Radiation Measurements 50 (2013) 71e73
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Fast neutron dosimetry using CR-39 track detectors with polyethylene as radiator F. Castillo a, G. Espinosa b, J.I. Golzarri b, D. Osorio c, J. Rangel a, P.G. Reyes c, J.J.E. Herrera a, * a
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Apartado Postal 70-543, 04510 México D. F., Mexico Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México D. F., Mexico c Facultad de Ciencias, Universidad Autónoma del Estado de México, Instituto Literario # 100, 50000 Col. Centro Toluca, Estado de México, Mexico b
h i g h l i g h t s < Optimum etching time for fast neutron irradiated CR-39 track detectors is found. < Relationship between dose and fluence obtained as a function of the track density. < Results are consistent with those reported elsewhere, and extend the dose range.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2011 Received in revised form 11 September 2012 Accepted 13 September 2012
The chemical etching parameters (etching time, temperature, normality of etchant, etc.) for the use of CR-39 (allyl diglycol carbonate e LantrackÒ) as a fast neutron dosimeter have been optimized. The CR-39 chips, placed under a 1.5 mm polyethylene radiator, were exposed for calibration to an 241Am-Be source at different time intervals for a given neutron fluence. After several chemical etching processes of the detectors with different conditions, the optimum characteristics for the chemical etching were found at 6N KOH solution, 60 1 C, for 12 h. An accurate relationship between the dose and fluence calculations was obtained as a function of the track density. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Nuclear tracks CR-39 Neutron dosimetry
1. Introduction Nuclear Track Detectors (NTDs) are agents with high potential for novel applications in radiation research and radiological protection dosimetry in many domains. They are able to register charged particles by the radiation induced damage caused along their interaction path. The damaged regions produced by radiation on the material are developed and amplified for visualization with an optical microscope using a well reported technique known as chemical etching. It has been observed that the zone of structural damage may be increased to 104 e 102 cm by etching with suitable chemical reagent. The versatility of NTDs in fast neutron detection and dosimetry has been well known for decades, since they have attractive characteristics compared to other detectors, such as non-fading of tracks, insensitivity to gamma, UV and X-rays etc. (Hashemi-Nezhad et al., 2002; Hermsdorf et al., 1999; Luszik-Bhadra et al., 1999). In the early days, optical and etching characteristics of polymeric detectors were mainly studied using heavy ions, neutrons
* Corresponding author. Tel.: þ52 5556224672. E-mail address:
[email protected] (J.J.E. Herrera). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.09.007
and gamma irradiation. However, limited studies have been reported on etching and optical properties for fast neutrons (Kalsi and Agarwal, 2008; Zaki, 2008; Singh and Prasher, 2004a, 2004b; Sharma et al., 2007; Khan et al., 2005; Tanner et al., 2005; Yamauchi et al., 2005; Chong et al., 1997; Gopalani et al., 1997; Marletta, 1990). Since neutrons do not directly cause any ionization in the detector, no tracks are produced by them. However, the interesting part lies in the secondary effects due to recoiling nuclei of the detector under neutron impact, leading to the production of charged particles that cause ionization, and consequently, etchable tracks. These tracks are the product of molecular chain breaking, and are related to cross-linking and free radicals (Marletta, 1990; Calcagno et al., 1992; Abel et al., 1995; Klaumiinzer et al., 1996; Nouh et al., 2003). The most probable way through which the tracks are produced, is through the recoil of hydrogen, carbon, and oxygen nuclei, and charged particles from nuclear reactions of the type (n, a) (Hasib, 1975). The registration “threshold” of the detecting material also has to be considered in order to establish the energy range of the neutrons to be recorded. For example, the energy range in which proton tracks can be observed in polycarbonate, is quite limited (Fleischer et al., 1975; Pyu and Fink, 2004). A better performance in the dosimetry using solid nuclear track detectors
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F. Castillo et al. / Radiation Measurements 50 (2013) 71e73
Fig. 1. Evolution of track density as etching time is increased, for a polyethylene CR-39 chip exposed to the 241Am-Be neutron source during 12 h.
can be obtained by adding an extra foil or radiator because secondary charged particles can also originate there. In this work we prove that assemblies of nuclear track detectors (CR-39) with polyethylene radiators, under optimized working conditions, concerning chemical etching and counting techniques, can be used as fast neutron dosimeters. The optimization for a specific purpose depends mainly on the standardization of chemical etching parameters such as etching time, temperature, and normality of etchant, with respect to the rate of energy loss of the track by radiation. Optimized etching conditions have been found in a previous work to be 6N KOH solution at 60 1 C (Espinosa et al., 1995; Tommasino et al., 1984). It was found in the present study the an etching time of 12 h is the most appropriate for the development of neutron induced protons, and this combination of etchant, temperature and etching time is used throughout the work. Hence, the effects of neutron radiation on track density and optical characteristics of CR-39 at optimized etching conditions have been investigated. 2. Experimental arrangement The CR-39 films with 500 mm thickness, and w1.3 g/cm3 density, used for this study, were commercially procured by LantrackÒ. All samples were irradiated at room temperature at the Instituto de
Fig. 2. Track density as a function of with irradiation times (hours.). The CR-39 samples were covered with 1.5 mm polyethylene plates as radiators.
