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Radiation Detectors Flashcards

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1
Q

What are different detection mechanisms for ionizing radiation?

A
  1. Ionization ⇒ Release of ion pairs by radiation
  2. Biological ⇒ Changes produced in a living system
  3. Chemical ⇒ Release of free radicals in a solution
  4. Heat ⇒ Temperature rise from deposited energy
  5. Scintillation ⇒ Light flash in a special phosphor
  6. Thermoluminescence ⇒ Light release on heating a phosphor
  7. Superheated Drop ⇒ Bubble formation in a gel matrix
  8. Radiochromic Dye ⇒ Color change after irradiation
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2
Q

Graph the characteristic curve for gas-filled radiation detectors.

A
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3
Q

Gas-Filled Characteristic Curve

Describe the recombination region.

A
  • The applied voltage is increased from zero.
  • More and more ions begin to evade other ions and make it to the anode or cathode.
  • Because the voltage is limited, not all of the ions reach the anode or cathode and recombine.
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4
Q

Gas-Filled Characteristic Curve

Describe the ioniziation chamber region.

A
  • With high enough voltage applied, all of the ions formed by the intial primary ray are able to avoid recombination.
  • With 100% collection results, the signal reaches a plateau and becomes constant even though the voltage continues to rise.
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5
Q

Gas-Filled Characteristic Curve

Describe the proportional region.

A
  • Due to gas multiplication, secondary ionizations are able to occur.
  • The increased potential difference does provides a strong enough Coulombic force to accelerate ions to energies above the W value.
  • The individual ions move with such high energy and velocity that they are capable of causing the ioniziation of gas molecules which they strike.
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6
Q

Gas-Filled Characteristic Curve

Describe the limited proportional region.

A
  • Same as the Proportional region, except the gas multiplication is not linear across voltage.
  • This range is not reliable for detection, and is not used.
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7
Q

Gas-Filled Characteristic Curve

Describe the Geiger-Mueller region.

A
  • A single ion pair injected into the counter is enough to cause complete discharge of the counter, creating an avalanche of electrons.
  • The output pulse amplitutde is of constant height regardless of the energy deposited in the counter.
  • When the avalanche reaches the collecting wire, the local energy density is so high that ultraviolet ligh photons are emitted.
  • These interact with the filling gas or tube wall to produce photoelectrons.
  • The photoelectron, being charged, intiates another avalanche at some other location on the collecting wire.
  • This process repeats several times until the collecting wire is eventually completely enveloped by ions.
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8
Q

Gas-Filled Characteristic Curve

Describe the continuous discharge region.

A
  • Extremely high voltage leading to the breakdown of insulating properties of the filling gas.
  • The gas is no longer an insulator, but has become a conductor.
  • This effectively causes a short circuit between the anode and cathode. The battery discharges across the tube.
  • This condition is damaging to detectors.
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9
Q

Ioniziation Chamber Detector

What two things can manufacturers do to ensure operation in the ion chamber region? (i.e., prevent recombination and prevent gas multiplication)

A
  1. Prevent secondary ionizations by limiting applied voltage to less than the W value. Typical applied voltage for ionization chamber detectors is 22 volts.
  2. Increase the physical diameter of the collecting electrode (which creates small electrical fields) and keep gas pressure high. The ions will collide with gas molecuels frequently enough to keep their average energy below the W critical value.
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10
Q

What is gas multiplication?

A
  • The increased potential difference in an ioniziation chamber is strong enough that the Coulombic force accelerates ions to energies above the W value.
  • The swarm of electrons created and converging at the central collecting electrode is termed an “avalanche”.
  • The size of the avalanche is dependent on the gas multiplication factor (M).
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11
Q

Ioniziation Chamber Detector

What is an example of a ioniziation chamber detector?

A
  • Eberline RO-20 (RO = “Rad Owl”)
  • Old version was called the “Cutie Pie”
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12
Q

Ioniziation Chamber Detector

What are the implications of an ion chamber being “unsealed”?

A
  • Unsealed ion chambers are vented to the atmosphere by a small hole drilled in the chamber wall.
  • This means that the chamber air pressure will vary with changes in barometric pressure over time depending on the altitude the meter is being used at.
  • If it calibrated at sea level and then used at 10,000 feet, the meter will read 30% below the true dose rate.
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13
Q

Ioniziation Chamber

What are advantages/disadvantage of a sealed ion chamber?

