Residual Radioactivity Measurements for High Purity Silicon Irradiated by Pile Neutrons
This paper describes residual radioactivity measurements for high purity silicon (purer than ten-nines) irradiated by pile neutrons to produce n-type semiconductor silicon. The silicon samples were irradiated in four different reactors with thermal neutron flux densities of 0.12, 1.45, 1.8, and 5.5 x 1013n/cm2/s. After irradiation, samples were dissolved in HF-HNO3 mixed solution, and the solutions dried. The residual radioactivity was measured with a 2 1 gas-flow low background G. M. counter. These results are listed in Table 1. The only radioactive species found was 32P, which was produced by the consecutive reactions, 3°Si (n, γ, β-)31P followed by 31P (n, γ)32P. The observed production rate for 32P was in good agreement with the calculated one, as shown in Fig. 1. This technique for residual radioactivity in irradiated silicon can be applied to estimate the amounts of not only doped phosphorus, but other trace impurities such as gold in irradiated silicon. Some samples, obtained from the tang end of the float zone purified silicon rod, contained about 0.01 atomic ppb of Au, but those from the middle of the rod contained no Au. The presented technique is easily applicable to check and characterize the rods for the production of neutron doped silicon.
KeywordsThermal Neutron Silicon Sample Consecutive Reaction Trace Impurity Neutron Dose
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