Nuclear physics for geo-neutrino studies 1Neutrino Geoscience 2010 Gran Sasso National Laboratory - Italy October, 2010 Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare Lino Miramonti Gianni Fiorentini, Aldo Ianni, George Korga, Marcello Lissia, Fabio Mantovani, Lino Miramonti, Stefano Nisi, Lothar Oberauer, Michel Obolensky, Oleg Smirnov, Corrado Salvo, Yury Suvorov

Lino MiramontiNeutrino Geoscience In all experiments that use hydrocarbons as detection media, the employed reaction for the geo-neutrino detection is the inverse β decay on free protons: Where  σ(E ν ) is the cross section  f(E ν ) is the neutrino decay spectra of geo-ν produced in each β decay. The specific signal is affected by unknown uncertainties a) Whereas σ(E ν ) is affected by uncertainties of less than 1%, b) It is difficult to assess the accuracy of neutrino decay spectra f(E ν ), which are determined from rather indirect measurements and questionable theoretical assumptions. Motivation The relevant quantity is the specific signals s i Free protons Incoming flux of geo-ν

Lino MiramontiNeutrino Geoscience Geo-neutrinos are produced through pure β and β-γ processes: Our goal is to provide a framework for a direct measurement of the neutrino decay spectra f(E ν ) in order to establish the accuracy of the specific signal s i.

Lino MiramontiNeutrino Geoscience The neutrino decay spectrum f(E ν ) is obtained as a superposition of spectra calculated assuming the “universal shape” approximation: To determine the geo-neutrino neutrino decay spectra f(E ν ), one has to know the: i) Feeding probabilities p n of the different energy states of the final nucleus and ii) Shape of the neutrino spectrum for each transition. Feeding probabilities p n are derived from measurements of the intensities I n,m of γ lines. The feeding probability of the lowest state p 0 (Which produce the most energetic geo-neutrinos) is determined indirectly by subtraction: Neutrino spectra should actually be measured ! Q − E n is the maximal energy that the neutrino can take

Lino MiramontiNeutrino Geoscience The natural radioactivity of present Earth arises mainly from the decay (chains) of nuclear isotopes with half-lives comparable to or longer than Earth's age: 238 U, 235 U, 232 Th, 40 K, and 87 Rb. “Effective” geo-neutrinos are produced only in 238 U, 232 Th decay chains. “Effective” Geo-neutrinos i.e. Antineutrinos with energy above threshold for inverse beta decay on free proton: E th = MeV

Neutrino Geoscience 2010Lino Miramonti 238 U chain 232 Th chain 6

Lino MiramontiNeutrino Geoscience U: 234m Pa and 214 Bi 232 Th: 228 Ac and 212 Bi Considering just the 3 decays of 234m Pa, 214 Bi and 212 Bi one have ≈ 97% of the total signal (the respective contributions being ≈ 31%, ≈ 46% and ≈ 20% respectively). Geo-neutrinos from U are expected to contribute ≈ 80% Th are expected to contribute ≈ 20% [ chondritic ratio for masses Th/U = 3.9 ] 7

Lino MiramontiNeutrino Geoscience How to measure geo-neutrino decay spectra When a nucleus X decays, whatever the transition involved, energy conservation provides a connection between the neutrino energy E ν, the kinetic energy of the electron T e, the total energy of the emitted gammas E γ. In order to measure the geo-neutrino spectrum, we need to measure the visible energy E vis deposited by electrons and gammas by mirror reflection, we obtain the number of events as a function of neutrino energy 214 Bi

Lino MiramontiNeutrino Geoscience In order to measure the visible energy E vis we need a detector able to collect the energy released by both electrons and gammas (with a similar response to both particles) calorimetric measurement. The Counting Test Facility (CTF) The CTF is composed by 4 tons of liquid scintillator (PC + PPO) enclosed in a transparent nylon vessel of 2 m in diameter. The vessel is surrounded by a volume of water contained in a cylindrical stainless steel tank. The light is recorded by 100 PM. The CTF photomultipliers and read-out electronics allow the measurement of arrival times and Pulse Shape Discrimination. The time correlation of events allows to identify specific decay sequences associated with: 214 Bi- 214 Po (in the 238 U chain), 212 Bi- 212 Po (in the 232 Th chain).

