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

3. Irradiation Facility 43Figure 3.8: Measured [36] (full line) and simulated [38] (open circles) neutron spectrum in theirradiation channel F19. In gure a) logarithmic scale is used to present the full energy rangewhile on the linear scale in gure b) only the fast neutron spectrum can be seen.The simulation uses an iterative method to determine the neutron eld in the reactorcore [39]. The initial step assumes a point neutron source with Watt spectrum [37] in eachrod and the eect of the neutrons from those sources on the generation of new sources inthe core elements is determined. The calculation is repeated with recalculated elementproperties and source distribution until an equilibrium condition is reached. The neutronux in the selected irradiation spot (in our case channel F19) is then determined fromthe neutron eld in the equilibrium. A more detailed description of the procedure can befound in [41, 42].Experimentally, measurement of the neutron spectrum was performed using theactivation method [43]. It exploits the activation of dierent materials in the neutroneld. Foil activation rate due to reaction i is determined byr i =Z Emax0 i (E)(E)dE : (3.3)Foils with well known reaction cross-section for reaction i ( i (E)) are used for the measurement.Several foils of dierent materials are irradiated and activation rates are determined.The neutron spectrum is obtained from the solution of the system of integralequations 3.3. The error on the resulting spectrum depends on the number of dierentactivated foils and on the sensitivity of the reaction cross-sections on neutron energy. Errorson the activation measurement, cross-section data, inuence of other reactions andsimilar eects also contribute to the precision of the spectrum determination. Ten dif-