33 mV preamp-output pulse (60 KeV photon energy) just exceeds the threshold.
The input current to the charge-pump is set by a series resistor. The output of the charge pump / filter is a DC current proportional to doserate in the detector which may be read by a meter, chart recorder, or computer data-acquisition system. In addition, a current-to-pulse-rate converter provides a TTL-compatible output for convenient interfacing to computer process-control and monitoring systems.
Spectroscopy Applications
In addition to general survey and monitoring applications, the same basic system detector and amplifier concept can be used for spectroscopy applications, where the detector diode is optically-coupled to a scintillating crystal such as CsI(Tl) for gamma-ray spectroscopy, or where the detector diode is used directly to detect x-rays below 60 KeV.
Spectroscopy applications demand the best in low-noise performance from the detector and its amplifying system. The circuit components and active devices must be selected specifically to match the application at hand. In particular, the amplifying transistors (JFET's) at the input of the charge-integrating preamplifier must be chosen to match the junction capacitance of the detector diode in order to achieve the lowest possible noise and best pulse-height resolution. In addition, the time constant(s) in the shaping amplifier must be optimized to minimize line-broadening effects due to noise and from "ballistic deficit" effects due to non-uniform charge-collection times in the diode detector.
Below is an example of a gamma-ray / x-ray spectrum from an 241Am check-source attached to the surface of a 2.7 mm x 2.7 mm diode (Cj = 15 pF, Vbias = 24 VDC). The prominent source peaks are:
Energy (KeV) Percent Abundance(2)
13.9 KeV_________13.2
17.8 KeV_________19.25
20.8 KeV__________4.85
26.35 KeV_________2.4
59.54 KeV________35.9
The large peak at 8 KeV is due to x-ray fluorescence from a thin copper foil attenuator placed between source and detector.
Spectral measurements are done at room temperature. The shaping-time is 8 µsec. The pulse-height resolution is 2.07 KeV (FWHM) at 59.5 KeV. The second trace, whose peak is shown centered at 43 KeV, is from a pulser and shows the line-broadening effects of electronic noise in the diode detector and preamplifier. The pulser-peak is 1.80 KeV wide (FWHM), which corresponds to an equivalent noise charge in the system "front end" of 212 e- rms.
References:
Knoll, Glenn F., Ch. 13 in Radiation Detection and Measurement John Wiley and Sons, New York, 1979
ibid.
Silicon Photodiodes and Charge Sensitive Amplifiers for Scintillation Counting and High Energy Physics Hamamatsu Photonics K.K., Solid State Division, Catalog #KOTH0002E02, June,1993
Evans, R.D., Ch. 23 in The Atomic Nucleus McGraw-Hill, New York, 1955
Fitzgerald, J.J., Brownell, G.L., Mahoney, F.J., Ch 2 in Mathematical Theory of Radiation Dosimetry Gordon and Breach Science Publishers, New York, 1967
op. cit., Chapter 4
118

119. Positrons emission tomography

120. SPIN 224

121. SPIN 224

122. Digital Radiography

123. Digital Radiography
Receptor Type Amorphous Silicon
Conversion Screen DRZ Plus
Pixel Area Total x 40.6 cm (11.5 in. x 16.0 in)
Active x 40.5 cm (11.5 in. x 16.0 in)
Pixel Matrix Total ,304 (h) x 3,200 (v)
Active ,304 (h) x 3,200 (v)
Pixel Pitch μm
Limiting Resolution lp/mm
DQE (with DRZ Plus) > 30%
MTF, X-Ray (with DRZ Plus) >45% (1 lp/mm)
Energy Range kVp
Fill Factor %
Contrast Ratio Large Area (12 cm): <2%
Small Area (1 cm): <10%
Scan Method Progressive
A/D Conversion bits
Frame Rate fps (1 x 1)
7 fps (2 x 2)
Data Output Gigabit Ethernet

124. Digital Radiography

125. Another type of Solid State / Scintillation system
Thermoluminescent Dosimeters

126. Thermoluminescence
(TL) is the ability to convert energy from radiation to a radiation of a different wavelength, normally in the visible light range.
Two categories
Fluorescence - emission of light during or immediately after irradiation
Not a particularly useful reaction for TLD use
Phosphorescence - emission of light after the irradiation period. Delay can be seconds to months. 
TLDs use phosphorescence to detect radiation. 

127. Thermoluminescence Radiation moves electrons into “traps”
Heating moves them out
Energy released is proportional to radiation
Response is ~ linear
High energy trap data is stored in TLD for a long time
Conduction band
Valence band
Forbidden energy gap
Trapping centres
127

128. Electron trap (metastable state)
TL Process
Conduction Band (unfilled shell)
Electron trap (metastable state) -
Phosphor atom
Valence Band (outermost electron shell)
Incident radiation

129. TL Process, continued - Conduction Band Thermoluminescent photon
Heat Applied
Phosphor atom
Valence Band (outermost electron shell)

130. Output – Glow Curves A glow curve is obtained from heating
Light output from TLis not easily interpreted
Multiple peaks result from electrons in "shallow" traps
Peak results as traps are emptied.
Light output drops off as these traps are depleted.
Heating continues
Electrons in deeper traps are released.
Highest peak is typically used to calculate dose
Area under represents the radiation energy deposited in the TLD

131. TLD Reader Construction
DC Amp
To High Voltage
To ground
PMT
Recorder or meter
Filter
TL material
Heated Cup
Power Supply

132. Advantages Advantages (as compared to film dosimeter badges) includes:
Able to measure a greater range of doses
Doses may be easily obtained
They can be read on site instead of being sent away for developing
Quicker turnaround time for readout
Reusable
Small size
Low cost

133. TLD Disadvantages Lack of uniformity – batch calibration needed
Storage instablity
Fading
Light sensitivity
Spurious TL (cracking, contamination)
Reader instability
No permanent record