Topics in..
.
Chemical Instrumentation Edited by
GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
I
These articles are intended to serve the readers O ~ T H EJOURNAL bv callino allention lo new developments i n the t h e w , depige, or availability of c k i c a l ' l a b ~ r a t o rinslrumentaiion, ~ or by presenting us&l insights and&olanalions of tooics lhal are of practical importance to lhose who use, or teach ihe use of, mod& instrurneniaiion and in~trumentaltechniques. The edilo7 invites correspondence from prospective conlributors.
LXXV.
Nuclear Radiation Detectors P. J. Ouseph and M. Schwartz, Physics Department, University o f Louisville, Louisvilie, Kentucky 40208
INTRODUCTION Since the perfection of a workable nuclear detector by Geiger in 1908, there have been several important advances in methods of detection. Table 1 lists the different types of detectors currently in use. One look a t the table will convince anyone of the truth of the statement made by MacKay ( I ) in 1953: "Whenever a nuclear ~hysicistobserves a new effect caused by an atomic particle, he tries to make a counter out of it." There are two types of counters, of which the "track type" counters are mainly of interest to high energy particle physicists. However, the "signal type" counters are of interest to several groups of scientists such as radiochemists, low energy nuclear physicists, biologists, biochemists, geologists using radioisotopic techniques. It is the purpose of this article to discuss and compare the characteristics of signal type counters. Table 1.
Name Ionization Counter Proportional Counter Geiger Counter Scintillation Counter
Figure 1 is a block diagram of a typical oulse counter. A radiation (charged partiele) incident an the counter produces a signal (output pulse). The proportionality between the a m ~ l i t u d eof the simral ( ~ u l s e height) and the energy of the'radiation, the efficiency, the energy resolution, and the time width of the pulse are some of the factors one considers in selecting a caunter. These characteristics which influence the selection of a counter are discussed below. If the amplitude of the signal is directly proportional to the energy expended hy the radiation in the detectors, determination of the amplitude of the signal will enable one to measure the energy of radiation. The efficiency can be defined as the ratio of the number of detectable signals produced to the number of incident radiations. Sometimes, especially far low energy radiations, the amplitude of the signals
Particle Detectors
Primary Interaction
Tvoe Signal
Ionization
Gas
Signal
Gas, liquid and solids
Photographic Emulslon Cloud Chamber (diffusion and expansion) Bubble Chamber Spark Chamber
Track Track
Ionization
Gas
Track Track
Ionization Ionization
Dielectric particle detector
Track
Ionization
Photochromic Detectors (2)
Track
Change in oxidation of Fe ions
Liquid Gas and solid Solid, tracks developed by etching Salid
Signal Signal
Manuel Schwartz, Prufersor and Chairman of the Department of Physics, University of Louisville, received his P h . D from the Illinois Institute of Teehnolaev. 1953. His present research inte&ts are the electrical characteristics of living and artificial membranes and the electron transport properties of thin film solid-state physics.
Medium
Excitation of electronic levels and associated production of photons Production of electrons and holes Production of photons by Cerenkov effect Ionization
Remiconduetor Counters Cerenkav Counter
P. J. Ouseph, Professor of Physics a t the University of Louisville, received his P h D . from Fordham University in 1961. He has puhlished several articles on Nuclear Radiation detectors and he has recently developed detectors using photochromic materials. His main research interests are the Mossbauer effect and angular eorrelation.
produced may be smaller than the noise, making it impossible to detect. Another factor is that the electronics used for analyzing the signals may have sensitivity Radiation
Solid
Gas, liquid and solids Solid
Detector
Figure 1. Block diagram of a pulse counter. Variation of the voltage (electrical pulse) following the incidence of the radiation ao a function of time is depicted.
