Tomoya Yamauchi

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.

Studies on the nuclear tracks in CR-39 plastics

A review was given for our recent studies on the latent tracks in CR-39 nuclear track detector. The radial size of track core has been determined through UV spectral measurements combined to the model of track overlapping and by AFM observations of slightly etched detectors. The track core radius was found to be about a few nano-meters and almost proportional to the cubic root of stopping power. As a control study, the etching properties of CR-39, irradiated by low-LET radiation, has been examined. The observed depth dependence and dose-rate dependence of the bulk etch rate of the irradiated CR-39 were explained that the damage formation process was governed by the reaction between the radiation induced radicals and the oxygen supplied from the air. This indicated that latent tracks in CR-39 are produced through local radiation induced oxidation process along the ion paths. Studies on vibration spectra, near-IR, FT-IR and Raman spectra, of CR-39 have also been carried out to estimate the chemical structure of the latent tracks. The creation of OH group in irradiated CR-39 has been confirmed.

Acceleration of highly charged GeV Fe ions from a low-Z substrate by intense femtosecond laser

Almost fully stripped Fe ions accelerated up to 0.9 GeV are demonstrated with a 200 TW femtosecond high-intensity laser irradiating a micron-thick Al foil with Fe impurity on the surface. An energetic low-emittance high-density beam of heavy ions with a large charge-to-mass ratio can be obtained, which is useful for many applications, such as a compact radio isotope source in combination with conventional technology.

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.

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.

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.