Radiator effect on plastic nuclear track detectors for high-energy neutrons

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Radiation Measurements 36 (2003) 119 – 124 www.elsevier.com/locate/radmeas

Radiator eect on plastic nuclear track detectors for high-energy neutrons K. Odaa;∗ , Y. Imasakaa , K. Tsukaharaa , T. Yamauchia , Y. Nakaneb , Y. Yamaguchib a Kobe

University of Mercantile Marine, Fukae-minamimachi, Higashinada-ku, Kobe 658-0022, Japan b Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan Received 21 October 2002; accepted 22 April 2003

Abstract The eect of radiators supplying charged particles to plastic nuclear track detectors has been investigated both experimentally and theoretically in order to apply them to personal dosimeters for high-energy neutrons. Performance of four types of radiator materials, CH2 , CD2 (deuterized hydrocarbon), LiF (lithium 8uoride) and C (graphite), was checked in a quasi-monoenergetic neutron :eld generated by p-Li reactions. The e;ciency has been numerically calculated based on a model with a special attention to the angular dependence of cross sections and data of characteristic response to light ions. The eect of respective radiator candidates has been evaluated as a function of the neutron energy. A two-layer radiator has also been proposed to adjust the energy dependence of the relative sensitivity to conversion factor for dose-equivalent. c 2003 Elsevier Ltd. All rights reserved.  Keywords: Plastic track detector; High-energy neutron; Radiator; Deuterized hydrocarbon

1. Introduction The risk associated with cosmic radiation to aircraft crew and frequent 8yers is a matter of concern, since International Commission of Radiological Protection (ICRP) included exposure to cosmic radiation as occupational in Publication 60 (ICRP, 1991). It is well known that the intensity of such radiation increases with altitude, and its monitoring is very important especially in space activities. A similar problem is often encountered in radiation protection around high-energy particle accelerators, in fruitful applications to cancer therapy, material sciences, life science, and so on. A plastic nuclear track detector (PNTD) is one of most promising elements for passive-type personal dosimeter for high-LET particles and energetic neutrons. Most types of neutron detectors, including applications of PNTDs (Spurny, 1995; Tanner et al., 2001) are designed to measure neutrons with energies below 20 MeV, and therefore unsuitable for such high-energy neutrons as are produced by primary ∗

Corresponding author. Tel./fax: +81-78-431-6304. E-mail address: [email protected] (K. Oda).

cosmic-rays or accelerated protons. There are few papers that deal with high-energy neutrons because of a lack of a mono-energetic neutron source and details of neutron cross sections (Nakane and Sakamoto, 2001). These situations motivated us to start a new project to develop personal dosimeter for high-energy neutrons up to 100 MeV. In the previous experiments, we tried to check the performance of new detector, “TNF-1” of a co-polymer of CR-39/ NIPAAm (N-isopropylacrylamide) developed by Ogura et al. (1997, 2001), together with “TD-1” of a CR-39 containing a small amount of antioxidant and “BARYOTRAK” of a pure CR-39, made by Fukuvi Chemical Co., Ltd., Japan. The total e;ciency for detection of 65-MeV neutrons with TNF-1 and TD-1 was found to be a few times as high as that with BARYOTRAK (Oda et al., 2001, 2002). Furthermore, the species of particles registered were identi:ed by the growth-curve method developed by the present author (Oda et al., 1988). It was pointed out that the fraction of protons recorded in BARYOTRAK was only 30%, whereas it amounted to about 70% for TNF-1, and that these results would be explained by the dierence in sensitivity to energetic protons (Oda et al., 2002).

c 2003 Elsevier Ltd. All rights reserved. 1350-4487/03/$ - see front matter
doi:10.1016/S1350-4487(03)00107-0

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K. Oda et al. / Radiation Measurements 36 (2003) 119 – 124