(ContinuedonpageA140)
Volume 51, Number 3, March 1974 /
A139
Chemical Instrumentation
R%
=
Full width a t half maximum
x 100. (1)
Pulse height a t the peak limits, i.e., pulses below the lower limit will not be detected. Therefore, one can see that the efficiency of a counter will vary with the lower limits of the electronics following the counter. A good example is the gamma detection efficiency of a scintillation counter. The energy resolution of a counter is the measure of its ability to distinguish between radiations of very close energies. This will depend on the spread in pulse heights (i.e., amplitudes of the signals) for radiations of the same energy. A typical pulse height distribution is Gaussian in shape. The resolution, R, is defined as follows:
Full width at half maximum (FWHM) is the width of the pulse distribution at half height of the peak. The power of a counter to resolve gamma rays increases as R decreases. The time width of the pulse is another important factor usually considered in selecting a counter. If the time width is large, overlappin'g of pulses will take place with a concomitant loss of count. The pulse has two parts: (1) the rising part and (2) the decay part. The first part is determined bv the . ~ r a.~ e r t i of e s the counter (such as the ion collection time) and the second part depends on the time eonstant (RC) of the system. The total width
Figure 2. Schematic representations of several counters. (a) Gas counter: 1, incident radiation: 2, central electrode; 3, outer electrode which also acts as the body of the counter. (b) Scintillation counter: 1, incident radiation: 2. scintillant crystai: 3, photomultiplier: 4, dynodes; 5, collector electrode. ( c ) Single crystal semiconductor counter: 7, incident radiation: 2, electrodes: 3, crystal. The incident radiation spends its energy producing electron-hale pairs in the crystal. can he reduced by reducing the time constant; however, a suitable relationship between the collection time and decay time should be maintained to keep the proportionality between the energy of the radiation and the nulse heieht. A fast rise time for the puke ic requmd far coincidence e x permenti w i t h good time resolution.
THREE BASIC COUNTER TYPES There are three different types of pulse counters as illustrated in Fig. 2. Even though scintillant ZnS-coated screen was used by Rutherford in his historic alpha scattering experiment, it was soon replaced by gas counters. The gas counters provided the advantage of radiation-producing electrical signals which were easy to manipulate. However, in 1944, Curran and Baker revived the scintillation counter; they replaced the eye by a photomultiplier to detect the flashes of light. The subsequent development of the thick organic and inorganic scintillant crystals brought the scintillation counter into wide use. The semiconductor counters came into use only in the last ten years, even though attempts were made in 1945 by Van Hearden and in 1952 hy Chynoweth to construct the socalled ''crystal counters."
Gas Counters In the gas counter (Fig. 2a), the incident radiation produces electrons and positive ions. If an electric field is a n ~ l i e dbetween the electrodes (the cylindr&l outer electrode is the cathode, and the central wire, (Continued onpageA146j A140
/
Journal of Chemical Education
Chemical Instrumentation the anode), the electrons and ions will be collected, producing a current between the cathode and the anode. The behavior of the ions and hence the characteristics of the counter, change significantly with the strength of the applied field. There are three distinct types of counters: 1) ionization counters where the voltage is just sufficient to collect the ions, 2) proportional counters where the applied field is large and hence the ions moving to the electrodes are capable of producing further ionization by collision, thus increasing the total number of ions, 3) Geiger counters where the voltage is still higher and the multiplication of ions by collision is such that the total number of ions collected is the same for all incident radiations irrespective of their energy. There are other regions such as that of limited proportianality between the proportional and the Geiger regions, and the discharge region which is above the Geiger voltages. The operations of these detectors are discussed in several hooks and review articles (3).
