Techniques to investigate the characteristic of CR-39 detectors using Am---Be neutron source

Two techniques have been developed to investigate the characteristic of the sensitivity of CR-39 (polyallyldiglycol carbonate) track detector using Am---Be neutron source. The firstis to measure volume track density created by fast neutrons in CR-39 detectors. The second is to measure areal asymptotic track density on the surface of CR-39 sheets. Two types of CR-39—SY-2 CR-39 made in China and another made in USA were calibrated using the techniques. It shows that the two types of CR-39 detectors have nearly the same sensitivity to detect fast neutrons. The techniques developed in this work can be applied to calibration of other track detectors.     

Thermal annealing of proton tracks with energies of 4 and 6 MeV in CR-39 polymer detectors

In this paper, we study the thermal annealing of proton tracks of 4 and 6 MeV at temperatures ranging from 150 to 240°C in CR-39 polymer detectors. A special experimental set-up for irradiating the detectors was arranged to obtain adequate proton beams from the Cyclotron CV-28 at IPEN/SP, Brazil. We report experimental data on track densities, track diameters, and activation energies based on current annealing models for the annealing of proton tracks in the energy range investigated. A value of (0.20±0.02)eV was determined as the mean activation energy of the annealing process in CR-39 detectors.     

On the effect of cure cycle parameters and dopants on pre and post irradiation annealing response of CR-39 track detectors

The effect of additives and cure cyle parameters on pre and post irradiation annealing characteristics of CR-39 detectors has been investigated. Effect of pre and post irradiation annealing on bulk etch rate, sensitivity, track diameter and etching behaviour of various CR-39 detectors has been investigated.     

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.     

Registration temperature effect on sensitivity of CR-39(DOP) and SR-90 polymer track detectors

The samples of CR-39(DOP) and SR-90 polymer track detectors have been exposed to -particles from ²⁴¹Am source in an exposure unit. The temperature of the detectors during irradiation has been varied from −30°C to 70°C. These exposed samples have been etched in 6.25 N NaOH solution at 60°C for various etching times. The variation of sensitivity of these detectors as a function of registration temperature has been studied. It has been observed that at the fixed registration temperature, the sensitivity of SR-90 is more than CR-39(DOP) polymer track detector. However, the enhancement in sensitivity with the decrease in registration temperature is more pronounced in case of CR-39(DOP) than SR-90.     

LET calibration for CR-39 detectors in different oxygen environments

High LET (linear energy transfer) radiation is the main contributor to the radiation field in low Earth orbit (LEO) in terms of dose equivalent. CR-39 plastic nuclear track detectors (PNTDs) can measure the LET spectrum and charge spectrum for the complicated radiation field in space. Previous research indicated that the sensitivity of CR-39 is different for CR-39 PNTDs working in different oxygen environments. LET calibration for CR-39 detectors in different oxygen environments is needed. Almost all the previous LET calibration work was carried out for CR-39 detectors in good-oxygen condition, LET calibration work for CR-39 in poor-oxygen condition has not been conducted until our work. Systematic LET calibrations were carried out by JSC-SRAG (Space Radiation Analysis Group) for CR-39 detectors working in different oxygen environments and abundant results of LET calibrations were obtained. This paper introduces the method for CR-39 LET calibration, presents and discusses the calibration results and some applications.     

Effects of radiations on the characteristics of alpha and fission tracks in CR-39 detectors

Abstract

The effects of neutron, gamma and alpha radiations on the alpha and fission fragment tracks registration and revelation properties of CR-39 detectors (CR-39 and CR-39(DOP) were studied. It was found that the ratio of the bulk etch rate of irradiated to unirradiated (V_G(irr.)/V_G(unirr.) detectors is linearly dependent on dose. An exponential decrease in fission track densities with increase in neutron fluence was observed. The ratio of V_G(irr.)/V_G(unirr.) was found to be high in CR-39 than that in CR-39(DOP) exposed to the same reactor neutron fluence. The decrease in fission track densities with increase in neutron fluence was observed to be faster in CR-39 than in CR-39(DOP). This indicates that doping with dioctyl phthalate improves the radiation resistance of CR-39 detectors. It was observed that in detectors exposed to an alpha flux of the order of 9.36 × 10⁶ / cm², the fission track density was reduced by 11% and thereafter it remained constant. The results also indicate that thermal neutron fluence up to 7.01 ×10¹¹ neutrons/cm² does not affect the alpha and fission track densities. I.R. spectra were also studied to find out the nature of chemical changes produced by these radiations on CR-39.     

Sensitization of polymeric track detectors with carbon dioxide

The effect of CO₂ on the sensitivity of polymeric track detectors was studied. It has been found that the sensitivity of SR-90 is dependent on the amount of CO₂ dissolved in the polymer at the time of etching. It has been also found that CR-39 can be sensitized to some extent with an aging in CO₂. Since the sensitization in CO₂ is possible even a long time after the irradiation, the mechanism of the latent track formation by CO₂ is quite different from that by O₂. This would be the key to achieve the long term stability of polymeric track detectors such as SR-90 and CR-39.     

