Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
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3. Ir<strong>radiation</strong> Facility 43Figure 3.8: Measured [36] (full l<strong>in</strong>e) and simulated [38] (open circles) neutron spectrum <strong>in</strong> their<strong>radiation</strong> channel F19. In gure a) logarithmic scale is used to present the full energy rangewhile on the l<strong>in</strong>ear scale <strong>in</strong> gure b) only the fast neutron spectrum can be seen.The simulation uses an iterative method to determ<strong>in</strong>e the neutron eld <strong>in</strong> the reactorcore [39]. The <strong>in</strong>itial step assumes a po<strong>in</strong>t neutron source with Watt spectrum [37] <strong>in</strong> eachrod and the eect <strong>of</strong> the neutrons from those sources on the generation <strong>of</strong> new sources <strong>in</strong>the core elements is determ<strong>in</strong>ed. The calculation is repeated with recalculated elementproperties and source distribution until an equilibrium condition is reached. The neutronux <strong>in</strong> the selected ir<strong>radiation</strong> spot (<strong>in</strong> our case channel F19) is then determ<strong>in</strong>ed fromthe neutron eld <strong>in</strong> the equilibrium. A more detailed description <strong>of</strong> the procedure can befound <strong>in</strong> [41, 42].Experimentally, measurement <strong>of</strong> the neutron spectrum was per<strong>for</strong>med us<strong>in</strong>g theactivation method [43]. It exploits the activation <strong>of</strong> dierent materials <strong>in</strong> the neutroneld. Foil activation rate due to reaction i is determ<strong>in</strong>ed byr i =Z Emax0 i (E)(E)dE : (3.3)Foils with well known reaction cross-section <strong>for</strong> reaction i ( i (E)) are used <strong>for</strong> the measurement.Several foils <strong>of</strong> dierent materials are irradiated and activation rates are determ<strong>in</strong>ed.The neutron spectrum is obta<strong>in</strong>ed from the solution <strong>of</strong> the system <strong>of</strong> <strong>in</strong>tegralequations 3.3. The error on the result<strong>in</strong>g spectrum depends on the number <strong>of</strong> dierentactivated foils and on the sensitivity <strong>of</strong> the reaction cross-sections on neutron energy. Errorson the activation measurement, cross-section data, <strong>in</strong>uence <strong>of</strong> other reactions andsimilar eects also contribute to the precision <strong>of</strong> the spectrum determ<strong>in</strong>ation. Ten dif-