Selecting Spark Based on the Difference of Specific lonization of High Energy Charged Particles Incident on a Self-triggering Spark Chamber
1975 Volume 24 Issue 5 Pages 305-311
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1975 Volume 24 Issue 5 Pages 305-311
In this paper the dependency of sparking voltage of a self-triggering spark chamber on the specific ionization of incident particles and its application are described.
The chamber was composed of an electroconductive glass anode and a gauze cathode with gap length d of 0.5 cm. The filling gas, a mixture of argon and a few per cent of ethylalcohol vapor, was used with pressure p of 300-760 Torr at room temperature. DC voltage was supplied on the anode through a quenching resistor. Spark pulses were taken out of the cathode and counted.
Using a RaDEF source, α- and β-rays, their specific ionizations are largely different, were shot into the gap nearly perpendicularly to the electrodes. Two step plateaus appeared on the curve of counting rate vs. applied voltage. It was found that the sparking voltage in the case of α-ray incidence was decreased more than 10% compared with that in the case of β-ray incidence. Setting the DC voltage to make sparks occur only in the case of α-ray incidence, it was possible to obtain an α-ray source image from an α- and β-ray mixing source.
In order to discuss theoretically about these results, an equation of sparking voltage Vs as a function of specific ionization N0was delived using the streamer theory of spark. Considering the reduction of Townsend's first ionization coefficient due to the space charge effect of an electron avalanche, the following was obtained.
Vs≅√C+1/2lnVs-lnNO√pd/A/√1-2pd/A⋅Vs/(Vs-Bpd)3+Bpd
where A, B and C are constants which depend on the sort of the gas. From this equation the sparking voltage Vsα for α-ray incidence and that Vsβfor β-ray incidence were calculated. Theoretical values of the decreasing rate of sparking voltage, (Vsβ-Vsα) /Vsβand experimental values agreed well. Then it was shown that the decrease of sparking voltage was well explained with the difference of specific ionization of incident particles.
