Nakahiro Yasuda

Relative optically stimulated luminescence and thermoluminescence efficiencies of Al2O3: C dosimeters to heavy charged particles with energies relevant to space and radiotherapy dosimetry

This article presents a comprehensive characterization of the thermoluminescence (TL) and optically stimulated luminescence (OSL) relative luminescence efficiencies of carbon-doped aluminum (Al2O3:C) for heavy charged particles (HCPs) with atomic numbers ranging from 1 (proton) to 54 (xenon) and energies ranging from 7 to 1000 MeV/u, and investigates the dependence of the Al2O3:C response on experimental conditions. Relative luminescence efficiency values are presented for 19 primary charge/energy combinations, plus 31 additional charge/energy combinations obtained by introducing absorbers in the primary beam. Our results show that for energies of hundreds of MeV/u the data can be described by a single curve of relative luminescence efficiency versus linear energy transfer (LET). This information is needed to compensate for the reduced OSL efficiency to high-LET particles in such applications as space dosimetry. For lower energies, the relative luminescence efficiency as function of LET cannot be describe...

Track core size estimation in CR-39 track detector using atomic force microscope and UV-visible spectrophotometer

Abstract The radial size of track cores in CR-39 plastics for several types of ions has been determined by two different methods. First, atomic force microscope observations were performed on the irradiated CR-39 subsequent to the slight chemical etchings. The track core radii for C, O, Ne and Xe ions evaluated from the intersections of the extrapolated lines, fitted to each growth curve of etch pit radius, were found to be in the range between 2.8 and 4.1 nm, independent of the ion species. Second, UV–visible spectra of the irradiated CR-39 were obtained at various fluences. Based on a track overlapping model, the track core radii are evaluated for H, He, C and O ions. Their core radii were almost proportional to the cube root of the stopping power. The results from the two different measurements are in good agreement with each other.

Development of a new multi-grid-type microstrip gas chamber

Abstract A new multi-grid-type MSGC (M-MSGC) has been designed and fabricated. This new MSGC has very narrow gaps between neighboring electrodes which can considerably reduce a surface charge effect. Several types of test detectors have been fabricated and the maximum gas gain was found to be beyond 10 4 for a 10 μm gap M-MSGC. The observed amplitude of the cathode signal was almost the same as the anode signal. This MSGC can be a promizing detector for the field, where both the high gain and the stable operation are required.

Structural Modification along Heavy Ion Tracks in Poly(allyl diglycol carbonate) Films

To identify the chemical modifications along nuclear tracks in poly(allyl diglycol carbonate) (PADC), we have made a series of Fourier transform infrared (FT-IR) measurements for films with a thickness of about 3 µm that have been exposed to C, Ne, Ar, and Fe ions in air. The amount of carbonated ester bonds lost due to the exposure was estimated from the changes in the absorbance of C=O and C–O–C bonds with the heavy ion fluence. The G-value for the breaking of carbonate ester bonds and the corresponding track core radii were obtained as a function of stopping power. The calculated radial dose distribution indicated that the core was formed at regions where the local dose was higher than 106 Gy.

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.

Irradiation system of ions (H–Xe) for biological studies near the Bragg peak

We have developed a new system for irradiating biological samples in air with ions from H to Xe below 6.0MeV∕nucleon near the Bragg peak. The irradiation system can provide ion beams with 20‐mm diameter of which the central area of 100mm2 is uniform in fluence rate within a standard deviation of ±10%. For each ion, the linear energy transfer is selectable by irradiation positions in air, from the lowest at the surface of a vacuum window to the highest at the Bragg peak, for example, from 281 to 977 keV/μm for C ions. A wide range of fluence rates, 10−3–104ions∕μm2∕s, can be provided by the system, which makes it possible to irradiate a variety of biological samples with different target sizes, from small plasmid DNA to living mammalian cells. The ion fluence irradiated to each sample is calculated from the output of the secondary electron monitor using the linear relationship between the output and ion fluence measured at the sample position by CR-39 track detectors. Survival curves and visualization of N...

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.

Heavy Ion and Proton Irradiation of Gas Electron Multipliers With Liquid Crystal Polymer Insulator: Evaluation Tests for Use in Space

Gas electron multipliers with a liquid crystal polymer insulator (LCP-GEMs) can be employed as X-ray polarimeters in several satellite missions in low-earth orbit. To evaluate their resistance to radiation, LCP-GEMs were exposed to Fe ion and proton beams from the heavy-ion accelerator facility HIMAC, and the gas gain was monitored. The LCP-GEMs survived 14.4-year-equivalent irradiation by fully stripped Fe ions with an energy of 500 MeV nuc-1, which simulated galactic cosmic-ray ions. We also studied the degradation of the insulator layer of the LCP-GEMs by irradiation with 160 MeV protons to simulate geomagnetically trapped protons. After irradiation with protons for 31.6 years equivalent, no significant change in gain was observed.

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.

Single particle irradiation system to cell (SPICE) at NIRS

Selective irradiation by an ionizing particle of a targeted cell organelle may disclose such mechanisms as signal transaction among cell organelles and cell-to-cell communication in the processes toward an endpoint observed. Bystander effect, existence of which has been clearly evidenced by application of the particle microbeam to biological experiments, suggests potential deviation from the conventional risk estimation at low particle fluence rates, such as in an environment of space radiation in International Space Station. To promote these studies we started the construction of a microbeam facility (named as SPICE) by using our HVEE Tandem accelerator (3.4 MeV proton and 5.1 MeV 4 He 2þ ). For our primary goal, ‘‘irradiation of cell organelle with a single particle with a position resolution of 2 l mi n a reasonable irradiation time’’, special features are considered. Usage of a triplet Q-magnet for focussing the beam to micrometer levels is an outstanding feature compared to facilities of other institutes. Other features are almost similar to those of other institutes. Those are precise position control of a cell dish holder, design of the cell dish, data acquisition of microscopic image of a cell organelle (cell nucleus), data processing, reliable particle detection, soft and hard wares to integrate all these related data and system to control and irradiate a targeted spot with exactly determined number of particles.