Angular response characteristic of pyramid shape fast neutron dosimeter using CR-39 films

CR-39 is a highly sensitive etched track detector for neutron monitoring and dosimetry applications but its dose equivalent response is strongly direction dependent with respect to the incident neutrons. This is considered to be a major drawback for their use. In the present study, an attempt has been made to develop a pyramid shaped dosimeter, which consists of polyethylene material of thickness 1 mm with the provision to hold three CR-39 films at an angle of 35° to each other. The response of CR-39 in this configuration under optimum electrochemical etching at elevated temperature have been found nearly angular independent and therefore the dosimeter can be used for neutron monitoring, i.e. personnel as well as area monitoring.     

A CR-39 based thermal neutron dosimeter

Both chemically and electrochemically etched CR-39 detectors are used for the detection of fast neutrons. In practical situations, fast neutrons are always accompanied by thermal neutrons. Therefore, the response of the CR-39 based dosimeter has to be extended to thermal neutrons. To do so, a radiator/converter like LiF is introduced in front of the CR-39 detector and an optimum thickness of the LiF film is determined so that a response similar to that of the fast neutrons is achieved. Thin films of natural LiF were prepared and the response of the CR-39 detector was studied as a function of the film thickness.     

Electrochemical etching technique for neutron dosimetry

In the present work we have employed allyl diglycol carbonate (CR-39) and cellulose triacetate (CTA) plastic for detection of neutron recoil tracks without radiator. For CR-39, the results reveal that registration efficiency is a function of duration of chemical pre-etching and the best results are obtained with chemical pre-etching of 3 hours. It was also investigated that the ac field strength of 28 kV/cm having 2.5 kHz frequency was optimum for revelation of tracks. Interestingly the sensitivity is fluence dependent and it was constant up to a fluence of about 10⁸ n/cm². The sensitivity abruptly decreased with increased fluence. At optimum experimental conditions the minimum detection limit for CR-39 was found to be 0.47 mSv. For CTA, the tracks have been revealed by electrochemical etching (ECE) only and the minimum detection limit was found to be 0.85 mSv at optimum experimental parameters.      

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.     

Development of a reader for track etch detectors based on a commercially available slide scanner

NRPB has operated a routine neutron personal dosimetry service based on the electrochemical etch of PADC elements since 1986. Since its inception it has used an automated reader based on a video camera and real time analysis. A new and more powerful replacement system has been developed using a commercially available photographic slide scanner. This permits a complete image of the dosemeter to be grabbed in a single scan, generating a 2592x3888 pixel file which is saved for subsequent analysis. This gives an effective pixel size of 10x10 μm with an image of the entire dosemeter in one field of view. Custom written software subsequently analyses the image to assess the number of etched pits on the dosemeter and read the detector identification number (code). Batch scanning of up to 40 detectors is also possible using an autofeed attachment. The system can be used for electrochemically etched tracks for neutron detectors and chemically etched tracks for radon detectors.     

Counting recoil proton tracks on PADC without a pre-etching step. A novel approach for neutron dosimeter application

At many laboratories involving in routine, individual neutron dosimetry, poly allyl diglycol carbonate (PADC) is utilized for particle registration. A pre-etching step is commonly applied in order to remove the alpha tracks of the environmental radon, so achieving the required lower limit of detection (LLD) performance. A novel approach is presented, which makes this pre-etching step unnecessary, but ensuring an excellent LLD performance, also providing a good throughput for a routine service. Exposing the PADC material to carbon dioxide atmosphere before the main etching step enhances the visibility of the microscopic track image. The enhancement is so efficient that the track size and image contrast between the alpha and the proton tracks are well distinctive. Customized image analyzer software is able to distinguish alpha and proton tracks, so providing the proton track density separately. A pilot study about the performance and potentials of this novel approach is presented. A complete application working with this approach was introduced to routine neutron dosimeter service 2 years ago and its good performance has been verified.     

