Studies on the Diffusion of 37Ar and the Thermal Annealing of 51Cr Recoil in Neutron Irradiated CaCrO4-Crystals

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Abstract
Summary: The apparent similarity of the rare gas diffusion and the recoil annealing processes both resulting in “pseudo-plateaux” leads to study these processes in CaCrO4-crystals in order to find out a possible relationship. The material used was crystallized from solution, dried at room temperature (sample A) and at 200° C (sample B), an average grain size of 130 μ was selected and irradiated for 8 hours with pile neutrons in a flux of 4 x 1012n/cm2 · sec. The diffusion of the homogenously distributed 37Ar, produced by 40Ca (n, ɑ) process was investigated in the temperature range from 100 to 800° C. The thermal annealing of (n, γ) 51Cr-recoil fragments could be measured between 100 and 300° C. - It was found that the percentage of released 37Ar is much dependent on the thermal treatment of the CaCrO4-crystals. For instance about 52% of the 37Ar diffused already in 3 hours at 300° C from sample (A), while from the tempered sample (B) only about 5% was lost. In both preparations the 37Ar-release reaches 100% at about 800° C possibly favoured by oxygen liberation. In the range of 300 ~ 700° C, linearity of log D with 1/T was observed. This part of the curve should be due to the normal volume diffusion. 37Ar-diffusion coefficients at 200° C were measured to be 6,0 · 10-11 cm2/sec for sample (A) resp. 7,7 · 10-15 cm2/sec for sample (B). At 300° C, 3,2 · 10-10 for (A) resp. 1,1 · 10-12cm2/sec for (B). The initial retention of recoil 51Cr was found to be 10% higher in sample Β which can be explained by the fact that the instantaneous recombination reaction is hindered by adsorbed molecules and lattice imperfection. The thermal annealing at the earlier stage formally follows first order kinetics, the reaction rate constants are almost the same for both samples at a given temperature. In multi-step annealing experiments R∞ reaches characteristic plateau-values which can be obtained also by a single-step heating at the respective temperature. This is not the case in the multi-step 37Ar-diffusion experiments which are found to be very sensitive to the "history" of the sample. The differences in the Ar-diffusion and the 51Cr-annealing-multi-step curves are explained by the pronounced physical and the more chemical nature of the processes involved.

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