Detection and Measurement of Radiationmindofisaac.com/data/The.One/Energy/Intro To Nuclear Engineering … · Detection and Measurement of Radiation Ionizing radiation is rarely detected

Chapter 8

Detection andMeasurement

of Radiation

Ionizing radiation is rarely detected directly. Instead, detectors usually measure thesecondary products arising from the interactions of the radiation with the detectormaterial. For example, as an alpha or beta particle traverses a detector's sensitivevolume, electron-ion pairs or electron-hole pairs are created and the subsequentmovement and collection of charges gives rise to an electrical pulse or current. In-directly ionizing radiation such as gamma photons and neutrons must first undergointeractions in the detector material that produce secondary charged particles, recoilatoms or electrons that, in turn, produce charge pairs as they slow down.

The collection of the ionization created by radiation in a detector volume canbe used simply to detect the passage of a radiation particle. The rate of generationof radiation-induced pulses can then be used to measure the rate at which radiationparticles traverse the detector. Such detectors are termed radiation counters. Insome detectors, the magnitude of the radiation induced pulse is related to the typeof radiation particle and its energy. By measuring both the number of pulses andthe distribution of pulse sizes produced by a given type of radiation, both thenumber and energy distribution of the incident radiation can be determined. Thesedetectors can then be used as energy spectrometers. In some detectors the averagecurrent can be used as a measure of the amount of ionization or energy deposition,per unit mass of detector material, caused by incident radiation. These detectorscan then be calibrated to measure radiation absorbed doses and are thus calleddosimeters. In this chapter, the properties of some of the most common radiationdetectors are reviewed.

An important aspect of radiation detection is an assessment of the uncertaintiesassociated with ionization measurements. Both the release of radiation by radioac-tive decay and the interactions of radiation with matter are stochastic in nature.Thus, repeated measurements of radiation emitted by a source of constant activity,in a given detector volume in a given time interval, exhibit random statistical fluctu-ations. The quantification of such statistical fluctuations is a necessity in radiationmeasurements. This topic also is addressed briefly in this chapter.

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

Cathode

Figure 8.1. Basic elements of a gas-filledradiation-detector tube. The cathode is oftenused to seal the gas cavity from the ambient en-vironment. The output voltage pulse is producedacross the load resistor RL. After Knoll [2000].

8.1 Gas-Filled Radiation DetectorsThe idea of measuring the radiation-induced ionization in a gas volumedates to the nineteenth-century.These early gas-filled detectors be-came known as ionization chambers.As ionizing radiation passes througha chamber, the motion within anelectric field of the ion pairs formedinside the chamber produces an elec-trical current. The magnitude ofthe current is measured and corre-lated (calibrated) to the intensity ofthe radiation field. A very com-mon geometry is a coaxial detectorthat consists of a thin, positively-charged, center wire anode (held inplace by insulators) surrounded by an outer, negatively-charged, cathode tube. Theouter tube contains the gas and defines the active volume of the chamber. Air filledchambers may or may not be sealed from the ambient environment. A gas-filledchamber is illustrated in Fig. 8.1.

Radiation either interacts in the wall of the chamber or directly in the filling-gas.For incident electromagnetic radiation the dominant interactions, photoelectric ef-fect, Compton scatter, and pair production, occur primarily in the chamber wallmaterial. If the electrons released from the atoms in the wall material escape fromthe wall and enter the active gas volume of the chamber, then these charged par-ticles (secondary radiation) produce ionization as they pass through the gas. Thepotential difference between the anode wire and the cathode establishes an electricfield that causes positive and negative charges to move in opposite directions. Elec-trons rapidly drift toward the anode and positive ions migrate more slowly towardthe cathode. The motion of these ion pairs causes a flow of current in the externalcircuit and establishes a voltage across the load resistance. If the incident radiationfield contains beta particles, for example, then the chamber must be constructed sothat these particles can enter the volume. This is achieved by placing a very thin"window" on one end of the tube.

There are three basic types of gas-filled radiation detectors: ionization chambers,proportional counters and Geiger-Mueller counters. All three are known as ioniza-tion chambers, but they each have a unique process for forming the total number ofion pairs that are collected at the electrodes. All three operate by forming initial ionpairs from the incident radiation. Once these ion pairs are formed, it is importantthat they do not recombine and thereby fail to contribute to the electrical signal.

Figure 8.2 shows the various operational regions of gas-filled chambers. RegionI represents the recombination region where the potential difference between theanode and cathode is not sufficient to collect all the initial ion pairs. Ion chambersoperating in this region are not useful radiation detectors.


recombination before collection

I I250 500

Voltage (volts)

750 1000

Figure 8.2. Operational regions for gas-filled radiation detectors.

As the potential difference between the anode and cathode is increased, theionization chamber Region II is entered. In this region the resulting output currentis referred to as the ionization chamber saturation current. Region III representsthe proportional counter region. In this region, electrons acquire sufficient energyto induce secondary ionization, and hence multiplication. This internal ion-pairmultiplication increases the total number of ion pairs in the active volume and,hence, the output current increases by a multiplicative factor M > 1. This regionis called the proportional counter region since the output current, or total collectedcharge per interaction, is proportional to the initial number of ion pairs created bythe incident radiation.

