40K Geoneutrino Detection Mark Chen Queen’s University and the Canadian Institute for Advanced Research Neutrino Geoscience 2015, Paris, France June 15, 2015 Why Potassium Geoneutrinos? n ~16% of Earth’s radiogenic heat is from 40K q would be good to quantify after 238U and 232Th n K/U ratio in chondrites > in the crust q would measurement help pin down abundances of other moderate volatiles in the Earth? n K may reside in the Earth’s core?? q e.g. Murthy, Lee and Jeanloz, Ohtani q does this help solve some core energetics issues (e.g. geodynamo, heat flow at CMB)? Th & U Volatility trend @ 1AU from Sun slide from Bill McDonough 40K Decay 40K→40Ca + e− +ν n 89.28% Qβ=1.311 MeV e 40 − 40 n 10.72% QEC=1.505 MeV K + e → Ar +ν e q 10.67% to 1.461 MeV state (Eν = 44 keV) q 0.05% to g.s. (Eν = 1.5 MeV) 0.0117% isotopic abundance 40K Spectrum + threshold for ν e + p → e + n [figure from KamLAND Nature paper] Potassium Geoneutrino Fluxes n (5-15) × 106 cm−2 s−1 for the antineutrinos 5 −2 −1 n (5-15) × 10 cm s for the 44 keV νe 3 −2 −1 n (2-6) × 10 cm s for the 1.5 MeV νe n compare to 1.44 MeV pep solar νe 8 −2 −1 1.42 × 10 cm s (and also CNO solar neutrinos at 1.5 MeV) You can probably forget about detecting the –5 1.5 MeV νe … only 63 keV away and 10 less intense! 40 K ν e Detection n ν e -e scattering q requires electron recoil directionality due to large flux of solar neutrinos (could imagine giant TPC) n 1/3 event per ton per year n NC nuclear excitation q not distinctive from νe or γ nuclear excitation n NC coherent neutrino-nucleus scattering q again, not distinctive from solar neutrinos unless you have nuclear recoil directionality [see Drukier talk] q energy resolution challenging (to distinguish 40K from U and Th geoneutrino in energy spectrum) n CC processes to be examined… CC Reactions for Antineutrinos n inverse β-decay + ν e + (A, Z) → (A, Z −1) + e q inverse β-decay requires Qβ + 1.022 MeV q 40K antineutrinos endpoint 1.311 MeV q need to find Qβ < 0.289 MeV n resonant orbital electron capture − ν e + e + (A, Z) → (A, Z −1) q resonant capture only useful over a very small range of energy…not useful for 40K spectrum CC Antineutrino Capture n e+ is produced [prompt] q detection 1.022 MeV minimum visible energy — n β decay follows [delayed] q long-lived: consider radiochemical (e.g. 3H, 35S) q short-lived: consider delayed detection of β– - n challenges being the distribution of β energies, and the low energies involved Krauss, Glashow & Schramm n Nature paper (1984) proposed radiochemical detection; listed several possible antineutrino targets with product lifetime > 1 day 3 3 q e.g. He → H, Qβ = 18.6 keV, t½ = 12.3 years q desirable to have small log ft for large ν e cross section 1 σ ~ ft n ~2000 atoms produced per year per kton n ~1/3 of those come from 40K 35 35 q Cl→ S, Qβ = 167 keV, t½ = 88 days n ~2 atoms produced by all geoneutrinos per year per kton On the topic of radiochemical experiments… n who performed the first radiochemical neutrino experiment? q Ray Davis, of course, right? q including non-detection of Cl to Ar with reactor antineutrinos in 1953 n who proposed the concept of radiochemical neutrino experiment (especially with chlorine target)? q Bruno Pontecorvo, right? ABSORPTION OF NEUTRI NOS A similar expression to (6) can be worked out (Z=30) is (4X0.53)/30=0.071A. With a screen- for the L electrons. However, there are two ing constant of about 6 this radius would be types of I electrons, the 2s and the 2p electrons. (4X0 53)/(30 —6) = 0.088A. The discrepancy be- The wave functions for the I electrons are tween yl, —0.020A and 0.071 or 0.088A is too given by quantum mechanics. ' From these the Z„ great. Putting it another way, if we make value for each kind of L electron may be calcu- yl, —0.088A in (9) and then calculate from (8), lated from the formula we obtain fr. =0 232. so that f=fx+f1.