- Email: [email protected]
Neutron dosimetry and fission-track age calibration: insights from intercalibration of uranium and thorium glass dosimeters
ABSTRACTS Neutron
d o s i m e t r y and f i s s i o n - t r a c k age c a l i b r a t i o n : i n s i g h t s from i n t e r c a l i b r a t i o n of u r a n i u m and t h o r i u m glass d o s i m e t e r s T a k a h i r o T A G A M I , Noriko HASEBE and S u s u m u N I S H I M U R A
Department of Geology and Mineralogy, Faculo' of Science, Kyoto University, Kyoto 606, Japan. Three glass dosimeters possessing various U isotopic and Th/U ratios, i.e. NBS-SRM612(612), Corning 1 (CNI) and Coming 3 (CN3), were intercalibrated at three reactor facilities of different neutron thermalisation, i.e. KUR-TCPn, KUR-Pn2 and MITR-IP facilities, to assess the influence of fast neutron induced fissions of 23SU and 232Th on fission-track (FT) age calibration. The induced track density ratios for two glass couples, pcN3/Pcsl and p612/PcNl, show systematic, negative correlation with increasing thermal to fast neutron flux ratios, ~th/Of. These facts demonstrate the significant contribution of 23SU and 232Th fissions to observed induced track Qensity at less thermalised facilities wherein 4#thlC~t is -10. For the quantitative assessment of 23Su and 232Th fissions, the intercalibra~ion concept was examined mathematically and thus their generalized descriptions were established. On the basis of the ideal pcN3/pcNi ratio determined at highly thermalised facility, the magnitude of influence of 232Th fission (or that of 238U fission) is accordingly given by the measured Pcs3/PCNi at each facility and Th/U (or 23SU/235U) ratio of an unknown sample. These results elucidate that in the context of FT age calibration, the criterion of "well-thermalised" neutron flux is controlled by the fluence monitor used, Th/U ratio of a sample to be dated and the minimum error allowed for analysis. Various constraints, implications and recommendations on neutron dosimetry and FT age calibration are discussed. Further researches will be presented to test other irradiation sites.
THERMAL MODELLING OF APATITE FISSION-TRACK DATA; CONSTRAINTS ON THERMO-MECHANICAL RIFTING MODELS P.A. van der Beck, M. Rohrman, P.A.M. Andriessen and S. Cloetingh (Vrije Universiteit, Instituut voor Aardwetenschappen, De Boclelaan 1085, 1081 HV Amsterdam, The Netherlands) Models for the evolution of continental rifts and rifted margins focus on the amount and timing of thermal permrbatious associated with rifting as well as on the mechanical response of the lithosphere to attenuation and thermal perturlmtion. different models predict different effects on the thermo-tectonic evolution of the rift, most noticeably on the timing, amount and rate of subsidence and uplift of the rift basin and shoulders, and on the thermal history of the rift. Quantification of the kinetics of apatite fission-track annealing allows the deten'nination of thermal history and can therefore be used to put constraints on different thermo-mechanical rifting models. Preliminary results of apatite fission-wack analysis from the Permo-Carboniferous Oslo rift region (S. Norway) are presented, and the cons~aints they put on rifting scenarios are discussed. The thermal effects of asymmelric (simple shear) versus symmelric (pure shear) rifting models are considered, as well as the effects of largescale magmatic activity.
601
d o s i m e t r y and f i s s i o n - t r a c k age c a l i b r a t i o n : i n s i g h t s from i n t e r c a l i b r a t i o n of u r a n i u m and t h o r i u m glass d o s i m e t e r s T a k a h i r o T A G A M I , Noriko HASEBE and S u s u m u N I S H I M U R A
Department of Geology and Mineralogy, Faculo' of Science, Kyoto University, Kyoto 606, Japan. Three glass dosimeters possessing various U isotopic and Th/U ratios, i.e. NBS-SRM612(612), Corning 1 (CNI) and Coming 3 (CN3), were intercalibrated at three reactor facilities of different neutron thermalisation, i.e. KUR-TCPn, KUR-Pn2 and MITR-IP facilities, to assess the influence of fast neutron induced fissions of 23SU and 232Th on fission-track (FT) age calibration. The induced track density ratios for two glass couples, pcN3/Pcsl and p612/PcNl, show systematic, negative correlation with increasing thermal to fast neutron flux ratios, ~th/Of. These facts demonstrate the significant contribution of 23SU and 232Th fissions to observed induced track Qensity at less thermalised facilities wherein 4#thlC~t is -10. For the quantitative assessment of 23Su and 232Th fissions, the intercalibra~ion concept was examined mathematically and thus their generalized descriptions were established. On the basis of the ideal pcN3/pcNi ratio determined at highly thermalised facility, the magnitude of influence of 232Th fission (or that of 238U fission) is accordingly given by the measured Pcs3/PCNi at each facility and Th/U (or 23SU/235U) ratio of an unknown sample. These results elucidate that in the context of FT age calibration, the criterion of "well-thermalised" neutron flux is controlled by the fluence monitor used, Th/U ratio of a sample to be dated and the minimum error allowed for analysis. Various constraints, implications and recommendations on neutron dosimetry and FT age calibration are discussed. Further researches will be presented to test other irradiation sites.
THERMAL MODELLING OF APATITE FISSION-TRACK DATA; CONSTRAINTS ON THERMO-MECHANICAL RIFTING MODELS P.A. van der Beck, M. Rohrman, P.A.M. Andriessen and S. Cloetingh (Vrije Universiteit, Instituut voor Aardwetenschappen, De Boclelaan 1085, 1081 HV Amsterdam, The Netherlands) Models for the evolution of continental rifts and rifted margins focus on the amount and timing of thermal permrbatious associated with rifting as well as on the mechanical response of the lithosphere to attenuation and thermal perturlmtion. different models predict different effects on the thermo-tectonic evolution of the rift, most noticeably on the timing, amount and rate of subsidence and uplift of the rift basin and shoulders, and on the thermal history of the rift. Quantification of the kinetics of apatite fission-track annealing allows the deten'nination of thermal history and can therefore be used to put constraints on different thermo-mechanical rifting models. Preliminary results of apatite fission-wack analysis from the Permo-Carboniferous Oslo rift region (S. Norway) are presented, and the cons~aints they put on rifting scenarios are discussed. The thermal effects of asymmelric (simple shear) versus symmelric (pure shear) rifting models are considered, as well as the effects of largescale magmatic activity.
601