Sentaro Takahashi

Evidence of deep-blue photon emission at high efficiency by common plastic

Various scintillation devices are used in many countries and wide scientific fields. Key elements that determine the performance of a scintillation device are the number of photons emitted per incident radiation event and the emission of easy-to-measure blue photons. It is generally known that only materials with very complex compositions perform well as scintillators. However, we demonstrated that the scintillation performance of a newly developed plastic such as 100 percent pure polyethylene naphthalate exceeds that of conventional organic scintillators. By measuring the light output spectra and emission spectra of several samples, we revealed that the plastic emits a high number of photons per incident radiation event (∼ 10500 photons/MeV), and, surprisingly, deep-blue photons (425nm). Even though the plastic has a simple composition, it could replace the expensive organic scintillators that have been used in many applications.

Development of polystyrene-based scintillation materials and its mechanisms

Scintillation materials based on polystyrene (PS) have been investigated. Para-terphenyl was employed as a fluorescent molecule (fluor) that functions as a wavelength shifter. A clear increase in photon yield of the scintillation materials relative to the pure PS was observed, which cannot be explained by the conventional theory of scintillation mechanism. Furthermore, the photon yield increased with flour concentration in accordance with a power-law. Here we reveal the emergence of a luminescence of PS-based scintillation materials and demonstrate that their photon yields can be controlled by the fluor concentration.

Mechanism of wavelength conversion in polystyrene doped with benzoxanthene: emergence of a complex

Fluorescent guest molecules doped in polymers have been used to convert ultraviolet light into visible light for applications ranging from optical fibres to filters for the cultivation of plants. The wavelength conversion process involves the absorption of light at short wavelengths followed by fluorescence emission at a longer wavelength. However, a precise understanding of the light conversion remains unclear. Here we show light responses for a purified polystyrene base substrates doped with fluorescent benzoxanthene in concentrations varied over four orders of magnitude. The shape of the excitation spectrum for fluorescence emission changes significantly with the concentration of the benzoxanthene, indicating formation of a base substrate/fluorescent molecule complex. Furthermore, the wavelength conversion light yield increases in three stages depending on the nature of the complex. These findings identify a mechanism that will have many applications in wavelength conversion materials.

Light propagation characteristics of high-purity polystyrene

Organic scintillation materials involve short wavelength light emitted from polymers containing aromatic ring moieties. We have characterized high-purity (>99.9%) polystyrene (PS) as a potential scintillator. It emits ultraviolet light with a 310-nm emission maximum. We demonstrate that the effective refractive index (1.67) for PS is a function of the emission spectrum. Light yield distributions generated by 137Cs and 207Bi radioactive sources were also characterized. The light attenuation length is 41.6u2009±u20090.5u2009mm, which is ten times than expected. These results demonstrate that high-purity PS has important light propagation characteristics needed for organic scintillation materials.

Optimized mounting of a polyethylene naphthalate scintillation material in a radiation detector.

Polyethylene naphthalate (PEN) has great potential as a scintillation material for radiation detection. Here the optimum mounting conditions to maximize the light collection efficiency from PEN in a radiation detector are discussed. To this end, we have determined light yields emitted from irradiated PEN for various optical couplings between the substrate and the photodetector, and for various substrate surface treatments. The results demonstrate that light extraction from PEN is more sensitive to the optical couplings due to its high refractive index. We also assessed the extent of radioactive impurities in PEN as background sources and found that the impurities are equivalent to the environmental background level.

DNA double-strand break induction in Ku80-deficient CHO cells following Boron Neutron Capture Reaction

