SrCl2 crystal for ECb search

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SrCl2 crystal for EC/b search

• Presented by J.H. So (KNU)

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Electron Capture/b

• It is a kind of double b decay.
• b decay has three decay mode, such as EC/EC for
  less than 1 MeV Q-value, EC/b for 1 MeV to 2
  MeV Q-value, b /b for more than 2 MeV Q-value.
• The Q-value of 84Sr is 1.787 MeV, so EC/ b is
  possible.

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• List of candidates

• EC/EC, EC/b, b/b had never been searched
  until today.
• We will search at 765 keV region for EC/b.

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Test setup for SrCl2

• Pure SrCl2 single crystal grown by CZ-method.
  (Jungin Kim)
• Cut and polished in the dimensions of 20 mm
  diameter and 15 mm height.
• Wrapped in teflon tape mounted on 2 inch PMT
  with bialkali photocathode with the help of
  optical grease.
• Whole assembly is encased in a light tight
  covering. (details in Gulroohs talk)

SrCl2 16.3 g (Sr 9 g)
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Relative light yield

• The light output of SrCl2 single crystal is 77
  of the light output of the CsI(Tl).

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Pulse height spectra of different radioisotopes
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Energy resolution
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a/ß light ratio
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Fluorescence decay time

• Fast decay time component is 91 ns (12) and slow
  decay time
• component is 907 ns(88)

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Pulse shape discrimination

• Good discrimination between a- and ?-pulses, for
  pulse height corresponding to
• 1MeV energy range

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Experimental Technique

• Putting source(EC/ ß) inside 4p CsI(Tl)
  crystals.
• Annihilation of ß gives the below possible
  signals.
• Using coincidence technique to tag the signals.
• Signal in figure 3 is the choice. Also we will
  use longitudinal back-to-back requirement with
  position measurement by attenuation length.

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Schematic
High Voltage ? -670 V for SrCl2 ? -1250
-1450 V for 4 p CsI Threshold 25 Trigger
pulse width 200 ns
For SrCl2
10 X
Preamp
400MHz FADC
ROOT
For 4 p CsI(Tl)
100 X
64MHz FADC
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Calibration (22Na)
We used 511 keV gamma ray for calibration and
calculation of preliminary limit.
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Signals from SrCl2 and 4 p CsI(Tl)
3 days data

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SrCl2 signals without 4 p CsI(Tl) signals
3 days data
We want to search here.
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Expected SrCl2 limits

• Q-value of 84Sr is 1.7868 MeV having natural
  abundance of 0.56.
• The interesting energy is 765 keV with 511 keV
  back to back gamma coincidence.
• Preliminary limit 1.84 X 1018 years (Previous
  limit was 7.3 X 1013 years.)
• This preliminary limit is calculated by CsI 0-hit
  data. After back to back coincidence analysis, it
  will be 10 times better than now.
• T0n1/2 (ln2/1.64) ? 6x1023? ? ? (a/A) ? ?(T/B
  R) at _at_ 90 C.L.
• B background index in cts/(keV kg year) ?
  42,000
• R FWHM energy resolution at Qbb in keV ?16
  at 511 keV (22Na)
• A mass number ?87.62, e detection
  efficiency at Qbb ? 100
• a EC/b isotope fraction ?0.05 g
• T measured time in year ? 1 year

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Summary and Future plan

• Pure SrCl2 single crystal developed for the first
  time of the neutrinoless EC/ß search.
• The scintillation properties of SrCl2 was good
  for EC/b search.
• The preliminary limit is 1.84 X 1018 years.
  (Previous limit was 7.3 X 1013 years.)
• We will take data for 3 months.
• Further background reduction is expected with
  triple coincidence between b signal and back to
  back g coincidence.
• We will investigate the contamination of 90Sr.(b
  decay Q value 2.27 MeV) from nucleon fall out.