The electron spin resonance of polycarbonate track detectors

Polycarbonate films are widely used as solid state track detectors (SSTDs) of radiation, but as yet our knowledge of the microscopic nature of the latent track is limited. The processes of chemical etching and thermal annealing are not fully understood. The lack of stability of track parameters bears on the accuracy of the charge determination of energetic heavy nuclei.

We have applied the technique of electron spin resonance, (ESR) in order to seek a correlation between the density of tracks and the growth and decay of free radicals detected by ESR. Samples of polycarbonate were irradiated with thermal neutrons and then etched progressively with NaOH. The ESR signal increased initially and then became weaker as the bulk of the polycarbonate was removed. Other samples were annealed after partial etching. ESR monitoring of this annealing showed that free radicals were recombining, probably due to diffusion processes.     

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.     

Response of CR-39 to medium energy electron irradiation

The effect of 5-15 keV electron irradiation on the etch rate of CR-39 nuclear track detectors has been investigated for surface doses from 0.09 to 13.85 J/cm2. Even before etching, electrons produce a surface depression proportional to the surface dose raised to the power 0.78 +/- 0.01. Etch rate enhancement was observed in the surface region (about 4 micrometers deep) and was well approximated by a Gaussian function of depth. The depth and thickness of the etch rate enhancement increase with increasing electron energy, but are independent of dose. The maximum etch rate is proportional to the peak volumetric dose raised to the power of 1.45 +/- 0.06, independent of electron energy. No signs of a saturated response were found in the etch rate. This enhanced etching rate may significantly affect nuclear track identification in situations with an electron background.     

Influence of intensive γ and electron radiation on tracks formation in the PM-355 detectors

Solid-state nuclear track detectors have found wide use in various domains of science and technology, e.g. in plasma experiments. The track detectors used in the fusion-oriented experiments encounter “harsh” conditions (intense X-ray and neutron radiation, heat impact, shock waves and plasma fluxes). Therefore, such detectors should be tested under similar conditions. This paper concerns influence of electron and γ radiation on the track formation in PM-355 detectors. Samples were irradiated with 1.5-MeV protons and ⁴He-ions, and with 5.5 MeV particles. After that they were exposed to γ and electron radiation. Irradiation doses were varied from 100 kGy up to 2000 kGy. The etching was performed in steps, and a bulk etching rate (V_B) was determined versus an absorbed dose of electron and gamma radiation.     

Effect of high gamma dose on the PADC properties

The effect of the properties of PADC nuclear track detectors after exposure to high doses of gamma absorbed doses up to 5×10⁵ Gray (50 Mrad) were studied. The gamma source was a 9.03 PBq (244 KCi), Co-60 source. Results indicate that each of the bulk etch rate (V_b), the tracks etch rate (V_t) and the sensitivity (V) of the detectors increases with the high gamma absorbed dose, but there is a drop in these parameters at the low gamma absorbed dose. Signs of surface roughness were observed by increasing the gamma absorbed doses and changes in color observed for doses larger than 2×10⁵ Gray. The temperature of detectors during irradiation time reached 41°C. The fission fragment tracks (from Cf-252 source) disappeared quickly within the etching time (minutes) for total absorbed doses greater than 3×10⁵ Gray due to their high bulk etch rate.     

On the formation of heavy ion latent tracks in polymeric detectors

Three kinds of polymeric track detectors have been examined by ESR spectroscopy following heavy ion or energetic electron bombardment: (1) Typical nuclear track detectors: makrofol, kapton, terphane; (2) Polymers whose radiolysis under γ-rays or electron beams is well established: PMMA, PE, PP; (3) Polymers in which etchable tracks have not so far been observed e.g. teflon. The influence on the various ESR spectra of the radiation dose, of annealing or chemical etching treatments leads us to suggest that existence of heavy ion latent tracks might be correlated with the formation of “carbon-like” radicals such as produced in polymer pyrolysis.     

