Hiroko Tawara

Absolute number of scintillation photons emitted by alpha-particles in rare gases

In order to determine absolute scintillation yields due to alpha particles in high pressure rare gases, the number of scintillation photons N/sub p/ was measured by using a VUV sensitive photodiode (PD) with a MgF/sub 2/ window and a Cs-Te photocathode with spectral quantum efficiency q/sub e/(/spl lambda/) measured as a function of wavelength /spl lambda/. The number of photoelectrons from the photocathode N/sub pe/ was measured absolutely by using charge-sensitive preamplifier calibrated in numbers of electrons. A collection efficiency F/sub ce/ at the photocathode for scintillation photons can be determined from solid angles subtended by the photocathode at a scintillation point under the condition that there is no photon reflected at surrounding wall. Then, N/sub p/ was determined from N/sub p/=N/sub pe//(Q/sub e/F/sub ce/), where Q/sub e/ is effective quantum efficiency calculated from q/sub e/(/spl lambda/) and a relative intensity I(/spl lambda/) of scintillation in rare gases at wavelength /spl lambda/. Although luminescence spectrums from rare gases emitted by radiation have been measured by many researchers, these spectrums were scarcely corrected by an efficiency of apparatus (e.g. efficiency of monochromator and scintillation detector) for /spl lambda/. In order to exactly determine the luminescence spectrums, these were also measured on our own terms. And, since it was reported that scintillation intensity from rare gases change with a pressure of rare gases, this experiments was carried out in a pressure range from 1.0/spl times/10/sup 5/ Pa to 1.0/spl times/10/sup 6/ Pa. The measurements were carried out in gaseous argon, krypton and xenon. In xenon of 1.0/spl times/10/sup 5/ Pa, N/sub p/ was measured to be 1.6/spl times/10/sup 5/.

Simultaneous measurements of absolute numbers of electrons and scintillation photons produced by 5.49 MeV alpha particles in rare gases

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/spl times/10/sup 5/ Pa to 1.01/spl times/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.

LET distributions from CR-39 plates on Space Shuttle missions STS-84 and STS-91 and a comparison of the results of the CR-39 plates with those of RRMD-II and RRMD-III telescopes.

The LET distributions during the Space Shuttle missions STS-84 (altitude 270-412 km, average 375 km; inclination angle, 51.6 degrees) and STS-91 (altitude 328-397 km, average 373 km; inclination angle, 51.6 degrees) were measured using CR-39 plastic nuclear track detectors. A correction for the dip-angle dependence of the track-formation sensitivity of the CR-39 plates was applied to the data analysis. The absorbed doses and the dose equivalents around RRMD Detector Units, estimated from the LET distributions in the LET region of 4-200 keV/micrometers, fluctuated with standard deviations of +/- 21% to +/- 35% in both flight experiments. The LET distributions obtained from the CR-39 plates agreed well with that obtained from RRMD-II in STS-91. However, the particle fluxes obtained from RRMD-III in STS-84 and STS-91 were two or three times higher than those obtained from RRMD-II and the CR-39 plates. It was concluded that the LET distributions obtained from RRMD-II and the CR-39 plates in the present flight experiments did not include the contribution of target-fragmented secondary heavy particles produced by low-LET particles, such as relativistic or semi-relativistic protons and helium ions, whereas RRMD-III was able to detect these secondary particles because of its low triggering level.

Improvement of mass resolution for iron isotopes in CR-39 track detector

The capability of the CR-39 track detector for isotope identification was verified using 56Fe and 55Fe beams from the heavy ion accelerator HIMAC at NIRS. A CR-39 stack was exposed to 56Fe and 55Fe beams with the energies of 460 MeV/nucleon. Drastic improvements in the accuracies of microscopic image analysis and detector thickness measurement enabled us to identify those iron isotopes with high mass resolution using the CR-39 track detector. As a consequence of the reduction of systematic errors, the mass resolution for iron isotopes in the CR-39 detector was obtained to be 0.22±0.03 amu in rms. The mass resolution newly obtained by the CR-39 detector was compared with the former result and found to be 66% better than the former value of mass resolution as a result of reducing the systematic errors of mass dispersion. Moreover, the ultimate mass resolution of the CR-39 detector was estimated by calculation using those experimental results. As a consequence of reducing the random error, it is expected that the mass resolution for iron isotopes can ultimately approach to ~0.10 amu in the CR-39 detector.