Some more new etchants for CR-39 detector

Recently, several new etchants have been reported for CR-39 detector (Molten Ba(OH)2. 8H2O as an etchant for CR-39 detector, Radiat. Meas. 37 (2003) 205; Discovery of new etchants for CR-39 detector, Radiat. Meas. (2004)). We have made further progress in this direction and have unveiled two more new etchants which are reported in this article. CR-39 detectors were irradiated with fission fragments and alpha particles from a thin 252Cf disc source. The irradiated detectors were then etched in our newly introduced etching solutions as well as in conventionally used 6 M NaOH aqueous solution at 70 degrees C. The newly prepared etching solutions included NaOH dissolved in methanol and NaOH dissolved in methanol + water. Optimum values of NaOH concentration in methanol as well as in methanol + water were determined. Optimum etching temperatures were also determined for both the above-mentioned etchants. From fission and alpha track diameters, bulk etching rate (VB), track etching rate (VT) and etching efficiency (eta) were determined and compared with that obtained for 6 M NaOH at 70 degrees C. Both the newly introduced etchants were found more efficient than the conventionally used 6 M aqueous NaOH (64%) at 70 degrees C and have relatively much smaller etching time.     

Thickness alterations of CR-39 plastic detectors due to the heating influence: Basic theory and experimental results

Heating experiments were conducted on CR-39 polymer/plastic detector samples at temperatures between 50 and 175 °C for various time intervals from 5 to 210 min to investigate the thermal behavior of detector material. Mass and thickness of un-heated and heated polymer samples were measured. Three distinct regimes have been observed showing different degrees of changes in thickness and mass of annealed samples. The percent change of thickness was considerably higher than the mass change during heating. Thermodynamic concepts are used to explain the experimentally observed heating regimes. Role of the difference between free energies of lateral and fold surfaces in detector thickening during heating is discussed. Scanning electron microscopy (SEM) was carried out on un-heated and heated CR-39 samples to examine the surface alterations due to heating. Results are useful for researchers who employ track detectors for radiation measurements in nuclear reactors and cosmic rays. They are also interesting for researchers in related fields, like fission track dating, which consider the thermal history of track recording materials. Our quantitative data and SEM imaging suggests that structural changes, especially surface changes in CR-39 polymer are very important and need to be explored carefully through joint SEM and the Atomic Force Microscopy (AFM).     

Gamma and neutron dose measurements with solid state nuclear track detectors

An attempt has been made to explore the possibility of using solid state nuclear track detectors for the estimation of gamma and neutron doses based on the use of changes in activation energy of degradation of these detectors, due to irradiation (gamma-neutron), as a means of dosimetry. Thermogravimetry (TG) has been applied as a tool for these studies carried out on Lexan and CR-39 track detectors. A linear relationship observed between the decrease in activation energy and the dose (gamma-neutron) received by the detectors suggests the possibility of the use of these detectors as gamma and neutron dosimeters.      

Comparison of Makrofol-E and CR-39 as Solid State Nuclear Track Detectors

A comparison of the etching parameters as well as the neutron response for Makrofol-E and CR-39 was considered as solid state nuclear track detectors. The calculated sensitivity for both detectors was compared with the measured one. It is found that CR-39 has higher response to fast neutrons than Makrofol-E. The response increase due to decreasing the rate of layer removed and increasing the etch-pits per unit area.     

Computer simulation of the processes in a CR-39 neutron detector

The application of CR-39 as a fast neutron dosimeter, based on counting elastic recoil nuclei tracks of H, C and O nuclei in a geometrically matched pair of detectors, was studied. A computer simulation of the processes during the exposure of a detector pair to neutrons is presented. The energy and spatial distribution of the recoil nuclei was calculated with the MCNP 4B program package. On the basis of this distribution, a Monte-Carlo procedure for the generation of latent tracks in the CR-39 detectors was run. Additionally, a procedure for calculation of the chemical track etching for revealing the visible track parameters was performed to obtain the detector response.     

The effect of post-exposure annealing on the track registeration characteristics of CR-39 nuclear track detectors

The effect of post exposure annealing on the track registeration characteristics of CR-39 (Pershore) Solid State Nuclear Track Detectors has been studied. The changes introduced in the bulk etch rates, track diameter, and the track density have been observed. The results indicate that the bulk etch rate of the detectors and the size of the tracks are considerably changed due to annealing. Variations in the track density are, however, not serious in the presently studied temperature range.     

Single-sheet identification method of heavy charged particles using solid state nuclear track detectors

The theoretical and experimental investigations of the penetration of charged particles in matter played a very important role in the development of modern physics. Solid state nuclear track detectors have become one of the most important tools for many branches of science and technology. An attempt has been made to examine the suitability of the single-sheet particle identification technique in CR-39 and CN-85 polycarbonate by plotting track cone length vs. residual range for different heavy ions in these detectors. So, the maximum etchable ranges of heavy ions such as ⁹³Nb, ⁸⁶Kr and ⁴He in CR-39 and ⁴He and ¹³²Xe in CN-85 polycarbonate have been determined. The ranges of these ions in these detectors have also been computed theoretically using the Henke-Benton program. A reasonably good agreement has been observed between the experimentally and theoretically computed values.      