Next is entered a limited proportionality region where the number of ion-electronpairs collected is relatively independent of the initial number of ion-electron pairscreated by the incident radiation. This region generally is not useful operationally.

The region of Geiger-Mueller (GM) counter operation, Region IV, exhibits a"plateau" over which M reaches a nearly constant value. In this region oneavalanche produces secondary photons whose interactions produce other local


avalanches until the entire anode is surrounded by ion-electron pairs. In this region,the charge collected per interaction is no longer proportional to the initial numberof ion pairs created. Therefore, it is very difficult to distinguish between differenttypes of incident radiation or to gain knowledge about the energy of the incidentradiation.

If the tube voltage is too large, the tube undergoes continuous avalanches aroundthe central anode, one leading to another. This region V, is known as the continuousdischarge region. Its entry normally leads to failure of the ion chamber.

8.1.1 lonization Chamberslonization chambers or ion chambers are widely used as radiation monitors. Theycan be designed to respond to alpha particles, beta particles, gamma rays, x rays,and neutrons. The ion chamber is the most basic type of gas-filled radiation detectorsince it operates without gas multiplication. These chambers are frequently referredto as current chambers since they are often operated by directly measuring theoutput current. When operated at the saturation current level (region II of Fig. 8.2),the output current level is proportional to the intensity of the incident radiationand permits a direct measurement of the exposure rate.

In an ion chamber with parallel-plate electrodes and a uniform electric field,current pulses from individual events can be registered, thereby allowing energyspectroscopy of the incident radiation. In their most common cylindrical geometry,however, ionization chambers cannot be used for this purpose since the pulse am-plitude is dependent upon where the ion-electron pairs are formed in the detector.

Gamma-Ray Ion ChambersGamma-ray ion chambers, operated in the current mode, are very stable and have along life. They can be fabricated in a variety of sizes and shapes. Large ion chambersare used as area monitors for ionizing radiation and high-pressure chambers offer avery high sensitivity, permitting measurement of exposure rates as low as 1 /iR/h.Small chambers with low gas pressures can be operated in radiation fields withexposure rates as great as 107 R/h.

Air-filled ionization chambers vented to atmospheric pressure are often usedto measure radiation exposure. After the formation of an electron-ion pair, theelectrons and ions begin to move through the gas towards their collecting electrodes.However, the electronegative oxygen quickly captures the free electron to produce aheavy negative ion. Because of this attachment most of the free electrons disappearand the chamber current is produced by the motion of negative and positive ions.This is not a problem when a detector is operated in the current mode as long as thepotential difference is sufficient to prevent significant recombination. However, sucha chamber is not suited for pulse-mode operation, since the positive and negativeions move very slowly compared to electron speeds in the same electric field, andvery broad and indistinct pulses are produced.

Neutron-Sensitive Ion ChambersBecause neutrons do not directly produce ion-electron pairs, an ion chamber sensi-tive to neutrons must incorporate some isotope with a large neutron cross sectionfor a nuclear reaction that produces charged particles, which, in turn, create ion-electron pairs in the chamber gas. The most common gases in use are 3He and BFs.


Alternatively, the inside of the tube may be lined with a neutron-sensitive materialsuch as boron or 235U which, upon absorbing a neutron, emits charged particlessuch as alpha particles or fission fragments into the chamber gas. Chambers linedwith a fissile isotope are called fission chambers. Such chambers are often operatedin the proportional-counter mode.

Fission chambers are often used where there is a mixed radiation field containinga large component of gamrna rays. Since these detectors are operated in the pulsemode, the large voltage pulse formed by the large charge produced by the ioniza-tion of the heavy charged fission products or their resulting radioactivity makes itpossible to discriminate against the much smaller gamma-ray voltage pulse. Fissionchambers containing 235U or 239Pu are efficient thermal-neutron detectors oftenused for in-core nuclear reactor startup channels.

A compensated ton chamber is a neutron detector that effectively discriminatesagainst the gamma ray induced current. Typically the chamber has three concentricelectrodes, one coated with a neutron sensitive material such as 23°U or a compoundcontaining 10B. Compensated ion chambers are widely used in nuclear reactorsbecause of their ability to respond to neutron fields that vary up to ten orders ofmagnitude; i.e., these detectors have a very large "dynamic range."

8.1.2 Proportional CountersProportional counters are gas-filled chambers operated in region III of Fig. 8.2. Inthis region, there is an internal multiplication in the chamber gas of the original ion-electron pairs created by the incident radiation. The physical mechanism for thismultiplication process is discussed in the next section. This internal multiplicationM is typically from 10 to 10,000. Because of the location of the gas multiplicationavalanche, near the central wire, the output voltage pulse is not only amplifiedby a factor of M but the pulse amplitude is proportional to the total ionizationenergy deposited inside the active volume of the detector. This feature allowsthe proportional counter to be used as a spectrometer by using the output pulseamplitude to infer the energy of the incident radiation. Also, since the number ofion pairs is proportional to the initial ionization caused by the incident radiation,proportional counters can be used to distinguish between different types of chargedparticles. For example, one common application is to use a gas-flow counter todistinguish between beta particles and alpha particles.

