Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Gas Multiplication Counters
Gas Multiplication Counters
Pages 75-87
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From page 75... ...
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.
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From page 76... ...
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.
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From page 77... ...
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.
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From page 78... ...
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.
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From page 79... ...
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 typical geometry, the anode is a fine wire of radius a, coaxial with a cylindrical cathode of radius b.
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From page 80... ...
The gas multiplication factor for a proportional counter may vary over a wide range. Actual values for particular filling gases are obtained experimentally as a function of applied voltage, chamber geometry, and gas pressure.
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From page 82... ...
Also, the anode is operated at ground potential, so there is no need for a highvoltage 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.
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From page 83... ...
SOURCE RING SUPPORT UPPER ANODE WIRE SLIDE HIGH VOLTAGE CONNECTORS Fig.
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From page 84... ...
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.
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From page 85... ...
Now, however, 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.
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From page 86... ...
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.
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From page 87... ...
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.
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