Gas multiplication

Although, gas multiplication factors as high as 10 to 10 can be achieved, the signal charges are of the order of 10 electrons or only 10" coulomb. Consequently, some electronic preampUfication has to be installed before the signals can be processed. 

Each ionization produces another electron, such that the gas multiplication is given by... 

Dead time is particularly significant for Geiger counters they have the largest dead times (up to 200 ps) because they have the most charge to collect (maximum gas multiplication, maximum size ionization avalanche), which takes time. 

The MWPC usually consists of three planes of wires as shown in figure 5.22 (a). One set of cathode wires is parallel to the anode wires, the wires in the other cathode are orthogonal to it. At the position where an X-ray photon is absorbed and gas ionisation occurs, the resulting electrons drift in the direction of the electric field towards the anode and gas multiplication takes place. A negative pulse is formed on the anode. Simultaneously, positive pulses are induced on a few of the neighbouring cathode wires in both the cathode planes. 

When the electric field strength inside a gas counter exceeds a certain value, the electrons that move in such a field acquire, between collisions, sufficient energy to produce new ions. Thus, more electrons will be liberated, which in turn will produce more ions. The net effect of this process is multiplication of the primary ionization. The phenomenon is called gas multiplication. To achieve the high field intensity needed for gas multiplication without excessive applied voltage, chambers operating in this mode are usually cylindrical with a very thin wire stretched axially at the center of the counter (Fig. 5.14). The wall of the counter is normally grounded and a positive voltage is applied to the central wire. In... 

The production of electrons by these processes results in the generation of successive avalanches of ionization because all the electrons, no matter how they are produced, migrate in the direction of the intense electric field and initiate additional ionization. The gas multiplication factor M, which is equal to the total number of free electrons produced in the counter when one pair is produced by the incident radiation, is calculated as follows. Let... 

A proportional counter is strictly proportional as long as the space charge due to the positive ions does not modify too much the electric field around the wire. The magnitude of the space charge is a function of the primary ionization and the gas multiplication. If the primary ionization is very small, the value of M may be 10 to 10 before the space charge affects the proportionality. On the other hand, if the primary ionization is too strong, the critical value of M is smaller. It has been reported that there is a critical maximum value of the charge produced by the multiplication process beyond which proportiontility does not hold. That number, obviously, depends on the counter (size, types of gas, etc.). 

A GM counter is a gas counter that operates in region IV of Fig. 5.3. Its construction and operation are in many ways similar to those of a proportional counter. The GM counter is usually cylindrical in shape, like most of the proportional counters. The electric field close to the central wire is so strong that NS 1 (see Sec. 5.5.1) and the gas multiplication factor M is extremely high. In a GM counter, a single primary electron-ion pair triggers a great number of successive avalanches. Therefore, the output signal is independent of the primary ionization. 

Geometry. Parallel-plate counters are almost exclusively ionization chambers. The intense fields needed for gas multiplication can be produced only in cylindrical or spherical geometry. 

Fission chambers are gas counters that detect the fragments produced by fission. The fission fragments, being massive charged particles with Z +20e and kinetic energy 60-100 MeV, have a short range even in a gas. They produce such an intense ionization that gas multiplication is not necessary. Thus, fission chambers operate in the ionization region. 

Electric field distortion. The gas multiplication in a proportional counter depends on the intensity of the electric field. Close to the ends of a cylindrical counter, the strength of the electric field becomes gradually less intense than in most of the counter volume. This effect produces lower pulses from proton recoils at the ends of the counter. Detectors with large length-to-diameter ratio are less affected by this problem. Theoreticcil corrections of this effect have been developed and successfully applied. ... 

The electric field strength at a distance x from the anode is proportional to Hx. If the applied voltage of a cylindrical chamber of 1 cm radius is 1000 V, the pot tial in die immediate vicinity of a center wire of 0.0025 cm diameter is approximately 7 X 10 V cm. As the primary electrons reach the vicinity of this high field and increase their kinetic energy, they cause secondary ionization which increases the pulse detected at the wire anode. The collected charge is given by (8.2), where a, the gas multiplication factor, is > 1. 

Values of a of 10 - 10 are commonly achieved in proportional counter operation. If a ==10, essentially all the gas multiplication occurs within 10 mean free path Iragths from the wire for the electron in the gas (2 = 1024). At 1 atm the mean free path lengdi is approximately 10 m, which means that the gas multiplication occurs within 0.01 mm of the wire. 

The gas multiplication factor varies with the applied voltage but for a given voltage a is constant so the detector pulse output is directly proportional to the primary ionization. As a result it is possible to use a proportional counter to distinguish between a- and 3-particles and between id tical particles of different energies inasmuch as different amounts of primary ionization are produced in these cases. 

Counting gases consists usually of one of the noble gases mixed with a small amount of polyatomic gas. The latter makes the gas multiplication factor less dependent on applied voltage, and increases the speed of electron collection. Typical counting gas mixtures are... 

Another technique for neutron detection uses a fission chamber. One design contains a stack of alternate anodes and cathodes, one of the electrodes being covered by a thin layer of uranium enriched in The fission fragments produce large ionization even though the gas multiplication is quite low. This detector is more sensitive to fast neutrons than the BF3 counter, and can be used for fast neutron fluxes up to 10 n s with a backgroimd of a few cps. 

Increasing the ratio of the gas- and liquid-phase introduction volume rates, wj W[ (the gas multiplicity of the disperse system), in the tubular turbulent reactor of diffusion-confusion construction to the value of 0.1, we observe an increase in the... 

Piper Alpha, North Sea 1988 167 deaths Fire/explosion of natural gas Multiple and root cause was management related. 

ASHRAE Handbook, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA. Multiple volumes updated regularly. 

When a proportional counter is operated at low temperature, the gas multiplication decreases, and some quenchers mixed in the He gas get condensed on the cold surface of the sample. A gas counter filled with He -I- 5% CO at atmospheric pressure showed good performance at 77 K (Isozumi et al. 1983 Cook and Agyekum 1985 Nomura and Ujihira 1985 Meisel 1994). Purified He, H2, Ne, and mixtures of two gases such as He -I- 5% N2 or He + several % CO may be used from temperatures as low as 15-300 K (Fukumura et al. 1991). 

This is the region where "proportional counters" operate, and current is proportional to radiation intensity. Total current flow per ionizing event is much larger than in the ion chamber region ("Gas multiplication or amplification"). Detectors operated in this region usually are "pulse counters" rather than... 

In this region the current flow is the same no matter how much initial ionization is caused by the radiation. There is tremendous gain from gas multiplication so measuring (counting) circuits can be quite insensitive. Pulses of volt sizes are achieved. 

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