Gas MultiplicationIn a proportional counter, the electricfield strength is large enough to increasethe energy of the electrons generated inthe active volume to a level sufficient tocause additional ionization of the cham-. . 25 |im diameterber gas. In a cylindrical counter, the elec- anode wiretrie field strength varies inversely withthe distance from the axis of the central Figure 8.3. An orthogonal view of ananode wire and essentially all of the mul- Blanche triggered by a single electron.

I he density ot the shading indicates thetiplication occurs very near the central concentration of electrons formed in theelectrode. This avalanche multiplication avalanche. After Knoll [2000].


process is illustrated in Fig. 8.3. Electrons produced by this avalanche are quicklycollected at the anode leaving a sheath of positive-ion space charge. Regardless ofwhere the initial ion pairs are formed in the active volume, most of the currentduring charge collection results from the motion of the positive ions moving fromthe anode wire to the cathode.

Applications

The proton recoil proportional counter is one of the few instruments available tomeasure fast neutron spectra in the energy range from 10 keV to about 800 keV.When a high energy neutron scatters from a hydrogen atom in the chamber gas,the recoil energy of the hydrogen nucleus (a proton) depends on the kinetic energyof the incident neutron. The recoil proton's energy is then registered as a currentpulse in the external circuit of the proportional counter. From the measured dis-tribution of the pulse magnitudes, the energy distribution of the incident neutronscan be inferred. Gas fillings for neutron energy spectroscopy are usually hydrogenor methane. Techniques have been developed to discriminate against gamma rays.Techniques are also available to infer tissue dose from neutrons by using countersfilled with a tissue equivalent gas.

Thin-window proportional counters are frequently used to measure low energyx-ray spectra. The efficiency is about 10% at 5 keV. To enhance the photoelectricresponse, the counters are filled with a counting gas mixture in which one componenthas a high atomic number, e.g., a mixture of xenon and CE^. Beryllium is a commonwindow material, and, because of its low atomic number, it has a low photoelectriccross section and is relatively transparent to incident low energy photons.

3He and boron trifluoride (BFs) gas-filled counters are mainly sensitive to slowneutrons based upon the two nuclear reactions: 3He(n,p)3H and 10B(n,a)7Li. Be-cause of the large output voltage pulse caused by the alpha particles and the pro-tons, these counters can be made essentially insensitive to gamma rays using pulseheight discrimination. These detectors have many applications including thermalneutron diffraction, moisture content measurements in soil and concrete, nuclearmaterial assay, and nuclear fuel fabrication monitoring. Many applications requirethat high-energy neutrons be moderated to increase the detector efficiency.

8.1.3 Geiger-Mueller CountersGeiger and Mueller developed the Geiger-Mueller (GM) detector in 1928. Theseradiation detectors are simple and robust devices that continue to be an impor-tant tool for sensing the presence of ionizing radiation. These gas filled detectorsoperate in region IV of Fig. 8.2, and have the remarkable property that the sizeof the pulse, or total charge produced in the active volume is independent of theionization energy deposited by the initial ion pairs. Consequently, these detectorsdo not have the inherent capability of distinguishing between different types of ra-diation or measuring radiation energy. They can, however, be configured so thatthey are sensitive to both charged particles (alpha particles and beta particles) andelectromagnetic radiation (x rays and gamma rays).

A GM detector's sensitivity to charged particles is limited only by the thicknessof the entrance window. The entrance window must be just thick enough to ensurethat the chamber's filling gas does not escape. Once charged particles enter the


active volume of the detector, they are detected with almost 100% efficiency sinceit takes only one ion-electron pair to initiate the pulse formation process. Typicalcounter characteristics are shown in Table 8.1.

Table 8.1. Typical GM counter specifications and characteristics.

primary gasquench gasgas multiplicationcathode (outer case) materialanode wire diameterwindow thickness (mg cm"2)window materialoperating voltageplateau slopedead timeenergy response for j3 particlesenergy response for x and gamma rayssensitivity for low energy /3 particlessensitivity for high energy beta particlessensitivity for x and gamma raysmode of operationspectroscopy applicationsadvantages

helium, argon or neonhalogen or organic104 - 108

stainless steel0.1 mm1.5 - 40mica500 - 1600 V2 - 1 0 %/100 V50 - 200 MSpoorpoorpoor due to window thickness limitationexcellent for thin window detectorsgoodpulsenonelow cost and large output voltage pulse

Pulse Formation in a GM CounterThe initial ion pairs produced inside a GM tube are quickly separated in the electri-cal field between the anode and the cathode. As electrons move toward the anode,they gain sufficient energy to produce additional ionization. At this stage, the pulseformation follows a pattern similar to the operation of a proportional counter. Veryquickly a dense collection of ion pairs forms near the anode, terminated when allof the electrons are collected. This initial avalanche is followed by a successionof other avalanches caused by the emission of light photons that are emitted fromgas molecules excited by electron impact. These photons, in turn, interact withthe walls of the detector and with the chamber gas to produce tertiary electrons,which spread the discharge over the whole volume near the positive electrode. Thiscascade of avalanches in a GM tube is shown in Fig. 8.4.

The operation of the GM counter, thus, depends upon the probability that anelectron in the gas produces a tertiary photoelectron. As the positive ion sheathgrows near the anode, a space charge builds up. This causes the electric field nearthe wire to weaken and the multiplication to decrease. The discharge becomesconvergent and ion pair production stops. During the collection of the electrons,the positive-ion sheath is essentially stationary because ion mobility is about 1000times less than that for electrons.


