Investigating%20Methods%20of%20Neutrinoless%20Double-Beta%20Decay%20Detection

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Investigating Methods of Neutrinoless Double-Beta
Decay Detection

• Matthew Rose
• Supervisor Dr. R. Saakyan
• 4C00 Project Talk
• 13th March 2007

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Talk Overview

• An explanation of 0nbb decay.
• What can be learnt from 0nbb decay?
• The Super-NEMO detector Calorimeter design.
• Why is Energy Resolution Important?
• How do we improve Energy Resolution?
• Studying Scintillators Photomultipliers.
• Results Achieved Energy Resolutions.
• Applications.
• Comparison with Previous Results.

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bb decay

• 2nbb decay is the simultaneous decay of two
  neutrons to two protons, by emission of 2 e- and
  2 ne.

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What can 0nbb decay teach us?

• Nature of the n (Majorana or Dirac)
• Place limits on the effective mass of the n, h mn
  i, by finding the half life of 0nbb events.
• (T1/20n)-1 (h mni /me)2G0n M0n2 / log(2)
• (uncertainties depend on matrix element
  calculations)
• T1/20n / h mn i-2

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Why is 0nbb so hard to find?

• 0nbb is very rare (T1/20n gt 1025yr), only 1 in
  105 bb events is estimated to be a 0nbb.

• The energies of 2nbb and 0nbb are quite distinct,
  however

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Why is 0nbb so hard to find?
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Detecting Events - Super-NEMO

• Super-NEMO will look for 0nbb decays
• bb source foil surrounded by tracking volume and
  Calorimeter (PMTs and Scintillators)

Light output (Nph)/ Ee Nph x Q.E. Npe
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DE/E, the Energy Resolution

• The energy resolution is related to the spread of
  the energy spectrum.

• Npe follows a poisson distribution, so

• Current DE/E 14 at 1 MeV.
• Aiming for 7 at 1 MeV, need an improvement in
  Npe by a factor of 4.

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PMTs Scintillators

• Must match Q.E. to wavelength of maximum
  emission.
• To do so, need to accurately know the emission
  spectra of the scintillators.
• Using a miniature spectrometer, can achieve this.
• First, does the spectrometer work?
• Can Laser or X-rays be used to approximate b
  decays?
• What are the W.O.M.E. for the scintillators?

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Spectrometer range 340-1000nm?
470nm
475nm

• Spectra of LEDs taken to test sensitivity around
  the 400-500nm region (region of scintillators)
• Consistent results give confidence in the
  sensitivity of spectrometer at these wavelengths.
• Now can take spectra of Scintillators

403.5nm
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Spectrometer Setup

• Laser hits scintillator, produces light
• Light travels along fibre to spectrometer
• Data from spectrometer is stored on Laptop
• Data analysed using ROOT
• Four different scintillator samples studied -
  Bicron because of high light output.
• gt80 spectra were taken for laser results alone,
  with various orientations of laser and
  scintillator.

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Laser Spectra
Each has 5 unscaled spectra, they are so similar
that any onecan be used for analysis. Background
light is negligible.
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Laser vs. X-ray spectra

• Repeated with X-rays for all but BC-408.
• Little difference between the spectra produced.
• Decided that Laser can be used to simulate
  ionizing radiation.
• Can therefore take wavelengths of maximum
  emission from Laser plots.

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Final Emission Spectra
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Finding DE/E

• 207Bi is used to produce b particles, as it has 2
  conversion electrons at 494 and 967 keV.
• 207Bi is a b AND g source.
• b can be stopped easily, so b g and g are
  taken.

• The two spectra are normalised about the region
  of g only. Subtracting the spectra should now
  give the b energy spectrum.

• A fit accounting for the K, L and M energies
  gives us sK and EK.

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Finding DE/E
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Finding DE/E
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Results
Scintillator l of max emission (nm) l of max emission (nm) DE/E () (with Hamamatsu R6233MOD PMT)
Scintillator Bicron Measured DE/E () (with Hamamatsu R6233MOD PMT)
BC-404 408 414-420 7.8
BC-408 425 426-468 8.2
BC-412 434 432-436 (424-8 also noted) 10.4
Karkhov - 418-425 -
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Comparison with Previous Results
Scintillator Coating DE/E,
BC-404 None 9.4
BC-404 Mylar 7.8
BC-404 Tyvec 8.2
BC-404 Mylar/Tyvec 7.4
BC-408 None 9.7
BC-408 Mylar 8.2
BC-408 Tyvec 8.5
BC-408 Mylar/Tyvec 7.7

• Previous investigations have seen better DE/E
  with other coverings.
• Have only investigated Mylar covering, variations
  may further improve DE/E.

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Results

• Target DE/E of 7 at 1 MeV seems within reach.
• The R6233 used has Q.E.max of 34.9 at 350 nm.

• Multiplying normalised spectra by Q.E. and Light
  Outputs can give interesting plots.
• The integral of this plot is proportional to Npe.

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Using the Integrals
DE/E / (Npe)-1/2 I Ng Q.E. Npe DE/E
(Npe)1/2 constant
Should find I404 ' I408 because DE/E404 '
DE/E408 I404 gt I412 because DE/E404 lt DE/E412
Using measured Karkhov spectra, can find light
output (55 Anthracene) and use this to scale
the spectrum before multiplying by Q.E. Can get a
(very) rough idea of DE/Ekarkhov using mean of
constants.
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Using the Integrals
Scintillator Light Output ( Anthracene) DE/E () Integral (I / Npe) DE/E (I)1/2
BC-404 68 7.8 17.183 32.33
BC-408 64 8.2 17.283 34.09
BC-412 60 10.4 12.819 37.24
Karkhov from spectra 55 9.1 14.261 mean 34.55
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Comparing integrals ( ?)

• 8.5, 13.2, 5.2 differences, acceptable for
  rough estimate of DE/E
• DE/Ekarkhov' 9.250.65

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Summary

• Aiming for 7 DE/E at 1 MeV.
• Have achieved 7.8 at 967 keV.
• This can be improved with change of scintillator
  covering and possibly through use of a
  Green-extended PMT.
• Have a convenient quick way to verify emission
  spectra of scintillators.
• Can estimate DE/E with reasonable precision from
  emission Q.E. spectra, which can be used to
  pre-judge suitability of scintillators before
  testing and also to check results.