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! 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?

40 K Decay 89.28% Q  =1.311 MeV 10.72% Q EC =1.505 MeV  10.67% to MeV state (E = 44 keV)  0.05% to g.s. (E = 1.5 MeV) % isotopic abundance

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

Potassium Geo-neutrino Fluxes (5-15) × 10 6 cm −2 s −1 for the antineutrinos (5-15) × 10 5 cm −2 s −1 for the 44 keV e (2-6) × 10 3 cm −2 s −1 for the 1.5 MeV e compare to 1.44 MeV pep solar neutrinos 1.42 × 10 8 cm −2 s −1 you can probably forget about the e ’s

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

CC Reactions for Antineutrinos inverse  -decay  inverse  -decay requires Q  MeV  40 K antineutrinos endpoint MeV  need to find Q   MeV resonant orbital electron capture  resonant capture only useful over a small range of energy…not for 40 K

Krauss, Glashow & Schramm Nature paper (1984) proposed radiochemical detection; listed several possible antineutrino targets with product lifetime > 1 day  e.g. 3 He → 3 H, Q  = 18.6 keV, t ½ = 12.3 years  desirable to have small log ft for large cross section ~2000 atoms produced per year per kton ~1/3 of those come from 40 K  35 Cl→ 35 S, Q  = 167 keV, t ½ = 88 days ~2 atoms produced by geo-neutrinos per year per kton

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

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

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

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

106 Cd for Potassium Geo-neutrinos isotopic abundance 1.25% 0 + → 1 + allowed transition to the 106 Ag g.s. Q  = 194 keV, detectable e + ( MeV) followed by a t ½ =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

Direct Detection or Radiochemical? (n,p) reactions produce background isotopes affecting a radiochemical measurement stopped  − 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!

Cadmium Detectors CdWO 4 scintillating crystals 106 Cd enrichment possible (Kiev group has enriched 116 Cd for double beta decay search)

More Cadmium Detectors CdZnTe semiconductor detectors COBRA experiment is testing pixelated anodes for vertex reconstruction and tracking 1 cm 3 array of CdZnTe makes a good positron identifier (separately detect 511 keV  ’s)? COBRA mentions 106 Cd as an interesting     candidate geo-neutrinos “catalyze” the 106 Cd     decay

Backgrounds from Double Beta? actual double beta decay of 106 Cd produces both positrons at once antineutrino capture produces two positrons separated by t ½ =24 min how about accidental coincidences (24 min window)  113 Cd (12.2% isotopic abundance)  decay (Q = 320 keV) 14.2 kHz (for 1 ton of 113 Cd)  116 Cd (7.5% isotopic abundance)  decay (Q = 2.8 MeV) 3.7 decays per second (for 1 ton of 116 Cd) high isotopic purity of 106 Cd is needed unless you have positron identification

nucl-ex/

Geo-neutrino Signal Rates 106 Cd log ft = 4.7  Q  = 194 keV   remember Q threshold = MeV; 40 K antineutrinos are emitted up to MeV in the few to ~ten events per year per kiloton

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