Anode wire

f Y/ r*—'-v—'-——v ^_x~v_x-x_x-v_x*"*

Y W Photon

Photon'

Cathode

Figure 8.4. The mechanism by which additional avalanches are triggered in a GMdischarge. After Knoll [2000].

After collection of the electrons at the central anode, the positive ions slowlymove toward the chamber's cathode. If they strike the wall, there is a finite proba-bility that these ions will cause new electrons to be released into the active volumeof the counter. If this happens, then the cascading avalanches begin anew. If theprobability of this happening is high, the counter can enter a continuous dischargemode and destroy itself. These secondary avalanches caused by the positive ionsare reduced to an acceptable level by adding a small quantity of a quench gas, nor-mally a halogen or organic compound. The final result of this complex process isan internal gas multiplication of 104 to 108, which produces a very large charge anda correspondingly large output voltage pulse.

Operating VoltageOne of the first steps in characterizing the operation of any gas-filled radiationdetector is establishing the correct operating voltage. GM counters have a fairlywide range over which they can be operated. However, each counter should beoperated near the center of its plateau within the GM region shown as region IVin Fig. 8.2. This assures the best long-term performance because, at this voltage,small changes in the applied voltage have an insignificant effect on the amplitudeof the output pulse (see Fig. 8.5). If the applied voltage is too low, then the gasmultiplication is less than desired and the counting rate is very sensitive to smallchanges in voltage.

Dead Time Corrections in GM CountersBecause of the relatively long time required for the ionization process to take placein a GM counter, the chamber becomes insensitive to additional incident radiationduring the ionization process. For large incident radiation intensities, the GMcounter is unresponsive or dead for a large fraction of the counting time. Theseslow detectors require that dead time corrections be made even when the count rate


2000

1500 -

1000

500 -

750 1150850 950 1050

Tube voltage (V)

Figure 8.5. Operational plateau curve for a Geiger-Mueller counter.

is relatively low. For any GM counter application, consideration must be given tosystematic errors introduced by this count rate dependent phenomenon. Dead timecorrections can be made to determine the true counting rate using the procedurediscussed in Section 8.5.3.

ElectronicsThese detectors do not require expensive electronic components since the outputsignal is very large. Simple analog meters and basic pulse counting circuits are quiteadequate.

ApplicationsGM counters are often the detectors of choice for applications requiring informationabout only the magnitude or intensity of the radiation field. As such they find wideapplication in hand-held survey meters used to detector radiation fields. Othertypes of radiation detectors are more suitable if information is needed on the typeor energy of the radiation. GM counters can be made in essentially any size andshape. Their low cost and high efficiency make them suitable as sensors of betaparticles, x-rays and gamma rays. This is especially true if the radiation level is lowenough that dead time losses are not a concern. These detectors are seldom usedto detect neutrons although, with a cadmium cover surrounding the tube, the GMdetector is sensitive to thermal neutrons and may be calibrated for thermal neutronflux density. Because of their poor energy response, inability to distinguish amongdifferent types of radiation, arid their large resolving times, care must be takento assure that some knowledge of the radiation field is available before makingmeasurements with these counters.


8.2 Scintillation DetectorsThere are two types of scintillation detectors, (1) solid crystals of inorganic mate-rial, and (2) plastics and liquids consisting of organic molecules. Their modes ofexcitation differ but the final result is the same. As charged particles pass throughthe material the energy that they lose is transferred into excitation energy of theinorganic crystals or molecular excitation of the organic molecules. The excitationenergy is released in fluorescence, i.e., scintillation. The number of light photonsemitted in any one event is proportional to the energy lost by the initial chargedparticle in that event. The time dependence of the fluorescence emission, and hencethe output pulse shape, is dependent upon the specific type of material.

Although scintillator material has been used for almost 100 years to detectionizing radiation, their widespread application dates from the development of thephotomultiplier tube (PMT) some 50 years ago. This vacuum tube device allowsthe measurement of extremely low levels of light. In a PMT the incident photonsstrike a photocathode thereby liberating photoelectrons. These photoelectrons arethen accelerated towards another electrode at a higher potential where the energeticimpinging electrons cause more electrons to be emitted. This electron multiplicationprocess continues along a series of electrodes, each at higher potential than theprevious and, at each, the electron population is increased. The number of electronsfinally collected at the last electrode may be millions of times greater than thenumber of electrons that began the cascade. In essence, the PMT is a photon toelectron amplifier.

A typical scintillator detector assembly consists of a hermetically sealed scin-tillation material optically mounted to the PMT's photocathode, a voltage dividerstring (YDS) in the PMT, and a preamplifier to produce a voltage pulse from theelectrons collected at the last PMT electrode. These components are usually boundtogether in a single assembly By using this common configuration, the detector as-sembly is a stand-alone device that only requires an external voltage for the PMTand an external power supply for the preamplifier. Such an assembly is often calleda scintillation detector.

