Swelling and structure of radiation induced near-surface damage in CR-39 and its chemical etching

Systematic measurements of swelling of CR-39 nuclear track detector (NTD) due to irradiation with fission fragments and alpha particles over a wide range of fluences from ²⁵²Cf are presented here. Precisely designed and optimized exposure and chemical etching experiments were employed to unfold the structure of radiation induced surface damage. Delays in the startup of the chemical etching of latent tracks in low radiation fluence detectors are measured and are found to contain important information about structure of the surface damage. Simple atomic scale pictures of radiation induced surface damage and its chemical etching are developed using measurements of radiation induced swelling of CR-39 detectors and nuclear track parameters. The computer code SRIM2010 was utilized for the calculations of basic features of latent tracks of fission fragments and alpha particles in CR-39. Another computer code TRACK_VISION was used to compute parameters of etched tracks. Computations and experimental findings in the paper coherently compose a realistic picture of radiation damage.     

High Energy Charged Particle Registration in CR-39 Polycarbonated Detector

Track etch rate characteristics of CR-39 plastic detector exposed to ²⁸Si ions of 670 MeV energy have been investigated. Experimental results were obtained in terms of frequency distribution of the track diameter, track density and bulk etching rate. A dependence of the mean track diameter on energy was found. The application of the radiation effect of heavy ions on CR-39 in the field of radiation detection and dosimetry are discussed. Results indicated that it is possible to produce etchable tracks of ²⁸Si in this energy range in CR-39. We also report the etching characteristics of these tracks in the CR-39 detector.     

Post-irradiation sensitization of CR-39 track detector in carbon dioxide atmosphere

The effect of post-irradiation treatment of CR-39 track detector in carbon dioxide atmosphere on the alpha particle registration sensitivity was investigated. Significant increase in etch rate ratio for 6.1 MeV ²⁵²Cf alpha particle was observed. The sensitization increasedboth as a function of trating time and as a function of carbon dioxide pressure in the treating vessel. Energy dependence of the sensitization was found to be small in the 2–9 MeV alpha particle energy interval. Fading of latent tracks after the sensitization process was slow in a one day period. Pre-irradiation treatment in carbon dioxide showed also some sensitization, though to a lesser extent than post-irradiation treatment. However, exposing CR-39 alpha particles in pure carbon dioxide atmosphere resulted in dramatic decrease in sensitivity. The role of dissolved oxygen and carbon dioxide in the primary latent track formation and in post-irradiation latent track transformation is discussed. Potential influence of the application of sensitization of CR-39 in radon, neutron and cosmic ray charged particle dosimetry is also discussed.     

Investigation of nuclear reaction products by atomic force microscopy

The use of Atomic Force Microscope (AFM, from Corporate Head, Santa Clara, California, USA) opened a new way to study latent nuclear tracks. In our experiments we used plastic track detectors of the type CR-39 (Columbia Resin No. 39) Impinging ions with energy above a threshold of 180 keV can alter the molecular structure forming latent tracks. Since nuclear latent tracks have diameters in the range of 10 to 1000 nm, they can be visualized by AFM with a slight chemical etching (6 min in 6 n NaOH solution at 70 °C). These tracks are significant for the energy, momentum and the mass of the incoming particles. In our study, passive CR-39 detectors were irradiated by secondary particles produced bombarding ¹⁰³Rh by ¹⁶O and ¹²C in a wide range of energy (1 MeV/amu to 33 MeV/amu) at the MP Tandem generator of the Laboratorio Nazionale del Sud in Catania, Italy. The experiment was carried out in order to identify the secondary particles and to determine their density and the spatial distribution.     

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.     

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     

ESR investigations of electron-beam irradiated cellulose nitrate

Electron spin resonance investigations on an electron-beam irradiated solid state nuclear track detector, based on cellulose nitrate (KODAK LR-311), are reported. The nature of free radicals induced in polymers by irradiation is discussed. The dependence of resonance spectral parameters on irradiation times, as well as on storage time and temperature, is studied. The experimental results are related to the stability of latent tracks and it is concluded that the free radicals induced by irradiation are located within the latent tracks. We have shown that both latent track and free radical thermal fading obey an Arrheniuslike dependence, with the same activation energy.     