We have proceeded the study on high-energy neutron dosimetry to the next step in this report. In general, total detection e;ciency is divided into two, intrinsic e;ciency and enhancement by radiator, according to the place where secondary charged particles are produced. It is known that the former becomes small compared with the latter for neutrons higher than several hundreds of keV (Oda et al., 1991; Hermsdorf et al., 1999), which implies that overall e;ciency will depend largely on the selection of radiator material. In this paper, our concern is directed to the radiator eect, and its e;ciency is evaluated both experimentally and theoretically. A possible technique is also discussed to adjust the sensitivity to a conversion factor from neutron 8uence into dose equivalent response. 2. Experimentals 2.1. Irradiation of 40 MeV neutrons A neutron irradiation experiment was carried out at TIARA (Takasaki Ion Accelerator for Advanced Radiation Application), Japan Atomic Energy Research Institute. An accelerated proton beam bombarded a lithium target, where quasi-monoenergetic neutrons with an energy of 40 MeV were generated by 7 Li(p; n) reactions. The neutrons were guided to an experimental room through a 220 cm thick iron collimator embedded in a shielding wall, and hit three types of PNTDs, BARYOTRAK, TD-1 and TNF-1. The neutron spectrum has already been evaluated by Baba et al. (1999), and recognized to have a lower energy component that amounts to about 40%. Although the following experimental results are not exact responses to monochromatic

40-MeV neutrons, they are useful to estimate the eect of radiators for high-energy neutrons. As reported in our previous paper (Oda et al., 2002), the sensitivity of BARYOTRAK to protons is much lower than that of latter two PNTDs, and there remains a problem in transparency after etching in TNF-1. The purpose of this study is focused on the radiator eect. So, we checked the performance of four types of materials as radiator candidates for high-energy neutrons. A CH2 material like polyethylene is the most popular radiator for fast neutron measurement, which supplies proton recoils to the detectors. As the neutron energy becomes higher, it is more di;cult for PNTDs to catch proton recoils. The kinematics and general registration characteristics of PNTD suggest another candidate of massive particle, deuteron. Among several deuterized hydrocarbons, we found dotriacontane (C32 D66 , denoted CD2 later for simplicity), which is a kind of para;n. Its melting point is 69:6◦ C, and is therefore easily reformed into a sheet from the powder originally supplied. The thickness of CD2 radiator was of the order of 100 mg=cm2 , because the small amount used is very expensive. In addition, 6 LiF tile, which was produced as a special shield for thermal neutrons without capture gamma production, and a graphite sheet, were also obtainable. These radiators were attached to a sheet of PNTD as shown in Fig. 1. Fission fragments from a 252 Cf source were irradiated on each PNTD sample in order to estimate the amount of bulk etch. After neutron irradiation experiments, PNTDs were chemically etched in a stirred NaOH solution at 70◦ C. The normality was 5 M for TNF-1 and 7 M for both TD-1 and BARYOTRAK, where the bulk etch rates were 1.15, 1.73 and 1:61
m=h, respectively.

Fig. 1. Four dierent types of radiator for PNTD and corresponding microphotographs of TD-1 surface.

K. Oda et al. / Radiation Measurements 36 (2003) 119 – 124

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100 mg=cm2 . The energy of proton recoils is too high, i.e. the etch rate ratio is too small, to be recorded by PNTD. On the other hand, deuteron recoils have a moderate energy and are registered more easily, in spite of a relatively small cross section for D(n; n) elastic scattering. It is also found in Table 1 that other radiators, LiF and C, have very little contribution although their recoil particles have much larger etch rate ratios.

3. Theoretical consideration of radiator eects 3.1. Calculation model Fig. 2. Dependence of etch-pit density on the thickness of layer removed.

Table 1 Summary of relative etch-pit density in three type PNTDs with dierent types of radiator Radiator

BARYOTRAK

TD-1

TNF-1

Absolute

CH2

4.9a

11:4a

12:9a

Relative

CH2 CD2 LiF C

1.00 1.07 0.74 0.77

1.00 1.12 0.76 0.48

1.00 1.14 0.74 0.39

a Etch-pit

density in pits=cm2 .