Semiconductor Counters Operation of semiconductor counters (Fig. 2e) is somewhat similar to that of ionization counters (4). Here the radiation produces electron-hole pairs and their col-
A146
/
Journal of Chemical Education
lection a t the electrodes causes the electric signal. However, a single crystal of semiconductor material such as Si or Ge will not make a suitable counter because of the DC current in the crystal. Random variations of this current may produce pulses similar to the radiation-induced pulses. To reduce this current a p-n junction in reverse bias is used. In thermal equilibrium, the conduction electrons contributed by the donor atoms (the impurity atoms in an n-type semiconductor) predominate in the n region. Here the conduction electrons neutralize the space charge of the donor atoms. Similarly the holes contributed by the acceptor atoms (the impurity atoms in the p-type semiconductor) neutralize the space charge of the acceptor atoms in the p region. In addition, at the junction between the two regions, diffusion allows electrons to move into the p region and holes into the n region. In this process the electrons leave behind positively charged donor ions and the holes leave negatively charged acceptor ions. These ions are fixed and form a double layer. The electric field of the double layer essentially prevents further diffusion across the junction.' In reverse bias, the holes accumulate closer to the negative electrode and the electrons t o the positive electrode. The region in the middle is practically free of charge and is, therefore, called the depletion region. Because of the concentration of the charges close to the electrodes, the potential drop
is essentially confined t o the depletion region. Consequently, if radiation enters the depletion space, electron-hole pairs produced in this region will be collected by the electrodes. If radiation falls outside this region, the electron-hole pairs, which are subjected to practically no potential difference, will rarely be collected a t the electrodes. Therefore, the sensitive area of the counter is the depletion region. It is interesting to contrast this with gas counters and scintillation counters in which the sensitive volume is equal to the physical volume afthe counter. T h e width of the sensitive volume (depletion region) depends on the applied bias voltage V and the resistivity p of the sample. For a p-type material, the width Wis approximately given by
where Vo is the potential barrier across the p-n junction a t zem bias voltage. The potentials V &d VOare expressed in volts; the resistivity p in ohm-cm; the width W in centimeters. The voltage Vo is of the order of 0.5 V. The ultimate maximum in width depends upon the effective purity of the material and upon the breakdown voltage of the junction. For example, a p-type silicon base material of 10,000 ohm-cm resistivity has about 4 x 1012 acceptors/cm3. At a reverse bias of 500 V, a width of 0.7 mm results. (Continued anpogeA148J
Chemical Instrumentation
& .li. E h C C ~ ~ ~ ."?ilFI.Thl" ..ilP' II"YII*LII 0.1 rn,Ck,
""
WTYP itng,. CV.S.> 4
used as the base material. Single crystals of high resistance are sliced into 1 mm pieces. Then phosphorous is diffused into one surface of these slices. A common method is to coat one side with phosphorous pentoaide dissolved in glycol and to heat the slice at 800°C in dry nitrogen for K hr. The phosphorous diffuses into the hase material while the glycol leaves the hase material covered with a black residual deposit. After diffusion, proper electric connections are made to the p and n sides. A typical counter arrangement is shown in Fig. 4. Special care is taken to reduce deterioration of the crystal and to minimize surface current, as distinct from volume current produced by electron-hole nnirs. The metal container is filled with
Can eoldered to detector
N_gion
surface
P region
Figure 3. Schematic diagram of a p-n junction counter.
Reverse biased p-n or n-p junctions are used as nuclear detecton. The schematic diagram of such a detector is shown in Fig. 3. The n layer is made extremely thin, typically 0.1 pm, so that the energy loss in this layer is very small. Most detectors currently in use are of three different types and they are discussed below.
Diffused Junction Detector This type is often produced by diffusing a high concentration of donor impurities into a p-type material. Silicon is usually
A1 48
/
Journal of Chemical Education
horan and gallium into n-type silicon crystals. Figure 4. Encapsulated diffused junction detec-
Barrier Layer Detectors
tor.
If n-type silicon is exposed to air, the surface layer oxidizes. The oxidized layer has the characteristics of a p layer. Since the layer is very thin, the junction is very close to the surface. Hence energy losses by the radiation outside the active volume is minimal. Since the oxidized layer is not conducting, a thin gold coating of about 40 pglcrnz is applied to the surface, and elec-
tric connection is made to this layer. Connection to the hack side is made through a nonreetifying metal contact. Surface harrier Si detectors can he operated at room temperature. Similarly, germanium surface harrier detectors can he made. Since the energy
gap is smaller, the counter cannot be used a t room temperatures. Energy resolution of germanium counters a t 77°K is almost equal to the energy resolution of the silicon detectors at room temperature.
Lithium-drifted Detectors Lithium is a donor atom. It does not go inta substitutional sites as do other donor atoms such as phosphorous. Instead, it enters interstitial sites. The diffusion coefficient is about 10' higher than that for phosphorous and, therefore, deep diffused junctions can be prepared. Diffusion is usually achieved in two steps. First, lithium is coated an single crystals and diffused into them by heating. In the second step the sample is heated and strong reverse bias is applied. This helps the lithium atoms t o diffuse deeper inta the erystal. There are three regions: the p region, the n region, and an intrinsic region. The last region results from the compensation or neutralization of the p-type impurities by the lithium atoms. The intrinsic region constitutes the active volume of the detector. Active volumes of 50 cm3 have been achieved. This type of drifted detector is also known as an n-i-p device. Lithium drift devices can be prepared in both germanium and silicon crystals.
