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Potassium Geo-neutrino Detection

Potassium Geo-neutrino Detection. Mark Chen Queen’s University Neutrino Geophysics, Honolulu, Hawaii December 15, 2005. Why Potassium Geo-neutrinos?. 16% of the radiogenic heat is from 40 K (based upon models) 3 rd isotope after 238 U and 232 Th largest flux!

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Potassium Geo-neutrino Detection

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  1. Potassium Geo-neutrino Detection Mark Chen Queen’s University Neutrino Geophysics, Honolulu, Hawaii December 15, 2005

  2. Why Potassium Geo-neutrinos? • 16% of the radiogenic heat is from 40K (based upon models) • 3rd isotope after 238U and 232Th • largest flux! • K may reside in the Earth’s core [V. Rama Murthy’s talk] • K/U ratio in chondrites > in the crust • where is the potassium? do we really know how much there is?

  3. 40K Decay • 89.28% Qb=1.311 MeV • 10.72% QEC=1.505 MeV • 10.67% to 1.461 MeV state (En = 44 keV) • 0.05% to g.s. (En = 1.5 MeV) 0.0117% isotopic abundance

  4. 40K Spectrum threshold for [figure from KamLAND Nature paper]

  5. Potassium Geo-neutrino Fluxes • (5-15) × 106 cm−2 s−1for the antineutrinos • (5-15) × 105 cm−2 s−1for the 44 keV ne • (2-6) × 103 cm−2 s−1for the 1.5 MeV ne • compare to 1.44 MeV pep solar neutrinos 1.42 × 108 cm−2 s−1 you can probably forget about the ne’s

  6. 40K Detection • -e scattering • not worth investigating due to solar ne (pep, CNO) • NC nuclear excitation • not distinctive from ne or g backgrounds • NC coherent neutrino-nucleus scattering [J. Collar’s talk] • again, not distinctive from solar neutrinos • CC processes to be examined…

  7. CC Reactions for Antineutrinos • inverse b-decay • inverse b-decay requires Qb + 1.022 MeV • 40K antineutrinos endpoint 1.311 MeV • need to find Qb < 0.289 MeV • resonant orbital electron capture • resonant capture only useful over a small range of energy…not for 40K

  8. Krauss, Glashow & Schramm • Nature paper (1984) proposed radiochemical detection; listed several possible antineutrino targets with product lifetime > 1 day • e.g. 3He →3H, Qb = 18.6 keV, t½ = 12.3 years • desirable to have small logft for large cross section • ~2000 atoms produced per year per kton • ~1/3 of those come from 40K • 35Cl→35S, Qb = 167 keV, t½ = 88 days • ~2 atoms produced by geo-neutrinos per year per kton

  9. CC Antineutrino Capture • e+ is produced • detection 1.022 MeV minimum visible energy • b--decay follows • long-lived: consider radiochemical (e.g. 3H, 35S) • short-lived: consider detection – disadvantage is the distribution of b- energies and low energies

  10. KGS Error • antineutrino captures on 64Zn (0+→1+ allowed transition) • 64Cu decays to 64Ni • KGS were thinking radiochemical detection of the stable 64Ni…mentioned in paper • error: sensitive to “40K, 238U, 232Th” X

  11. Low Qb Targets for 40K • 3He, 14N, 33S, 35Cl, 63Cu • potentially sensitive to 40K geo-neutrinos • allowed transitions to ground state • KGS also identified some allowed transitions to excited states for antineutrino capture • e.g. 79Br, 151Eu have low enough Q

  12. KGS Missed One! this one is sensitive to 40K geo-neutrinos!

  13. 106Cd for Potassium Geo-neutrinos • isotopic abundance 1.25% • 0+→1+ allowed transition to the 106Ag g.s. • Qb = 194 keV, detectable e+ (1.02-1.12 MeV) • followed by a t½=24 min EC decay (a big one) • can consider direct detection of reaction • could also consider radiochemical detection of Pd • it’s a positron decay also! (not a tiny branch) • “double-positron” signature potentially distinctive

  14. Direct Detection or Radiochemical? • (n,p) reactions produce background isotopes affecting a radiochemical measurement • stopped m− capture makes a background that affects only radiochemical it’s the prompt positron that rejects the above backgrounds → deep underground location certainly helps with the above • potassium geo-neutrino event rates are going to be so small you really want zero backgrounds…direct detection is better, if possible • delayed coincidence positron-positron!

  15. Cadmium Detectors • CdWO4 scintillating crystals • 106Cd enrichment possible (Kiev group has enriched 116Cd for double beta decay search)

  16. More Cadmium Detectors • CdZnTe semiconductor detectors • COBRA experiment is testing pixelated anodes for vertex reconstruction and tracking • 1 cm3 array of CdZnTe makes a good positron identifier (separately detect 511 keV g’s)? • COBRA mentions 106Cd as an interesting b+b+ candidate geo-neutrinos “catalyze” the 106Cd b+b+ decay

  17. Backgrounds from Double Beta? • actual double beta decay of 106Cd produces both positrons at once • antineutrino capture produces two positrons separated by t½=24 min • how about accidental coincidences (24 min window) • 113Cd (12.2% isotopic abundance) b decay (Q = 320 keV) • 14.2 kHz (for 1 ton of 113Cd) • 116Cd (7.5% isotopic abundance) bb decay (Q = 2.8 MeV) • 3.7 decays per second (for 1 ton of 116Cd) high isotopic purity of 106Cd is needed unless you have positron identification

  18. nucl-ex/0508016

  19. Geo-neutrino Signal Rates 106Cd • log ft = 4.7  • Qb = 194 keV  • remember Qthreshold = 1.216 MeV; 40K antineutrinos are emitted up to 1.311 MeV in the few to ~ten events per year per kiloton

  20. Summary • going beyond the Krauss, Glashow and Schramm paper…there is a new idea for 40K geo-neutrino detection using 106Cd • 106Cd could be made into scintillating crystals or semiconductor detectors • distinctive “double-positron” signature

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