Design of a hybrid gas proportional counter with CdTe guard counters for <f>14</f>C dating system

Nowadays uniform, low-cost and large-size compound semiconductor detectors are available up to several square centimeters. We are trying to combine this technology with conventional gas detectors to upgrade an anticoincidence type proportional counter, Oeschger-type thin wall counter of <f>2.2 l</f>, used for a <f>14C</f> dating facility at the University of Tokyo. In order to increase the ratio of the signal to the background for smaller quantity of samples less than <f>1 g</f>, an effective approach is to minimize the detector volume at higher gas pressure. However, the anticoincidence function suffers from such a small volume. Therefore we designed a new active wall gas counter of <f>0.13 l</f> counting volume using CdTe compound semiconductor detectors as the wall of the gas proportional counter to perform anticoincidence. Simulation study showed that at noise thresholds less than <f>70 keV,</f> the wall counters can reject above 99.8% of events arising from outer gamma rays. Measured noise levels of CdTe detectors were smaller than <f>24 keV</f> which is low enough for 99.8% anticoincidence efficiency. The experiment showed an anticoincidence efficiency for outer gamma rays from 70% to 80%, similar to that of the old <f>14C</f> counter. The lost anticoincidence efficiency results from the area of 21.74% which was not covered with CdTe due to two holes for the path of the center anode wire and slots between every two sides of CdTe detectors. [Copyright &y& Elsevier]