2.2. Etch-pit density In Fig. 1 are also shown four microphotographs at each radiator region after 15±0:2
m etching. The etch-pits were counted semi-automatically under a size condition of 5 ± 0:5
m or larger in diameter of equivalent circle. Fig. 2 shows the dependence of the number of counted etch-pits upon the thickness of layer removed for TD-1 detector. If more charged particles are generated inside the detector than outside, the number of tracks will increase with the bulk etch. On the other hand, the etch-pit density becomes constant for longer etching duration when all tracks of charged particles coming from radiator grows large enough to be recognized as etch-pits. It is expected from increase pattern in Fig. 2 that the radiator eect is dominant for CD2 , and graphite radiator contributes. A comparison among the three PNTDs is made in Table 1. The absolute values of etch-pit density at a bulk etch of 15
m are in the following order: TNF-1, TD-1 and BARYOTRAK, which is the same as that reported previously (Oda et al., 2002). The density for dierent radiator materials is normalized by that for CH2 , and it is easily found that CD2 radiator is most eective at a thickness of the order of

The detection e;ciency for a radiator, r , is de:ned here as the number of charged particles per incident neutron, which are generated in the radiator, reach PNTD and can be observed as etch-pits. The number of charged particles generated by a neutron, , is expressed in the following formula of double integral with respect to the solid angle,  of charged particle generation and the depth, z, in the radiator:

d =N d d z; (1) d where N is the density of atoms interacting with neutrons and is the cross section. In order to calculate the e;ciency, r , the integral ranges in Eq. (1) should be limited by two major conditions characteristic of chemically etched PNTDs. One is the well-known condition with respect to the critical angle, which is expressed by the ratio of the bulk etching rate, Vb , to the track etching rate, Vt , as follows: cos  ¿ cos c = Vb =Vt :

(2)

For example, an inclined latent track A illustrated in Fig. 3 is unetchable. The other condition is related with etch-pit identi:cation under a microscope. Experimentally, it is dif:cult to discriminate very small or shallow etch-pits from roughness on a post-etched surface. So, we set up a certain criterion for the diameter of equivalent circle, d, in the following calculations; d ¿ dc ;

(3)

which re8ects on a reduction of the range of z-integral. For example, latent track B of very short-range and D of extremely low etch-rate ratio are both uncountable in a measurement system. 3.2. Procedure of numerical calculations High-energy neutrons interact with atoms constituting the radiators used in this experiment, H, D, Li, C and F, in several ways such as elastic and inelastic scattering, (n,p), (n,t),

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K. Oda et al. / Radiation Measurements 36 (2003) 119 – 124 post-etched surface

Radiator

PNTD A

B Neutrons

C D

Fig. 3. Latent tracks in PNTD exposed to energetic neutrons (solid lines) and post-etched surface (dotted line).

(n; ) reactions, and so on. Among them, particles recoiled through elastic collisions are considered to be most eective as radiator owing to their ranges. Thus, recoil proton, deuteron, 6 Li and 12 C ions are treated in the calculation, as a :rst step. The elastic collision with 19 F atoms is expected to have a much smaller eect. A contribution of other reactions will be discussed in another opportunity. The cross section data have been evaluated well below 20 MeV, but there are few libraries which systematically contain data for higher energies. In this calculation, we referred to the nuclear data :les, ENDF/B-VI (Rose, 1991) and their newest version, which have neutron cross section data up to 150 MeV for H, D and C. Since dierential cross sections are usually given in the :les as a function of neutron scattering angle in the center of mass system, (d =dn )c , they are converted into those with respect to the angle of recoil particles in laboratory system by the following relations: (cos n )c = 1 − 2 cos2 ; and d = d



d dNn

(4)

Fig. 4. Calculated etch-pit diameter as a function of particle energy. Lower and upper energies are determined with which latent tracked are countable.

