Afterpulse effect in SiPM and neutron irradiation studies

Radiation hard SiPMs are strongly demanded for the application in the LHC detectors. Examples are the LHCb fiber tracker, the CMS outer calorimeter, and for future application in detectors for ILC, as well as for space detectors. Radiation doses up to a few times 1012neq/cm² are expected at the positions of the SiPMs at LHC. Dedicated studies on the effects of radiation on SiPMs are required to improve the performance of the existing devices. The effects of proton and neutron radiation on silicon crystals are well known. Macroscopically, they lead to the reduction of the signal, the increase of noise and the alteration of the electric field. In SiPMs defects formed in the vicinity of the narrow multiplication region (up to 2-3 μm from the surface) have an impact on the device operation. In this paper we focus on the investigation of various noise sources, generation and recombination (or dark noise), noise induced by de-trapping (or afterpulses), and optical cross-talk between pixels. We attempt to establish a method to separate the afterpulse contribution of noise and determine the de-trapping time constant and the afterpulse probability as a function of temperature and excess bias voltage. We found that afterpulses in a particular KETEK SiPM can be described with the use of one characteristic de-trapping time constant of the order of 75 ns. We present a series of irradiation studies performed on few samples of SiPMs from KETEK exposed to a neutron fluence of 1-10×10¹⁰n_eq/cm². After such irradiations, while the SiPM static parameters do not change significantly, the noise increases by 3 orders of magnitude, posing serious limitations to their usage. Unfortunately, with such an increase we have not managed to disentangle the afterpulse contribution from the noise.