ELIMINATION OF END EFFECTS IN COUNTERS

The mounting of the central wire in proportional counters and Geiger–Müller counters is usually carried out by means of tubes or rods with a diameter substantially larger than that of the wire itself. In this case, as is known, a nonuniformity of the electric field appears, extending over a considerable part of the counter1 (of the order of its radius). This distortion leads to a decrease in the effective volume of the counter and to a change in the value of the gas amplification coefficient along the wire. The paper reviewed2 describes an effective method that makes it possible, with comparative simplicity, to eliminate end effects in counters. A guard ring, inside which the wire is located, is placed in a tube having a potential corresponding to its radius (see figure). The operation of such a counter, filled with a mixture of argon ($p = 60\ \text{cm Hg}$) and methane ($p = 10\ \text{cm Hg}$), was investigated by means of a collimated source of $\gamma$ rays Ge 81 ($E = 9.23\ \text{keV}$), mounted in various

Arrangement of a counter with a correcting tube.

positions along the counter filament. At the same time, the authors calculated the distribution of the electric field of a counter with a correcting tube and found that its length should be approximately equal to the radius of the cathode cylinder. The experiments showed that, when the appropriate voltage is applied to the correcting tube, pulses of the same magnitude are obtained at various distances of the source from the end of the filament.

The almost complete elimination of end effects by means of a correcting tube makes it possible to construct counters of small length. In the case of a Geiger counter, the counter design is simplified, since the need for a guard ring is eliminated. The correcting potential was applied to a thin conducting layer deposited on the insulator supporting the counter filament. First the tube was connected to the filament and measurements were made with the $\gamma$-rays of Ra. Then the same measurements were made with a potential applied to the tube. In this case the counting rate increased by 30%, the threshold voltage decreased from 1300 volts to 1200 volts, and the slope of the plateau decreased to 0.044% per volt from 0.074% per volt.

B. R.

CITED LITERATURE

1. B. Rossi and G. Staub, Ionization Chambers and Counters, IL, 1951, p. 89.
2. A. L. Cockroft and S. C. Curran, Rev. Sci. Instr. 22, 37 (1951).