Fig. 5. Dependence of enhanced e;ciency by the radiator on the thickness of CH2 and CD2 materials. A broken line represents the e;ciency for two layers consisting of CD2 with a constant thickness of 0:8 g=cm2 and CH2 with an arbitrary thickness.

 × 4 cos :

(5)

c

Next, we consider the integral with respect to the depth for a given recoil angle. The condition of Eq. (3) suggests that there exist lower and upper limits in the energy of recoil particles incident on PNTD. Fig. 4 demonstrates these limits, where the etch-pit diameter after 15
m etching, calculated by using fundamental data for the etch rate ratios for light ions (Yamauchi et al., 1999, 2001) and the particle range evaluated by STOPOW code (Henniger et al., 1988), was plotted against the particle energy when cos  = 0:9. Introducing the range of particles, RL and RU , corresponding to above energy limits and the maximum range, R0 of recoil particle, then z-integral limit, zmax is the smaller one of (R0 cos  − RL cos ) and the thickness of an actual radiator, T , and zmin is determined similarly. Hence, Eq. (1) for

radiator eect becomes
d ∗ d(cos ): r = 2 (zmax − zmin ) d

(6)

zmax = Min(R0 cos  − RL cos ; T ); zmin = Min(R0 cos  − RU cos ; T )

(7)

where  is the macroscopic cross section and d =d∗ is normalized angular dierential cross section. 3.3. Results of numerical calculations An example of numerical calculations of Eqs. (6) and (7) is shown in Fig. 5, where the e;ciency for 40 MeV neutrons of normal incidence is plotted as a function of

K. Oda et al. / Radiation Measurements 36 (2003) 119 – 124

Fig. 6. The e;ciency for radiator, ∗ as a function of neutron energy. An uppermost dotted line represents personal dose equivalent given by ICRP74.

the thickness of two types of radiator, CH2 and CD2 . The radiator, in general, becomes more eective with increasing the thickness, and the e;ciency reaches the maximum near the range of recoil particles with highest energy. Because deuteron recoils have lower initial energy, the e;ciency saturates in a thinner region compared with protons. As to maximum value, CD2 radiator has less e;ciency than CH2 contrary to our expectation. In the experiment described in the previous section, the thickness of the radiators were 170 and 190 mg=cm2 . In such a thin region the e;ciency for CD2 radiator is higher than for CH2 according to Fig. 5, which is consistent with the experimental results of Fig. 2. Of course, there is another possible cause of contamination of lower energy neutrons. The saturated value, ∗ , i.e. maximum value under an ideal condition of no attenuation of incident neutrons in the radiator, is shown as a function of the neutron energy for respective radiators in Fig. 6. A CH2 radiator is still most eective for all energies shown in this :gure. It is also con:rmed that carbon ions from graphite and 6 Li ions from LiF radiator have negligibly small contribution of two :gures lower than protons because of low total cross section and a distorted angular dependence. 4. Discussion In order to practically apply PNTDs to personal dosimetry of high-energy neutrons, overall sensitivity to radiation dose and its energy dependence should be carefully con:rmed. In particular, it is important to adjust the detector response to the factor converting from neutron 8uence to dose equivalent. An uppermost line in Fig. 6 represents the personal dose equivalent per unit neutron 8uence tabulated in ICRP74 (ICRP, 1996) where the values for ambient dose equivalent were substituted above 20 MeV because no data for personal dose equivalent have been evaluated yet. There is an argument for using personal dose equivalent, Hp (10)

