National Academies Press: OpenBook

Detection and Measurement of Nuclear Radiation (1962)

Chapter: Gas Multiplication Counters

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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Suggested Citation:"Gas Multiplication Counters." National Research Council. 1962. Detection and Measurement of Nuclear Radiation. Washington, DC: The National Academies Press. doi: 10.17226/18670.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

The technique to be followed was treated in detail by Chetham-Strode, et al., who discussed the problem of eliminating background counts in order to realize the inherently low background possible with silicon detectors. Materials of construction used in the counting chamber should have very small alpha contamination. In cases where the recoil daughter nuclei formed in alpha decay are radioactive and may contaminate the silicon detector these authors suggest introducing a low gas pressure into the chamber to slow the recoils to an energy such that they can be returned to the source plate by application of an electric field. An alpha spectrum of Cm244 obtained with a surface barrier detector is shown in Fig. 35. The full-width at half-maximum counting rate is only 16 kev, and so the two main alpha groups, 43 kev apart, are well resolved. In addition to the groups shown, a third group, 142 kev below the intense peak (0^42) is known to be present in an abundance of 0.017%. Since the resolution is adequate it would be hoped that measurement of such low-abundance groups could be accomplished on sources with too low an intensity to measure by magnetic deflection. Such weak alpha groups may often be obscured by a non-gaussian, low- energy tail on the peaks. The height of this tail at the expected position for the a,42 peak of Cm244 is 0.15% the height of the a0 peak. From information available to date, it appears that a substantial portion of this tail is associated with processes within the detector and not with environmental effects such as source properties, scattering, or energy loss in the gold detector film. V. GAS MULTIPLICATION COUNTERS 1. Introduction The operation of an ionization chamber depends upon the collection of the charge produced when an ionizing particle traverses the sensitive volume. Any enhancement of the electri- cal signal produced must be done in the electronic amplifier system, as the chamber possesses no internal amplification properties. On the other hand, proportional counters and Geiger counters make use of gas multiplication, which enables them to produce output pulses many times larger than would be 75

1000 — ir </> Cm2 ALPHA SPECTRUM 5.801 Mev 76.7% 5x5 mm Si Surface Barrier Detector 90 Volts Reverse Bias 258C FWHM 0.28% (16 kev) 5.759 Mev 23.3 % 500- CHANNEL NUMBER Fig. 35. Pulse-height distribution measured with a 5 x 5-mm silicon surface-barrier detector at 25 C and 90 volts reverse bias. The energies and abundances of the alpha groups to the ground and 43-kev states of the Pu240 daughter nucleus are literature values (A. Chetham-Strode, Oak Ridge National Laboratory). obtained if only the primary ionization were collected. The distinction between simple ionization chambers and gas multiplication counters may be seen by considering the number of ion pair collected (or pulse height) as a function of applied voltage. For example, Fig. 36 shows the behavior of a gas- filled counter with coaxial electrodes; the inner electrode is a fine wire, which serves as the anode. As was shown in Section III.2., the number of ion pairs collected rises with applied voltage, until a saturation region (B of Fig. 36) is reached. When the voltage is advanced beyond region B, the electrons acquire sufficient energy near the anode to produce 76

APPLIED VOLTAGE Fig. 36. Illustration of the relative number of ion pairs collected in a counter as a function of the applied voltage, showing the relationship between the ionization chamber, proportional counter, and Geiger-MQller counter regions of operation. additional ionization of the gas by collision, and the pulse height rises. Throughout region C, each electron produced initially will produce an avalanche of rr secondary electrons; m is called the gas multiplication. Region C is referred to as the proportional counter region, because the pulse size is proportional to the initial ionization; hence, the pulse height from the alpha particle of Fig. 36 remains a factor of 10 higher than the electron pulse height. This suggests that alpha particles can be counted in the presence of electrons which usually have a lower energy: a pulse-height discriminator (cf., Section VI. below) may be set to reject the low-amplitude electron pulses and count only the alpha particles. Eventually, a further increase in voltage creates such a