The amplitudes of the output voltage pulses are proportional to the energy de-posited by charged particles produced in the scintillation material. A gamma raypenetrating the scintillator material may give up its energy to the scintillator ma-terial through photoelectric interactions, Compton scattering and pair productionreactions. If all of the incident gamma-ray energy is deposited in the scintillatormaterial, the number of scintillation photons produced is proportional to the in-cident gamma-ray energy. Thus, by measuring the distribution of pulse sizes orthe pulse height distribution (PHD) produced by the scintillation detector, the en-ergy distribution of the incident gamma rays can be determined. Thus, one of themost important applications of scintillation detectors is gamma-ray spectroscopy.Characteristics of some common scintillation detectors are shown in Table 8.2.

Nal Scintillation DetectorsThe most popular inorganic scintillation material is Nal(Tl). These detectors areavailable in a variety of sizes and shapes. Because the maximum wavelength oflight emitted by this material is 415 nm, it is easy to find commercially availablePMTs whose maximum sensitivity matches the fluorescence emission spectrum. The


Table 8.2. Common scintillation materials.

MaterialDecay

constant Primary applications Comments

Inorganic:

Nal(Tl) 230

Organic:

BC-400NE-102plastic

2.4

(1) x- and 7-ray detectors(2) x- and 7-ray

spectrometers

(1) x- and 7-ray detectors(2) used in portal and large

surface area detectors(3) a. (3 and fast n detectors(4) j3 spectrometers

(1) hygroscopic(2) high Z material(3) 3 x 3 inch most common for 7(4) 7-ray resolution of 6 - 8%(5) right cylindrical shape most common(6) well-type have outstanding efficiency

(1) non-hydroscopic(2) inexpensive(3) low Z; minimal photoelectric effect(4) high efficiency for charge particles(5) available in many sizes and shapes

relatively large decay time constant is normally not a problem since a very highefficiency for x- rays and gamma rays dominates their radiation response. Of allthe different Nal(Tl) detectors available to characterize gamma-ray radiation fields,the 3 x 3 inch right circular cylindrical detector, has historically been the favorite.This is the most extensively characterized Nal(Tl) detector and extensive efficiencydata are available in the literature. A typical Nal(Tl) pulse height distribution isshown in Fig. 8.6.

Because of its very high efficiency for electromagnetic radiation (see Fig. 8.7),Nal(Tl) is widely used to measure x rays and gamma rays. X-ray detectors witha thin entrance window containing a very thin Nal(Tl) detector are often usedto measure the intensity and/or spectrum of low energy electromagnetic radiation.Because Nal(Tl) detectors do not require cooling, they can be used in a great varietyof applications. Field applications are possible since they can operate over a longtime period in warm and humid environments, resist a reasonable level of mechanicalshock, and are resistant to radiation damage. Basically, for any application requiringa detector with a high gamma-ray efficiency and a modest resolution, the Nal(Tl)detector is clearly a good choice.

Comparison of Inorganic ScintillatorsAdvances in x-ray computed tomography and positron emission tomography haveplaced new demands on speed, efficiency, and effective atomic number of scintillatorsused for x-ray and annihilation photon detection. Table 8.3 compares with Nalseveral new inorganic scintillators.

Organic Scintillation DetectorsOrganic scintillation detectors are also very popular. They provide an excellentoption when a larger less expensive detector is needed. They can't be used for high-energy electromagnetic radiation spectroscopy since their low atomic number limits


SingleDouble escapeescape r— peak

peak \

500 1000 1500 2000

Energy (keV)

2500 3000 3500

Figure 8.6. Pulse height distribution of the gamma rays emittedby the radioactive decay of 24Na as measured by a Nal(Tl) scintilla-tion detector. In addition to the two photopeaks, corresponding tothe complete absorption of the 24Na 2.754 and 1.369-MeV gammarays, several other peaks are also apparent. The single and doubleescape peaks arise from the escape of one and both 0.511-MeV an-nihilation photons generated in the Nal crystal by the annihilationof positrons created in pair-production interactions. The annihila-tion peak is caused by 0.511-MeV annihilation photons producedoutside the Nal crystal and subsequently depositing their energyin the crystal. The backscatter peak arises from photons scatteredfrom the source material into the Nal detector.

0.7

0.6

0.5

«u 0.4

0.3

0.2

0.1

0.0

3x1 Solid Crystal

2x2 I source 9.3cm3x2 [ on axis

j

0.20 0.40 0.60 0.80 1.0 1.2

Photon energy (MeV)

1.4

Figure 8.7. Intrinsic peak efficiency for Nal(Tl) detectors.


Table 8.3. Comparison of inorganic scintillation materials."

Scintillator

Relative light yieldPeak wavelength (nm)Decay constant (ns)Density (g/crn'^)Effective Z

Index of refraction

Nal(Tl)

100

410

230

3.67

51

1.85

BGO

Bi4Ge3Oi2

15-20

480

300

7.13

75

2.15

LSO

Lu2(SiO4)O

75

420

12, 42

7.40

66

1.82

GSO

Gd2SiO5

20-25440

30-606.71

59

1.85

"Compiled by Dr. Frederic H. Fahey. Wake Forest University Medical School.

their photoelectric sensitivity. Thus, Compton scattering dominates from about 60keV to 10 MeV. Moreover, since these detectors are composed of only hydrogenand oxygen with a density of 1.032 g cm~3 they make a unear tissue equivalent"detector. Typical applications include portable survey instruments, portal monitors,and beta-particle spectrometers.

