Fission Track Dating at RA-6

HERNANDEZ. R; HERNANDEZ, J; BECCHIO, RAÚL; RICARDO PEREYRA; GLASSMACHER, U.; RUEDA, G.; ARRIBERE, M; SOLADATTI, A; MARCEL, C.

BAriloche

Simposio; 16th IGORR 2014/IAEA Technical Meeting; 2014

CNEA_IGORR

Public institutions CONICET and Y-TEC, and the private enterprise GEOMAP S.A. joint efforts to create the first Thermochronology Laboratory in Argentina, the La-Te-Andes Thermochronology Development Center. This institute will comprise a Heavy Mineral Separation Unit (HMSU), a Fission-Track dating Unit (FTU) and a Numerical Modeling Unit (NMU). Aiming to develop the complete technological path in the country, and associated with German institutes (University of Heidelberg and FRM II Reactor in Munich), the group will be involved in knowledge transfer and staff training. Within this framework, the Centro Atómico Bariloche was invited to participate with the irradiation facility, the MTR type 1 MW reactor RA-6.Total Uranium concentration and 235U induced fissions are necessary data to calculate the mineral´s age through the fission-track analysis method. For that, the samples must be irradiated in a thermal neutron flux with at least 1015 n/cm2 fluence. The thermal to fast neutrons ratio (th/f ratio) must be >100. The most suitable irradiation place in the RA-6 reactor to approach that requirement is the J7 peripherial position. This place is characterized by a thermal neutron flux of th = (1.26 ± 0.05)×1012 n/cm2s and a th/f ratio = 26 [1]. With this fluxes, irradiation times are in the order of 15-20 min, which are acceptable values. However, the presence of fast neutrons will induce the fission of 238U and 232Th. If an atomic Thorium/Uranium ratio of less than 75 occur in minerals like apatite and zircon, which are used for fission-track dating, the correction presented above will introduce an additional uncertainty of about 5.8 %. Normally, 20 or more apatite and zircon grains are used simoultaneusly to determine the age of a geological sample. The complexity of the technique increases because each of these grains has very different U and Th contents. Additional fissioning of these two isotopes create high-induced track areal densities. As the uranium and thorium content of the minerals are not known a priori, another correction has to be applied. To generate such correction values it is necessary to irradiate 238U and 232Th foils of known 238U and 232Th content each time a minerals set is irradiated. In this case, this correction is less significant when the neutron flux is more themalized. Therefore, we present in addition the preliminar thechnical aspects of a reactor desing modification that is under discussion at the moment: the design and construction of a thermal irradiation column in the periphery of the RA-6 core, that would provide better irradiation conditions for this kind of geological analyses.