Determination of impurities concentration inside the silicon monocrystal by the neutron activation analysis

The contribution described a measurement of impurities in a silicon sample realized by means of neutron activation analysis. The measurement was done on the LVR-15 research reactor situated in q̌ež near Prague in the Czech Republic. The silicon sample in form of a single crystal (a 30 cm high cylinder with 5 cm in diameter) was irradiated inside the reactor core of the LVR-15 reactor for approximately 90 minutes. After the irradiation the sample was transported and the gamma spectrum was measured at a HPGe detector. Activation of ³⁰Si to ³¹Si with half live of 157 min. does not allow to measure isotopes with short half life. The total efficiency of the HPGe detector had to be calculated in the transport code MCNP(X) v.2.6 due to the non-standard geometry of the set detector-sample and also due to large dimensions of the silicon crystal. The evaluation of the measured data showed the presence of almost all detectable elements, although none of them exceeded expected concentration. The measurement is connected with regular irradiation of silicon in the reactor core for neutron transmutation dopping during semiconductor production which is performed on the LVR-15 reactor regularly and on a large scale. Therefore, the purity of the crystal is controlled occasionally and the technique of neutron activation analysis has several advantages as it is very sensitive (for some elements) and it can measure the impurities concentration even inside large crystals (not only on the surface). Also, the most expensive part of the analysis - silicon irradiation - can be a part of the standard production process in our case, so the whole NAA is very inexpensive, fast, can be performed on a regular basis, and the crystal can still be processed later for industrial purposes due to non-destructive behavior of the NAA method.