A

Advantages

  1. The chamber can be made much smaller.
  2. Can be used in an environment that includes radioactive gases.

Disadvantages

  1. Sometimes lose their seal, chamber gas leaks out, and meter loses sensitivity.
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14
Q

Proportional Counter

What characteristics are desired in this detector?

A
  1. Large gas multiplication factor
  2. Very small diameter collecting electrodes
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15
Q

Proportional Counter

What is a common fill gas?

A
  • P-10
  • 10% methane and 90% argon
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16
Q

Proportional Counter

Graph Count Rate vs Applied Voltage in a mixed alpha/beta field.

A
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17
Q

Proportional Counter

Describe the detection of thermal neutrons.

A
  • Neutrons are not charged, so they will not directly produce ioniziations in the detector.
  • 10B is employed as its nucleus will capture thermal neutrons and emit an alpha particle.
  • When this reaction takes place inside a proportional counter, the alpha can be easily counted.
  • 10B is included as gas molecules or a lining inside the wall of the proportional counter.
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18
Q

What is the reaction for thermal neutron capture in 10B?

Why will 10B only capture thermal neutrons and not fast neutrons?

A

The thermal neutron interaction cross-section is very large, whereas it is very small for fast neutrons.

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19
Q

Proportional Counter

Describe the detection of fast neutrons.

A
  • Cadmium sheet is included outside the counter to stop thermal neutrons.
  • Beyond the cadmium, a wax moderator thermalizes the fast neutrons.
  • The new thermal neutrons are measured by the alphas emitted by the 10B capture of the thermal neutrons.
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20
Q

Geiger-Mueller Counter

What are common quenching gases?

A
  • Alcohol is an organic quench gas and is used up in the process (does not replenish itself).
  • Chlorine is an inorganic quench gas and recombines to provide a continuous supply.
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21
Q

Geiger-Mueller Counter

What are regular fill gases?

A
  • Argon
  • Neon
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22
Q

Geiger-Mueller Counter

What is the typical range for gas multiplication factors (M)?

A
  • 108 - 1010
  • Output pulses are of the order of a few volts in height, so no preamplifiers are usually required for GM circuits.
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23
Q

Geiger-Mueller Counter

What is dead tme?

A

The minimum length of time that must elapse between two ionizing events occurring in a GM counter such that they are distinguishable.

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24
Q

Geiger-Mueller Counter

What is the typical range for dead time?

A

300 - 600 microseconds

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25
Q

Geiger-Mueller Counter

Graph dead time and related parameters.

A
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26
Q

Geiger-Mueller Counter

What is resolving time?

A
  • The minimum time that elapses from the moment of detection of a first ray until the electronics connected to the tube are able to count a second ray.
  • It is longer than the dead time because the electronics package always includes a pulse height discriminator set higher than the background noise pulses.
  • Therefore, the resolving time includes enough time beyond the dead time for the second, partially formed pulse to grow big enough to trip the electronics into recording a second event.
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27
Q

Geiger-Mueller Counter

What is recovery time?

A
  • The time from the point on the tail of the pulse when a second tiny pulse is just dinstinguishable as arriving at the end of the dead time.
  • It is measured out to the time when a second detected ray produces a full amplitude pulse.
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28
Q

Geiger-Mueller Counter

What are advantages and disadvantages of this type of detector?

A

Advantages

  1. Useful for detecting low-level radiation fields
  2. Useful for detecting contamination

Disadvantages

  1. Saturation will miss ionization events in high radiation fields
  2. Energy dependence
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29
Q

Geiger-Mueller Counter

What is big problem for GM detectors?

A
  • Saturation.
  • It is related to dead time and refers to the behavior of some GM survey instruments when exposed to a very high exposure rate.
  • The ionizing events are interacting with the counter tube at an average separation in time much closer together than the counter dead time.
  • Most of these rays will be missed since the tube is “dead”.
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30
Q

Geiger-Mueller Counter

Explain energy dependence in a GM counter.