Scintillation Counters The third type of pulse counter is a scintillation counter which consists of a seintillant crystal, a photomultiplier: a
power supply, and an amplifier-analyzerscaler system (Fig. 2b). When an ionizing radiation passes through the seintillant, it produces light photons. The mechanism involved in the production of light is often complicated and not well understood. Also the mechanism is different for different types of scintillants. For a discussion of this and other properties of scintillant materials, the reader is referred to "The Theory and Practice of Scintillation Counting," by Birks (5). The number of photons produced, n p , is proportional to the energy absorbed in the seintillant. The scintillant is covered with a reflector ereept on the side connected to the photomultiplier. The photomultiplier has a photoelectric film (usually coated onto the photomultiplier tube) as its first element. When light h l l s on it, electrons are released. The number of electrons produced is proportional to the number of photons, and is equal to n,r where c is the efficiency of the photoelectric material. In most cases r is about 10%. The electrons pmduced are focused onto the first dynode. The dynodes are coated with materials like cesium-antimonide so that, when high energy eketrons hit them, secondary electrons are produced and hence electron multiplication occurs. The photomultipliers have ten or more dynodes. When an electric potential is applied between any two dynodes, the secondary electrons produced in preceding dynodes gain energy before they strike the next dynode and produce more secondary electrons. Thus multiplication occurs in each dynode and the final num-
ber of electrons, N., collected by the last electrode called the collector, will be given by
N,.
=
n,c.m,.m,---
(3)
.
where m,.mz . . , are multiplication factors for successive dynodes. The factors m1.m . , are dependent on the potential between the dynodes and usually independent of the number of incident electrons on any dynode. From Eq. (3), one can see that N. is proportional to np and, consequently, proportional to the energy which the incident radiation can supply to the scintillant. The electrons finally pass through a resistance producing a voltage drop across this resistance. The voltage drop is of short duration and is proportional t o N,. This electrical pulse has an amplitude proportional to the energy of the radiation and can be amplified and analyzed.
..
PULSE HEIGHT VS ENERGY AND ENERGY RESOLUTION For all the counters discussed above, except gas counters operating in the Geiger region, the pulse height has some relationship to the energy of the radiation. In gas counters and semiconductor counters this dependence is linear. In scintillation counters, behavior of the different scintillants varies in this respect. For inorganic scintillants aver a wide range of energy, the pulse height is linear(Continued onpageA152)
Volume 51. Number 3, March 1974
/
A149
Table 2.
Absorption process
Contrlbutions to Pulse
eight Distribution Due to Events Occurring in the Scintillant
Photoelectric effect ma escapes)
ed gamma is absorbed)
Pair Production annihilation (gamma rays escape)
Pair Production (One of the anmhilation radiations escapes)
Pair Praduetion (Both radiations absorbed)
'air Produeion (One of he annihilaion radia.ions produce S Compton) tlectron
-
Kinetic energy of the electron positron pair = E, - 1.02 MeV
Varies from 0 to
Energy absorbed in the crystal
Tmax; T,., = E:
Pulse height
Pulses in the photo peak
Pulses in the Compton distribution
Pulses in the photo peak
Proportional t o E. - 1.02 MeV a i d the peak is called double nenk r
E, - 0.51 MeV and peak is called single es-
I I LNotes: E, is theI energy of the incident gamma ray. In the photoelectric effect, the electron produced takes all the energy,
'ulses in the Zornptan :ontinuurn
h e gamma ray. ? electron gains In the Compton effect, the energy gained by the electron varies from 0 to T,,,.., depending on the angle of scattering. T,,,.,when the gamma is scattered back (scattering angle s radians). T,.. is given by the e uation in Column B, where i is the rest mass of the electron, and mocZis equal to 0.51 MeV. In pair production, 1.02 MeV is converted to tRe mass of the electron-pos~tl pair. In the opposite process, where the positron annihilates with an electron, two gamma rays of 0.61 MeV each are produced. (Continued onpogeA154) ~~~
A152
/
Journal of Chemical Education
Table 3.