BackgroundBoron neutron capture reaction (BNCR) is based on irradiation of tumors after accumulation of boron compound. 10B captures neutrons and produces an alpha (4He) particle and a recoiled lithium nucleus (7Li). These particles have the characteristics of high linear energy transfer (LET) radiation and have marked biological effects. The purpose of this study is to verify that BNCR will increase cell killing and slow disappearance of repair protein-related foci to a greater extent in DNA repair-deficient cells than in wild-type cells.MethodsChinese hamster ovary (CHO-K1) cells and a DNA double-strand break (DSB) repair deficient mutant derivative, xrs-5 (Ku80 deficient CHO mutant cells), were irradiated by thermal neutrons. The quantity of DNA-DSBs following BNCR was evaluated by measuring the phosphorylation of histone protein H2AX (gamma-H2AX) and 53BP1 foci using immunofluorescence intensity.ResultsTwo hours after neutron irradiation, the number of gamma-H2AX and 53BP1 foci in the CHO-K1 cells was decreased to 36.5-42.8% of the levels seen 30 min after irradiation. In contrast, two hours after irradiation, foci levels in the xrs-5 cells were 58.4-69.5% of those observed 30 min after irradiation. The number of gamma-H2AX foci in xrs-5 cells at 60-120 min after BNCT correlated with the cell killing effect of BNCR. However, in CHO-K1 cells, the RBE (relative biological effectiveness) estimated by the number of foci following BNCR was increased depending on the repair time and was not always correlated with the RBE of cytotoxicity.ConclusionMutant xrs-5 cells show extreme sensitivity to ionizing radiation, because xrs-5 cells lack functional Ku-protein. Our results suggest that the DNA-DSBs induced by BNCR were not well repaired in the Ku80 deficient cells. The RBE following BNCR of radio-sensitive mutant cells was not increased but was lower than that of radio-resistant cells. These results suggest that gamma-ray resistant cells have an advantage over gamma-ray sensitive cells in BNCR.

Poly (ether sulfone) as a scintillation material for radiation detection

Considerable attention has been drawn to the advantages of using aromatic ring polymers for scintillation materials in radiation detection. Thus, it is important to identify and characterise those with the best potential. Here, we characterise poly (ether sulfone) (PES), which is an amber-coloured transparent resin that possesses sulfur as a main component and has a density of 1.37 g/cm(3). PES emits short-wavelength light with a 350-nm maximum. By taking into account its emission spectrum, we demonstrate that its effective refractive index is 1.74. Light yield distributions generated by (137)Cs and (207)Bi radioactive sources were obtained. PES has a light yield that is 2.21 times that of poly (ethylene terephthalate), and 0.31 times that of poly (ethylene naphthalate). The energy response to 5,486 keV alpha particles emitted from (241)Am was 546 ± 81 keV electron equivalents (keVee), while the energy resolution was 17.0 ± 0.1%. The energy response to 6,118 keV alpha particles emitted from (252)Cf was 598 ± 83 keVee, while the energy resolution was 16.0 ± 0.1%. Overall, PES has potential for use as a scintillation material in radiation detection.

Characterizing radiation spectra with stacked plastic sheets

Radiation spectra are demonstrated for educational purposes using inexpensive and readily available materials.

Relative biological effects of neutron mixed-beam irradiation for boron neutron capture therapy on cell survival and DNA double-strand breaks in cultured mammalian cells

Understanding the biological effects of neutron mixed-beam irradiation used for boron neutron capture therapy (BNCT) is important in order to improve the efficacy of the therapy and to reduce side effects. In the present study, cell viability and DNA double-strand breaks (DNA-DSBs) were examined in Chinese hamster ovary cells (CHO-K1) and their radiosensitive mutant cells (xrs5, Ku80-deficient), following neutron mixed-beam irradiation for BNCT. Cell viability was significantly impaired in the neutron irradiation groups compared to the reference gamma-ray irradiation group. The relative biological effectiveness for 10% cell survival was 3.3 and 1.2 for CHO-K1 and xrs5 cells, respectively. There were a similar number of 53BP1 foci, indicators of DNA-DSBs, in the neutron mixed-beam and the gamma-ray groups. In addition, the size of the foci did not differ between groups. However, neutron mixed-beam irradiation resulted in foci with different spatial distributions. The foci were more proximal to each other in the neutron mixed-beam groups than the gamma-ray irradiation groups. These findings suggest that neutron beams may induce another type of DNA damage, such as clustered DNA-DSBs, as has been indicated for other high-LET irradiation.

Optimised mounting conditions for poly (ether sulfone) in radiation detection

Poly (ether sulfone) (PES) is a candidate for use as a scintillation material in radiation detection. Its characteristics, such as its emission spectrum and its effective refractive index (based on the emission spectrum), directly affect the propagation of light generated to external photodetectors. It is also important to examine the presence of background radiation sources in manufactured PES. Here, we optimise the optical coupling and surface treatment of the PES, and characterise its background. Optical grease was used to enhance the optical coupling between the PES and the photodetector; absorption by the grease of short-wavelength light emitted from PES was negligible. Diffuse reflection induced by surface roughening increased the light yield for PES, despite the high effective refractive index. Background radiation derived from the PES sample and its impurities was negligible above the ambient, natural level. Overall, these results serve to optimise the mounting conditions for PES in radiation detection.