Optimization of CR-39 for fast neutron dosimetry applications

We present the results of an experimental work aimed at improving the performances of the CR-39^® (Registered Trademark of PPG Industries Inc.) nuclear track detector for neutron dosimetry applications. The work was done in collaboration with the Intercast Europe S.p.A., producer of CR-39 for commercial and scientific applications. We compare the CR-39 made with different additives concentrations and different polymerization processes. We evaluate the response of the CR-39 to fast neutrons from three sources: ²⁴¹Am---Be, ²⁵²Cf and ²³⁸Pu---Li. Particular attention was paid to background fluctuations that limit the lower detectable dose.     

Fast neutron leakage in 18 MeV medical electron accelerator

In this work the neutron fluence of the Varian Clinac 2100C Medical Accelerator has been evaluated using CR39 track dosimeter. The assessment of fast neutron dose to a patient for typical treatment of 200 cGy with an 18 MV photons beam is performed at surface-source distance of 100 cm with a field size of 20×20 cm². Fast neutron leakage around of the accelerator head is evaluated.     

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.     

Improvement of neutron detection efficiency with high sensitive CR-39 track detector

Two subjects have been studied for establishing an efficient detection technique applicable to neutrons with an energy higher than 20 MeV, for which personnel dosimetry becomes more important especially in space activities and radiation protection around high-energy accelerators.

A performance of a new sensitive detector, recently developed copolymer of CR-39 and N-isopropylacrylamide, was checked in a D–T neutron field. It was confirmed experimentally that its sensitivity was several times as high as a pure CR-39 in the radiator effect and more than twice in the total efficiency. In the other experiment, CR-39 samples were exposed to 65 MeV quasi-monoenergetic neutrons, and the registration characteristics have been investigated from a fundamental point of view. The particles recorded were divided into three groups of proton relatives, -particles and heavy ions by applying the growth curve method and the multi-step etching technique. The measured fractions were in good agreement with the values calculated by SSNERS code previously developed.     

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.      

Operational and dosimetric characteristics of etched-track neutron detectors in routine neutron radiation protection dosimetry

There are a number of etched-track neutron dosimetry systems in routine use for personal monitoring. In this paper, the operational and dosimetric characteristics of these systems are summarized. Brief details are given of the dosemeter design, the material used, its quality control procedures, background, processing and read methods, neutron energy range, energy and angle dependence of response, decision threshold, linearity, signal storage stability, calibration methods including normalization, effect of influence quantities, and the advantages and disadvantages of the systems in routine application.     

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.     

Study of neutron generation from relativistic heavy ion interactions

Study has been carried out on neutron generation in the interactions of 44 and 18 GeV ¹²C with Cu-and Pb-targets. The properties studied include, (1) Relative yields of secondary neutrons generated in 44 GeV versus in 18 GeV ¹²C bombardment to Cu target and also to Pb target. The ratios of these are 2.12±0.19 and 2.04±0.15, respectively, which is nearly in consistence with but somewhat larger than the values of theoretical calculations for Cu target. (2) Relative yields of secondary neutrons produced in Pb target versus in Cu target. The ratios obtained are 2.54±0.20 and 2.56±0.20 for 44 GeV and 18 GeV, respectively. More neutrons are generated in Pb target than in Cu target.     

Study of nuclear tracks on different polycarbonate foils and their feasibility for use in personnel monitoring

CR-39 (PADC) a Solid State Nuclear Track Detector., is used as a routine personnel monitor. 1800 workers are being monitored quarterly for fast neutron using CR-39 foils. These foils procured from Pershore Mouldings, UK are very expensive and indigenous development will make the foils cost effective. The aim of this paper is to find a suitable alternative to the imported CR-39 foils for use in personnel monitoring. The foils from three different manufacturers have been compared with the CR-39 foils from Pershore Moulding, UK, presently in use. Out of the three, only sample no 1 is promising. It has a background and sensitivity comparable with CR-39 presently used. The sample 2 is CR-39 being developed in India, has a relatively high background and poor sensitivity. Efforts are being made to improve the quality of this sample. The sample 3 was a poly carbonate from local manufacturer which produced very few tracks and the standard deviation of track counts was very large and hence not useful for Personnel Monitoring.      

