GLAST LAT Collaboration Meeting

1
MIPs in your Photodiodes
Davids SVAC task How well do we find MIPs?
(Darios doing cool stuff) (MIP Minimum
Ionizing Particle) Benoits task Make images of
the CAL layers. Sashas task What is the light
output of tracks crossing the photodiodes? Benoi
t is sharing his tools with me Ill get serious
about my task late August, when I settle into
Palo Alto. In the meantime Benoit explained the
standard model of photodiode MIP response to
me, and we concurred that with our DAQ setup we
could do a quick reality check. Weve taken muon
(and 22Na) data in CsI, in a photodiode, and in
a CDE. Benoit calculated simulated expected
response. Preliminary result data apparently x5
lower than predicted. Work-in-progress explain
the phenomenon.
2
Setup (1 of 2)
2-scintillator telescope
Photos by J. Bregeon
3
Setup (2 of 2)
Same pre-amps, amps, adc, daq as for Ganil,
GSI, and CERN (testbeam CsI stack at left)
Smaller (top) scintillator 2cm x 2cm

• We also used
• A 2cm long CDE
• A naked photodiode
• (thanks to G. Bogaert for providing the latter)

4
Muons look normal in the CDE
1 MIP 12 MeV 1400 dc 1.4 volts (very
roughly 1 kev per dc)
5
Muons in a naked photodiode -- data
In the CDE we said 1 MIP 12 MeV 1400 dc 1.4
volts For the photodiode without CsI, we see
320 dc.
6
Muons in a naked photodiode -- expected
The standard calculation is Dx (dE/dx) 300
mm (1.6 MeV-cm2/gm) 0.12 MeV for
Silicon (Geant confirms this expected energy
deposit.) Furthermore, it takes 3.6 eV to
liberate an electron-hole pair. So we expect
1.2E5/3.6 30000 electrons for a MIP crossing
just the photodiode without CsI . Recall, 60000
electrons for a MIP in the CsI. That is, a muon
in a photodiode would give half what you see for
a muon in a CDE. This is what Sasha coded into
the Glast simulation. We also took data with a 2
cm mini-CDE (DAQ triggered by phototube
coincidence) muon passing CsI only gives a
3200 dc peak muon passing photodiodeCsI also
gives a 3200 dc peak (Photodiode contribution
smaller than geometrical ambiguity) Apparent
contradiction data gives MIP/10 where we expect
MIP/2
7
Muons in a naked photodiode -- expected
Geant4 simulations by Benoit top, muon in CsI.
Bottom, muon in silicon of photodiode.
MeV, in CsI
MeV, in Silicon
The ambiguity is, how does energy deposit
convert into pulseheight?
8
Radioactive sodium (22Na) -- Data
340 keV (see next slide)
Left - the CDE Right naked photodiode
511 keV 1275 keV
9
Radioactive sodium (22Na) -- simulations
More Benoit Geant 4
positron in 300 mm silicon 340 keV edge
340 keV (see next slide)
Left - the CDE Right naked photodiode
511 keV 1275 keV
1275 keV gamma in 300 mm silicon
10
Conclusions

• Explanation of what weve seen will have to wait
  for after vacation
• It will be fun to compare the Merit Ntuple with
  the real data well be getting
• from the first towers Davids and Sashas
  tasks are genuinely motivating.