From piilonen@vt.edu Wed Jan 18 16:41:13 2012
Date: Wed, 18 Jan 2012 21:40:57 -0500
From: Leo Piilonen <piilonen@vt.edu>
To: Yutaka Ushiroda <yutaka.ushiroda@kek.jp>, Gary Varner <varner@phys.hawaii.edu>, Dmitri Liventsev <livent@itep.ru>
Subject: "25 year" SiPM lifetime in barrel modules

Dear all:

Gary asked me to summarize where I came up with the "25 year" projected lifetime for the SiPMs in the barrel.  Here are the details.

Endcap SiPMs (Hamamatsu S10362-13-050C), from pp. 331-332 of TDR:
  o expected dose after 10 years @ L=8x10^{35}/cm^2/s is < 40Sv
  o dark current increases by x150 from 0.1 uA to 14 uA after 10 years @ L=8x10^{35}/cm^2/s (extrapolated from Belle experience)
  o dark current increases by x170 from 0.1 uA to 16 uA after 2.0x10^{11} protons/cm^2 in ITEP beam
  o dark current increases by x70 from 0.1 uA to 6.5 uA after 0.74x10^{11} protns/cm^2 in ITEP beam
  o using 7.5-pixel threshold, intrinsic noise increases after 10 years @ L=8x10^{35}/cm^2/s to 10 kHz, but this is still 1/20 of the neutron hit rate.
  o projected lifetime of Hamamatsu SiPM in endcap environment is > 10 years (but upper limit estimate is not given)

Barrel SiPMs (Hamamatsu S10362-13-050C):
  o Touschek 1-MeV-equivalent neutron flux at barrel layer 0 is max of 8 neutrons/mm^2/s.
  o at this rate, assuming 200MeV proton is equivalent to 1MeV neutron [the weighting factor for 200MeV neutron is 0.8], it will require 25 years of this flux (using 1 year = 1E7 seconds) to reach an integrated flux of 2.0x10^{11} neutrons/cm^2, the flux quoted above for the 10-year-min lifetime of the endcap SiPMs.
  o Therefore, the barrel SiPMs would live > 25 years (but upper limit estimate is not given)

Caveat:  the background estimation for Bhabha-scattered and beam-gas-scattered neutrons are not included yet.  If we were to assume that these increase the neutron rate by a factor of 2.5 over Touschek-only, then barrel SiPMs would live > 10 years (but upper limit estimate is not given)

Regards,
  Leo