A
  • It does not produce the same pulse output rate when exposed to the same exposure rate produced by gamma rays of different energies.
  • At low energies, the gamma rays undergo a photoelectric interaction, while higher energies undergo Compton scattering.
  • The GM counter is calibrated to read mR hr-1 and the roentgen is defined only for air as the absorber.
  • Because photoelectric effect is proportional to Z3, the tube will read high.
  • At higher energies, the tube will read correctly because Compton interactions are indpendent of Z.
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31
Q

Geiger-Mueller Counter

Graph Response vs. Gamma Ray Energy

A

The graph shows that at low energies the GM tube will over respond to radiation.

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32
Q

Liquid Scintillation Counting

Block diagram for a liquid scintillation counter

A
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33
Q

Liquid Scintillation Counting

Describe the operation of liquid scintillation counting.

A
  • The solvent dissolves both the source radioactivity and scintillating solute.
  • The solute absorbs the decay energy from the solvent and re-emits the energy as light.
  • Often a secondary solute is added which shifts the wavelength of the emitted light to a more desirable wavelength to something more sensitive to the photomultiplier tubes used.
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34
Q

Liquid Scintillation Counting

What is the purpose of an emulsifier?

A

Facilitates the mixing of the aqueous samples in the organic solvents.

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35
Q

Liquid Scintillation Counting

Advantages and Disadvantages

A

Advantages

  1. High sensitivity
  2. High accuracy
  3. Can count multiple radioisotopes in one sample

Disadvantages

  1. Cost per sample is relatively high
  2. High voltage applied to PMT cause high background noise.
36
Q

Liquid Scintillation Counting

What are the two components of a liquid scintillation cocktail?

A
  1. Solvent
  2. Scintillating solute
37
Q

Liquid Scintillation Counting

What are common solvents?

A
  • Toluene
  • Dioxane
38
Q

Liquid Scintillation Counting

What are the different categories of quench?

A
  • Chemical
  • Optical (color)
  • Ionization
39
Q

Liquid Scintillation Counting

What is chemical quench?

A

The sample being counted contains atoms which trap some of the emitted energy and releases it as thermal energy rather than measurable light.

40
Q

Liquid Scintillation Counting

What is optical (color) quench?

A
  • Absorption of some of the light before it leaves the solution to be measured.
  • Many samples such as urine, contain molecules which strongly absorb certain wavelengths of light, thus degrading the signal.
41
Q

Liquid Scintillation Counting

What is ionization quench?

A
  • Energy from the charged particle goes to ionizing the cocktail molecules and does not contribute to light output.
  • Common with alphas, where 90% of the alpha energy goes to ionizing other molecules.
  • A typical 5 MeV alpha will produce the light output of 500 keV beta, therefore decaytime discrimination is required.
42
Q

Liquid Scintillation Counting

What are the typical energy channels in a liquid scintillation counter?

A
  • Lowest (3H)
  • Middle (14C, 35S)
  • Highest (32P)
43
Q

Liquid Scintillation Counting

What is the purpose of coincidence circuitry?

A
  • The liquid scintillation counter will have random counts caused by thermionic noise pulses in the photocathodes of the two PM tubes.
  • The coincidence circuit only processses a pulse if it receives a pulse from both photomultiplier tubes at the same time.
44
Q

Liquid Scintillation Counting

What is a decay time discriminator?

A

Separates beta counts from alpha counts.

45
Q

Solid Scintillation Detectors

How does a NaI(Tl) detector work?

A
  • When energy is depsoited in the form of photoelectrons, Compton electrons, or an electron-positron pair, the phsophor converts it into a light flash (photons).
  • The individual light photons are in the blue region of the optical spectrum (410 nm) with a time duration of about 0.25 microseconds.
  • The intensity of the light flash is directly proportional to the energy deposited by the gamma ray in the crystal.
  • The sealed crystal is cemented to a the entrance window of a photomultiplier tube, an electronic device that amplifies weak light pules into a large elctrical signal.
46
Q

Solid Scintillation Detectors

NaI(Tl) detector advantages and disadvantages

A

Advantages

  1. Relatively dense, therefore it is a good absorber and efficient detector of penetrating radiation.
  2. Output from PMT is proportional to energy, so can be used for counting.
  3. Higher counting efficiency that gas-filled and HPGe detectors.

Disadvantages

  1. Exclusively used for recording gamma rays.
  2. Hygroscopic ⇒ The crystal readily absorbs moisture out of the environment, and will self destruct if subjected to prolonged exposure to humidity.
47
Q

Solid Scintillation Detectors

What is the preferred packaging technique for a NaI(Tl) detector?

