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In this paper we report results from a neutron irradiation campaign ofUltra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15,3e15, 6e15 n/cm2. The UFSD used in this study are circular 50 micro-meter thickLow-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. Theyhave been produced by Hamamatsu Photonics (HPK), Japan, with pre-radiationinternal gain in the range 10-100 depending on the bias voltage. The sensorswere tested pre-irradiation and post-irradiation with minimum ionizing particle(MIPs) from a 90Sr based \b{eta}-source. The leakage current, internal gain andthe timing resolution were measured as a function of bias voltage at -20C and-30C. The timing resolution was extracted from the time difference with asecond calibrated UFSD in coincidence, using the constant fraction method forboth. The dependence of the gain upon the irradiation fluence is consistentwith the concept of acceptor removal and the gain decreases from about 80pre-irradiation to 7 after a fluence of 6e15 n/cm2. Consequently, the timingresolution was found to deteriorate from 20 ps to 50 ps. The results indicatethat the most accurate time resolution is obtained at a value of the constantfraction discriminator (CFD) threshold used to determine the time of arrivalvarying with fluence, from 10% pre-radiation to 60% at the highest fluence. Keychanges to the pulse shape induced by irradiation, i.e. (i) a reducesensitivity of the pulse shape on the initial non-uniform charge deposition,(ii) the shortening of the rise time and (iii) the reduced pulse height, werecompared with the WF2 simulation program and found to be in agreement.

Properties of HPK UFSD after neutron irradiation up to 6e15 n/cm2