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

3. Irradiation Facility 39Fission platerRzza)Figure 3.4: Dependence of the fast neutron ux on the distance from the ssion plate. a)Schematical presentation of the setup. b) Comparison of measurements [36] and calculations.of a calibration and an irradiation measurement. During short calibration measurementsan In foil in a Cd shield 18 was placed at the sample position and a Zn foil was putin the middle of the box. After the irradiation, the activation of Zn (thermal neutroncapture, decay time 244 days) was measured and compared to the uence of fast neutronsas determined from the activation of In with a well known activation cross-section. In theirradiation run only the Zn foil was placed at the same position as in the calibration run.By comparison of Zn activations in both runs, the irradiation uence was determined.The error of this method was estimated to be about 15%.Due to the limited size of the ssion plate, the fast neutron ux depends on thedistance from the plate. Dependance on the distance can be determined by geometricalconsiderations (g. 3.4) 19 . The following relation is obtained(z 1 )(z 2 )R2ln(1 + )z= 2 1ln(1 + )R2z 2 2(3.1)where R is the radius of the ssion plate and z the on-axis distance from it (g. 3.4a).Measurements were performed at three dierent distances from the ssion plate: 3.5cm(218Cd shield was used to reduce In activation due to thermal neutrons.19From the collision probability P 1 = 4rwhere is a reaction cross-section and r distance from2the neutron source one obtains reaction probability as its integral over the surface of the ssion plateP = R dNf:pl: dS P 1dS. From this ux dependence can be determined by (z) = P = R dN 1f:pl: dS 4rdS =2dN 1dSln z2 +R24 z 2: