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

44 3. Irradiation Facilityferent reactions were used in the neutron spectrum measurement at channel F19. Theywere 197 Au(n,), 197 Au(n,)+Cd, 115 In(n,n'), 115 In(n,)+Cd, 27 Al(n,), 58 Fe(n,)+Cd,63Cu(n,)+Cd, 63 Cu(n,), 59 Co(n,) and 59 Co(n,p) [43]. The resulting spectrum is shownin gure 3.8.Both simulation and measurement datawere used to determine the value for thisirradiation site. The same energy cuts were used as in the case of irradiation chamber,i.e. 0.1 - 15 MeV. The damage functions of Ougouag [27] and Grin [44] were used todetermine the value on measured data: OugouagF 19=0:90 (3.4) GriffinF 19=0:90 : (3.5)Due to good agreement of both calculations only the Ougouag damage function was usedwith the simulated spectrum simF 19=0:93 : (3.6)From these results, the value for irradiation site F19 was determined to be F 19 =0:90 0:03 (3.7)with error conservatively estimated from the dierence between values 3.4 and 3.6.To determine the contribution of neutrons with energy below 0.1 MeV, calculationswithin wider energy ranges were also performed. They showed that neutrons with energybelow 0.1 MeV contribute an additional 1.5% to NIEL damage. Thus to determineneutron uence, equivalent to 1MeV neutrons (eq. 2.49) the following equation was usedthroughout this work F 19eq = F 19 (1 + 0:015) (E > 0:1MeV) : (3.8)The dosimetry in the irradiation channel was performed using Au activation (thermalneutron capture, decay time 2.7 days). Since this was also one of the foils used todetermine the neutron spectrum it was possible to make a calibration of Au foil activationto ux above 0.1 MeV. The saturated activity for the reaction 197 Au(n,) of4:8 10 ;10 Bqper 197 Au atom was determined to correspond to fast neutron ux of 1:8 10 12 n/cm 2 s.The systematic error of the method was estimated to about 15%.This method is highly sensitive to changes of the spectrum since Au activation issensitive to slow neutrons. Therefore uence measurements for irradiations performed