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The scintillation response to an α source (241Am) and the ionization yield in pure gaseous xenon and its mixtures with 0.2–2% of methane were measured at a pressure of 26 atm and different values of reduced electric field E/N. Both light and ionization yields in xenon were shown to decrease with increasing percentage of methane.

We summarize the present R&D status of high-pressure xenon time projection chamber (HPXe-TPC) which is a new gamma-ray camera for planetary science mission.

We have obtained the ratio of the transverse diffusion coefficient to the mobility of electrons multiplied by the elementary charge, in Xe and Xe+H2 under electric fields at a higher pressure of 1 MPa in comparison with those in preceding experiments. The results show that the density effect (nonlinear effect) of the ratio in both pure Xe and Xe+H2 is ≤15% below 1 MPa over the reduced electric field range from 0.08 to 0.6×10-17 V·cm2. We also found that the diffusion of an electron swarm is suppressed by adding trace amount of hydrogen to high-pressure xenon gas. We discuss the application of Xe+H2 gas to a new gamma-ray camera.

High-pressure xenon time projection chamber (HPXe-TPC) is a sophisticated detector, which can determine not only an energy of gamma ray but also its arrival direction by using Compton scattering. Diffusion of electrons and scintillation yield in Xe and Xe+H/sub 2/ mixture have been investigated to realize a HPXe-TPC. The transverse characteristic energy of electrons in Xe-H/sub 2/ mixture (H/sub 2/ 0.44%) was reduced to about 40% against that of pure Xe at 1.0 MPa. Scintillation yields by alpha-particles (/sup 241/Am) in Xe+H/sub 2/ mixture (H/sub 2/ /spl sim/ 0.23-5.7%) at 2.6 MPa were obtained as a function of E/N. The scintillation yields in Xe+H/sub 2/ mixture at 2.6 MPa and at E/N = 0.3/spl times/10/sup -17/ Vcm/sup 2/ were luminous enough to generate a trigger signal of a HPXe-TPC. It is found that doping hydrogen into xenon is effective to prevent the degradation of angular resolution of a HPXe-TPC.