A
  • Use an aluminum cylinder with an aluminum cap at one end to form a close-fitting “cup”.
  • The inside is then coated with a white reflective paint to increase light output sent to the PMT.
48
Q

Solid Scintillation Detectors

How do you perform neutron detection with a scintillation detector?

A
  • Thermal neutrons are detected by using lithium or boron to produce an alpha particle and then detecting the alpha with a scintillator.
  • The most common phosphor used as an alpha scintillator is silver activated zinc sulphide, ZnS(Ag).
49
Q

Solid Scintillation Detectors

What is a Phoswich?

A
  • “Phosphor sandwich” allows one type of radiation to be counted in the presence of another type.
  • Consistes of two different scintillation cyrstals bonded together on the same photomultiplier tube. One is thin, and one is thick.
  • The different scintillation phosphors have different pulse decay times, allowing for decay time discrimination.
50
Q

Solid Scintillation Detectors

Describe a microR meter.

A
  • Used to make quantitative readings at background levels, so require the sensitivity of a solid state counter.
  • Usually employ NaI(Tl) detectors.
  • Good gamma sensitivity
  • Energy response is poor due to the high effective atomic number of the scintillator, so useless for measuring actual dose equivalent rates.
51
Q

Semi-Conductor Detectors

What are three types of semiconductor detectors?

A
  1. Surface barrier diode detector
  2. Germanium detector family
  3. Cadmium telluride with zinc alloy (CZT)
52
Q

Semi-Conductor Detectors

Difference between P-type and N-type semiconductor slabs?

A
  • P-type, current is carried by positively charged holes
  • N-type, current is carried by negatively charged electrons
53
Q

Semi-Conductor Detectors

How do semi-conductor detectors operate?

A
  • The semiconductor slab is configured with a reverse bias to attract the electron-holes away from the central region of the slab to leave the region “depleted” in charge carriers.
  • This depletion region plays the same role as the filling gas in an ion chamber.
  • The incoming ray interacts and causes ionization which produces electron-hole pairs rather than + and - ion pairs.
54
Q

Semi-Conductor Detectors

What is the W-value for silicon?

A

W = 3.6 eV per electron-hole pair

55
Q

Semi-Conductor Detectors

Describe surface barrier detector.

A
  • Useful only for particulate radiation such as alphas and betas.
  • Commonly used in modern alpha air samplers to distinguish alpha contamination from the radon background activity.
56
Q

Semi-Conductor Detectors

State two types of germanium semiconductor detectors.

A
  • Lithium drifted germanium counter, Ge(Li)
  • High purity germanium detector, HPGe
57
Q

Semi-Conductor Detectors

What is the W-value for Germanium?

A

2.9 eV per electron-hole pair

58
Q

Semi-Conductor Detectors

What is the typical resolution of a germanium detector?

A
  • 0.15%
  • Less than 2 keV for 60Co energies
59
Q

Semi-Conductor Detectors

Germanium detectors advantages and disadvantages.

A

Advantages

  1. Higher atomic number than silicon means more chance for gamma interactions
  2. Good energy resolution
  3. Large crystals allow for “total absorption” detection of photons

Disadvantages

  1. High cost
  2. Must be used at liquid nitrogen temperature
  3. Ge(Li) msut always be cooled, even during storage, or the lithium ions will drift out and the detector will be useless.
60
Q

Semi-Conductor Detectors

Describe the cadmium telluride detector.

A
  • Cadmium telluride is alloyed with a small amount of zinc (hence the name CZT).
  • It is a small gamma and X-ray detector which operates at room temperature and does not need a PMT.
  • Because of a high atomic number (Zeff = 50.2), the sensitivity is much higher than a germanium counter (Z = 32).
  • CZT also exhibits high electron mobility and has a large bandgap.
  • These properties lead to a high efficiency for electric charge collection and that produces good energy resolution.
  • Energy resolution better than Na(Tl) but worse than Ge.
61
Q

Pulse Height Spectrum

Graph a typical pulse height spectrum produced by a mutli-channel analyzer.

A
62
Q

Pulse Height Spectrum

What is the backscatter peak?

