Keiji Oda

Formation of CO2 gas and OH groups in CR-39 plastics due to gamma-ray and ions irradiation

Abstract FT-IR spectral studies have been made for CR-39 detectors irradiated by gamma-rays, protons and carbon ions. An absorption peak for CO 2 appeared and grew with the fluence. The peak height was found to decrease gradually by storing in air. Absorbance of the OH group produced by ion bombardment was observed separately from that of adsorbed water using an FT-IR system on the beam line. The concentration of OH group would govern the rate of penetration of chemical agents along the track.

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.

Dose-rate effects on the bulk etch-rate of CR-39 track detector exposed to low-let radiations

Abstract The effect of gamma-rays and pulsed electrons has been investigated on the bulk etch rate of CR-39 detector at doses up to 100 kGy under various dose-rate between 0.0044 and 35.0 Gy/s. The bulk etch rate increased exponentially with the dose at every examined dose-rates. It was reveled to be strongly depend on the dose-rate: the bulk etch rate was decreased with increasing dose-rate at the same total dose. A primitive model was proposed to explain the dose-rate effect in which oxygen dissolved was assumed to dominate the damage formation process.

Dose-equivalent response CR-39 track detector for personnel neutron dosimetry

Abstract A dose-equivalent response detector based on CR-39 has been designed to be applied for personnel neutron dosimetry. The intrinsic detection efficiency of bare CR-39 was first evaluated from irradiation experiments with monoenergetic neutrons and theoretical calculations. In the second step, the radiator effect was investigated for the purpose of sensitization to fast neutrons. A two-layer radiator consisting of deuterized dotriacontane (C32D66) and polyethylene (CH2) was designed. Finally, we made the CR-39 detector sensitive to thermal neutrons by doping with orthocarborane (B10H122C2), and also estimated the contribution of albedo neutrons. It was found that the new detector — boron-doped CR-39 with the two-layer radiator — would have a flat response with an error of about 70% in a wide energy region, ranging from thermal to 15 MeV.

Effects of low-LET radiations on CR-39 track detector

Abstract The effects of Co-60 gamma-rays and about 25-MeV pulsed electrons have been investigated on CR-39 plastic nuclear track detector. The bulk etch rate was measured as a function of the absorbed dose under three different irradiation conditions; gamma-ray exposure in air at the dose rate of 6 kGy/h, that in vacuum and electron shower in air at 126 kGy/h on average. It was found that the bulk etch rate ( V ) increased nearly exponentially with the dose ( D ); V = V 0 exp( kD ), and the characteristic parameter ( k ) was 35, 9 and 12 MGy −1 , respectively. It is considered that a lack of oxygen in damage formation processes could occur momentarily during a short electron pulse, just similarly to the vacuum effect of reduction in track etch rates in outgassed CR-39 plastic.

An evaluation of the radical life-time in CR-39 track detectors outgassed under vacuum

Abstract The property of latent track registration for CR-39 plastic track detectors outgassed in vacuum has been examined. It was found that the sensitivity of CR-39 detectors recovered partially when exposed to air after α-particle irradiation. The life-time of radicals produced by the irradiation could be evaluated from the dependence of recovery on the time between irradiation and air exposure. At least two components of radicals with life-times of about 40 min and a few min were found to be formed.

Study on response of CR-39 detector to light ions

Abstract The response of CR-39 detector has been obtained precisely for protons, deuterons, tritons, alpha-particles and lithiumions with energies of a few MeV/nucleon. The track etch rates were determined from the growth curves of pit radius and summarized as the response functions. The accuracy of this method is high enough to discriminate among hydrogen isotopes under a certain condition. The correlation between the response and LET with various cut-off energies has also been examined. It was found that LET is not an unique parameter describing the response function adequately for different ions by the same expression.

Inter-comparison of geometrical track parameters and depth dependent track etch rates measured for Li-7 ions in two types of CR-39

Abstract The depth dependent track etch rates of two types of CR-39, TASTRAK and BARYOTRAK, for Li-7 ions with incident energies of 4.82, 6.75 and 10.77 MeV have been evaluated from track length measurements. The pit lengths versus the etching time obtained by three different laboratories have the same trend, but did not agree completely. The differences can be attributed to an error margin of less than one decimal point in the etching temperature. Significant deviations were also present for the assessed track etch rates. Most of the deviation was attributed to the derivation step of the growth curves. A sufficient amount of data and reasonably smooth growth curves were required near the Bragg peak in order to make a precise estimation.

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.

Raman and near-IR study on proton irradiated CR-39 detector and the effect of air-leak on damage formation

Abstract Raman and near-IR spectral studies have been made on the proton irradiated CR-39 plastics with 3.4 MeV at fluences ranging from 1010 to 10 14 ions cm −2 . The density of –CH2– group began to decrease after the air leak subsequent to the irradiation. This implies an important role of the oxygen on forming the permanent damage along the latent track. Production of OH group and some types of radicals was confirmed by near-IR observations in the air.