8.3 Semiconductor lonizing-Radiation DetectorsFunctioning of these radiation detectors is based upon the newest technologies. Theimpact on radiation detection and measurement has been revolutionary because ofunique semiconductor properties, especially outstanding energy resolution. Newsemiconductor detectors continue to be introduced into the market place. Thethree most common types are described in Table 8.4.

Germanium Semiconductor DetectorsThere are two main types of germanium semiconductor detectors: (1) Ge(Li) agermanium crystal doped with interstitially drifted lithium ions to cancel the effectof natural impurities in the germanium crystal, and (2) the more recent HPGehigh purity germanium crystal in which impurity atom concentration are less than1010 cm~3. The more expensive HPGe detectors have replaced the older Ge(Li)technology since they can be kept at room temperature when not in use whereasGe(Li) crystals must always be kept at liquid nitrogen temperatures.

Germanium detectors, besides having exceptional energy resolution, are veryefficient for detecting photons. Their efficiency ranges from excellent for low energyx-rays to good for medium to high-energy gamma rays over an energy range of 1 keVto 10 MeV. The performance of these detectors is often compared to Nal(Tl) andCd/Zn telluride (CZT) detectors. Because of the higher atomic number arid largersize, Nal(Tl) detectors often have a higher efficiency for high energy gamma raysthan do germanium detectors, but a much poorer energy resolution. The dramaticdifference in the energy resolution between Na(Tl) and Ge(Li) spectrometers isshown in Fig. 8.8.


0)c0)co-(->oJC.0.

i i

Iniii i i

oinO

OnO

CMO

oo

uiq ja

d

sq.unoo

S w)

o •<

00 O

£ §s 5M) X


Table 8.4. Common semiconductor ionizing-radiation detectors.

Material

HPGe

Si(Li)

Cd/Zn/Telluride(CZT)

Resolution(FWHM)

2.0 keV at 1.33 MeV1.0 keV at 122 keV0.15 keV at 5.9 keV

0.14 keV at 5.9 keVfrom °°Fe

1.5 keV at 122 keVfrom 5TCo

Principal application

x- and 7-ray energymeasurements

x-ray (low keV) andcharged particles(especially a particles)energy measurements

x-ray and low energygamma-ray energymeasurements

Comments

(1) excellent resolution(2) intermediate atomic number(3) high cost-varies with 7 eff.(4) cooling needed when used(5) small volume

(1) excellent resolution(2) low atomic number(3) low cost(4) doesn't require cooling(5) small volume(6) thin entrance windows

(1) good resolution(2) high atomic number(3) expensive(4) minimum cooling needs(5) available only in small sizes

Silicon Semiconductor DetectorsSi (Li) detectors with thin entrance windows are commonly used in alpha- and beta-particle spectrometers. They can be configured to achieve essentially 100% intrinsicefficiency and have excellent resolution. They also offer an inexpensive option forx-ray spectroscopy. Since Si(Li) detectors have a much lower atomic number thanCZT. Nal(Tl), and HPGe, their relative efficiency per unit thickness is significantlylower for electromagnetic radiation. However, for x-ray or gamma ray energies lessthan about 30 keV, commercially available Si(Li) detectors are thick enough toprovide performance which is superior to CZT, Nal(Tl), and HPGe and detectormaterials. For example, as shown in Fig. 8.9, a 3 to 5 mm-thick detector with athin entrance window has an efficiency of 100% near 10 keV. Based upon the factthat a majority of the applications require a thin window, Si(Li) detectors are oftenmanufactured with very thin beryllium windows.

Cadmium Zinc Telluride DetectorsCadmium zinc telluride (CZT) is a new high-resolution and high-atomic numbersemiconductor detector material. A reasonable degree of cooling for the detectorand the directly coupled preamplifier enhances detector system performance. CZTdetectors offer an excellent option for low energy x-ray spectroscopy where coolingis not possible. Keeping the detector and preamplifier at about -30 °C, is adequateto achieve optimum energy resolution. By contrast, HPGe detectors must be cooledat liquid nitrogen temperatures to achieve optimum resolution.

These detectors are not available in large sizes. Their small size diminishes thepossibility of making detectors with large efficiencies for high-energy electromagneticradiation (see Fig. 8.10). Therefore, the major application is low energy x- orgamma-ray spectroscopy.


100

80

60

40

20

010" 10'

Photon energy (keV)

Figure 8.9. Full energy peak efficiency of Si(Li) detectors.

10'

hotoelectric + Compton

0 200 400 600 800 1000

Gamma-ray energy (keV)

Figure 8.10. Gamma-ray efficiency for a 2 mm thick CZT detector.


When x-rays or gamma rays interact in a CZT detector, an average of oneelectron/hole pair is produced for every 5 eV of energy lost by the photoelectronor Compton electron. This is greater than in Ge or Si, so the resolution of thesedetectors is not as good as HPGe or Si(Li) detectors. However, they are the detectorof choice for many applications requiring good energy resolution. For example, theyperform better than HPGe when it is not feasible to provide liquid nitrogen cooling.Likewise, when detector sensitivity is the dominant consideration, they are muchbetter than Si(Li).

