Transcript ppt

GLAST LAT Project
Gamma-ray Large
Area Space
Telescope
CAL Recon rewriting meeting, May, 11, 2005
In what cases we need external
position estimation ?
Alexandre Chekhtman
NRL/GMU
A.Chekhtman
1
GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
“Normal” single crystal energy reconstruction
• We have the measured adc signal for both ends of crystal:
•
adcP and adcM
• We subtract pedestals and correct for nonlinearity by converting
to DAC scale (using charge injection calibration):
• dacP=adc2dac(adcP-pedP)
• dacM=adc2dac(adcM-pedM)
• We calculate the energy estimator based on the data from each
crystal end, considering that energy is deposited in the crystal
center:
• ctrEneP=MeVperDac*dacP
• ctrEneM=MeVperDac*dacM
• We calculate “geomeric mean” of two ends, which doesn’t depend
on position along the crystal:
• Ene = sqrt(ctrEneP*ctrEneM)
• We do not need to know the position along the crystal for energy
reconstruction.
A.Chekhtman
2
GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
Sometimes this scheme doesn’t work
• If one end of crystal has a dead channel
• In case of direct energy deposition to one diode
• Energy deposition is closer than 1 cm to a diode :
– Energy measurement by this diode is affected by light
collection non-uniformity
• In all these cases we should use only the energy
measurement from one diode and the external position
estimation, to correct for light attenuation. For
example, if we know only ctrEneM, then using ext_pos
we can find ctrEneP:
• Asym = pos2asym(ext_pos)
• ctrEneP = ctrEneM*exp(asym)
• Then we calculate energy like in normal case (see
previous slide)
A.Chekhtman
3
GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
Non-uniformity near the diode up to 20%
• This effect could
be significant only
for narrow energy
depositions: muons,
protons, heavy ions,
first few X0’s of
high energy
showers (>100 GeV).
• For high energy
shower near and
after the maximum
and for low energy
showers the
effect of this nonuniformity will be
small.
A.Chekhtman
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GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
Direct energy deposition in a diode
• The signal from direct energy deposition by MIP in a
big diode is close to the MIP scintillation signal
• For small diode direct energy deposition will be ~6
times bigger than from scintillation.
• In this case of direct energy deposition the position is
known ( it is near the end of the crystal), but to
decide that there is a direct energy deposition we
need an external position estimation.
– “external” means “not from this crystal” – it could be from
the fit, based on position measurements in other CAL layers
A.Chekhtman
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GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
Requirements to external position estimation
• we do not need very good precision
– Asymmetry is ~0.3 at 15 cm from crystal center => ratio of two ends
changes by 2% per 1cm => error of 1 cm in the external position
estimation contributes 1% to the energy measurement in this crystal.
• We can use the linear fit to the transversal positions of the hit
crystals (“hodoscopic” position)
– precision is ~1 cm
– Always correct, while longitudinal position measurement could be
wrong or unavailable
– Provides information about the width of the energy deposition
• Probably we need 2nd pass in CAL reconstruction
– Calculate rough energies in all crystals
– Make a “hodoscopic” fit to get rough position estimation
– Refine enegy calculation in the crystals with narrow energy
deposition close to the crystal end
– Make “full” position fit – including longitudinal position measurements
where they are available
A.Chekhtman
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GLAST LAT Project
CAL Recon rewriting meeting, May, 11, 2005
Conclusion
– The are situations when only one end of crystal could be used
for energy measurement
– in these cases we need “external” position estimation to
calculate the crystal energy
– Hodoscopic fit could be used for this purpose: requires
modification in CalRecon sequence
A.Chekhtman
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