High-resolution nuclear track mapping in detailed cellular histology using CR-39 with the contact microscopy technique

In radiation cancer therapies using energetic charged particles such as proton/heavy-ion therapy and boron neutron capture therapy (BNCT), studies on radiation-induced biological response at cellular level are important because the radiation damage from energetic charged particles is highly localized along their paths and the radiation sensitivity is quite different in each cellular organelle. In such studies the information on the position of charged particle impact in biological cells is necessary. A novel method for high-resolution nuclear track mapping in detailed cellular histology has been developed. In this technique, biological specimens mounted on CR-39 plates are exposed to energetic charged particles. The irradiated samples are exposed to UV, and then etched for a short time. Both etch pits of nuclear tracks and relief for transmission UV image of the specimen can be observed on the CR-39 surface with an atomic force microscope (AFM) at about 100 nm resolution.     

Effect of Transferred Electronic Energy Density on Optical,Electrical and Structural Properties of Polyallyl-Diglycol Carbonate (CR-39) Polymer

Commercially available polyallyl-diglycol carbonate (CR-39) track detectors have attracted wide interest in many fields of science and technology. This is because of their low cost, relatively easy handling, and being more similar to human tissue than other passive detectors. After the Fukushima accident, there was a need to study the impact of the released alpha particles from radionuclides. In this study, CR-39 polymers were irradiated with α-particles with different linear energy transfers 11.11, 19.70, and 28.77 eV/Å, at fluences of 49,490/cm² and 2,482,763/cm². The modifications in the optical, electrical, and structural properties induced by the radiation were measured. The results showed that low values of transferred electronic energy density along the alpha particle tracks led to a slight increase in the optical band gap energy and a small reduction in the conductivity. The amorphous nature of the CR-39 samples was not affected by the changes of the transferred electronic energy density.     

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.     

Neutron dosimetry at low and high fluences with CR-39

In the Laboratory of Nuclear Microanalysis, we have developed two techniques for neutron dosimetry; the first for low fluence, the second for high fluence. These two techniques use a Solid State Nuclear Track Detector (SSNTD): the CR-39. The low fluence technique is based on the measurement of etched tracks resulting from a neutron-proton conversion. A Monte Carlo code performs a simulation of the (n,p) collision in the detector, and a numerical computer code of latent track etching allows the evaluation of the etched track parameters. The object of this is to obtain characteristics of a neutron fluence from the measured etched track parameters. When there is a high fluence and high doses, CR-39 is unsuited for optical microscopy analysis. This is because of track overlapping which makes it impossible to carry out counting and exploitation. We have therefore developed a new method permitting the reading of samples based on the measurement of the angular distribution of coherent light (He---Ne laser) transmitted through the irradiated etched SSNTD. We present these two techniques and our initial results.     

The Effects of Storage on CR-39

The effects of storage temperatures on the track registration of CR-39 as a solid state nuclear track detectors have been investigated. The results show that the storage CR-39 detectors cause partial or complete removal of tracks depending on storaged time and temperatures. The bulk activation energy for CR-39 has been found to be 0.883 eV.     

Investigation of neutron emission from relativistic heavy ion interactions by nuclear tracks

Fast neutrons produced in 44 and 22 GeV ¹²C+Cu interactions have been recorded and analysed with a CR-39 detector stack. The irradiation of the CR-39 stack to fast neutrons was carried out at the accelerator Synchrophasotron, Joint Institute for Nuclear Research, Dubna, Russia. Areal and volume densities of tracks induced by fast neutrons in the CR-39 at different positions and for different etching time have been measured. The neutron production ratio of 44 GeV to 22 GeV ¹²C+Cu interactions has been obtained, which is 2.17 ± 0.30 by areal track density, or 2. 12±0.33 by step etch technique, or 2.03±0.34 by volume track density measurement. These results confirm that the production rate at 44 GeV ¹²C+Cu interactions is more than theoretical estimation.     