Fig. 37. Shape of a proportional counter pulse. The dotted curve shows the shape obtained with an input time constant RC = 5 psec (Staub44). density of secondary charges near the anode that the positive- ion space charge from one primary electron begins to interfere with the formation of an avalanche by a neighboring electron. This effect leads to the situation in D of Fig. 36 where the different amounts of primary ionization produce slightly differing pulse heights, but the strict proportionality is lost. Region E is the Geiger-M'uller region, in which the detector produces a pulse of almost constant height, regardless of the initial ionization. In this mode of operation the discharge is not localized, but rather a single electron initiates an avalanche which propagates throughout the anode's length. Although the Geiger-Miiller counter, or Geiger counter as it is often called, cannot be used for spectrometry, its large-amplitude output pulse makes it useful for many appli- cations where simple counting will suffice. 2. Proportional Counters Proportional counters are especially useful where pulse counting of beta radiation is required. Because of their shorter resolving time, proportional counters can be used at much higher counting rates than a Geiger counter, and they See references 39, 40, 41, 65, 66, and 67. 78

exhibit excellent long-term stability. When used as a spectrometer for low-energy beta rays and X-rays, the pro- portional counter is capable of much better resolution than a scintillation spectrometer. A. Conditions for Gas Multiplication. To achieve gas multiplication it is necessary to provide an electric field strength capable of accelerating the primary electrons to an energy sufficient to produce additional ionization. In a typi- cal geometry, the anode is a fine wire of radius a, coaxial with a cylindrical cathode of radius b. The electric field strength £ , at a radial distance r, and an applied voltage V, is (14) r loge (b/a) It is apparent from Eq. 14 that the high field is confined to the region near the central wire; hence, most of the gas multiplication occurs within a few mean free paths of the anode. Since the electrons move such a short distance before they are collected, the voltage pulse which appears on the anode arises from induction by the positive ions as they move away from the central region. The formation of a voltage pulse in this way is not as slow as might be expected, because the positive ions move through most of the voltage drop while still in the high- field region near the central wire. The time to collect all of the ions depends on the geometry, applied voltage, the gas 41 chosen, and its pressure; generally, collection times are a few hundred psec. 44 The pulse shape for a single ion or a group of ions pro- duced at the same place is shown in Fig. 37. The output voltage rises very rapidly at first, being nearly linear with time; later, the pulse shape becomes logarithmic. In the example of Fig. 37, the pulse reaches half of its maximum amplitude in about 5 jjsec; the total collection time in this case would be 590 usec. If the time constant is reduced to 5 psec, the maxi- mum pulse height drops to about 1/3 of the former value but is much narrower. By "clipping" or "differentiating" the pulses in this way, the pulse width can be made small enough to permit high counting rates (c.f., Section VI.2.). Note that in this procedure all pulses are reduced by the same factor, since the final pulse height in a proportional counter does not depend on the location of the primary ionization. However, there will be 79

some variation in the rising part of the voltage pulse if the primary electrons are distributed radially through the chamber, since the primary electrons will then require a varying amount of time to drift into the multiplication region. For this reason, it is customary to integrate the signal with a time constant equal to several times the signal rise time to reduce / Q the effect of rise time variations on output amplitude. The gas multiplication factor for a proportional counter may vary over a wide range. Actual values for particular fill- ing gases are obtained experimentally as a function of applied voltage, chamber geometry, and gas pressure. Usually, gas multiplications range from unity up to about 10*, with values as high as 106 possible for events with low primary ionization; the onset of nonproportionality is observed to occur with parti- cles of high primary ionization at lower multiplications than minimum ionizing particles (see Fig. 36). B. Construction and Use. Many practical forms of propor- tional counters have been employed. The design chosen usually depends on the application - for spectroscopy, the requirements may be quite exacting, while for beta counting, the design is not so critical. When the proportional counter is to be used for spec- troscopy of low-energy electrons or X-rays, the electric field should be uniform and the electrical noise generated by insu- lator leakage must be very small. The former requirement can best be met by using a cylindrical geometry, and then only if the central wire diameter is very uniform. It can be seen from Eq. 13 that small variations in the diameter of the central wire will cause large variations in the electric field strength. Any practical design will lead to some distortion of the electric field at the ends, the so-called end effect. However, if the radiation to be analyzed can be collimated so that only the center portion of the sensitive volume is illuminated, then the end effect will not be very serious, providing that the counter tube is long with respect to its diameter. Such a design is shown in Fig. 38; low-energy X-rays are permitted to enter the counter through a beryllium window, midway along the wall. Occasionally it may be inconvenient to make a long counter; a considerable latitude in dimensions is possible without field distortion if electrically insulated field tubes are placed 80

