Radiation induced M-centre production in NaCl at and above 15 °C

Abstract
From a series of experiments studying F and M-centre production at low gamma dose rates ~ 10⁴ Rh^-1) and at temperatures in the range 15-95 °C, it was found that M-centre production could be described by two distinct processes, one purely statistical, and another due to F-centre thermal diffusion [l, 2]. Experiments are now being carried out irradiating samples with 1.2 MeV electrons (I ~ 10⁴ nA cm^-2, equivalent to ~ 10⁵ Rh^-1), with the aim to bridging the gap between the low dose rate work (~ 10⁴ Rh^-1, F-centre density ~ 10¹⁶ cm^-3) and the high dose rate data (≥ 10⁶ Rh^-1, F-centre density ≥ 10¹⁷ cm^-3). In this way, one should be able to separate the individual processes involved in F-aggregation and determine the dominant process at any given dose rate. At the medium dose rates a non-thermal diffusion process is observed, which is directly proportional to the dose rate. The M-centre production rate may be written, dM/dt = 2K₁ F dF/dt + K₂(T) F² + K₃(φ) F² - f (M). The first term represents the statistical M-centre production, the second thermal diffusion, the third radiation induced diffusion and the last term all possible M-centre annihilation processes [2]. This last term is necessary at and above about 75 °C where R-centre production is observed to be important. Whereas the thermal diffusion process is important even at low F-centre densities (< 10¹⁶ cm^-3), the radiation induced diffusion appears to be short range, only being observed for F-centre densities above about 10¹⁷ cm^-3. As in the case of the thermal diffusion process, the radiation induced diffusion process also does not lead to an M = K₁ F² relation. The fact that this relationship is observed to hold at very high dose rates could be due to the importance of the term f(M) giving rise to an equilibrium situation, or to the term dF/dt which increases rapidly in third stage coloration.