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Design of counters. Device gamma counters


   Over its half-century existence of the structure of gas-discharge counters have undergone a variety of changes. For the purpose of self-extinguishing counters can be divided into several groups: gamma counters, counters are "hard" β-particles or the so-called beta-gamma-counters, the counters for soft β-particles which in their design should have a window to pass a soft β-particles into the counter, the counters of photons (quanta of light) etc.
SENSORS OF PHYSICAL PARAMETERS
A schematic sectional view of the counter of STS-2

Fig. 1. A schematic sectional view of the counter of STS-2

   By design, the gamma counters is very simple. One type is characterized in that the gamma-counter. Another type gamma counter with a halogen content is shown in figure 1. In this thin-walled (0.2 mm) cylinder - the cathode is a stainless steel spacer plate spring is inserted into a glass cylinder. From the cylinder using the molybdenum wire is output to the outside. The thread is strengthened in the same way as in alcohol meter, but in this case, the thread is made of Kovar. The counter is filled with halogen mixture (neon - argon - bromine) and can be used in a pulse mode (counting), and current modes. This counter refers to a series of STS (steel counter).
   In pulse mode, the counter has a very long service life. Under the technical conditions determined by counting a billion (109) pulses without noticeable deterioration of the counting characteristics. One of the advantages of halogen counters is their performance at high count rates. Counters series STS (СТС) will allow for speeds account one hundred thousand (105) pulses per minute and above; this preserves the proportionality between the irradiation intensity and speed counting up to two hundred thousand pulses per minute.
   This count rate corresponds to a dose of about 0.25 R / h. The magnitude of the dose to which the count rate remains directly proportional to her as well as the value of the maximum dose counter operation in the current mode depends on the geometric dimensions. The smaller the diameter of the cathode and the working length of the thread, the more limiting the dose and the longer it is maintained the proportionality between count rate and dose rate radiation.
   When choosing a working length in the meter are governed by the ratio of the working length of the thread shall not be less than twice the diameter of the cathode. With a smaller working length of the thread counter works too, but the potential value of spark discharge, it increases dramatically. For example, for a counter with a cathode diameter of 10 mm at the working length equal to 5 diameters (50 mm), at a certain gas filling, the start voltage is 400 V, when the working length equal to two diameters of the cathode - 405, at the working length equal to one diameter of the cathode - 450 V, and when the working length of the thread equal to 0.5 of the diameter of the cathode, the potential of ignition increases to 500 V.
   When reducing the working length of the thread decreases as the length of the counting plateau characteristics. So, for meters with a diameter of 6 mm when reducing the working length of the thread from seven diameters of the cathode to one the length of the plateau decreases from 200 to 30 V.
Therefore, of all the counters that are commercially available, only prototypes of the gamma counter of the type SI 2BG (СИ 2БГ) have a working length of thread less one diameter.
   With a decrease in working length, it is desirable to increase the diameter of the "security" of straws, as in this case, the total count rate has a smaller number of pulses, due to the end portions of the counter. For example, for a counter with a cathode diameter of 6 mm, the diameter of the guard "straw" 1.5 mm and the working length of the thread equal to one diameter of the cathode (6 mm), the count rate is 480 pulses/min, and for the same counter, but with the diameter of the "straws" of 4 mm from the same source and at a constant location relative to the meter the count rate is 280 pulse/min.
Meter with a smaller diameter of the guard "straw", the slope of the plateau is much more than the counters with a large diameter straw. This is due to the increase in the working volume of the counter with the increase in voltage on the meter due to the "blowups" of the electric field, i.e. leaving it outside of the cathode.