Kazunobu Fujitaka

LET and Ion Species Dependence for Cell Killing in Normal Human Skin Fibroblasts

Abstract Tsuruoka, C., Suzuki, M., Kanai, T. and Fujitaka, K. LET and Ion Species Dependence for Cell Killing in Normal Human Skin Fibroblasts. Radiat. Res. 163, 494–500 (2005). We studied the LET and ion species dependence of the RBE for cell killing to clarify the differences in the biological effects caused by the differences in the track structure that result from the different energy depositions for different ions. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon, neon, silicon and iron ions that were generated by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS) in Japan. Cell killing was measured as reproductive cell death using a colony formation assay. The RBE-LET curves were different for carbon ions and for the other ions. The curve for carbon ions increased steeply up to around 98 keV/μm. The RBE of carbon ions at 98 keV/μm was 4.07. In contrast, the curves for neon, silicon and iron ions had maximum peaks around 180 keV/μm, and the RBEs at the peak position ranged from 3.03 to 3.39. When the RBEs were plotted as a function of Z*2/β2 (where Z* is the effective charge and β is the relative velocity of the ion) instead of LET, the discrepancies between the RBE-LET curves for the different ion beams were reduced, but branching of the RBE-Z*2/β2 curves still remained. When the inactivation cross section was plotted as a function of either LET or Z*2/β2, it increased with increasing LET. However, the inactivation cross section was always smaller than the geometrical cross section. These results suggest that the differences in the energy deposition track structures of the different ion sources have an effect on cell killing.

Analysis of the calibration results obtained with Liulin-4J spectrometer-dosimeter on protons and heavy ions.

We are developing a portable dosimeter (Liulin-4J) based on a silicon semiconductor detector for use in measuring the absorbed dose from primary or secondary cosmic rays to astronauts and airplane crews. The dosimeter can measure not only the flux and dose rate, but also the deposited energy spectrum for silicon in per unit time. In order to calibrate the dosimeter, we have carried out exposures at the NIRS cyclotron and HIMAC heavy ion synchrotron facilities. We obtained a detector response function for using in measuring energy deposition and LET.

Effective Dose Equivalent on the Ninth Shuttle–Mir Mission (STS-91)

Abstract Yasuda, H., Badhwar, G. D., Komiyama, T. and Fujitaka, K. Effective Dose Equivalent on the Ninth Shuttle–Mir Mission (STS-91). Organ and tissue doses and effective dose equivalent were measured using a life-size human phantom on the ninth Shuttle–Mir Mission (STS-91, June 1998), a 9.8-day spaceflight at low-Earth orbit (about 400 km in altitude and 51.65° in inclination). The doses were measured at 59 positions using a combination of thermoluminescent dosimeters of Mg2SiO4:Tb (TDMS) and plastic nuclear track detectors (PNTD). In correcting the change in efficiency of the TDMS, it was assumed that reduction of efficiency is attributed predominantly to HZE particles with energy greater than 100 MeV nucleon–1. A conservative calibration curve was chosen for determining LET from the PNTD track-formation sensitivities. The organ and tissue absorbed doses during the mission ranged from 1.7 to 2.7 mGy and varied by a factor of 1.6. The dose equivalent ranged from 3.4 to 5.2 mSv and varied by a factor of 1.5 on the basis of the dependence of Q on LET in the 1990 recommendations of the ICRP. The effective quality factor (Qe) varied from 1.7 to 2.4. The dose equivalents for several radiation-sensitive organs, such as the stomach, lung, gonad and breast, were not significantly different from the skin dose equivalent (Hskin). The effective dose equivalent was evaluated as 4.1 mSv, which was about 90% of the Hskin.

Non-linearity of the high temperature peak area ratio of 6LiF:Mg,Ti (TLD-600)

Abstract Thermoluminescent dosemeters of 6 LiF:Mg,Ti (TLD-600) were exposed to high-energy heavy ion beams (He, C, Ne, and Ar) and 137 Cs γ-rays and the glow curves were analyzed. The height of the main peak (peak 5) at about 200°C slightly increased for He from γ-rays and decreased for heavier ions with increasing Linear Energy Transfer (LET). Whereas the high temperature peak (peak 7) area around 260°C simply increased. Non-linearity was seen between the LET and the high temperature peak (peak 7) area ratio (HTR) which was calculated as the TL integrated over 225–275°C from the peak 5 normalized glow curve. Based on these results, average LET and quality factor were estimated for an assumed LET spectra of space radiation by using the HTR method. Considerable differences were found between these estimations and true values.

Characteristics of a phoswich detector to measure the neutron spectrum in a mixed field of neutrons and charged particles

Abstract A phoswich detector composed of NE115 and NE213 scintillators has been developed to distinguish gamma-ray, neutron, and proton events. The detector performance was investigated in a neutron–proton mixed field at National Institute of Radiological Sciences (NIRS), which showed satisfactory particle identification.