Use of chemical etching of CR-39 foils at elevated temperature for fast neutron personnel monitoring in India

CR-39 Solid State Nuclear Track Detecting foils (SSNTD), along with 1 mm thick polyethylene radiator, sealed in triple laminated pouches, are used for country wide Fast Neutron Personnel Monitoring in India. With the present system of processing by elevated temperature electrochemical etching (ETECE) and evaluation using automatic image analysis, only 16 foils are processed at a time and it is useful over the dose equivalent range 0.2 mSv to 10 mSv. It has been reported that, by processing CR-39 of good detection efficiency by chemical etching at elevated temperature, more numbers of foils can be processed simultaneously. In the present study, CR-39 foils from Pershore Moulding (UK) have been chemically etched using 7 N KOH under various conditions of temperature and etching durations and evaluated using high magnification microscope. The duration of chemical etching, has been optimized at a constant temperature of 60°C for chemical etching process. The characteristics of the chemically etched CR-39 foils are compared with the characteristics of the CR-39 foils processed by the existing system of ETECE and the detailed results are presented in the full text of the paper. It has been observed that by chemical etching process, the dose equivalent range of CR-39 foils can be extended above 60 mSv.      

Comparison of SEM and optical analyses of DT neutron tracks in CR-39 detectors

A solid state nuclear track detector, CR-39, was exposed to DT neutrons. After etching, the resultant tracks were analyzed using both an optical microscope and a scanning electron microscope (SEM). In this communication, both methods of analyzing DT neutron tracks are discussed.     

用于n,γ混合场的新型脉冲中子探测器研究

新型脉冲中子探测器采用特殊工艺将两个PIN半导体组合而成.利用脉冲γ辐射研究了探测器对γ的响应；利用脉冲中子源研究了探测器对DT中了的响应，并与闪烁探测器进行了比较 .结果表明：脉冲中子探测器对脉冲γ辐射基本不灵敏，对脉冲中子辐射的灵敏程度依赖于中子辐射体，是一种用于n，γ混合脉冲辐射场中子测量的新型探测器. 关键词： 硅半导体 差分补偿 脉冲中子探测器 n γ混合场     

Application of CR-39 detectors for study of corpuscular emission from Prague capillary pinch

The paper describes experimental studies of the corpuscular emission from fast Z-pinch discharges, which were performed within the CAPEX capillary pinch facility in Prague, Czech Republic. Time-integrated measurements were carried out with an ion pinhole camera equipped with CR-39 nuclear track detectors. Irradiated detectors were etched at the standard conditions, and developed tracks were analysed with an optical microscope coupled with a CCD camera. It was found that the fast capillary discharges emit strong corpuscular streams. On the basis of the optical analysis of the etched detectors it was estimated that the time-integrated particle flux, at the distance of 20 cm from the collimator outlet, was about 3.5×10⁷ particles/cm²/pulse. Particular attention was paid to analysis of ion-track photos, which were obtained by means of an electron microscope. Details of the ion micro-craters have been observed in the photos of fractures of the CR-39 detectors after their 7 h etching.     

用于脉冲中子探测器的小型化高压模块性能研究

高压电源的性能特性对闪烁中子探测器的性能有着十分重要的影响。为克服传统高压供电设备体积大、便携性差的问题,在便携式脉冲中子探测器研制中拟采用内部集成小型化高压模块为光电倍增管提供高压的技术路线。根据实验结果,选用的高压模块最高输出高压可达2.5 kV并且具有良好的输出一致性,可适应16~36 V的工作电压,连续工作稳定性及温度稳定性优良,可以满足脉冲中子探测器研制的要求。