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as operational quantity for such high-energy neutrons (Ferrari et al., 1997). We should be ready at any time for corresponding to possible change of conversion factors. It is apparent from a comparison between the e;ciencies and the conversion factor that there remain some improvements of the detector response. Especially, a rapid decrease at higher energies is a severe problem for establishing dose-equivalent response. With any type of single radiator, the e;ciency could never exceed that of CH2 . A technique of multi-layer (Oda et al., 1991), however, will make it possible to increase the e;ciency at high energies with the sacri:ce of a decrease at lower energies, and to control the shape of the energy dependence. The thickness dependence for the CH2 radiator shown in Fig. 5 indicates very little contribution below a few hundreds of mg/cm2 , which will be understood more easily by dierentiating the curve. The protons recoiled in the radiator in such a thin region are too energetic to be registered by the PNTD. It can be said that this layer does not work as a radiator but as a degrader for protons generated in a thicker region. If so, the radiator eect should be unchanged by substituting this layer by any other material. Thus, we can propose a two-layer type radiator, which, for example, consists of CD2 of 0:8 g=cm2 and CH2 of 1:7 g=cm2 thick for 40 MeV neutrons. The CD2 layer plays roles of both radiator for deuterons and degrader for protons recoiled in deeper CH2 layer. The total e;ciency increases as shown by an uppermost line in Fig. 5. This technique brings about a decrease in the sensitivity for lower energies. We believe, however, there should exist a most suitable combination of thickness or mixture ratio of both materials to adjust the energy dependence of PNTD. Such a control technique is now in progress, together with continuous search for another useful radiator like (n; )-reactive material. 5. Conclusion Personal dosimetry of high-energy neutrons becomes more important in space activities, air 8ights and research works around particle accelerators. A PNTD is considered to be one of the most promising detectors for personal and long-term monitoring, but its characteristic response is still unclear in such a high-energy region. In this report, the eect of the radiator was investigated both experimentally and theoretically. A performance of radiators, CH2 of 170 g=cm2 , CD2 of 190 g=cm2 , LiF of 470 mg=cm2 and C of 150 mg=cm2 , was checked in a quasi-monoenergetic :eld of 40-MeV neutron at TIARA, JAERI, Japan. The etch-pit density for the CD2 radiator exceeded that for the CH2 one, which suggested that CD2 should be another candidate, as in CH2 . In order to estimate the eect theoretically, numerical calculations have been carried out based on a model with a

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K. Oda et al. / Radiation Measurements 36 (2003) 119 – 124

special attention to angular dierential cross sections and fundamental data for response to light ions. The dependence of the e;ciency was obtained as a function of the radiator thickness or the neutron energy. We also pointed out an effectiveness of two-layer type radiator composed of CD2 and CH2 to control the energy dependence of PNTD sensitivity to the dose equivalent. Acknowledgements The present work was undertaken as part of the special project research between universities and JEARI in Japan, and partially supported by a Grant-in-Aid for Scienti:c Research from the Ministry of Education, Culture, Sports, Science and Technology. References Baba, M., Nauchi, Y., Iwasaki, T., Kiyosumi, T., Yoshioka, S., Hirakawa, N., Nakamura, T., Tanaka, S., Meigo, S., Nakashoma, H., Tanaka, S., Nakao, N., 1999. Characterization of a 40 – 90 MeV 7 Li(p; n) neutron source at TIARA using a proton recoil telescope and a TOF method. Nucl. Instrum. Methods A 428 (2), 454–465. Ferrari, A., Pelliccioni, M., Pillon, M., 1997. Fluence to eective dose conversion coe;cients for neutrons up to 10 TeV. Radiat. Prot. Dosim. 71 (3), 165–173. Henniger, J., Enghardt, W., Gippner, P., 1988. Stopping power and range tables of low and medium energy ions with Z ¡ 16. Rossendorf Central Institute of Nuclear Research, ZfK-648, Rossendorf. Hermsdorf, D., Bretschneider, R., Dorschel, B., Henniger, J., 1999. Neutron response calculation on the basic of variable track etch rates along the secondary particle trajectories in CR-39. Radiat. Meas. 31 (1– 6), 431–436. ICRP, 1991. International Commission on Radiological Protection. 1990 recommendation of the International Commission on Radiological Protection. ICRP Publication 60, Pergamon Press, Oxford.

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