Investigating Methods of Neutrinoless Double-Beta

Decay Detection

Matthew Rose

Supervisor: Dr. R. Saakyan

4C00 Project Talk

13th March 2007

Matthew Rose 4C00 Project Talk 2

Talk Overview

• An explanation of 0 decay. • What can be learnt from 0 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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2 decay is the simultaneous decay of

two neutrons to two protons, by emission of

2 e- and 2 e.

decay

0 decay does the same, but by

simultaneous emission of a e and absorption of a e, to conserve lepton

number.

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What can 0 decay teach us?

• Nature of the (Majorana or Dirac)• Place limits on the effective mass of the

, h m i, by finding the half life of 0 events.

(T1/20)-1 = (h mi /me)2G0 |M0|2 / log(2)

(uncertainties depend on matrix element calculations)

T1/20 / h m i-2

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Why is 0 so hard to find?

• 0 is very rare (T1/2

0> 1025yr), only ~1 in 105 events is estimated to be a 0

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

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Why is 0 so hard to find?

Tiny energy signature, easily lost amongst background radiation

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Detecting Events - Super-NEMO

• Super-NEMO will look for 0 decays

• source foil surrounded by tracking volume and Calorimeter (PMTs and Scintillators)

Light output (Nph)/ Ee

Nph x Q.E. = Npe

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E/E, the Energy Resolution

• Npe follows a poisson distribution, so

EE

=FWHM

E=

2.35σ

E

σ = mean = N pe

EE

=2.35

N pe

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

• Current E/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

decays?• What are the W.O.M.E. for the scintillators?

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Spectrometer range = 340-1000nm?

• 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…

470nm470nm

403.5nm403.5nm

475nm475nm

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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.• >80 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 E/E

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

• 207Bi is used to produce particles, as it has 2 conversion electrons at 494 and 967 keV.

• 207Bi is a AND source.• can be stopped easily, so

+ and are taken.

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

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Finding E/E

2.35σ K

EK

=7.90%2.35σ K

EK

=819%

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Finding E/E

2.35σ K

EK

=104%

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Results

Scintillator

of max emission (nm) E/E (%)

(with Hamamatsu R6233MOD PMT)Bicron Measured

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 E/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 E/E with other coverings.

• Have only investigated Mylar covering, variations may further improve E/E.

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Results

• Target E/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 E/E / (Npe)-1/2; I = N£ Q.E. = Npe

E/E £ (Npe)1/2 = constant

Should find:

I404 ' I408 because E/E404 ' E/E408

I404 > I412 because E/E404 < E/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 E/Ekarkhov using mean of constants.

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Using the Integrals

ScintillatorLight Output

(% Anthracene)E/E (%)

Integral

(I / Npe)E/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 E/E:

E/Ekarkhov' 9.25§0.65%

E / E408

ΔE / E412

= 0.788,N pe412

N pe408

= 0.861

E / E404

ΔE / E412

= 0.750,N pe412

N pe404

= 0.864

E / E404

ΔE / E408

= 0.951,N pe408

N pe404

= 1.003

E/E (A)

ΔE/E (B)=

N pe(B)

N pe(A)

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Summary• Aiming for 7% E/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 E/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.