Fig. 3. Relationship between the track density and the estimated amount of neutrons that reach the dosimeter, obtained from source flux, the size of the dosimeters, and the exposure time.
Ciencias Nucleares, UNAM, with an 241Am-Be water shielded fast neutron source. The CR-39 chips were exposed at different time lengths within the experimental channel, 20 mm away from the source. The neutrons from this kind of sources span a wide energy spectrum in the range between thermal and 11 MeV, with broad peaks at 3 and 5 MeV (Geiger, 1970). The neutron emission rate in our source is 2 106 neutrons s1. The samples were covered with a polyethylene radiator 1.5 mm thick, in order to produce (n, p) reactions. Since it has been previously established that CR-39 detectors respond well to protons in the range from 0.92 to 9.28 MeV (Sinenian et al., 2011), we assess that the region of the source spectrum is being properly detected by the chips. Considering the chips’ size is 5 mm 6 mm, therefore covering a 30 mm2 area, at 200 mm from the source, the neutron flux they are exposed to is 1.19 102 neutrons s1. The use of a radiator in contact with the CR-39 chip can enhance its response because of the proton recoils generated within it. As mentioned earlier, the samples were etched in a 6N KOH solution contained in a beaker at a 60 1 C temperature, with the constant stirring of a magnet inside it, in order to keep the thermal homogeneity of the solution. For a chip
Fig. 4. Track density as function of nominal dose for the
241
AmeBe source.
F. Castillo et al. / Radiation Measurements 50 (2013) 71e73
irradiated during 12 h, it is found that the track density increases with the etching time, which becomes optimum at about 12 h. While the density does not increase significantly for longer etching times, as shown in Fig. 1, the pits tend to deteriorate. This etching time was therefore established for the rest of the work. After every etching, the detectors were washed with deionized water and dried in dry air. The tracks were analyzed and counted in 10 10 reticules, covering the 30 mm2 using a Carl Zeiss optical microscope with a 200 magnification. Tracks were counted manually in different fields around the centre of the detector, covering the most exposed area. Unirradiated foils were etched simultaneously with irradiated foils, for background determination. The measurements were performed by a single operator. 3. Results and discussion The samples were irradiated at different time intervals, in order to test for various fluences (yield time/4p r2, where r is the distance between the neutron source and the irradiated chip.) The mean values of track density given as a function of exposition time, are shown in Fig. 2; the error bars correspond to standard deviations from the readings in the 10 10 reticule. Apart from this, we recorded the calibration curve to check the sensitivity of the detector for neutron detection. The curve between neutron fluence and track density after 12 h of etching is shown in Fig. 3. This plays an important role in determining the equivalent dose in the field of neutron dosimetry (Kumar et al., 2010) Such type of studies may be important for the future use of CR-39 as dosimeters in particle accelerators and Radioactive Ion Beam Facilities. Fig. 4 shows the dose as a function of track density, once the background is subtracted. A calibration was previously made with an IAEA (1993) certified 241Am-Be at the Oak Ridge National Laboratory (ORNL, 1985). These results are consistent with those reported in Saint Martin et al. (2011), and extend the dose range, indicating the possibility of using CR-39 for the estimation of neutron doses or fluence. 4. Conclusions Our preliminary study shows that it is possible in the traditional method of neutron dosimetry to correlate the total number of tracks per unit area with the dose and neutron fluence. Furthermore, our experimental results with varying neutron fluence will contribute to have an exact picture of fast neutron dosimetry. The experimental results suggest the potentiality of using CR-39 detectors as fast neutron dosimeters in future high energy accelerators and Radioactive Ion Beam Facilities. The use of a system of radiator-CR-39 as a nuclear track detector makes it possible to obtain a permanent registration of neutron dose, which has legal importance. It must be noticed that the detection efficiency is good, and is adequate for radiological protection applications. Acknowledgments This work was partially supported by DGAPA-UNAM grants IN120409-3, IN101910 and IN103013.
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