40K Geoneutrino Detection

Mark Chen Queen’s University and the Canadian Institute for Advanced Research

Neutrino Geoscience 2015, Paris, France June 15, 2015 Why Geoneutrinos? n ~16% of ’s radiogenic heat is from 40K q would be good to quantify after 238U and 232Th n K/U ratio in > 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 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 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 -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 , 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 . 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 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 additions must have been a test, in spite of the fact that the theory pre- followed by the emission of a 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. Although calculations What was attempted80 days and whose beta-ray inspectrum 1939has an byof the energy Aux or heat transport within a upper limit of about 0.3 Mev. ' The minimum star have not reached this degree of refinement, energy of neutrino which can transform Cl" into it is nevertheless interesting to know whether the Crane… S" is therefore 1.3 Mev, a value well within the neutrinos escape from the center of the star range of energy of neutrinos emitted from without further collision with matter, or whether mesothorium and its products. In an attempt they constitute a rapid means of transport of to transform Cl" into S"I placed a capsule con- part of the star's energy from the core to the taining one mC of MsTh and its products in the outer regions. center of a bag containing three pounds of The question of whether the absorption process NaCl. After 90 days of irradiation I extracted the under discussion is great enough to prevent the sulphur' and measured its activity in an ioniza- escape of neutrinos from the sun can be decided tion chamber which was capable of detecting the by performing the experiment described, with emission of 10 electrons per second. No measur- 1000 mC of radioactive material, either meso- able activity was found. By taking into account or radium (with products). This will the large factor by which the sulphur was con- detect a cross section of 10 "cm'. In a column centrated in the chemical separation, it appears of the sun's matter one cm' in cross section, ex-' that a cross section of 10—"cm' for the absorp- tending from the center to the surface, there are tion of neutrinos by Cl" could have been de- about 2 &(1033 atoms (average atomic weight assumed to be that of air). There are consider- 2 E. B. Andersen, Zeits. f. physik. Chemic 32, 237 less than 2)&1033 atoms which favor- (1936); M. S. Livingston and H. A. Bethe, Rev. Mod. ably are as Phys. 9, 359 (1937). able as Cl" for the neutrino absorption process. 'The salt was dissolved in water and H202 added to We can therefore with some confidence oxidize the sulphur (which may have existed in various say that degrees of oxidation) to the sulphate form. The sulphate if the process is not detected in the laboratory, it was precipitated as barium sulphate. No sulphate carrier will not be of in was added because the NaCl already containd a trace of importance the question of the sulphate as an impurity. escape of neutrinos from the sun. Here’s a bold idea – Geo Chlorine Experiment n many ktons of NaCl n “expose” to the global geoneutrino flux q and reactor antineutrinos n chemical extraction of 35S q dissolve in water q hydrogen peroxide to oxidize q barium to precipitate the sulfate n collect small amount of precipitate and count in sensitive beta counter n …more on radiochemical experiments later! Another crazy idea… n many kilotons of liquid nitrogen (14N) in very clean cryostat n “expose” to geoneutrino flux n allow the N2 to evaporate n collect residue of atoms of 14C, try to count with sensitive beta counter n …what level of impurities are in LN2? n note: low-background experiments know how to purify LN2, typically with charcoal (probably bad!) KGS Error

n antineutrino captures on 64Zn (0+→1+ allowed transition) n 64Cu decays to 64Ni n KGS were thinking radiochemical detection of the stable 64Ni…mentioned in paper n error in paper: sensitive to “40XK, 238U, 232Th” 40 Low Qβ Targets for K ν e n 3He, 14N, 33S, 35Cl, 63Cu q all potentially sensitive to 40K geo-neutrinos q allowed transitions to ground state n KGS also identified some allowed transitions to excited states for antineutrino capture q e.g. 79Br, 151Eu have low enough Q Here’s the one I found… (KGS didn’t have in their list)

this one is sensitive to 40K geoneutrinos! 106Cd for Potassium Geoneutrinos n isotopic abundance 1.25% n 0+→1+ allowed transition to the 106Ag g.s. + n Qβ = 194 keV, detectable e (1.02-1.12 MeV) n followed by a t½=24 min EC decay (a big one) q can consider direct detection of reaction q could also consider radiochemical detection of Pd q it’s a positron decay also! (not a tiny branch) q “double-positron” signature potentially distinctive Cadmium Detectors

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

n CdZnTe semiconductor detectors n C0BRA experiment tested pixelated anodes for vertex reconstruction and tracking n array of 1 cm3 CdZnTe could make a good positron identifier q by separately detecting the 511 keV gammas…possible? n has been noted that 106Cd is an interesting β+β+ candidate

geoneutrinos “catalyze” 106Cd double positron decay Backgrounds from Double Beta?

n actual double beta+ decay of 106Cd produces both at once n antineutrino capture produces two positrons separated by t½=24 min n how about accidental coincidences? – q 113Cd (12.2% isotopic abundance) β decay (Q = 320 keV) n 14.2 kHz (for 1 ton of 113Cd) – – q 116Cd (7.5% isotopic abundance) β β decay (Q = 2.8 MeV) n 3.7 decays per second (for 1 ton of 116Cd)

high isotopic purity of 106Cd is needed and/or energy resolution, unless you have positron identification Geoneutrino Signal Rates 106Cd n log ft = 4.7 K n Qβ = 194 keV L 40 q remember Qthreshold = 1.216 MeV; K antineutrinos are emitted up to 1.311 MeV

in the few to 10 events per year per kiloton Positron Detection or Radiochemical? (in general) n (n,p) reactions produce background isotopes affecting a radiochemical measurement n stopped µ− capture makes a background that affects only radiochemical prompt positron detection rejects the above two backgrounds → deep underground location certainly helps with the above n potassium geoneutrino event rates are going to be so small you really want near zero backgrounds…direct positron detection is better, if possible n delayed coincidence positron-positron (like 106Cd) even better! My additional thoughts on radiochemical approach n don’t forget (unfortunately) that if you lower the threshold, you get more U and Th that you need to subtract away q doable, but incurs a statistical penalty n ideally, locate geoneutrino radiochemical experiment near large liquid geoneutrino detector…and deep underground q like at Jinping underground lab in China q the liquid scintillator geoneutrino detector quantifies the U and Th at the location of the integrating radiochemical experiment n of course, also have to subtract the reactor-antineutrino-induced radiochemical events q want a low reactor background site (like at CJPL) Summary n 10 years later…still no easily achievable approach for 40K geoneutrino detection n best ideas (I think) are: q 106Cd q radiochemical 35Cl or 14N q in distant third, directional antineutrino-electron detector with recoil directionality