C 8 o w 60 £3 *4 O in 4-> - d) O E ^ O rt fH fc •P O O co rH >> rt rt C C O I -H O <H 0) bfi h T3 <D -H O rt O O eS -n a x O Ifl +J O O 'M o, o O CQ oo <J co ^ • C be O .H - rT. ^J O M •P w 81

over the guard tubes. Their voltage is adjusted so that the lines of force are radial at the end of the sensitive volume. Figure 38 also serves to illustrate how the requirement of low leakage noise is met. First, a guard electrode is used in the same way as for ionization chambers. Also, the anode is operated at ground potential, so there is no need for a high- voltage blocking capacitor, which often can become a source of noise. Because they are so much more convenient and reliable than Geiger counters, most routine beta assay work is performed with proportional counters. Often it is neither practical nor neces- sary to approximate a cylindrical geometry for an end-window counter; in this case, the design of Fig. 39 is useful. A wire loop is the anode and may be placed inside a cylinder as shown, or inside a hemisphere [Fig. 51(b)]. These counters are usually GAS V8OD COPPER TUBE-~ HIGH VOLTAGE CONNECTOR SS CAPILLARY TUBE 1-mil SS WIRE 7/i6 DIA V8OD COPPER TUBE — GAS 0-RING ALUMINIZED MYLAR WINDOW, 1mg/cm2 Fig. 39. End-window proportional counter for routine beta-ray counting. 82

operated at one atmosphere pressure of filling gas, which flows continuously. If pure methane is used as the counter gas, a rather high voltage (typically 3500 volts) is needed; a mixture of 90% argon and 10% methane ("P-10 Gas") permits operation at about half the methane voltage. In a flow counter the window may be eliminated entirely. This is especially helpful in the counting of alpha particles and low-energy electrons. The sample is introduced into the chamber by means of a slide. The 47T counter has enjoyed increasingly widespread use in the standardization of radioactive sources (c.f., Section VIII.2.D.). A useful version of this type of counter is shown in Fig. 40. Two identical proportional counters view the source with almost an exact 47T geometry. The chambers are of the "pillbox" type with 1-mil stainless steel anode wires. When used for absolute beta counting, the anodes of the two halves are connected together and operated as a single counter. A slide is provided for rapid changing of samples. The carrier-free source is mounted on a thin plastic film, metallized to ensure electri- cal conductivity; thus, the source film cannot acquire an electrostatic charge but remains a part of the ground plane dividing the two counters. SOURCE RING SUPPORT UPPER ANODE WIRE SLIDE HIGH VOLTAGE CONNECTORS Fig. UO. Proportional counter for kff beta counting (Cak Ridge National Laboratory Model Q-1632). 83

C. Plateau Characteristics. For alpha and beta counting the required voltage sensitivity is about 1 mv. This is obtained, for example, if the amplifier voltage gain is 250, and the scaler or other recording equipment records all pulses above 0.25 v. As the voltage is increased, the counting rate rises until all particles yield a pulse large enough to count; increasing the high voltage still further produces an essenti- ally constant counting rate, since all particles are now being recorded. This region of almost constant counting rate is called the plateau. In Fig. 41 is shown a counting rate-voltage curve obtained with a source containing both an alpha and a beta emitter. The BETA PLATEAU _^ /*"* /' ALPH A PLA TEAU Z / / 2200 2600 3000 3400 APPLIED VOLTAGE 3800 4200 Fig. 41. A counting rate-voltage curve obtained with an end-window proportional counter, and a source containing both an alpha and a beta emitter. alpha particles deposit more initial ionization than the beta particles, and so a smaller gas multiplication is needed to count them. The figure shows that the alpha-particle plateau is reached several hundred volts before the beta-particle plateau. A well-designed proportional counter will have a plateau of negligible slope (< 0.2%/100 volts) for several hundred volts, when counting beta particles having energies greater than about 200 kev. This performance can be obtained only with an amplifier having low noise and good overload properties. (See Section VI.2., below). 84

