Investigating Methods of Neutrinoless Double-Beta Decay Detection

1. Investigating Methods of Neutrinoless Double-Beta Decay Detection Matthew Rose Supervisor: Dr. R. Saakyan 4C00 Project Talk 13th March 2007

2. 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. Matthew Rose 4C00 Project Talk

3. bb decay 0nbb decay does the same, but by simultaneous emission of a ne and absorption of a ne, to conserve lepton number. 2nbb decay is the simultaneous decay of two neutrons to two protons, by emission of 2 e- and 2 ne. Matthew Rose 4C00 Project Talk

4. What can 0nbb decay teach us? • Nature of the n (Majorana or Dirac) • Place limits on the effective mass of the n, h mni, by finding the half life of 0nbb events. (T1/20n)-1 = (h mni /me)2G0n|M0n|2 / log(2) (uncertainties depend on matrix element calculations) T1/20n/h mni-2 Matthew Rose 4C00 Project Talk

5. Why is 0nbb so hard to find? • 0nbb is very rare (T1/20n > 1025yr), only ~1 in 105bb events is estimated to be a 0nbb. • The energies of 2nbb and 0nbb are quite distinct, however… Matthew Rose 4C00 Project Talk

6. Why is 0nbb so hard to find? Tiny energy signature, easily lost amongst background radiation Matthew Rose 4C00 Project Talk

7. 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 Matthew Rose 4C00 Project Talk

8. 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. Matthew Rose 4C00 Project Talk

9. 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? Matthew Rose 4C00 Project Talk

10. 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 Matthew Rose 4C00 Project Talk

11. 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. Matthew Rose 4C00 Project Talk

12. Laser Spectra Each has 5 unscaled spectra, they are so similar that any onecan be used for analysis. Background light is negligible. Matthew Rose 4C00 Project Talk

13. 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. Matthew Rose 4C00 Project Talk

14. Final Emission Spectra Matthew Rose 4C00 Project Talk

15. Finding DE/E • A fit accounting for the K, L and M energies gives us sK and EK. • 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. Matthew Rose 4C00 Project Talk

16. Finding DE/E Matthew Rose 4C00 Project Talk

17. Finding DE/E Matthew Rose 4C00 Project Talk

18. Results Matthew Rose 4C00 Project Talk

19. Comparison with Previous Results • Previous investigations have seen better DE/E with other coverings. • Have only investigated Mylar covering, variations may further improve DE/E. Matthew Rose 4C00 Project Talk

20. 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. Matthew Rose 4C00 Project Talk

21. 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 > I412 because DE/E404 < DE/E412 Using measured Karkhov spectra, can find light output (55 % Anthracene) and use this to scale the spectrumbefore multiplying by Q.E. Can get a (very) rough idea of DE/Ekarkhov using mean of constants. Matthew Rose 4C00 Project Talk

22. Using the Integrals Matthew Rose 4C00 Project Talk

23. 8.5%, 13.2%, 5.2% differences, acceptable for rough estimate of DE/E: DE/Ekarkhov' 9.25§0.65% Comparing integrals ( ?) Matthew Rose 4C00 Project Talk

24. 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. Matthew Rose 4C00 Project Talk