8.4 Personal Dosimeters

8.4.1 The Pocket Ion ChamberVery familiar to radiation workers over many, many years are the self-reading pocketionization chamber (PIC) and the film badge dosimeter. The pocket ionizationchamber is an ion chamber, as described in Section 8.1, in the form of a cylinderabout the size of a fountain pen. A charge is placed on the electrodes of the ionchamber and the corresponding voltage is displayed through an eyepiece using anelectroscope. As the ion chamber receives radiation exposure, the electrodes aredischarged and the voltage change of the electroscope is presented in a reticulescaled to radiation dose or exposure. More commonly, the PIC is sensitive only togamma radiation. However, neutron sensitive ionization chambers are also used,calibrated in dose equivalent.

8.4.2 The Film BadgeThe film badge consists of a packet of photographic film sealed in a holder withattenuating filters. Ionizing radiation darkens the film, as in the production of anx-ray image. The filtration is designed to render the degree of film darkening asnearly as possible a known function of gamma-ray exposure, independent of theenergy of the incident gamma rays. After the badge is carried by a radiation workerfor a period of time, the film is processed, along with calibration films with the sameemulsion batch exposed to known radiation doses. The worker's radiation dose forthe period is assessed and ordinarily maintained in a lifetime record of exposure. Insome cases special attenuation filters are used to relate the darkening of portions ofthe film to beta-particle or even neutron dose. Special badge holders in the form ofrings or bracelets are used to monitor the radiation exposure of hands, wrists, andankles.

8.4.3 The Thermoluminescent DosimeterThe thermolumiriescent dosimeter (TLD) is a solid state radiation detector, whoseradiation dose may be gauged by measurement of the release of light upon heatingof the detector after exposure. TLDs are inorganic crystals such as LiF or CaSO4to which impurity elements, or dopants, such as Mg. P, Mn, or Cu are added insmall concentrations. The TLD matrix is an insulator but the dopant adds holeand electron traps with energy levels within the insulator band gap. Upon radia-tion exposure, traps are filled. They remain filled until the TLD is heated, therebyreleasing the trapped charge carriers. Recombination of newly mobile charged car-riers leads to light emission. Natural Li, in which the abundance of 6Li is 7.4%, is


somewhat sensitive to thermal neutrons. TLD neutron sensitivity may be enhancedby increasing the abundance of the lighter isotope. Similarly, neutron insensitiveLiF employs only 7Li. Dosimetry of mixed neutron and gamma-ray radiation maybe accomplished by using pairs of neutron-sensitive and neutron-insensitive TLDs.

8.5 Measurement Theory

8.5.1 Types of Measurement UncertaintiesThe analysis of any type of experimental data always requires that an assessmentbe made of the uncertainties associated with each measurement. Without suchan estimate, the data have very limited value. There are several types of uncer-tainties associated with any measurement. These include stochastic and samplinguncertainties and errors as well as systematic errors. For example, the decay ofradioactive atoms occurs randomly or stochastically so that a measurement of thenumber of decays in a fixed time interval has an inherent stochastic uncertainty.Repeated measurements would give slightly different results. Systematic errors areintroduced by some constant bias or error in the measuring system and are oftenvery difficult to assess since they arise from biases unknown to the experimenter.Sampling errors arise from making measurements on a different population fromthe one desired. These biases too are hard to detect, let alone quantify.

Engineers and scientists must always be aware of the difference between accu-racy and precision, even though popular usage often blurs or ignores the importantdistinction. Precision refers to the degree of measurement quantification as deter-mined, for example, by the number of significant figures. Accuracy is a measure ofhow closely the measured value is to the true (and usually unknown) value. A veryprecise measurement may also be very inaccurate.

8.5.2 Uncertainty Assignment Based Upon Counting StatisticsData measured with an ionizing-radiation detection system embody both randomuncertainties and systematic errors. Uncertainty assignment requires knowledge ofboth. Even with a perfect measurement system capable of operating over a long timeperiod without introducing significant systematic error, one must always considerthe random nature of the data caused by the stochastic decay of radioactive nuclei.

This section deals only with one component of the total uncertainty—the randomor statistical uncertainty, which can be estimated using the binomial distributionsince it is the basic statistical distribution describing the stochastic decay processof radionuclides. It is usually possible to approximate the binomial distribution bya Poisson distribution, which, for a sufficiently large number of decays or measuredevents (> 40), can be well approximated by a Gaussian distribution. This approx-imation simplifies the process of assigning the statistical uncertainty since, for thePoisson distribution, the variance cr2 is equal to the mean value x, the value usuallymeasured.

Data are routinely reported as x ± s where x is the number of measured countsduring a specified time period and s is the standard deviation of x. When an erroris reported as one standard deviation (or "one sigma") it is called the standarderror. For a single measurement x of a radioactive sample, it can be shown that


the standard deviation, for x greater than about 40, is

s = v/x (8.1)

Thus, for a single count measurement, the expected or mean value is estimated as xwith an estimated standard deviation of s = ^/x, so the reported value is x±s. Don'tarbitrarily use this concept to assign uncertainties to other types of experimentalmeasurements such as pressure or temperature measurement, which have totallydifferent statistical distributions from the Poisson distribution governing radioactivedecay.

