Response of multiwire proportional counter for energy loss measurement

A multiwire proportional counter has been developed in order to experimentally simulate the energy deposited by high-energy heavy ions, such as C, Ne, and Ar of several hundred MeV/nucleon, in length as small as 5 μm in soft tissue of a human body. Four sensors with wire spacings of 2, 3, 4, and 5 mm have been prepared for comparison. To estimate the performance of these sensors, the collimated alpha particle beam from 241Am is used as a substitute for heavy ion beams in this work. The relative gas multiplication factor and the response to the position of the incident particles are measured for these four sensors with a tissue equivalent gas, and for the 5 mm spacing sensor with various gases. The signal-to-noise ratio is also estimated. For the 5 mm spacing sensor, P-10 gas shows the best performance among the gases examined in this work. The uniformity of the response is not so good for a tissue equivalent gas that is often used to simulate measurements in a human body. The response of the sensor, which depends strongly on the gas used, can be understood qualitatively by the electronegativity of the gas. The energy loss in soft tissue is obtained by multiplying the value measured in P-10 gas by the ratio of the energy loss in a tissue equivalent gas to that in P-10 gas.