Gas-filled counter

TOWNSEND AVALANCHE. A term used in gas-filled counter technology to describe a process which is essentially a cascade multiplication of ions. In this process an ion produces another ion by collision, and the new and original ions produce still others by further collisions, resulting finally in an avalanche of ions (or electrons). The terms cumulative ionization and cascade are also used to describe this process. It occurs in a nonself-maintained gas discharge, where ions have sufficient energy. 

Gas-filled counters (ionization, proportional, Geiger-Muller counters)... 

By adding the three above components, the resulting dead time of a scintillation counter is of the order of 1-5 fxs. This is much shorter than the dead time of gas-filled counters, which is of the order of tens to hundreds of microseconds. 

Figure 8.18 shows a gas-filled counter and a source of radiation placed outside it. Usually the particles enter the detector through a window made of a... 

To discuss the effect of the statistical fluctuations on energy resolution, consider a monoenergetic source of charged partieles being detected by a silicon semiconductor detector. (The discussion would apply to a gas-filled counter as well.) The average energy w needed to produce one electron-hole pair in silicon is... 

Fano factors have been calculated and also measured. For semiconductor detectors, F values as low as 0.06 have been reported. For gas-filled counters, reported F values lie between 0.2 and 0.5. Values of f < 1 mean that the generation of electron-hole pairs does not exactly follow Poisson statistics. Since Poisson statistics applies to outcomes that are independent, it seems that the ionization events in a counter are interdependent. 

It can be seen from Eq. 9.7 that the resolution is better for the detector with the smaller average energy needed for the creation of a charge carrier pair (and smaller Fano factor). Thus, the energy resolution of a semiconductor detector (w 3 eV, F < 0.1) should be expected to be much better than the resolution of a gas-filled counter (w = 30 eV, F 0.2), and indeed it is (see Chaps. 12 and 13). 

A variation of the detector described above is the so-called drift chamber. The drift chamber determines the position from the time it takes the electrons produced by the incoming particle to drift to the nearest anode wire. A two-dimensional MWPC has also been constructed for detection of neutrons scattered from biological samples. It is a He gas-filled counter that detects neutrons through the (n, p) reaction. 

Response functions of proportional counters have been measured and calculated by several people. Verbinski and Giovannini gave a critical study of response functions of gas-filled counters as well as a comparative study of the different codes used to unfold their spectra. Figures 14.12 and 14.13 show measured and calculated response functions for methane- and hydrogen-filled proportional counters. 

PIC-6 PAC-4G Proportional gas-filled counter Proportional gas-filled counter y a 1 mR/h to 1000 R/h Measures y dose rate... 

Chapters 5-7 describe the different types of radiation detectors. Full chapters have been devoted to gas-filled counters, scintillation detectors, and semiconductor detectors. Detectors with special functions are discussed in Chap. 17. 

All gas-filled counters are in principle ion chambers (with the exception of the less common gas scintillation counters). The ionization produced in an ion chamber by a single nuclear particle produces too low a charge pulse to be easily detectable exc t for a-particles. However, an ion chamber can be designed so that the munber of ion pairs formed in each event is multiplied greatly. 

Detectors for particular purposes, e.g. gas-filled counters, or counters for very low activities... 

Figure 5 (A) Gas-filled counter with associated electronics. (B) Number of ion pairs n collected at central electrode as a function of voltage (V) for X-ray photons of low (curve 1) and high energy (curve 2). Regions A-F are described in the text.

Benchtop X-ray energy dispersive analyzer BRA-17-02 based on a gas-filled electroluminescent detector with an x-ray tube excitation and range of the elements to be determined from K (Z=19) to U (Z=92) an electroluminescent detector ensures two times better resolution compared with traditional proportional counters and possesses 20 times greater x-ray efficiency compared with semiconductor detectors. The device is used usually for grits concentration determination when analysing of aviation oils (certified analysis procedures are available) and in mining industry. 

Counter, Geiger-Mueller (GM counter)—Highly sensitive, gas-filled radiation-measuring device to detect (count) individual photons or particulate radiation. 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

B10 lined or BF3 gas-filled proportional counters are normally used as source range detectors. Proportional counter output is in the form of one pulse for every ionizing event therefore, there is a series of random pulses varying in magnitude representing neutron and gamma ionizing events. 

A detailed procedure for combustion, purification, and filling of the small gas proportional counters may be obtained from the Brookhaven authors. Since the apparatus for combustion had a maximum capacity of 10 grams it was necessary to burn three samples of Frobisher iron in order to get enough carbon dioxide to fill the counter. The carbon dioxide from the three samples was... 

The detector of choice for the efficient detection of 14-KeV y-rays is the gas proportional counter with a (mostly) krypton filling. (Methane, which we use, or carbon dioxide are typically added in a concentration of ca. 10% for quenching secondary discharges.)... 

Time variations in the intensity of the flux during irradiation. This is an important consideration only when a single sample transfer system is used. Gas-filled BF3 neutron counter tubes are often used to monitor the neutron flux in order to normalize the data when the sample and the standard are not irradiated simultaneously. Gain shifts and dead-time effects associated with the use of neutron monitoring detectors also contribute to the errors associated with a single sample transfer system. 

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