Numbers of scintillation photons produced in NaI(Tl) and plastic scintillator by gamma-rays

The W/sub S/-value, which is defined as an average energy expended per scintillation photon, is determined to be 17.2+or-0.40 eV for a NaI(Tl) phosphor and 60.8+or-4.3 eV for a plastic scintillator (NE-102A). These are obtained from the numbers of photoelectrons measured with several combinations of a photomultiplier tube and a NaI(Tl) or a NE-102A scintillator. The number of photoelectrons, which are measured by the photomultiplier tube as a vacuum photodiode, are converted to the number of scintillation photons by using an averaged quantum efficiency of each photomultiplier photocathode and a calculated collection efficiency of the scintillation photons at the photocathode. The above values do not include the uncertainties due to the unknown exact emission spectra and the photomultiplier response curves. >

CR-39 plastic for massive magnetic monopole search

Abstract A new CR-39 plastic containing a small amount of antioxidants (Naugard-445) has been developed for use in a search for massive magnetic monopoles. Its characteristics have been investigated from the standpoint of the “heavy etching method”, which was proposed several years ago. The results are compared with those of pure CR-39 plastics measured under the same conditions.

Average Energies Required per Scintillation Photon and Energy Resolutions in NaI(Tl) and CsI(Tl) Crystals for Gamma Rays

The average energies required to produce one scintillation photon Ws were determined for 662 keV gamma rays in NaI(Tl) and CsI(Tl) crystals to be 15.0±1.3 and 13.3±1.1 eV, respectively, from the absolute numbers of photoelectrons measured for several combinations of a crystal and a photomultiplier tube (PMT) used as a vacuum photodiode. The numbers of scintillation photons were obtained by calculating the collection efficiency of scintillation photons at the photocathode using Monte Carlo simulations and by determining experimentally the photon-to-photoelectron conversion efficiency at PMT photocathode. The values of Ws determined in the present study are in good agreement with the theoretical values presented recently. The factors affecting energy resolutions were also examined. The calculated resolution agrees well with that obtained experimentally.

A program for the precise observations of ultra heavy nuclei in galactic cosmic rays

The origin of galactic cosmic rays (GCRs) nuclei is still unknown. Precise observation of ultra heavy GCRs would be an important step in resolving their origin including the remaining problems in cosmic ray astrophysics. A program to observe UH nuclei in GCRs is proposed which involves the use of a high performance solid-state track detector on board a long-duration balloon. The program focuses measuring the elemental and isotopic compositions of UH nuclei up to the actinides. The observation of nuclear composition covers a wide range of scientific themes including studies of nucleosynthesis in cosmic ray sources, chemical evolution of galactic material, the characteristic time of cosmic rays, and heating and acceleration mechanisms of cosmic ray particles.

Mass resolution for iron isotopes in CR-39 track detector

We have verified the capability of a CR-39 track detector for isotope identification using iron beam from the heavy-ion accelerator of HIMAC at NIRS. The same CR-39 stack was independently exposed to 56Fe and 55Fe ions with an energy of 460 MeV/nucleon. Mass resolutions for 56Fe and 55Fe ions were determined to be 0.28±0.12 amu and 0.28±0.11 amu in rms, respectively. A detailed analysis of the sources of the mass dispersion in the present mass identification experiment showed that significant errors were caused in the measurements of the surface position and the thickness of CR-39 sheets. By eliminating these errors as much as possible, the mass resolution is expected to be improved to ~0.20 amu.

Measurement of a Linear Energy Transfer Distribution with Antioxidant Doped CR-39 Correcting for the Dip Angle Dependence of Track Formation Sensitivity

Antioxidant doped CR-39 detectors were loaded onto the STS-95 space shuttle mission (altitude: 574 km; inclination: 28.45°; flight duration: 8.9 days) for measuring the linear energy transfer (LET) distribution above 10 keV/µm for space radiation dosimetry. It is known that the track formation sensitivity of antioxidant doped CR-39 detectors depends on the dip angle of the incident particle. We investigated this dip angle dependence for a wide range of LET values and dip angles. The track formation sensitivities at lower dip angles were obviously decreased in the LET region below 100 keV/µm. We introduced minimum-cutoff dip angles in order to correct for such dip-angle dependence. The LET distribution of the STS-95 mission was obtained from the measurements of etch pits having dip angles larger than the minimum-cutoff dip angles. This new correction method increased the absorbed dose and dose equivalent above 10 keV/µm by 54 and 28%, respectively.