Determination of zeta values for fission-track age calibration using thermal neutron irradiation at the JRR- reactor of JAEA, Japan

Transcription

1 Jour. Geol. Soc. Japan, Vol. 115, No. 3, p , March 2009 note Determination of zeta values for fission-track age calibration using thermal neutron irradiation at the JRR- reactor of JAEA, Japan Tohru Danhara and Hideki Iwano Received October 30, 2008 Accepted January 26, 2009 Kyoto Fission-Track Co., Ltd., 44-4 Minamitajiri-cho, Omiya, Kita-ku, Kyoto, , Japan Method In our laboratory, zircon and apatite minerals were analyzed using the conventional external detector method, in which spontaneous tracks are counted in the internal surface, and the induced tracks are counted in an external detector ED1 method: Gleadow, External natural Corresponding author; T. Danhara, Abstract: We recently determined new zeta values of zircon and apatite for fission-track dating using diallyl phthalate DAP detectors. Thermal neutron irradiation was performed using the pneumatic tube No. at the JRR- reactor of Japan Atomic Energy Agency JAEA, for which the cadmium ratio for Au is. The resulting zeta values are ED and ED for zircon with Q-DAP detectors by an observer TD, ED and ED for zircon with Q-DAP detectors, and ED for apatite with S-DAP detectors by a second observer HI. These values are - higher than those obtained for the pneumatic tube at the JRR- reactor of JAEA, with a cadmium ratio of.. The significant difference in zeta values between the two irradiation sites can be partly attributed to the effect of the fast neutron-induced fissions of Th and U in SRM glass at JRR-, with its relatively poor thermalisation. Table. Summary of the observation system and experimental conditions for zeta value determination. Key words: fission-track dating, zeta value, zircon, apatite, JRR-3 reactor Introduction We have routinely reported the zeta values for fissiontrack age calibration whenever a part of our dating systems was changed; for example a reactor, dosimeter glasses, a microscope and external detectors Danhara et al., 1991, 2003, 2007; Iwano and Danhara, 1997, 1998; Danhara and Iwano, This is a report of new zeta values determined for zircon and apatite combined with diallyl phthalate DAP resin as an induced-fission-track detector Tsuruta, 1999, 2000; Yoshioka et al., 2002, 2003; Danhara et al., 2003, 2007 using the JRR-3 reactor of Japan Atomic Energy Agency JAEA following the international recommendation by the I.U.G.S. Subcommission on Geoch ronology Hurford, CThe Geological Society of Japan

2 142 T. Danhara and H. Iwano Table. Fission track analytical data of zircon age-standard samples combined with the Q-DAP detector by using the direct surfaces of zircon used for spontaneous-track counting ED2 method: Gleadow, 1981 were also analyzed. Induced tracks were produced by thermal neutron irradiation in a research reactor. Since 2001, samples have been irradiated at the pneumatic tube PN of the JRR-4 reactor of JAEA with the SRM612 dosimetry glass and DAP external detectors Danhara et al, 2003, Track counting was done using two types of observation ways: the direct observation through oculars of a microscope and the high-resolution monitor observation. Tracks in DAP detectors were identified by using two criteria: 1 cone-shape and 2 width over 1 m. The laboratory procedure and zeta-value calculation have been described in detail elsewhere Danhara et al., 2003, In this study, we used the pneumatic tube No. 2 PN-2 in the JRR-3 reactor of JAEA instead of the JRR-4 reactor. Experimental conditions for zeta value determination are summarized in Table 1. As an additional experiment, the experiment 1 performed by Danhara and Iwano 2001 was re-investigated to intercalibrate zeta values against SRM612 glass between JRR-3 and JRR-4 reactors and to estimate the influence of fastneutron fission of 232 Th and 238 U in SRM612 glass. The ratios of induced track densities that are produced by the same thermal neutron irradiation between two dosimetry glasses, SRM962a and IRMM-540 were re-determined. The SRM962a, which is named a pre-irradiated SRM612 and equivalent to the SRM612, contains ppm Th and ppm U having a depleted 235 U/ 238 U ratio of

3 Jour. Geol. Soc. Japan Determination of zeta values for JRR Table. Fission track analytical data of zircon age-standard samples combined with the Q-DAP detector by using the monitor atom Carpenter, The IRMM-540 contains ppm U of natural 235 U/ 238 U ratio of atom and less than ppm Th De Corte et al., The glass couples with mica external detector were irradiated at site PN of JRR-4 with cadmium ratio for Au 3.6 and site PN-2 of JRR-3 with cadmium ratio for Au 24. The mica detectors were etched with 46 HF solution at 20 for 44 minutes after irradiation, and the induced track densities were measured with the monitor Results The fission track data of zeta value determinations are shown in Tables 2 and 3 zircon and Table 4 apatite. Errors represent 1. The weighted mean zeta values of zircon measured by an observer TD using the direct observation system are and for ED1 and ED2 data, respectively. The weighted mean zeta values of zircon measured by another observer HI using the monitor obser- vation system are and for ED1 and ED2 data, respectively. Failures of 2 -test Galbraith, 1981 at the 5 significance level were found mainly in ED1 data of Fish Canyon Tuff zircon, probably because of uranium heterogeneity within a crystal. However, these data were also included in the calculation of weighted mean value in order to compare the zeta value for JRR-4 determined by Danhara et al. 2003, 2007 on the same criterion. In fact, we confirmed that there is no significant difference between the weighted mean zeta values for all data and those of the passed-data in this study. For apatite, all data passed the 2 -test and no difference in zeta values between the Fish Canyon Tuff apatite and the Durango Apatite standards was found, that is in contrast to the results of PN at JRR-4 with less thermalisation, where the Durango Apatite exhibited higher zeta values than the Fish Canyon Tuff. The weighted mean zeta value for ED1 is determined to be These values for JRR-3 are 6-11 higher than those of

