Study of radiation damage in silicon detectors for high ... - F9

40 3. Irradiation FacilityFigure 3.5: a) Fast neutron spectrum as measured at 11 cm in front of the ssion plate [36].b) Watt spectrum [37].meas.), 8.2 cm and 33 cm. The measured uxes were 4.310 8 n/cm 2 s and 4.210 8 n/cm 2 s,2.110 8 n/cm 2 s and 2.510 7 n/cm 2 s, respectively. As can be seen in g. 3.4b the measuredvalues can be tted well with the calculated curve.To determine the relative damage factor (eq. 2.48), it is necessary to know theshape of the neutron spectrum. For the calculation of the value in the irradiationchamber behind the ssion plate, the result from an existing measurement [36] (g. 3.5a)was used. The measurement was performed in front of the ssion plate at the distance of11 cm, while samples were irradiated behind the ssion plate. Due to large absorption ofslow neutrons in the ssion plate, only the fast part of the measured spectrum is valid forthe irradiations behind the plate. Using the Ougouag damage function values [27] (g.2.5) a value IC =0:99 (3.2)was determined, for the energy interval (0.1 - 15 MeV). Contribution of the spectrumbelow 0.1 MeV was estimated to contribute less than 1% to NIEL damage.To estimate the possible error, the IC value (3.2) was compared with the valuefor a pure ssion (Watt [37], g. 3.5b). The value obtained was W att = 1:05. Wattspectrum gives the energy distribution of neutrons produced in the ssion plate and thediscrepancy is caused by slow neutrons, coming directly from the core. The dierencebetween both values was used to estimate the error on the value from the measuredspectrum to less than 6%.