Takuji Kojima

Polymer-alanine dosimeter and compact reader

Abstract An alanine dosimeter moulded with polystyrene (PS) was developed for highly precise and practical free-radical dosimetry. Use of polymers produced flexibility of the dosimeter shape and capability of mass-production which are required for practical dosimetry. A compact and automatically controlled reader has been also developed to establish a convenient and reproducible free-radical dosimetry system. The alanine/ESR dosimetry system composed of the PS-alanine dosimeter and the reader provide a highly precise dose evaluation within ±1% at doses above 100 Gy.

Investigation of applicability of alanine and radiochromic detectors to dosimetry of proton clinical beams.

Cancer therapy studies using proton accelerators are underway in several major medical centers in the U.S., Russia, Japan and elsewhere. To facilitate dosimetry intercomparisons between these laboratories, alanine-based detectors produced at the National Institute of Standards and Technology and commercially available radiochromic films were studied for their possible use as passive transfer dosimeters for clinical proton beams. Evaluation of characteristics of these instruments, including the LET dependence of their response of proton energy, was carried out at the Institute of Theoretical and Experimental Physics. Results of absolute dose measurements were regarded as a preliminary step of dose intercomparison between ITEP and NIST. Measurements made in a number of experiments showed average agreement between the ITEP and NIST dosimetry standards to 2.5%.

Yield of OH radicals in water under high-density energy deposition by heavy-ion irradiation

Abstract Taguchi, M. and Kojima, T. Yield of OH Radicals in Water under High-Density Energy Deposition by Heavy-Ion Irradiation. Radiat. Res. 163, 455–461 (2005). The purpose of the present study was to evaluate the dependence of the OH radical yield on the atomic number and the energy of the heavy ions to understand chemical reactions of aqueous solutions. The total yields of oxidized products from phenol in water increased superlinearly as the incident energy increased from 5 MeV/nucleon to 18 MeV/nucleon for carbon and neon ions. The radiolytic yields of OH radicals produced by the ions were determined by analyzing the relationships of the oxidation yields of phenol to the incident energies up to 18 MeV/nucleon for ions in the range of LET from 110 eV/nm to 550 eV/nm and from 320 eV/nm to 1100 eV/nm for carbon and neon ions, respectively. The yields of the OH radicals increased with the specific energy for the same kind of ion and decreased with the atomic number for different ions used at the same specific energy.

Pretreatment of lignocellulosic wastes by combination of irradiation and mechanical crushing

Abstract Pretreatment of lignocellulosic wastes such as sawdust, chaff and paper was studied using a combination of irradiation and mechanical crushing. An acceleration effect of irradiation on crushing of these wastes was observed. The time taken to crush the irradiated samples to a size below 250 mesh decreased markedly with increasing enzymatic hydrolysis owing to the increased susceptibility of the irradiated samples to cellulase.

Effect of low irradiation temperature on the gamma-ray response of dyed and undyed PMMA dosimeters

Abstract Temperature effects on dose responses of undyed and dyed polymethylmethacrylate (PMMA)_dosimeters were studied at doses of 2 and 25 kGy under irradiation temperatures from −196°C to +45°C. At 25 kGy, Radix RN15 has linear dependence with a coefficient of +0.15%/°C and +0.25%/°C for irradiation temperature below and above −15°C respectively, and Red 4034 and Amber 3042 have smaller dependence in this temperature range, although Red 4034 has relatively high responses at −196°C and −78°C. At a dose of 2 kGy, dose response of Amber 3042 increases nearly linearly with temperature above −78°C with a coefficient of about 0.5%/°C. The dose response of Gammachrome YR below −78°C is almost 30% higher than that at 20°C and decreases with increasing temperature above −78°C with temperature coefficient of −0.3%/°C.

Radical formation in the radiolysis of solid alanine by heavy ions

Abstract Radical formation in solid α-alanine irradiated with 175-MeV Ar8+ and 460-MeV Ar13+, 220-MeV C5+, and 350-MeV Ne8+ ions were studied by the ESR method. The radical yield (number of radicals per incident ion) is constant below the critical fluence of about 1010 ions cm−2 for the Ar ions, 1011 ions cm−2 for the C ion, and 5 × 1011ions cm−2 for the Ne ion. Above the critical fluence, the yield decreases with increasing ion fluence. G-value of the radical formation was obtained from the constant yield at the low fluences. The G-value is not a simple function of LET. This is ascribed to the difference in lateral dose distribution of ion tracks. Assuming a simple cylindrical shape of the ion tracks, the average dose in and the radius of the ion tracks were estimated from the G-values. The radius is 8–25 nm, which is larger than the radius of 2–5 nm for 0.5–3 MeV H+ and He+ ion-irradiations. The fluence-yield curves were simulated with the cylindrical tracks and with the dose-yield relationship for the radical formation in γ-irradiated alanine. The simulated curves agree well with the experimental ones. With the cylindrical model of ion tracks, the variation of the radical yield at the high fluences can be estimated for solid alanine irradiated with several hundreds of MeV heavy ions.