=2.140. Our experimental value f= 3.5 is thus con- sin kr siderably larger than what would seem to be a n(r) dr, (7) kr reasonable value of f on theoretical grounds. We have carefully repeated the diffuse scattering where k=(4s. sin —',p)/X and u(r)dr is the proba- experiment at @=90' several times but we bility of the electron being between spheres of consistently obtain values of (Sp/p), ~ which radii r and r+dr. The formulas for the 8,'s so necessitate high values of f Our r. esults require obtained contain a parameter y. If it is assumed that f decrease more slowly at high values of that y is the same for each type of electron, the (sin —,'4)/X than would otherwise be expected. formulas may be added so as to give Either the X or the L electrons or both are on the average concentrated more closely to the = — — — f 1=2E,, , +68, , , 8(1 y)(1 y/2)(1+y) ', (8) nucleus than on theoretical grounds we had where expected. In Fig. 1 it is interesting to note that all y = (16m'yl, ' sin' —',y)/X'. (9) curves and points approach each other closely at &=90'. At large angles the effect of the Now = at (sin ~~/)/X 2.2, fr, = 8X0.211= 1.688. atomic vibrations becomes negligible and the Solving for (8) y, we obtain y=0.30. From (9) zinc crystal scatters x-rays in the same way as this gives y~=0.020A. The radius of the I orbit gaseous zinc atoms would scatter the rays. in the Bohr model of a hydrogen-like atom In conclusion we wish to thank Mr. J. E. Nafe Radiochemical' See Neutrinofor assistance Absorptionin the calculation of the curves L. Pauling and E. B. Wilson, Introduction to Quan- tum Mechamcs (McGraw-Hill Book Co., 1935),pp. 134, 135. in Fig. i. Experiment on Chlorine-35 MARCH 1, 1939 P H YS ICAL REVIEW V GLUM E 55 An Attempt to Observe the Absorption of Neutrinos H. R. CRANE Vn7,versity of 3A'chigan, Ann Arbor, Michigan (Received January 10, 1939) ' 'T HAS been quite conclusively demonstrated' example of this is that the presence of neutrinos cannot be Cl" p~S35+ e+. detected by an ionization effect, of the kind which + results from the passage of charged particles, The product S" is a radioactive isotope (as neutrons or gamma-rays through matter. At would be true of the product in general), and least one possibility of detecting them remains, decays back to C13~ with the emission of a nega- however, and that is by a process which is the tive electron and a neutrino: reverse of the E-electron capture process. An — S35~C135+e + I M. E. Nahmias, Proc. Cambridge Phil. Soc. 31, 99 (1935}. Adding the equations we see that the energy of acknowledging C. Peña-Garay for showing me this paper 502 H. R. the neutrino producing the transformation must tected. This is based upon the estimate that be greater than 2mc'+ 8'0, where 8'0 is the upper 3 X10' neutrinos of energy greater than 1.3 Mev limit of the beta-ray spectrum of the radioactive (the threshold energy for the transformation) are isotope produced. The process is a cyclic one, emitted per second from the source used. in which nothing but the creation of electron The way in which this result applies to an pairs is accomplished. astrophysical question is as follows. On the A rough estimate for high energies on the basis that nuclear transformations of the familiar basis of the Fermi theory of the cross section for kind constitute the energy source, and that the above eA'ect has been made for me by Pro- hydrogen is the main building material, we sup- fessor G. Breit, and is 0~10 "(E„/mc')' This pose that a star generates about six percent of indicates that neutrinos of cosmic-ray energies are its energy in the form of neutrinos. When a pro- readily absorbed, but that those of only a few ton is added to a nucleus about eight Mev Mev will not be absorbed in a detectable amount. energy is liberated, on the average. Since in all It is possible to perform an experiment which the nuclei built up the atomic weight is roughly will test for a cross section as small as 10 "cm', twice the atomic number, we can assume that at and it has seemed to me worth while making such least half the proton additions must have been a test, in spite of the fact that the theory pre- followed by the emission of a positron and a dicts a much smaller cross section. The experi- neutrino. If the average energy of the neutrinos ment is of interest especially because the results is taken as one Mev, we have about six percent have an application in astrophysics. for the fraction of the energy which is generated S" is a radioactive isotope whose half-life is in the form of neutrinos.