3. Geiger Counters The Geiger counter once was the most popular type of radi- ation detector, since it was capable of detecting any ionizing radiation with adequate sensitivity, and because of its large pulse size, it did not require a high-gain amplifier. Now, how- ever, Geiger counters are not often used in laboratory counting for a number of reasons: their plateaus have a greater slope than proportional counters, and so a Geiger counting setup is not as stable as the proportional type; they possess a long dead time which arises from the mechanism of the discharge, and cannot be reduced; finally, they produce a pulse of constant amplitude regardless of the initial ionization, and therefore cannot distinguish between alpha and beta particles. Geiger counters can be made into very rugged assemblies, and, because they require only very simple ancillary equipment, they are widely used for survey devices and other field applications. In general appearance the Geiger counters resemble their relatives, the proportional counters, but they differ in the nature of the filling gas and the pressure. Design require- ments for a Geiger counter are rather critical, and a cylindri- cal geometry is almost always used because the necessary parameters are easily controlled. A. Mechanism of the Geiger Counter. As mentioned above in Part V.I., if the voltage applied to a proportional counter is increased, all pulses eventually become of the same high amplitude, regardless of the initial ionization (see Fig. 36). This region of operation is called the Geiger region, and will be discussed briefly. Detailed treatments of the subject have been given by Wilkinson41 and by Korff.69 Just as in a proportional counter, the electrons drift toward the anode and become accelerated in the electric field Their high energy enables them to release more electrons by ionization, each new electron releasing further ionization. Some of the excited atoms emit photons, and occasionally a photoelectron is produced. As the electric field strength increases, the number of photons produced in an avalanche grows until each avalanche produces a photoelectron. Since a single electron can start a new avalanche, the avalanche region spreads until it envelops the entire central wire. See references 39, 41, 65, and 69. 65

As in a proportional counter, collection of all the electrons is a fast but small contribution to the voltage pulse. The larger part comes from electrostatic induction by the positive-ion sheath as it crosses the high-field region on its way to the cathode. If a positively charged rare-gas ion strikes the cathode, a secondary electron may be produced. This additional electron will result in another discharge unless provision is made to prevent multiple discharges by quenching. Almost all modern Geiger tubes are self-quenching; that is, they contain some polyatomic gas which brings a halt to the process when the positive-ion sheath reaches the cathode. Argon-filled Geiger tubes frequently use alcohol as the quenching gas; in such a tube, an argon ion makes frequent collisions with argon atoms and alcohol molecules on its way to the cathode. The proba- bility is very high that an argon ion will be neutralized in an encounter with an alcohol molecule, but the opposite transfer of charge is not energetically possible. Therefore, the positive-ion sheath which finally reaches the cathode is composed only of alcohol molecular ions. These ions cannot release secondary electrons; instead, they become neutralized at the cathode surface and dissociate harmlessly. The organic gas eventually becomes depleted after about 109 counts have occurred, and the counter is no longer usable. The consumption of quenching gas increases with operating voltage. The life of Geiger counters has been extended greatly through the use of halogens as quenching gases. A common fill- ing is about 0.1% chlorine in neon. The quenching mechanism is the same as just described for organics, except that after the diatomic chlorine molecules dissociate at the cathode, they eventually recombine; thus, the quenching gas is continually replenished. This makes it possible to operate the tube at very high voltages without harm, and so very large signal pulses are obtainable. B. Plateaus. The counting rate-voltage plateaus for Geiger counters do not, as a rule, have a very small slope. The organically quenched tubes exhibit plateaus 200-300 volts long, with a slope of 1-2% per 100 volts. Because of the organic quenching gas, their life is limited to only about 109 counts, and therefore should not be subjected to high counting rates for extended periods. The halogen-quenched tubes are character- 86

ized by shorter plateaus (100-200 volts), and steeper slopes of 3-4% per 100 volts. Halogen-quenched tubes have a much longer counting life, extending to perhaps 1011 counts. An early objection to the halogen-quenched tubes was that, because of the large anode used, the detection efficiency was not uniform over the window area. Recent improvements in design have largely eliminated this effect and have improved the plateau performance to the extent quoted above. C. Resolving Time. Although the time constant of the counting equipment may be made quite short so that only the initial part of the output pulse is utilized, the Geiger tube does not immediately recover from the discharge. Unlike the proportional counter, the positive ions form a nearly cylindrical sheath around the anode, which profoundly disrupts the electric field and prevents the initiation of new avalanches near the anode. This situation prevails until the ion sheath has migrated out of the high-field region. The time interval during which the tube is completely insensitive to additional ionizing parti- cles is called the dead time; the time interval which follows is called the recovery time, because once the tube begins to count again, it requires a considerable period before the pulse size regains the original amplitude. This is illustrated in Fig. 42. In a practical measuring system, it is necessary to know the resolving time, or average time interval for which the I- DEAD TIME Fig. 42. Illustration of pulse shapes in a typical Geiger tube operating at a high counting rate. The dead time and recovery times are determined by the Geiger tube charac- teristics, but the resolving time depends on the triggering level of the electronic recording system. 87

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