Reduction in Life Span of Normal Human Fibroblasts Exposed to Very Low-Dose-Rate Charged Particles

Abstract Suzuki, M., Tsuruoka, C., Uchihori, Y., Ebisawa, S., Yasuda, H. and Fujitaka, K. Reduction in Life Span of Normal Human Fibroblasts Exposed to Very Low-Dose-Rate Charged Particles. Radiat. Res. 164, 505–508 (2005). We studied the effect of chronic low-dose irradiation with heavy ions on the life span of normal human fibroblasts in vitro. Cells were cultured in a CO2 incubator that was placed in the irradiation room for biological studies of heavy ions in the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS) and were exposed to scattered radiations produced by heavy-ion beams for the life span of the cell population. The absorbed dose, which was measured using a thermoluminescence dosimeter (TLD) and a silicon semiconductor detector, was 1.4 mGy per day when the HIMAC was operated for biological experiments. The total number of population doublings of the exposed cells as reduced to 79–93% of that of nonexposed control cells. However, the life span of cells exposed to low-dose 137Cs γ rays (∼1 mGy/day) in the CO2 incubator in the γ-irradiation room in NIRS was prolonged to 104–106% of that of nonexposed control cells. Thus there is evidence that exposure to chronic low-dose heavy-ion radiation reduces the life span of cells.

Neutron spectrometry in a mixed field of neutrons and protons with a phoswich neutron detector Part I: response functions for photons and neutrons of the phoswich neutron detector

Abstract We have developed a phoswich neutron detector consisting of an NE213 liquid scintillator surrounded by an NE115 plastic scintillator to distinguish photon and neutron events in a charged-particle mixed field. To obtain the energy spectra by unfolding, the response functions to neutrons and photons were obtained by the experiment and calculation. The response functions to photons were measured with radionuclide sources, and were calculated with the EGS4-PRESTA code. The response functions to neutrons were measured with a white neutron source produced by the bombardment of 135 MeV protons onto a Be+C target using a TOF method, and were calculated with the SCINFUL code, which we revised in order to calculate neutron response functions up to 135 MeV . Based on these experimental and calculated results, response matrices for photons up to 20 MeV and neutrons up to 132 MeV could finally be obtained.

Neutron spectrometry in a mixed field of neutrons and protons with a phoswich neutron detector: Part II: application of the phoswich neutron detector to neutron spectrum measurements

Abstract Using the developed phoswich neutron detector, the neutron energy spectra in a neutron–proton mixed field and in a neutron and heavy-charged-particle mixed field were measured using an unfolding method coupled with the obtained neutron response functions. The photon and proton energy spectra were obtained separately in these mixed fields using the photon response functions and the proton light yield calibration given in the preceding work. The measured spectra were compared with the spectra obtained by the TOF (time-of-flight) method.

Solid-state integrating detectors as an indicator of biological doses from HZE particles.

For interpretation of results obtained in future biological experiments in the International Space Station (ISS), biologically equivalent doses have to be determined using small-scale detectors without disturbing the surrounding radiation field. The detectors should be lightweight, stable, safe, and simple in handling. Solid-state integrating detectors (SSID) can satisfy these requirements. This paper demonstrates that combination of SSID such as thermoluminescence dosimeters and radiophotoluminescence glasses can be practically used for the evaluation of biologically equivalent doses. Statistical errors (type-A uncertainty) of this method will be satisfactorily small relative to those generally observed in biological responses. Permissible levels of systematic errors (type-B uncertainty) depend on dosimetry purposes (most-probable or conventional) and variability of biological responses.

Probability of cell hits in selected organs and tissues by high-LET particles at the ISS orbit.

The fluence of high-LET particles (HLP) with LET infinity H2O greater than 15 keV micrometers-1 in selected organs and tissues were measured with plastic nuclear track detectors using a life-size human phantom on the 9th Shuttle-Mir Mission (STS-91). The planar-track fluence of HLP during the 9.8-day mission ranged from 1.9 x 10(3) n cm-2 (bladder) to 5.1 x 10(3) n cm-2 (brain) by a factor of 2.7. Based on these data, a probability of HLP hits to a matured cell of each organ or tissue was roughly estimated for a 90-day ISS mission. In the calculation, all cells were assumed to be spheres with a geometric cross-sectional area of 500 micrometers2 and the cell-hit frequency from isotropic space radiation can be described by the Poisson-distribution function. As results, the probability of one or more than 1 hit to a single cell by HLP for 90 days ranged from 17% to 38%; that of two or more than 2 hits was estimated to be 1.3-8.2%.