H. Tachibana

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.

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.

Orientation effects of ESR analysis of alanine-polymer dosimeters

Abstract Orientation effects during electron spin resonance (ESR) measurement are studied on alanine-polymer dosimeters prepared by different molding procedures (press-molding or extrusion), and also according to different shapes (rods with different lengths and thin-films). The variation in ESR signal amplitude was measured by rotating a dosimeter around the vertical mid-axis and at a right angle to the magnetic field in the ESR cavity, at a temperature of 22°C. Orientation effects for rod dosimeters with both molding procedures (press and extrusion), and with different lengths, were negligible. Thin-film dosimeters molded by extrusion show a pronounced orientation effect when the samples are cut into disks and positioned horizontally (parallel to the magnetic field) during readout in the ESR cavity. Orientation effects on the film dosimeters can be eliminated, however, when they are positioned vertically and parallel to the mid-axis of the ESR cavity.

Development of portable ESR spectrometer as a reader for alanine dosimeters

Abstract A prototype portable electron spin resonance (ESR) spectrometer was designed and tested, and its feasibility as a reader of alanine dosimeters was studied from the two standpoints of reproducibility of readings and sensitivity sufficient for dosimetry in the absorbed dose range 1–100 kGy. It has two main components: (1) A permanent magnet and resonator; and (2) a unit box with a microwave and auto-frequency control (AFC) circuit, a sweep controller of magnetic field, display, etc. In the present preliminary study, reproducibility values are measured with the same ESR parameters and alanine-polystyrene (alanine-PS) dosimeter at a dose of 1 kGy: (1) Repeatedly measuring without removing dosimeter from the cavity (±0.5%, 1 σ ); (2) individual measurement with removing and inserting again into the cavity with readjustment of ESR parameters (±3%, 1 σ ). Alanine/ESR dosimetry using this spectrometer has a measurable dose range from 1 to 100 kGy with relatively high precision within ±3% (1σ) as a preliminary result. The portable ESR spectrometer may also be modified as an automatic, more precise, dedicated alanine dosimeter reader.

Uncertainty estimation in 60Co gamma-ray dosimetry at JAERI involving a two-way dose intercomparison study with NPL in the dose range 1–50 kGy

The uncertainties in 60Co gamma-ray dosimetry at JAERI have been estimated by reviewing irradiation and calibration procedures. The uncertainty in dose delivered by the JAERI calibration system has been estimated to be 2.2% (2σ) and the uncertainty in dose measurement using JAERI alanine/ESR transfer dosimeters has been estimated to be 3.4% (2σ). A two-way dose intercomparison study between NPL and JAERI employing alanine-PS, alanine-paraffin, and dichromate dosimeters as transfer dosimeters was performed to confirm the estimated uncertainty over the dose range 1–50 kGy. Dose intercomparison results using both alanine and dichromate dosimeters show agreement within about 2%, which gives confidence in the estimated uncertainty in 60Co gamma-ray dosimetry at JAERI.

Recent progress in JAERI alanine⧸ESR dosimetry system

Abstract Alanine/ESR (electron spin resonance) dosimetry system has been developed on the basis of precise, mass-productive alanine dosimeters of various shapes, and easy-handling ESR reading system. Polymers as binders enable us to mass-produce homogeneous and dimensionally uniform alanine dosimeters of different shapes and thickness relative to penetration range of radiations, for instance, rods for gamma and X-rays, and thin films for electrons and heavy charged particles. They also allow us to simplify parameters of ESR operation exclusively for specific dosimeters and to achieve good reproducibility of measurement. An easy-handling ESR spectrometric reading system was developed on the basis of custom-made ESR spectrometer with a permanent magnet, and automated with auxiliary interfaces and a personal computer. Alanine/ESR dosimetry in JAERI works for domestic and international transfer dosimetry, for simple and reliable routine dosimetry in radiation processing, and for dosimetry of short-penetration range radiations.

Effects of temperature during irradiation and spectrophotometry analysis on the dose response of aqueous dichromate dosimeters

Abstract The irradiation temperature dependence of the 60 Co gamma-ray dose response of a silver-dichromate dosimeter was studied to verify temperature coefficients over 5–60°C, using both low and high range standard-type dosimeters. The temperature coefficients in the temperature range 25–60°C are estimated to be −0.20 and −0.26%/°C respectively at doses 2–10 kGy and 10–50 kGy, although there is a slight tendency of dose dependence. The coefficients covering temperatures of 25–50°C estimated to low and high dose ranges are respectively −0.20 and −0.23%/°C which lead us to confirm the previous data including the ASTM Standards E1401-96, 1997 . The effect of temperature during spectrophotometry analysis on molar extinction coefficients was not appreciably observed in the temperature range 5–50°C.

The gamma-ray response of clear polymethylmethacrylate dosimeter radix RN15®

Abstract Basic characteristics of different batches of Radix RN15® dosimeter, a commercially available undyed polymethylmethacrylate dosimeter, was studied for its application to process control of radiation sterilization. Radix has relatively small deviation of thickness, optical absorption spectrum, and optical density values before irradiation. The response curves, in terms of net optical density per unit thickness (ΔOD/mm) as a function of absorbed dose, D PMMA , are nearly linear up to 15 kGy and become sublinear at higher doses. Scattering of dose response in 5 dosimeter replicates is ± 1% (1 σ). The dose-response slope at 25 kGy while held at irradiation temperatures in the range 0–60°C, relative to those under 25°C, increases with temperature up to 40°C, the maximum point, and decreases at higher temperatures. Over the temperature range of 10–50°C, the variation with temperature of evaluated dose derived from the calibration curve at 25°C is less than 5% when using a dose rate of 6 kGy/h. At lower dose rates (e.g. 0.7 kGy/h) the irradiation temperature dependence is negligible over this temperature range. The temperature dependence at 40 kGy is less severe than that at 25 kGy. The post-irradiation stability of dose response is less than 5% for more than 100 h after 25 kGy irradiation and storage at 0–35°C. The change in optical density of unirradiated dosimeters 3 years after manufacturing is negligible when the dosimeters are stored at controlled laboratory conditions of temperature (25°C) and relative humidity (40%). The 3-year-old dosimeters by irradiation to a dose of 25 kGy show about 2% lower response than that at 25 kGy at the beginning of the 3-year period.

Alanine⧸ESR Dosimetry system for routine use in radiation processing

Abstract A new alanine-polystyrene (PS) dosimeter prepared with simplified molding procedure and an automatic desk-top dose-reader of alanine dosimeter were developed for the purpose of routine use. Combination of these two allows us to apply a reliable alanine/ESR dosimetry system to routine dosimetric process control in industrial gamma radiation processing.

Study on dosimetry of bremsstrahlung radiation processing

Abstract The dosimetry in high-power bremsstrahlung irradiation for the industrial processing has been studied. The dosimeter systems used are cylindrical ionization chamber to measure average exposure rate and CTA, clear PMMA and alanine dosimeters for routine dosimetry. The results gave some useful information on the measurement of average exposure rate using the ionization chamber. Clear PMMA and alanine dosimeters showed good characteristics for their usefulness as routine dosimeters for X-ray irradiation in the industrial processing.