Charged nanoparticle formation from humidified gases with and without dilute benzene under electron beam irradiation

Abstract The nucleation of aerosols in a field of high-density free radicals, ions, and secondary thermalized electrons was studied by irradiating air, N 2 , O 2 , and Ar containing different water contents with electron beams. The charged nanoparticles in the irradiating gases were analyzed by a cluster differential mobility analyzer equipped with a Faraday cup electrometer. All experiments with humidified gases formed both positively and negatively charged particles with mean mobil. equiv. diameters ( D m ) range of 7–10 nm and with the same number concentration ( N ) as well as water ion clusters with D m range of 1.0– 1.1 nm . The N of these large particles increased with water content and absorbed dose. The experimental result showed that hydrogen peroxide was contained in these large charged particles as its part. In the presence of ppbv-level benzene, the D m and N of large charged particles increased with benzene concentration, although their D m were constant at different doses.

Thin film alanine-polyethylene dosimeter

A thin-film dosimeter was prepared by homogeneously mixing ground DL-α-alanine crystalline powder and melted low-density polyethylene and by subsequent extrusion into a long tape with 8 mm width. The basic dosimetric characteristics of the 200-μm thick film dosimeter were studied for 60Co gamma-rays and 2.0-MeV electron beams. Dose responses were measured using a conventional electron spin resonance (ESR) spectrometer as well as a custom-made compact reader. The ESR measurement procedure was improved by the design of a dosimeter holder for high reproducibility, which avoided the orientation effect of samples during ESR readings. The random uncertainty of dose response among 20 irradiated replicate dosimeters is ±1% (1σ) for absorbed doses of 103–105 Gy, with correction being made for the weight differences of the dosimeters. The measurable dose range using the conventional ESR spectrometer is from 0.05 to 100 kGy. The compact reader can also be applied for dose evaluation above 1 kGy, with a precision within 1%, covering the measurable dose ranges of both FWT-60 radiochromic film and cellulose-triacetate film dosimeters. The estimated dose derived from two calibration curves for gamma-rays and electron beams was in good agreement. The thin-film polyethylene-alanine dosimeter can be useful as a reference dosimeter for medium energies of electron beams (∼2 MeV) as well as for comparison of radiation effects on various materials for photons and electron beams.

Fading characteristics of an alanine-polystyrene dosimeter☆

Abstract Alanine dosimetry is useful for transfer dosimetry by long distance mailing, because of its stability. It has the advantage that the measurement of electron spin resonance (ESR) spectral signal is non-destructive to the dosimeter, with the promise that the method may supply archival dosimetry data, depending on the degree of post-irradiation stability of the signal. The effects of temperature during irradiation and storage on fading of the ESR signal were studied using an alanine dosimeter molded with polystyrene (alanine-PS dosimeter). This investigation covered a long range of storage time (up to 160 days) after irradiation to absorbed doses in the range 1 to 100 kGy, for application to transfer dosimetry between Japan and neighboring Asian countries. Dose response of an alanine-PS dosimeter depends on the temperature during irradiation. The same temperature coefficient of +0.24%/°C was measured at different dose levels of 1, 10 and 100 kGy administered at a constant dose rate of 7 kGy/h. Fading of the dose response was measured under storage at various temperatures (5–40°C). The fading curve generally has two phases with fast and slow fading rates. The response of an alanine dosimeter is relatively stable for doses of 1.4 and 14 kGy, when stored at temperatures below 25°C. However, the degree of fading was roughly 3 and 5% under a storage temperature of 40°C for 5 and 100 days, respectively, after irradiation to 14 kGy. The fading percentages at 100 kGy were 2 and 4% (after 5 days) and 6 and 15% (after 100 days) under the storage temperature of 25 and 40°C, respectively. The fading rates have a relatively small dependence on irradiation temperature. This is observed even when irradiation are made at high temperatures (60°C) and for the doses 100 kGy and above. The mechanism of decay of radicals is discussed to explain the fading characteristics of the two phases of fading. The alanine-PS dosimeter is useful for transfer standard dosimetry up to a dose level of 10 kGy when stored after irradiation at temperature below 40°C. However, consideration of temperature effects during and after irradiation is vital for accurate transfer dosimetry of high doses, especially in the southern Asian countries.

Estimation of keV submicron ion beam width using a knife-edge method

Abstract A beam width measurement system has been developed for keV submicron ion beams of 0.1 μm or less in width assuming a round shape beam. The system enables to measure beam current change as a function of knife-edge position by cutting a beam focusing point (beam spot) with the sharp edge within a spatial resolution of 0.02 μm. The width of 30 keV order submicron H + ion beam was estimated by fitting current change curves based on three different ion density models: uniform, flat-top and Gaussian. Among these models, the flat-top model provide the most reasonable beam width of 0.56 μm interpreting contribution of halo around the beam spot to beam width estimation. The beam width measurement system with the high spatial resolution and the data analysis based on the flat-top ion density model should contribute to accelerate developments of submicron ion beam production technologies.