On the application of silicon for fast neutron dosimetry

The defect structure of neutron-damaged silicon as a function of irradiation dose has been investigated. Two types of defects were detected in the neutron-irradiated n-Si (FZ) crystals studied: divacancies (V2), from the IR absorption (1.8 μm band), and four-vacancy clusters (P3) from EPR spectrum, their concentration increasing linearly with fast neutron dose, ΦN. In a recent paper both V2 and P3 centres had been simultaneously observed in neutron-irradiated Si. The production efficiency of P3 clusters was found equal to 1 per 20 divacancies, independently of ΦN. From our estimate, approximately two P3 centres per primary cascade were produced (irrespective of ΦN). This suggests that both the above defects are mainly associated with the displacement spike which terminates the cascade. Excellent agreement of our results with partial literature data indicates that the concentration NV2 and NP3 depend only ΦN, irrespective of the shape of the neutron spectrum. Therefore, the divacancy-related 1.8 μm absorption band of silicon can be used for simple determination of fast neutron dose, up to ΦN = 1018 cm-2. Proper high temperature annealing of radiation defects enables multiple use of Si crystal as a fast neutron dosemeter. (author)