Preliminary results of the PSI beam test: energy resolution

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Preliminary results of the PSI beam test energy
resolution

• Fabrizio Cei
• INFN University of Pisa
• for the MEG Pisa group

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Outline

• Data sample
• Data selection criteria
• Calibrations
• Results
• Comparison with MC calculations
• Possible improvements
• Conclusions

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• Data sample
• Runs with low beam intensity (FSH52 125)
• Trigger S1 Xe (Front Back) NaI
• LXe calorimeter collimator 95 mm
• Gain 106
• Data selection criteria
• RF cut when available
• gt 50 triggered PMTs (beam cut)
• NaI energy in the range
• 70 MeV lt ENaI lt 110 MeV for 54.9 MeV gs
• 30 MeV lt ENaI lt 70 MeV for 82.9 MeV gs.
• No position/topological corrections.

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Reconstruction techniques

• Position
• MINUIT fit on the PMT distribution in the
• entrance face
• NaI energy
• QSUM
• Lxe Energy
• Linear fit trained by Monte Carlo simulation
  of the LP or QSUM (not shown, the former works
  better !)

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Calibrations

• PMT gains determined using LED
• Q.E. using a sources in gas or liquid
• Usual procedures, but different
• results for beam on or off
• further corrections needed

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Comparison between beam on/off for LED and a
sources
a sources
LED
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LED/a source comparison
a sources
LED
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Data selection

• NaI energy (MeV) RF/NPMT

Green RF selected Red NPMT selected
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Lxe-NaI energy correlation
All events (52k)
Beam events (46k)
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Reconstruction quality

• Energy vs depth (X,Y) coordinates

Collimator
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Energy resolution _at_55 MeV

• No selections gt2.5 cm from the wall

9.4k events (69 ) FWHM 6.5
13.5k events FWHM 7.4
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Energy resolution _at_83 MeV
No selections gt2.5 cm from the wall
FWHM 6.9
FWHM 6.9
No effect (deeper events)
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MC predictions (labs 3 m)
55 MeV FWHM 4.0
83 MeV FWHM 3.8
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Linearity

• With the same set of coefficients
• 55 MeV 83 MeV 4.4 MeV (Am/Be)

Erec 4.2 MeV
Erec 83.7 MeV
Erec 55.3 MeV
Erec 55.3 MeV
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Possible improvements - I

• Q.E.

Green MC Red data with Q.E. in
gas Charge distribution for data is not so
symmetric as it should be Q.E. could be
measured individually in the Pisa cryogenic test
facility.
PMT relative charge
Beam
PMT number
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Possible improvements - II
The quality of the linear fit depends very
strongly on the MC/data agreement ! MC
refinements - light collection/transport
algorithms - Fresnel and/or total
reflection - LXe scintillation light spectrum,
labs .
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Time instability
ltEgt between 50 60 MeV
Ordering number of run
DE 0.4 MeV
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The best that one can do

• Assume that the MC is correct
• determine a Q.E. set comparing the expected and
  measured charge PMT by PMT
• apply position cuts to select the signal in a
  restricted region.

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And this is the result
No cuts
R lt 3cm
R lt 3cm D gt 2.5 cm
Best result 4.8 FWHM with R lt 1.5 cm D from
wall gt 2.5 cm (efficiency 6.8 )
R lt 1.5 cm
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Conclusions

• The PSI beam test data were fully reconstructed
  using MINUIT and linear fit
• The energy resolution is generally rather worse
  than that expected by MC, but comparable
  resolutions are obtained with strict position
  cuts
• The calibrations (i.e. Q.E.) should be refined
  and effects like the beam intensity must be taken
  into account
• The MC simulation must be further refined also.

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