Depth-dependence of the bulk etch rate of gamma-ray irradiated CR-39 track detector

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Abstract

The depth-dependent bulk etch rate has been examined for the gamma-irradiated CR-39 at doses ranging from 20 to 100kGy. The thickness of the damaged region in gamma-irradiated CR-39 plastics, in which the bulk etch rate was significantly enhanced, was found to be limited in the thin layer near the surface and decreases with increasing the dose-rate, while it barely depend on the total dose. This indicates that it is possible to apply CR-39 plastics as high dose gamma-dosimeter by assessing both the bulk etch rate in the damaged region and its thickness in principle.

Introduction

In our last study, we found a bottle-like structure in etched tracks of fission fragments in gamma-irradiated CR-39 track detectors (Yamauchi et al., 2001). In this structure, two cylindrical parts of etched track with different radius are joined along the ion path, as shown in Fig. 1. The etched track has a larger opening on the surface and has relatively small radius around the pit-tip in the deep layer. We named the larger part in radius as Body and the smaller part as Neck by analogy with a wine bottle. Such unique structure could be produced when the damage, in which the bulk etch rate was significantly enhanced, was limited near the detector surface. Oxygen supplied from the air was inferred to take an important role to form the damage. On the other hand, the dissolved oxygen would be exhausted in the deeper layer and the damage formation should be suppressed, in which the Neck formed (Yamauchi et al., 1999).

Our previous studies have been carried out in terms of the changes in the bulk etch rate of the damaged region, where the Body was formed, near the surface after irradiation (Oda et al., 1997; Yamauchi et al., 1999). It has been confirmed that the bulk etch rate increased exponentially with absorbed dose, D, in the region under certain conditions of a constant dose-rate, R, and the surrounding atmosphere (O'Sullivan et al., 1982; Charvat and Spurny, 1988; Oda et al., 1997), as follows:Virra=Vbexp(gD),where Virra is the bulk etch rate of irradiated CR-39 plastics, Vb is that of unirradiated one, D is in Gy and g is a constant in Gy−1. The factor g could be regarded to consist of two component; the first is related with the oxygen, gox, and the second is independent with the oxygen, gvac(g=gox+gvac). The latter has a constant values of 9.0×10−6Gy−1 in our etching condition (stirred 6NKOH, 70°C). The farmer was found to be expressed well by a simple model, in which the rate of damage formation was proportional to the concentration of dissolved oxygen as well as the dose-rate. As a result, for the in-air irradiation, we have succeed in giving the experimental equation for the factor g as follows (Yamauchi et al., 1999),g=gvac+48.0×10−6/(R+0.2),where R is in Gy/s. As expressed in , , the bulk etch rate of irradiated CR-39 depends on the absorbed dose as well as the dose-rate. Consequently, it is impossible to use CR-39 plastics as a gamma-ray dosimeter only by measuring its bulk etch rate, because there exists various combinations of total dose and dose-rate for a given bulk etch rate.

In this study, we have investigated the depth-dependence of the bulk etch rate of gamma-ray irradiated CR-39 detector. The thickness of the damaged area was strongly dependent on the dose-rate but hardly on the total absorbed dose. This implies the possibility of using CR-39 as a gamma-ray dosimeter by measuring both the bulk etch rate in the damaged region and its thickness.

Section snippets

Experimentals

The CR-39 plastics, BARYOTRAK (Fukuvi Chemical Ltd., Japan), were used as samples in this study. The excellent feature of the plastics is that it was made from the purified monomer more than 99.9% of diethylene glycol bis(allyl carbonate). The thickness was 950μm. Before gamma-ray irradiations, the samples were bombarded normally by fission fragments from Cf-252 source on the top and on the polished side surfaces. The later irradiation was made for the side view observation. Each set of

Typical views of bottle-like etched track

In Fig. 2, photographs of etched track of fission fragments are shown at various etching times. The total dose and the dose-rate are 80kGy and 3.66Gy/s, respectively. In general, the roughness of the detector surface was enhanced by gamma-ray irradiation. With increasing etching time, the radius of etched track increases as shown in these photos. At longer etching times more than 30min, smaller circles appeared at the center of the etched track. The similar dark part was also observed clearly

Conclusions

In this study, we have assessed the dose and dose-rate dependence on the thickness of the damaged region as well as the bulk etch rate. As far as we have examined, the thickness has little dependence on the total dose. This implies the possibility of applying CR-39 detector to high-dose gamma-ray dosimeter, by evaluating both the bulk etch rate and the thickness of the damaged regain.

Acknowledgements

The authors express heartfelt thanks to Mr. T. Ikeda, Dr. Y. Honda and Prof. S. Tagawa on the help of gamma-ray irradiation.

References (5)

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