Track revelation and optical properties of pentaerythritol tetrakis (allyl carbonate) plastic for application as nuclear track detector: effects of gamma radiations

The etching conditions of an indigenously prepared thin film of pentaerythritol tetrakis(allyl carbonate) (PETAC) were standardised for the use as a nuclear track detector. The optimum etching times in 6?N NaOH at 70°C for the appearance of fission and alpha tracks recorded in this detector from a ²⁵²Cf solid source were found to be 30 min and 1.50?h, respectively. The experimentally determined values for the bulk and track-etch rates for this detector in 6?N NaOH at 70°C were found to be 1.7?±?0.1 and 88.4?±?10.7?µm/h, respectively. From these results, the important track etching properties such as the critical angle of etching, the sensitivity and the fission track registration efficiency were calculated and compared with the commercially available detectors. The activation energy value for bulk etching calculated by applying Arrhenius equation to the bulk etch rates of the detector determined at different etching temperatures was found to be 0.86?±?0.02?eV. This compares very well with the value of about 1.0?eV reported for most commonly used track detectors. The effects of gamma irradiation on this new detector in the dose range of 200–1000?kGy have also been studied using bulk etch rate technique. The activation energy values for bulk etching calculated from bulk etch rates measurements at different temperatures were found to decrease with the increase in gamma dose indicating scission of the detector due to gamma irradiation. The optical band gap of this detector was also determined using UV–visible spectrometry and the value was found to be 4.37?±?0.05?eV.     

CR-39 and Lexan calibrated as low-LET radiation dosimeter, for three Mexican irradiation facilities

An experimental calibration of two common nuclear track detectors, CR-39 and Lexan, for gamma and electrons radiation, was performed using various irradiation facilities. The dose response was obtained as a function of two parameters, the bulk etch rate and the UV absorbance for a wide dose range from 10 to 1000 kGy. The bulk etch rate sensitivity, for gamma and electrons, in CR-39 detector is higher than for Lexan detector. Lexan has a well-defined UV absorbance spectrum, but presents saturation for doses higher than 500 kGy, the same saturation characteristic is observed for the corresponding bulk etch rate response. For electron and gamma radiation, CR-39 shows a good response for doses from 10 kGy up to 1000 kGy, where data fit well an exponential curve for electrons and a lineal curve for gamma radiation.     

Modeling the fission track etching process in apatite: Segmentation or crystallography influence

In this study, two factors which can influence fission track etching in apatite are considered: track segmentation (induced by thermal annealing) and variable radial etching speed (due to the reagent diffusion during the etching process).

During the latent track annealing, two distinguishable steps can be identified by measuring track lengths or diameters. A length reduction is firstly observed, followed by a segmentation process which leads to the emergence of disrupted regions (gaps).

At present time, electron microscopy studies on fission tracks in apatite show profiles which lead to hypotheses of a variable radial etching speed versus depth. These variations can be interpreted in terms of acid diffusion along the track. Moreover, the existence of several bulk etching speeds related to crystallographic orientation is approached.

Taking into account these different points, a software program, integrating parameters as original track orientation and depth, number of gaps, etc., is developed in order to model the track profile evolution during the etching process. Comparison with experiments in Durango apatite (Mexico) are also undertaken.     

A study on the etching properties of electron-irradiated plastic track detectors

The variations of the bulk etching rate of electron-bombarded CN, CA, CAB, PC and PET sheets were studied as a function of electron dose and etching temperature. A conclusion was made that in the plastics under study a radiation-damaged region produced, either by an electron beam or a heavily ionizing nuclear particle display, to a certain extent, analogous etching properties. It was shown that the ratio of the etching rates of an electron-bombarded and an unirradiated sheet increases with the increase of the etching temperature for PC and PET, and with the decrease of etching temperature for CN and CA. Similar temperature dependences were obtained for the ratio of track– and bulk-etching rates (i.e. for registration sensitivity) if the plastic sheets were irradiated with heavier nuclear particles. Our results seem to confirm the assumption that the delta electrons may play an important role in the formation of primary nuclear particle tracks in plastic track detectors.     

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.     

A comparative study of gamma radiation effects on track properties of different PADC detectors

Track properties of gamma-irradiated polyallyldiglycol carbonate (PADC) detectors (Homalite, Pershore and Trastrack) are studied in the dose range of 10¹–10⁶ Gy. Results show that the bulk and track etch rates of all three types of PADC detector increase at doses higher than 10⁴ Gy. This increase is more pronounced in post-gamma exposed detectors. The change in etch rate ratio (S) for post-gamma exposed PADC–Homalite detectors is more significant when compared with the other detectors. The critical angle for etching shows that etching at a low temperature of 60 °C is more effective, and the critical angle value is decreased to a significant extent for all three types of PADC detectors. Etching efficiency of post-gamma exposed samples is found to be much higher than the pre-gamma exposed sample at the highest dose of 10⁶ Gy. When compared, it was found that of the three PADC detectors, PADC–Homalite detectors seem to be more sensitive to gamma irradiation.     

