?-ray production by the reactions Li(p,?)Be and B(p,?)C tested at the Legnaro INFN Laboratory

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Updates on the Calibrations of the MEG detector
Giovanni Signorelli INFN Sezione di Pisa

1. ?-ray production by the reactions Li(p,?)Be and
   B(p,?)C tested at the Legnaro INFN Laboratory
2. Monte Carlo simulation of point-like Americium
   a-sources

BVR, 2006 February 15
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Gamma line measurements

• Main method to check the energy scale and
  stability of the calorimeter on almost-daily
  basis
• We tested the calibration method by means of
  p(N,g)N reactions with the Legnaro VdG
  accelerator coupled to a custom target tube with
  different home made targets
• We studied the reliability of the method paying
  attention to
• Reactions rates at different energies
• Different target thickness
• Quality of the ?-lines

Reaction Resonance energy s peak g-lines
Li(p,?)Be 440 keV 5 mb 17.6 MeV, 14.6 MeV
B(p,?)C 163 keV 2 10-1 mb 4.4 MeV, 11.7 MeV, 16.1 MeV
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Legnaro VdG Properties

• The Legnaro Van de Graaff proton accelerator has
  characteristics somewhat different from those of
  the foreseen MEG Cockroft-Walton.
• Presence of a bending and focusing system

Legnaro VdG
MEG CW
Energy keV 400-2000 300-900
Energy spread (FWHM) keV 15 lt0.5
Angular divergence (FWHM)mrad2 - lt 3 x 3
Spot size at 3 m (FWHM) cm lt 0.5 x 0.5 lt 1x 1
Energy setting reproducibility 0.2 0.1
Energy stability (FWHM) 0.2 0.1
Range of the average current ?A 0.1-1 1-100
Current stability 10 3
Current reproducibility 10 10
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Experimental set-up

• Large square NaI detector (28 x 28 x 35 cm3)
• 6.3 solid angle on average
• Small cylinder NaI detector (4 inch f, 4 inch h)
• 1.5 solid angle
• Thin Al target tube (9 cm f, 1 mm thick)
• Target at 45o wrt the proton beam
• Multichannel analyzer

p beam
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Target production
Boron target

• Targets deposited on polished copper discs
• Thermal evaporation
• Lithium Fluoride
• High vapor pressure _at_ low temperature
• Good uniformity
• Electron gun evaporation
• Boron
• High melting point
• Slow deposition - tends to explode

LiF target
Quartz balance
Target support
Requested Requested Requested Produced
material Thickness (mm ) Energy loss (keV)
LiF 0.12 10 0.110.02
LiF 1.41 120 1.340.05
LiF 4.74 500 4.720.12
B 3 300 1.840.18
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Target supporting pipe

• Beam monitoring and current measurements
  (normalization)
• Isolated tube (Faraday cup)
• Series of the diaphragms
• Preliminary centering of the beam
• Light from protons on CsI with perspex window

Target holder

Diaphragms
Tube
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Li(p,?)Be reaction

• Target LiF easier to prepare compared to Li
  alone
• Fluorine has a large cross section for gamma
  production
• The raw spectrum shows radioactivity, F lines and
  Li lines

I 90 nA Target LiF Thickness 4.78 ?m Tp
500 keV
Natural radioactivity
Fluorine lines
Li(p, ?1) at 14.6 MeV
Li(p, ?0) at 17.6 MeV
Cosmics in NaI
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LiF target excitation curve

• Number of collected photons in Li peak as a
  function of the proton energy
• We checked the energy scale and resolution of
  Legnaro VdG!

Thick target during slowing down in target all
protons eventually reach the resonance Thickness
1.34 ?m
Thin target only resonant protons do
react Thickness 0.11 ?m
?(keV) 10 1 ? (keV) 446 1
?(keV) 17.97 0.03 ? (keV) 452.4 0.5
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The 17.6 MeV ?-line

• Gamma lines from natural radioactivity are used
  to calibrate the energy scale
• 40K (1.460 MeV) 214Bi (1.764 MeV) 214Bi
  (2.204 MeV) 208Tl (2.601 MeV)

Large NaI Energy Resolution ?(E)/E 3.09
0.03 (at 17.6 MeV)
I 90 nA Tp 500 keV
Rate(17.6 MeV) on LXe 1.8 kHz / mA
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B(p,?)C reaction

• From the de-excitation of Carbon 94 of the
  times the 16.1 level decays in two photons
• Three energetic gamma lines
• Powerful tool to explore the capability of the
  MEG calorimeter to reject pile-up events.

gt16.1 MeV
gt11.7 MeV 4.4 MeV
I 240 nA Thickness 1.84 ?m Tp 500 keV
Background subtracted
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Boron single rates

• The Legnaro VdG could not reach at the correct
  energy (too low)
• Production rate increases with energy (see cross
  section in previous slide)
• The 11.6 MeV and 16.1 MeV lines undergo
  Doppler-shift
• No good energy reference for this test
• MEG CW accelerator will be operated at the
  correct energy!
• Foreseen single rate of the 16.1 MeV line 1
  Hz/mA in MEG calorimeter

Natural radioactivity
B ?-line
R 16 Hz _at_600 keV R 5 Hz _at_500 keV R 3 Hz
_at_400 keV
F ?-line
Li ?-line
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Coincident ?-lines

• We triggered on the 11.6 MeV line on one detector
  and recorded the spectrum on the other NaI
• Almost all coincidences were 4.4 MeV - 11.6 MeV
  pairs!
• Coincidence rate compatible with expectations
• Foreseen coincidence rate in MEG calorimeter 1
  Hz/mA

4.4 MeV Spectrum on small NaI
4.44 MeV
1st escape
2th escape
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Conclusions

• Good quality of the 17.6 MeV ?-line for the MEG
  calibration
• Bad quality of the 16.1 MeV ?-line at Tp 500
  keV
• Good quality of the 4.4 MeV ?-line
• The MEG CW will be operated at lower energy
• Boron as a source of coincident ?s
• Study of pile-up rejection capability
• Good agreement of the rates between predictions
  and experimental data

The use of an electrostatic machine for several
days, under conditions similar to the ones
foreseen for MEG, was rich in suggestions useful
to the design of the final MEG calibration
equipments (New MEG internal note)
Full success of the Legnaro test
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New MEG internal note
MEG TN032
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SORAD a-source Photos

• Am sources much larger half-life (kyears instead
  of 130 days)
• Difficult to prepare
• 210Po electrodeposited
• Not possible for 241Am
• Clipping of Au foils on thin wire

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241Am in Gas Xenon

• In gas xenon there is no difference between
  americium and polonium sources.
• QE determination in gas ok.

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...but in liquid

• No more rings as in the 210Po case

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simulated!

• Our MC simulation is good!
• An investigation with the factory is in progress
  to improve the symmetry.

50 mm thick gold plate clipped around the wire
100 mm thick tungsten wire
200 mm
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in Italy it is carnival time

• Can you guess how I am going to be dressed?

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I will be a Lxe detector!
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