A model for annealing of nuclear tracks in solids

A mathematical formula is derived for annealing of radiation induced damage in solids starting with the empirical equation having a form of the Arrhenius equation. This formula is transformed into a practical relationship in the usable form to fit the experimental track annealing data by making use of a relationship between fractional defect density in the latent track and etched track length. Calculated lengths of fission fragment tracks in annealed CR-39 based on the proposed model are compared with corresponding experimental measurements (our previously published results). A very good agreement is found between calculated results and experimental measurements. Physical meanings of parameters involved in the proposed model are given with appropriate theoretical explanation guided by analysis based on the model-fit on track annealing measurements. The results in this paper are useful for a wide spectrum of researchers including cosmic-ray physicists, geologists, nuclear track methodologists and semiconductor/accelerator physicists using ions implantation for doping semiconductors and materials modifications.     

A photometry method for measuring tracks density on SSNTDs

The purpose of this work is to determine alpha track density on solid state nuclear track detectors (SSNTDs) such as CR-39 efficiently with minimal time. Samples of CR-39 were irradiated with different doses from an alpha emitter. These samples were chemically etched and the tracks counted using an optical microscope. The intensity of the transmitted light (λ=400–1200 nm) through irradiated and etched CR-39 was measured using a photomultiplier. For all wavelengths used in this study, it is found that the transmittance of light decreases as irradiation time increases. In addition, the calibration factor was calculated from the relation between track density on CR-39 and light transmittance. The procedure appeared to be a good and fast technique for determining radon concentration in the environment and consequently dose reading.     

Yields of formation and scissions at ether bonds along nuclear tracks in CR-39

Yields of CO₂ gas formation and the scissions at ether bonds have been determined along ion tracks in CR-39 plastics. Thin CR-39 films with thickness were irradiated by 130 MeV C-13 ions at 45 incidence up to , and then IR spectra were obtained without removing the samples from the vacuum chamber. The absorption band for CO₂ was observed clearly around , whose absorbance increased proportional to the fluence up to 3×10¹² ions/cm² and then saturated. Using a mole absorption coefficient for gaseous CO₂, the yield along the track was determined to be molecules/μm. This means that the G-value of CO₂ formation in CR-39 was about 3.6 molecules/100 eV. The scission yield of ether bonds was found to be 11,000 scissions/μm. This corresponds to a G-value of about 5.5 scissions/100 eV. These values suggest that the parts between the two carbonate ester bonds in each repeat unit of CR-39 can be segmented into two CO₂ and other small molecules along the nuclear tracks.     

Angular and dose dependence of CR-39 neutron response for shape-selected tracks

A shape selection method corresponding to an energy discrimination was used to eliminate unwanted events disturbing evaluation of CR-39 detectors in detecting tracks induced by particles both of perpendicular and oblique incidence. The angular dependence of the response was examined, detecting fast neutrons from ²⁵²Cf with shape selection technique at various angles and distances. Also, the CR-39 track detectors with the ²⁵²Cf source were exposed to high γ-intensity of a ⁶⁰Co irradiation facility in the range 0.1 to 4.5 kGy, similar to the exposures inside spent fuel assemblies. Using the two functions the lower limit of burnup could be determined by the method.     

The effect of infrared radiation on etching characteristics of CR-39 plastic track recorder

The effect of infrared radiation on the etching characteristics of CR-39 plastic track recorder has been studied. CR-39, which is commonly employed in cosmic rays studies is found to be affected by IR radiations. The changes in the bulk etch rate, in the track etch rate and in etching efficiency due to infrared radiation exposure of CR-39 are discussed on the basis of scission and cross linking during infrared exposure.     

Identification of heavy ions by means of latent tracks in the passive detector CR-39

The use of the Scanning Probe Microscope for analyzing latent tracks induced in CR-39 by ions of carbon, oxygen and A～110 ions is described.     

Estimation of the latent track size of CR-39 using atomic force microscope

As a frame work of the study for the latent track size measurement using atomic force microscope, we have measured the minute etch pits and the extremely small amount of bulk etch of CR-39 at the beginning of chemical etching, and obtained its growth curves in nanometer dimensions. The pieces of CR-39 were exposed to 6 MeV/nC and Fe ions with normal incidence angle and were etched in 70°C 7 N NaOH solution for 0.5,1,2,3,5 min. The diameters of latent track were estimated to be 17 nm for Fe ions and 8 nm for C ions, respectively. These values are comparable to the experimental data on the average ‘track core diameters’ that have been obtained by various experimental techniques.