The effects of impurities on the ionization yield in helium

The experimental determination of the ionization yield in helium is affected by impurities due to the Penning effect. The excited helium atom He^* ionizes the impurity atom or molecule in the collision, which increases the ionization yield as additional ion pairs are generated. To enable measurements not affected by the impurities, it is important to maintain a high gas purity during measurement as the impurities are continuously released from the surface of an ionization chamber. In this study, we investigated two purification methods namely, the gas flow and purifier, to measure the ionization yield in helium. High purity gas was supplied to a chamber continuously in the gas flow method, which can inhibit the impurities from increasing. The purifier method employed a purifier containing Zr-Ba-Fe getters. The chamber was connected to a purifier to achieve continuous gas purification. The chamber was pumped out to a pressure less than 4.6 x 10^-5 Pa before filling with gas. The gases used in this study have a purity of 99.9999%. Measurements were performed under a pressure of 40 kPa in helium. The ionization yield in helium was measured using a gridded ionization chamber, which has a cathode, grid made of wire, and collector. A negative high voltage was applied to bias the grid and cathode. The ²⁴¹Am α-particles were irradiated in helium, and the reading from the collector was used to observe the ionized electrons. The signal pulses from the collector were fed to a charge-sensitive preamplifier (CP580H, Clear Pulse) and amplifier (CP403, Clear Pulse). A multi-channel analyzer system was used to analyze the output signals from the amplifier. The number of electrons was determined from the pulse heights using a charge terminator and pulse generator. The variation in the ionization yield in helium was measured over time. The ionization yield in helium increased over time, and finally settled at a constant value. The increase in the ionization yield was lower when using a purifier, as the purification process reduces the impurities. The change in the ionization yield over time in the case of gas flow also showed a similar trend to when using a purifier. Thus, this indicates that the gas flow method can also prevent the impurities from increasing in the chamber. (author)