Frequently, more than one count measurement is recorded. For this case, assum-ing the radiation source has constant activity, estimates of the mean and standarddeviation can be obtained directly from the TV measured values, Xi , £2, • • • , ^TV- Themean or expected count is the average value of the counts, i.e.,

N-l

— - — \^x~j^2^Xi

1=1and the corresponding estimate of the standard deviation of the mean is

s = \/x/N - I

Note that for replicate measurements the error is reduced by the square root ofN. This assumes, of course that the measurements are independent and that nochanges occur in the radiation field or with the detector's response during the timethat the N data points were being recorded.

If a Gaussian distribution can approximate the data, then s has a specific mean-ing, namely, there is a 68% probability that the mean value falls within the rangex ± s. This means that if the measurement were repeated a large number of times,68% of the observed counts would fall within x±s. Table 8.5 shows the relationshipbetween the percent standard deviation and the number of counts recorded. Forexample, a standard error of one percent or less would require recording at least10,000 counts.

Uncertainties are not always reported as one standard deviation. Sometimes alarger uncertainty e is reported in order to increase the probability that the meanwill be included within the range between x — e and x + e. As shown in Table 8.6,if the error is reported as one standard deviation, the probability is 68%. Howeverfor an error range of x —1.65s to x + 1.65s the probability increases to 90%. Thiswould be reported as the 90% confidence interval. The convention is to report errorsas one standard deviation. For anything other than one standard deviation, oneshould specify the number of standard deviations or the probability interval.

8.5.3 Dead TimeAll radiation detection systems operating in the pulse mode have a limit on themaximum rate at which data can be recorded. The limiting component may beeither the time response of the radiation detector, which is the case for the Geigercounter, or the resolving capability of the electronics. The true counting rate for a


Table 8.5. Standard deviation (%) ofcount data measured with a radiation de-tector operating in the pulse mode.

Table 8.6. Probability interval for dif-ferent number of standard deviationsbased upon a Gaussian distribution.

Observedcounts x

100400

11002500

10000

Percent standard error(1 sigma error)

10%5%3%2%1%

No. of standarddeviations

0.681.001.651.963.00

Probability

0.500.680.900.950.997

detector with zero dead time losses (n) is related to the recorded counting rate fora detector with significant dead time losses (m) by

ran =

1 mr(8.4)

where T is the dead time of the detector. Note that the term mr is the fractionof the time that the detector is unable to respond to additional ionization in theactive volume of the detector. When designing an experiment, it is advisable tokeep these losses to a minimum. If possible, this means that mr should be less than0.05. For example, for a GM counter with a typical dead time of r = 100 yus, themaximum count rate would be 500 counts/s.

8.5.4 Energy ResolutionOne important application for radiation detectors is radiation spectroscopy, mea-surement of the energy distribution of a given type of radiation. Such spectrometersrequires measuring some quantity that is proportional to the energy of the incidentradiation. Typically, that quantity is the amplitude of a voltage pulse producedfrom collection of the separated charge deposited in the detector. Raw spectroscopydata, known as pulse height distributions (PHD), display count rate vs. pulse height.They contain peaks whose relative positions are related to the energy of the incidentradiation. By processing a PHD, it is possible to determine the energy distributionof the incident radiation.

BIBLIOGRAPHYBEVINGTON P.R. AND D.K. ROBINSON, Data Reduction and Error Analysis for the Phys-

ical Sciences, 2nd ed., McGraw-Hill, New York, 1992.

EiCHHOLZ, G.G. AND J.W. POSTON, Principles of Radiation Detection, Ann Arbor Sci-ence, Ann Arbor, MI, 1979.

KLEINKNECHT, K., Detectors for Particle Radiation, Cambridge University Press, U.K.,1999.

KNOLL, G.F., Radiation Detection and Measurement, 3rd ed., Wiley, New York, 2000.

PRICE, W.J., Nuclear Radiation Detection, McGraw-Hill, New York, 1964.

TSOULFANIDIS, N., Measurement and Detection of Radiation, Hemisphere, New York,1983.


PROBLEMS

1. Should the quenching gas in a GM tube have a higher or lower ionizationpotential than the major tube gas? Why?

2. What effect does each of the following changes have on the performance of aproportional counter? (a) The diameter of the anode wire is increased, (b)The pressure of the fill gas is increased, (c) The atomic number of the wallmaterial is increased.

3. For 1-MeV photons, sketch how the detector efficiency of a proportional or GMchamber varies with the thickness of the cylindrical wall.

4. A given GM tube has a dead time of 0.25 ms. If the measured count rate is 900counts per second, what would be the count rate if there were no dead time?

5. The decay constant for Nal(Tl) fluorescence radiation is about 230 /its. Howlong must one wait to collect 90% of the scintillation photons?

6. In anthracene, scintillation photons have a wavelength of 447 nm. If 1-MeV ofenergy is deposited in an anthracene crystal and 20,000 scintillation photonsare produced, what is the scintillation efficiency?

7. Why is air often used as a tube fill gas in an ionization chamber, but not in aproportional counter?

8. If the energy resolution of a Nal(Tl) detector is 8%, what is the FWHM of thefull-energy peak for a 137Cs source?

9. Which detector has the greatest full-energy peak efficiency: Nal(Tl), HPGe,Si(Li). or CZT for (a) 100-keV gamma rays, and (b) for 1-MeV gamma rays?Justify your answer.


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