CR-39核径迹探测器及其在核科学等研究领域中的应用

介绍了一种核径迹探测器ＣＲ－３９塑料的特性,这种探测器对逞电粒子非常灵敏,它还具有稳定、透明等特点,可记录ｐ、ａ粒子,裂变碎片和其他带电粒子,它是现有固体核径迹探测器中能量沉积密度探测阈最低的材料,介绍了ＣＲ－３９对带电粒子的响应,给出了各种带电粒子的ｖＴ对限定能量损失（ＲＥＬ）的响应曲线,利用ＣＲ－３９与转换屏的组合还可测定能量范围广的中子能谱,可作为方便的个人中子剂量计,介绍了ＣＲ－３９在研究     

Measurement of LET distribution and dose equivalent on board the space shuttle STS-65

Space radiation dosimetry measurements have been made on board the Space Shuttle STS-65 in the Second International Microgravity Laboratory (IML-2). In these measurements, three kinds of detectors were used; one is a newly developed active detector telescope called “Real-time Radiation Monitoring Device (RRMD)” utilizing silicon semi-conductor detectors and others are conventional detectors of thermoluminescence dosimeters (TLDs) and CR-39 plastic track detectors. Using the RRMD detector, the first attempt of real-time monitoring of space radiation has been achieved successfully for a continuous period of 251.3 h, giving the temporal variations of LET distribution, particle count rates, and rates of absorbed dose and dose equivalent. The RRMD results indicate that a clear enhancement of the number of trapped particles is seen at the South Atlantic Anomaly (SAA) without clear enhancement of dose equivalent, while some daily periodic enhancements of dose equivalent due to high LET particles are seen at the lower geomagnetic cutoff regions for galactic cosmic ray particles (GCRs). Therefore, the main contribution to dose equivalent is seen to be due to GCRs in this low altitude mission (300 km). Also, the dose equivalent rates obtained by TLDs and CR-39 ranged from 146.9 to 165.2 μSv/day and the average quality factors from 1.45 to 1.57 depending on the locations and directions of detectors inside the Space-lab at this highly protected orbit for space radiation with a small inclination (28.5°) and a low altitude (300 km). The LET distributions obtained by two different detectors, RRMD and CR-39, are in good agreement in the region of 15–200 keV/mm and difference of these distributions in the regions of LET < 15 keV/mm and LET > 200 keV/mm can be explained by considering characteristics of CR-39 etched track formation especially for the low LET tracks.     

DIP angle dependence on track formation sensitivity in antioxidant doped CR-39 plates

Recently, space radiation dosimetry measurements were made by passive and active detectors inside the Spacelab [STS-47 (FMPT): 300km, 57°, STS-65 (IML-2 mission): 300km, 28.5°]. The LET distributions obtained by antioxidant doped CR-39 inside the Spacelab were compared with those measured by the tissue equivalent proportional counter (TEPC) and the real time radiation monitoring device (RRMD) consisting of eight silicon detectors. While both distributions by CR-39 are in good agreement with those obtained by active detectors over the region of LET of several tens to 200 keV/μm, a significant difference in the LET region of smaller than several tens keV/μm is seen. It is considered to be caused by the dip angle dependence of track formation sensitivity in CR-39. The track formation sensitivity for different dip angle were measured for several high heavy energy ions. Using these results, the correction for the dip angle was made for the LET distribution. The corrected result is consistent with the results obtained by active detectors.     

Membrane-based thermal effects in 222Rn dosimetry

Radon detector cups fitted with polyethylene anti-thoron membranes and exposed to a fixed radon concentration in a chamber containing a thermal gradient gave erratic track densities — ones that do not correlate with the temperatures of the CR-39 detectors. There is, however, a simple, monotonic variation of track density with the temperatures of the membranes. The results are used to infer radon permeabilities of polyethylene versus temperature. This variation needs to be considered in interpreting secular variations of inferred ²²²Rn when using this detection system during times of sizable temperature variations or in considering absolute ²²²Rn values measured at unusual temperatures.     

Effect of vacuum, oxygen and carbon dioxide on the track registration in SR-90 and CR-39

The molecular structure of SR-90 and CR-39 is very similar except that the average length of carbonate linkages in SR-90 is 1.6 times longer than in CR-39. However, the effect of vacuum and carbon dioxide (CO₂) on the track registration of these two detectors is very different: 1) the sensitivity of CR-39 becomes very low in vacuum; the sensitivity of SR-90 does not go down to zero even after storage in vacuum for a long time; 2) the sensitivity of SR-90 decreases both in air and in vacuum but the original sensitivity can be restored by storing it in CO₂ of 13 atm for several days. We also found that the sensitization of latent tracks by oxygen (O₂) is effective only at the time of irradiation but the sensitization by CO₂ is effective after irradiation.