A
  • Results from the capture in the crystal of photons which have Compton-scattered from shielding or other objects near the detector.
  • Since they have lost some energy already, they show up energies less than the photo peak (full energy peak).
63
Q

Pulse Height Spectrum

How do you determine energy resolution of a scintillation counter?

A

The width of the peak at half-amplitude, divided by the energy, times 100%.

64
Q

Define and Calculate

Absolute detector efficiency

A
  • Ratio of the number of pulses recorded by the detector to the number of radiation quanta emitted by the source.
  • Optimization relies on detector properties (e.g., size of detection volume, fill gas, etc.) and detector geometry (distance from source to detector).
65
Q

Describe

P-type material

A
  • Acceptor site doped impurities have one free (missing electron)
  • Like saying it has an extra “hole”
  • Shortens the forbidden energy gap by increasing the acceptor levels within the valence band
  • Example ⇒ Boron impurities in silicon semiconductor
66
Q

Describe

N-type material

A
  • Donor electron elements used as impurities are pentavalent and have one free electron.
  • Shortens the forbidden energy gap by increasing the donor levels within the conduction band.
  • Example ⇒ Phosphorous impurities in silicon semiconductor
67
Q

Describe two monitoring techniques that you would use to measure airborne tritium concentrations.

Give an advantage and disadvantage for each.

A

F_l_ow through Kanne ionization chamber

  • (+) Provides instantaneous measurement of the total airborne concentration of HTO & T2
  • (–) Does not differentiate between HTO & T2, important because HTO has more dose significance

Measurement of HTO in the form of tritiated water vapor w/ LSC to obtain specific activity

  • (+) Provides accurate measurement of HTO concentration
  • (–) Requires sampling and analysis which is opposite of continuous monitoring
68
Q

If a free-air ionization chamber was calibrated in rad hr-1 for photons in air, would a correction factor have to be applied to determine a gamma skin dose rate in rad hr-1?

A
  • Yes
  • For most photons (Eγ > 50 keV), the correction factor would be tissue to air collision stopping power ratio, based on the electrons produced by photon interactions and present at a tissue depth of 7 mg cm-2.
  • For lower energy photons, the correction factor would be the tissue to air ratio of the mass absorption coefficients.
  • Both ratios ≈ 1.1, so the reading would be multiplied by 1.1
69
Q

Explain how to determine the flux and average energy of an unknown neutron field using the Bonner Sphere method with a 6LiI(Eu) scintillator.

A
  • Several polyethylene spheres (~5) ranging in size from 2 – 18 inches in diameter are exposed individually to a field of interest.
  • The 6Li(n, α)3H reactions in the center are captured by a photomultiplier tube, and are sent to a preamp → amplifier → scaler to be recorded as counts.
  • The counting rates from different spheres are compared w/ predetermined counting efficiency vs. neutron energy matrix to determine shape of the energy distribution.
  • Energy can then be obtained in the form of fluence rate per unit energy distribution 𝜙(E).
  • Total fluence can be obtained by integrating the distribution of the entire energy range.
70
Q

Explain the basis for neutron detection by the foil activation method.

What is the key advantage when used for criticality dosimetry?

A
  • Foil activation can evaluate neutron fluence rates and shape of energy distribution
  • Specific foils have energy thresholds for neutron induced reactions
  • Radioactivity of products are determined through measurements of their radiation emission rates.

Benefits

  • No electronics are needed for reading after irradiation.
  • Good for criticality dosimetry and can be read immediately after suspected exposure.
71
Q

Which uranium isotope (235U or 238U) provides for detection of thermal neutrons?

A
  • 235U
  • Thermal neutrons cannot induce fission in 238, which has a neutron energy fission threshold of about 1 MeV.
72
Q

What are “blank” samples?

Why are they used to determine instrument background?

A
  • Representation of media and containers in which actual radioactive samples are counted.
  • Used because no net activity from the medium sampled is present
  • Blank samples may contain varying amounts of radioactivity The blanks themselves cause variation in the gross counting rate, which should be subtracted from the sample
73
Q

Define FWHM

Why is the FWHM of an HPGe detector smaller than that of a NaI(Tl) detector?

