Potassium Geo-neutrino Detection

Mark ChenQueen’s University

Neutrino Geophysics, Honolulu, HawaiiDecember 15, 2005

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?

40K Decay

89.28% Q=1.311 MeV 10.72% QEC=1.505 MeV

10.67% to 1.461 MeV state (E = 44 keV) 0.05% to g.s. (E = 1.5 MeV)

e

e

AreK

eCaK

4040

4040

0.0117% isotopic abundance

40K Spectrum

[figure from KamLAND Nature paper]

threshold for nepe

Potassium Geo-neutrino Fluxes (5-15) × 106 cm−2 s−1 for the antineutrinos (5-15) × 105 cm−2 s−1 for the 44 keV e

(2-6) × 103 cm−2 s−1 for the 1.5 MeV e

compare to 1.44 MeV pep solar neutrinos 1.42 × 108 cm−2 s−1

you can probably forget about the e’s

40K 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…

e

e

CC Reactions for Antineutrinos inverse -decay

inverse -decay requires Q + 1.022 MeV 40K antineutrinos endpoint 1.311 MeV need to find Q 0.289 MeV

resonant orbital electron capture

resonant capture only useful over a small range of energy…not for 40K

eZAZAe )1,(),(

)1,(),( ZAZAee

Krauss, Glashow & Schramm

Nature paper (1984) proposed radiochemical detection; listed several possible antineutrino targets with product lifetime > 1 day e.g. 3He →3H, Q = 18.6 keV, t½ = 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 40K

35Cl→35S, Q = 167 keV, t½ = 88 days ~2 atoms produced by geo-neutrinos per year per kton

e

ft1~

CC Antineutrino Capture

e+ is produced detection 1.022 MeV minimum visible energy

-decay follows long-lived: consider radiochemical (e.g. 3H, 35S) short-lived: consider detection – disadvantage is the

distribution of energies and low energies

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

Low Q 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

e

KGS Missed One!

this one is sensitive to 40K geo-neutrinos!

106Cd for Potassium Geo-neutrinos isotopic abundance 1.25% 0+→1+ allowed transition to the 106Ag g.s. Q = 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

Direct Detection or Radiochemical? (n,p) reactions produce background isotopes

affecting a radiochemical measurement stopped − capture makes a background that

affects only radiochemicalit’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

CdWO4 scintillating crystals

106Cd enrichment possible (Kiev group has enriched 116Cd for double beta decay search)

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 ’s)?

COBRA mentions 106Cd as an interesting candidate

geo-neutrinos “catalyze” the 106Cd decay

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) decay (Q = 320 keV)

14.2 kHz (for 1 ton of 113Cd) 116Cd (7.5% isotopic abundance) 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

nucl-ex/0508016

Geo-neutrino Signal Rates 106Cd log ft = 4.7 Q = 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

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