4 144 T. Danhara and H. Iwano Table. Fission track analytical data of apatite age-standard samples combined with the S-DAP detector using the monitor Table. Analytical data and induced track density ratios of SRM962a and IRMM-540 glass dosimeters. JRR-4 determined by Danhara et al. 2003, Table 5 shows the results on the induced track density determination of the glass dosimeters and the track density ratios of SRM962a/IRMM-540. Errors represent 1. The weighted mean ratios for the glass pair were obtained to be for JRR-3 and for JRR-4. The former value is in agreement with an ideal value of Danhara and Iwano, 2001 that can be calculated from uranium contents and 235 U/ 238 U ratios in the two dosimetry glasses. On the other hand, the latter for JRR-4 is 7 higher than the ideal value, of which the difference is expected based on calculation of the effect of fast-neutron induced tracks of 232 Th and 238 U in SRM612 glass Honda et al., 1990; Ohira and Honda, Therefore, the difference of zeta values, which was found between JRR-3 and JRR-4 reactors, can be mostly or partly attributed to the effect of 232 Th and 238 U fission in the SRM612 glass at less thermalised JRR-4 reactor as Tagami and Nishimura 1992 pointed out.

5 Jour. Geol. Soc. Japan Determination of zeta values for JRR Especially for fission track dating of apatite with various Th/U ratios, we should use a well-thermalised reactor with thermal to fast neutron flux ratio th / f of 100 Tagami and Nishimura, 1992; Wagner and Van den haute, In addition, the number of shallow etch-pits background noises due to fast-neutron-induced recoil tracks in DAP detectors: Yoshioka et al., 2002, 2003 is decreased when irradiated at JRR-3 reactor. Therefore use of a well-thermalised reactor is recommended especially when DAP detectors are used. Acknowledgements: Standard zircons the Buluk Member Tuff and the Tardree Rhyolite were kindly provided to us by Drs. A. J. Hurford and A. Carter. Mr. S. Ichimura and Mr. A. Hirose helped our thermal neutron irradiation in the reactor at the Japan Atomic Energy Agency. Comments from an anonymous reviewer and Dr. N. Hasebe were helpful. References Carpenter, B. S., 1984, Calibrated glass standards for fission track use-supplement. Natl. Bur. Stand. Spec. Publ., no Danhara, T., Iwano, H., 2001, System calibration for fission track dating of zircon using the new standard glass dosimeter IRMM-540, Fission Track News Lett., no. 14, Danhara, T., Iwano, H., Kobayashi, H. and Tsuruta, T., 2007, A report on fission-track data of zircon and apatite age-standards for age calibration using diallyl phthalate DAP resin detectors and a minor revision of zeta values by Danhara et al Jour. Geol. Soc. Japan,, Danhara, T., Iwano, H., Yoshioka, T. and Tsuruta, T., 2003, Zeta calibration values for fission track dating with a diallyl phthalate detector. Jour. Geol. Soc. Japan,, Danhara, T., Kasuya, M., Iwano, H. and Yamashita, T., 1991, Fission-track age calibration using internal and external surfaces of zircon. Jour. Geol. Soc. Japan,, De Corte, F., Bellemans, F., Van den haute, P., Ingelbrecht, C. and Nicholl, C., 1998, A new U-doped glass certified by the European Commission for the calibration of fission-track dating. In Van den haute, P. and De Corte, F., eds., Advances in Fission-Track Geochronology, Kluwer Academic Publishers, Galbraith, R. F., 1981, On statistical models for fission track counts. Jour. Math. Geol.,, Gleadow, A. J. W., 1981, Fission-track dating methods: what are the real alternatives Nucl. Tracks., Honda, T., Danhara, T. and Nozaki, T., 1990, Th and U contents in age standard samples for fission track dating: Implications for zeta-values. Fission Track News Lett., no. 3, Hurford, A. J., 1990, Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology. Chem. Geol. Isot. Geosci. Sec., Iwano, H. and Danhara, T., 1997, Zeta calibration of fission-track dating system using high-resolution monitor and comparison of reported zeta values, Fission Track News Letter. no. 10, Iwano, H. and Danhara, T., 1998, A re-investigation of the geometry factors for fission-track dating of apatite, sphene and zircon. In Van den haute, P. and De Corte, F., eds., Advances in Fission-Track Geochronology, Kluwer Academic Publishers, Ohira, H. and Honda, T. 1992, Fission track dating using sphene An example of the Hanawa pluton. Jour. Min. Petr. Econ. Geol.,, Tagami, T. and Nishimura, S., 1992, Neutron dosimetry and fission-track age calibration: insights from intercalibration of uranium and thorium glass dosimeters, Chem. Geol. Isot. Geosci. Sect., Tsuruta, T., 1999, Characteristics of diallyl phthalate resin as a fission track detector. Radiat. Meas.,, Tsuruta, T., 2000, Diallyl phthalate resin and its copolymers containing allyl diglycol carbonate as nuclear track detectors. Radiat. Meas.,, Yoshioka, T., Tsuruta, T., Iwano, H. and Danhara, T., 2002, Use of diallyl phthalate detector for fission-track dating. Fission Track News Lett., no.15, 1-8. Yoshioka, T., Tsuruta, T., Iwano, H., Danhara, T. and Koguchi, Y. 2003, Fission-fragment registration and etching properties of diallyl phthalate with reference to its use as an external detector in fission track dating. Nucl. Instr. and Meth. In Phys. Res. B,, Wagner, G. A. and Van den haute, P. 1992, Fission-Track Dating. Enke, Stuttgart, 285p. : in Japanese : in Japanese with English abstract