A
  • The full width of the total absorption peak at half the maximum peak height
  • The FWHM is smaller than that of a NaI(Tl) detector because of better resolution.
  • This results from the larger number of electron-hole pairs in the HPGe detector compared to the # of electrons released from the photocathode surface of the PMT
74
Q

In an HPGe detector’s crystal was increased in size, would the height of the photopeak in relation to the height of the Compton edge be higher or lower?

A
  • The height of the total absorption peak would increase because the larger crystal would have more deposition events.
  • More Compton scattered photons produced in the larger detector would have a higher probability of interacting by photoelectric effect.
75
Q

Why are escape peaks generally more prominent in HPGe detectors in comparison with NaI(Tl) detectors?

A

Typically HPGe detectors have a lower intrinsic photon detection efficiency than NaI(Tl) detectors

76
Q

What are three different methods to analyze a neutron spectrum?

A
  • Bonner sphere
  • 3He neutron spectrometer
  • Threshold activation coils
77
Q

Describe the operation 3He neutron spectrometer

A
  • Evaluating the pulse height distribution from the proton & triton produced in the 3He(n, p)3H exothermic reaction it is possible to determine the neutron energy distribution.
  • When neutrons above 2 MeV are present, the confounding influence of the pulses from 3He recoils produced through elastic scattering with neutrons may have to be dealt with through pulse height discrimination.
78
Q

Describe the operation of threshold activation coils

A
  • Irradiation of various target nuclides that exhibit energy thresholds for the production of a radioactive product
  • Example ⇒ 32S(n,p)32P
  • Select different target nuclides in foils with different neutron energy thresholds and known effective cross sections
  • Measure the amount of radioactive product formed. One can infer using computational analysis, the shape of the neutron energy distribution.
79
Q

Compare GM and Thin window NaI(Tl) detectors for dealing with a potential iodine uptake

A

GM

  • (+) High efficiency for detecting beta radiation emitted from contamination of the skin
  • (–) Low efficiency for gamma-rays emitted from the thyroid
  • // Measurements w/ and w/o an absorber thick enough to stop beta particles can be used to distinguish skin contamination and 131I γs

Thin window NaI(Tl)

  • (+) High efficiency for detecting gamma-rays emitted from the thyroid
  • (–) Depending on thickness of window, it may not be able to detect beta radiation from skin contamination
80
Q

When is Kerma of indirectly ionizing radiation is equal to dose?

A
  • Bremsstrahlung losses are negligible (all energy must be deposited in the material)
  • Electronic equilibrium is reached (attained at the point of the maximum range of a primary electron created at the surface
  • Medium is uniform (because the range and distribution of the primary electrons must be uniform)
81
Q

Is a dose measured by an ion chamber the “true” skin dose?

Why or why not?

A
  • No
  • In measuring dose, the source chamber geometry is commonly such that the chamber volume is not irradiated uniformly.
  • Depending on chamber design characteristics, some beta radiation may be attenuated (e.g., a distributed source may have some betas incident on the side of the detector)
  • The conversion of beta instrument response to tissue dose often requires different conversion factors that are implicit in gamma exposure to tissue dose conversion (e.g., mass collision stopping power vs. mass energy absorption coefficient)
82
Q

Explain how lead shielding affects the response of a shielded GM detector.

A
  • The response for photons calibrated will decrease with lead shielding thickness because of attenuation of primary photons by photoelectric and Compton scattering.
  • Electrons arising from these reactions, primarily in the lead and the wall of the detector, produce the response in the GM detector.
  • For the same source strength, dead time losses are expected to be smaller in the shielded detector because of its lower response.
  • The response to background radiation is expected to be lower in the shielded detector.
83
Q

List thee parameters that affect the correction factor needed to convert an ionization chamber reading to an actual beta dose rate.

A
  • Gamma dose rate response parameter without the shield in place
  • Energies of beta particles
  • Density of air in ion chamber
84
Q

Define

Electronic equilibrium

A

Condition achieved when the number, direction, and energies of charged particles entering a small volume element of material are equal to the respective values for charged particles leaving the volume element.

85
Q

What is the consequence if full electronic equilibrium is not established in a meter?

A
  • An instrument is calibrated under electronic equilibrium conditions at one photon energy.
  • If the same instrument is used to measure a higher energy photon radiation field and electronic equilibrium is not established for those higher energy photons, the measured dose rate will indicate less than the actual dose rate.

Health Physics (17 decks)

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