Discovery of new etchants for CR-39 detector

After reporting the excellent etching properties of molten Ba(OH)2 8H2O as an etchant, we now report some more new and efficient etchants for CR-39 detector. CR-39 detectors were irradiated with fission fragments and alpha particles with a thin 252Cf source. The irradiated detectors were etched in a number of our newly introduced etching solutions as well as in conventionally used 6 M NaOH at 70 degrees C. The newly prepared etching solutions included NaOH/ethanol and NaOH/1-propanol. Processing conditions were optimized for these etchants. From fission and alpha track diameters, bulk etching velocity (VB), track etching velocity (VT), etching efficiency (eta) and their activation energies were determined and compared with that obtained for 6M NaOH at 70 degrees C.     

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.     

Proton dose-dependent modification in track etching response in some polymers

In the present work, the effect of four different doses of 62 MeV protons, on the fission fragment track etching characteristics of two polymers, viz. polycarbonate (Makrofol-N (MFN)) and polyimide (PI) are studied by nuclear track technique. The bulk etch-rate of PI increased by around 30% at the highest proton dose, whereas the activation energy of etching remained almost constant for the same. A considerable increase in the bulk etch-rate of MFN was observed (75%) at the highest proton dose. The activation energy of etching of the fission fragment tracks in MFN was also found to be an inverse function of dose.     

Submicroscopic analysis of tracks in mica

Tracks of 49 MeV ³⁵Cl ions in muscovite mica were analyzed by transmission electron microscopy up to 10 Å resolution applying the track replica method. The enlargement of the track diameter is measured as a function of etching time. The track shape evolution shows circular tracks for very short etching times. By increasing the etching time the tracks gradually become ovals. For longer etching times, tracks show the characteristic rhombic shape of the bulk region. A non monotonous decrease of the track velocity vs. radius is reported. Formation of a low velocity shell in the intermediate region between the central amorphous zone and the non-perturbed crystalline lattice is suggested.     

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.     

Impact of electron irradiation on particle track etching response in polyallyl diglycol carbonate (PADC)

In the present work, attempts have been made to investigate the modification in particle track etching response of polyallyl diglycol carbonate (PADC) due to impact of 2 MeV electrons. PADC samples pre-irradiated to 1, 10, 20, 40, 60, 80 and 100 Mrad doses of 2 MeV electrons were further exposed to 140 MeV²⁸ Si beam and dose-dependent track registration properties of PADC have been studied. Etch-rate values of the PADC irradiated to 100 Mrad dose electron was found to increase by nearly 4 times that of pristine PADC. The electron irradiation has promoted chain scissioning in PADC, thereby converting the polymer into an easily etchable polymer. Moreover, the etching response and the detection efficiency were found to improve by electron irradiation. Scanning electron microscopy of etched samples further revealed the surface damage in these irradiated PADCs.     

Etchability of Latent Fission Fragment Tracks in CR-39

We report the chemical etching behaviour of the CR-39 polymer detector exposed to fission fragments of 252Cf describing etchability of latent tracks, which are like nanocylinders. The fission fragment exposed detectors were etched in 1-7 N NaOH water solutions at temperatures 50-80℃ for 45 min in the case of track length and 180 min in the case of track diameter measurements. The reduced etch rate S （called here etchability） is determined using experimental results for all etching conditions and the etching conditions with the highest reduced etch are obtained. Physics and energetics of bulk and track etching are discussed. Possible effects causing spurious changes in determination of activation energy of etching are investigated.     

Annealing of heavy ion tracks in cellulose nitrate plastic track detectors

The effect of heat treatment on the latent tracks in cellulose nitrate plastic track detectors has been studied. The bulk etch rate increases with annealing temperature while the track diameters of different ions in cellulose nitrate decrease with increase in annealing time and temperature. Experimental results show that for heavier ions higher temperatures are needed for their complete erasure. The track length and track etch rate are decreased by the application of heat. Experiments reveal that annealing reduces track density. The vertical tracks are more stable than the oblique tracks and require higher temperature for their complete erasure.