Improvements of uniformity of sensitivity and stability of operation on self-triggering spark chambers

To make the sensitivity of a self-triggering spark chamber more uniform and its operation were stable, a hybrid spark chamber was developed which lowers the sparking voltage by increasing the number of initial electrons which trigger the spark. The chamber has four plane parallel electrodes in flowing gas, a mixture of argon and ethyl alcohol. The first and second electrodes constitute a spark chamber (SC), the second and third a proportional chamber (PC) and the third and fourth an ionization chamber (IC). The second and third electrodes are gauzes for electrons to pass through. Anger electrons or x-rays from Fe deposited on the fourth electrode ionize the gas locally in the IC. The electrons produced drift toward the third electrode and some pass through it. In the PC these electrons create an avalanche of electrons some of which, greatly multiplied, pass through the second electrode and trigger a spark in the SC. When the voltage applied to the PC was increased. Plateau characteristics were improved and greater stability was obtained. It was assumed that non-uniformity was caused by the slight difference of gap distance in the sensitive area, and that it could be improved by increasing over-voltage (difference between operational voltage and sparking voltage). It was also assumed that instability resulted from the refiring of sparks due to imperfect quenching, and that it could be improved by decreasing the total number of charges in the spark. The sensitivity was improved without refiring, even at relatively high over-voltage. Good images were obtained for β emitters, and the usefulness was proved for radiochromatography. (Evans, G.)