Contributions to the Gamma Spectrum by Gamma Rays Scattered From the Source and Surroundings into the Scintillant
Event
Campton back-scattering
Compton a t angle other than r
Pair production in the environment and one gammaseattered into the scintillant.
Energy
E, - T , . is the max energy of the electron*
Energy between E, - T , . and E,
0.51 MeV
Pulse height
Peskdue to bnck.scattered gnmma, c a h d oara-scattered oesk. owurnat a well defined
Pulse height distribution in-
photo peak.
lar Compton distribution.
.
source decays by positron decay.
*See footnote toTable 2.
ly proportional to the energy for electrons, protons, and deuterons incident upon the scintillant material (nonlinearity produced hy hackscattering for @ rays is discussed helaw). In the case of gamma rays, scintillants receive energy from the electrons produced by gamma rays, and therefore.
A154
/
Journal of Chemical Education
the same linear relationship holds between gamma ray energy and the pulse height. Slight nonlinearity has been observed for electrons produced by gamma rays below a few hundred keV. For heavier particles, nonlinearity occurs over a wide range of energy. In the ease of organic seintillants
nonlinearity for heta and gamma rays occur a t very low energies, approximately around a few keV, while nonlinearity for heavier particles extends over a wider range of energy. Another point to consider is the relationship between the pulse height and the nature of the particle. The height of the pulses produced by heavily ionizing particles such as alpha particles may differ considerably from that produced by electrons of the same energy. This difference depends on the nature of the counters, being generally small in gas counters and semiconductor counters. The energies required to produce an electran-hole pair in Si for alpha and heta particles are 3.62 eV and 3.67 eV respectively so that the difference here is of the order of 1%.Similar results have been obtained for Ge detectors. The energy per ion pair for alpha particles and beta particles are slightly different for all the commonly used counter gases. Beta particles have to expend 42.3 eV and alpha particles 42.7 eV to produce an ion pair in helium; the corresponding values are 27.3 eV and 29.2 eV in CH*. However, this difference is much higher in seintillation counters, and i t is usually given as the a/@ ratio, that is, the ratio of pulse height produced by en alpha particle to that produced by a beta particle of the same energy. For Na(TI)I, a/@ is 0.5; for anthracene, 0.1; and for a plastic scintillant, 0.09. Pulse height distribution of alpha particles can be obtained with ionization, proportional, scintillation and semiconductor counters. Since semiconductor counters give the best resolution, they are mainly used these days. The next best resolution is obtained from an ionization counter. However, because of the very slaw drift velocity of the positive ions, the pulse height depends very much on thedirection of the alpha particle in the chamber. Frisch (6) has overcome this difficulty by introducing a grid between the two plates of the ionization chamber, thereby divid-
Figure 6. Block diagram of a particle spectrometer system. With a semiconductor counter, a iow-noise FET-inpuf preamplifier must be used. CRT, cathode-ray tube oscilloscope: MCA, multichannel analyzer; H-V, high-voltage power SUPPlY.
Figure 5 . Pulse-height spectra of 1.368 and
2.75 MeV gamma rays from 24Na; (a) spectrum obtained in a Na(TI1I detector fbl spectrum obtained with a Ge(Li) detector. Tremendous improvement in resolution in the Ge(Li) is clearly seen. The lefters A , B, etc., correspond t0Tables 2 and 3.
ing it into two parts. The source is placed such that the ionization occurs in one part of the counter and the pulse is developed by the flow of electrons in the other part between the grid and anode. Since the pulses are due to the flow of fast electrons through a fixed potential, their heights are proportional to the numher of primary electrons and they are independent of the direction of the alpha particles. hoportional counters and thin crystal scintillation counters can be used for cl-spectroscopy in cases where resolution is not of prime concern. When diffused junction and barrier-layer detectors are employed it is necessary to have counters with the thinnest electrode and dead layer so that practically all the energy is expended in the active volume of the counter. In studies involving heta rays, the pulse height distribution is distorted due to hackscattering, especially for energies
