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For the experimental determination of an average energy to produce an ion pair in gases, W, by heavy ions from accelerators, an apparatus consisting of a particle-energy degrader, an ionization chamber, and a time-of-flight (TOF) energy spectrometer was designed and constructed, where two types of pulse operation modes in an ionization chamber were employed. Using this system, the values of W were measured for He and C ions in pure argon and air as a function of the energy of ions. For the C ions, the energy dependence of W was clearly observed, while W for He ions was approximately constant over a wide rage of energy (the mean value was 26.5 [+ or -]0.9 eV in argon and 34.3 [+ or -] 1.3 eV in air, respectively) and agreed well with the value for 5.49 MeV alpha particles from 241 Am in each gas. The value of W for C ions obtained at the highest energy was also in good agreement with the value for 5.49 MeV alpha particles in each gas. Index Terms--Energy dependence, heavy ions, ionization yield, W-value.

The absolute numbers of scintillation photons and electrons produced by 5.49 MeV alpha particles were measured simultaneously in argon, krypton and xenon in the gas pressure range from 1.01 x [10.sup.5] Pa to 1.01 x [10.sup.6] Pa. The ratio of the number of excited atoms to the number of electron-ion pairs is an important quantity for understanding the energy pathway of the absorbed radiation energy and was found to be 0.52, 0.55, and 0.60 in argon, krypton and xenon, respectively. The ratios were determined by measuring the number of scintillation photons originating from the excited atoms and the number of electrons. The value of [W.sub.s], which is defined as the average energy to produce one photon, in the case that all of the electron-ion pairs recombine was estimated to be 17.5, 15.4, and 13.0 eV in argon, krypton and xenon, respectively. From the relation between the numbers of electrons escaping from the recombination with ions and the numbers of scintillation photons, it is confirmed experimentally that one scintillation photon is emitted from one recombination process. This means that an excited molecule caused by three-body collisions is not de-excited without emitting a scintillation photon in the vacuum ultraviolet region. Index Terms--Absolute number of scintillation photon, ionization, radiation detector, rare gas, scintillation yields.

To determine absolute scintillation yields due to alpha particles in high-pressure rare gases, the number of scintillation photons [N.sub.p] was measured using a vacuum ultraviolet (VUV) sensitive photodiode with spectral quantum efficiency [q.sub.e]([lambda]) measured as a function of wavelength [lambda]. The absolute number of photoelectrons from the photocathode [N.sub.pe] was measured using a charge-sensitive preamplifier calibrated with respect to charge number. The collection efficiency for scintillation photons [F.sub.ce] at the photocathode was determined from the solid angle subtended by the photocathode at a scintillation point under the condition that there are no photons reflected off the surrounding walls. [N.sub.p] was determined from [N.sub.p] = [N.sub.pe]/([Q.sub.e][F.sub.ce]), where [Q.sub.e] is the average quantum efficiency calculated from [q.sub.e]([lambda]) and a relative intensity I([lamba]) of scintillation in rare gases. We measured the luminescence spectra using a VUV monochromator of known efficiency in order to obtain I([lambda]). Measurements were performed in gaseous argon, krypton, and xenon in the pressure range from 1.01 x [10.sup.5] Pa to 1.01 x [10.sup.6] Pa. [N.sub.p] in xenon at 1.01 x [10.sup.5] Pa is 1.60 x [10.sup.5] for 5.49-MeV alpha particles. Index Terms--Rare gas, scintillation yield.