Lino MiramontiNeutrino Geoscience Results from a diffuse Rn source in 2005 during some operations with the CTF there was a limited radon contamination due to an accidental air leak. Radon is the progenitor of both 214 Bi and 212 Bi In this case is possible to study only 214 Bi. ( τ 1/2 of 220 Rn is too short ) Data from this period have been used in order to: 1.determine the probability p 0 of populating the lowest energy state (assuming the universal allowed shape). 2.discuss the implications on the specific geo-neutrino signal s( 214 Bi). 3.determine if the spectrum of the pure β transition (that to the lowest state) is deformed with respect to the universal allowed shape. 222 Rn (in the 238 U chain -> 214 Bi) has a τ 1/2 = days 220 Rn (in the 232 Th chain -> 212 Bi) has a τ 1/2 = 55.6 s

Lino MiramontiNeutrino Geoscience Data selection Bi-Po coincidence The main selection criterion is that the coincidence time between the prompt β decay of 214 Bi and the delayed α decay of 214 Po, must be 2 μs < Δt 1/2 <602 μs. This yield 4.54 × 10 5 events. Others quality cuts such as energy cuts on the prompt and delayed signal, radial position of the two signals and α/β discrimination give an acceptance efficiency of 99.4%. This yield 4.46 × 10 5 events. To reduce systematic effects because the gammas are only partially contained and because of deviations from spherical symmetry, a Fiducial Volume cut at R=42cm is applied leaving 3.14×10 4 events.

Lino MiramontiNeutrino Geoscience Data have been fitted from 3 to 65 leaving free 3 parameters: i.p 0, ii.light yield, iii.normalization. 1) Feeding probability of the lowest state Events have been grouped into 65 bins from 0 to 3.4 MeV. The populations of the 82 excited 214 Po states are fixed at the values given in the table of isotopes (ToI)

Lino MiramontiNeutrino Geoscience At the minimum χ 2 /degrees of freedom = 61.7/(63 − 3) the best-fit value is p 0 = 0.177, with a statistical 1σ error of ± The total systematic error is estimated as −0.001 The value is consistent with the one reported in Table of Isotope.  CTF measures p 0  whereas Table of Isotope deduces p 0 from what was not observed! The largest systematic uncertainties originate from Imperfect spherical symmetry of the detector (because of the deformations of the inner vessel), Non uniform distribution of the active PMTs.

Lino MiramontiNeutrino Geoscience ) Implications for the specific signal The geo-neutrino signal s( 214 Bi) can be written as the sum of 2 contributions: where the cross section is averaged over the neutrino energy distribution. Assuming universal shape, that is the cross sections are From the previous analysis, we find that with errors of an order of half a percent

Lino MiramontiNeutrino Geoscience ) Shape factor for the pure β transition we release the assumption that the spectrum for the transition to the ground state has the universal shape. The electron kinetic-energy distribution ф(T e ) is We constrained p 0 and p 1 to the ToI values leaving unconstrained only y. This result shows the potentiality of detecting spectral deformations. Interesting results can be obtained in reducing statistical and systematic errors. A large improvement will be obtained by positioning suitable sources in the center of the CTF. The best fit, gives  p 0 = 0.177,  p 1 = 0.008,  y = −0.11 ± 0.06 (stat). At minimum: χ 2 = 51.6/(65-5) The statistical evidence of deformed shape is 2.4σ The dimensionless shape parameter y describes the deviation from the universal formula.

Lino MiramontiNeutrino Geoscience So far, we have estimated the 214 Bi geo-neutrino specific signal by using CTF data resulting from a limited radon contamination. Our estimate has a comparable error with the one derived from Table of Isotope. Our method has 2 advantages: The pure β transition can be detected in CTF and its probability can be measured directly. One can check the validity of the universal shape approximation for the most important decay mode. for more info see: Physical Review C (2010) Next step is to reduce both statistical and systematic errors (i.e.): Statistical error Δp 0 /p 0 ≈ 0.5% (being Δ 0 / 0 ≈ 0.5% ); this requires a statistics larger by a factor of about 20 or some 6 × 10 5 selected events. Systematic error; the largest improvement should be obtained by concentrating the source near the center of the detector. Dedicated Radon Source

Lino MiramontiNeutrino Geoscience Radon generator Preparation of a concentrated Rn source: We have built some quartz vials ( transparent to UV light ) with an external diameter of 50 mm ( that is the maximum diameter we can introduce in CTF from the insertion system ). In a vial we have introduced scintillator (i.e. PC+PPO) spilled from the CTF itself. We have “contaminated” the scintillator with Radon gas starting from a Radon generator. Because the Rn spike source has to be oxygen free we have built a system permitting to introduce Rn under nitrogen pressure. The total activity must be below 10 Hz ( because the electronic of the CTF ) to avoid pile up events. oxygen free system

Lino MiramontiNeutrino Geoscience The total number of collected Bi-Po events (≈ 2 weeks) are 3.25 × 10 5 Data obtained with the diffused Rn (blue) are within 50 cm from the CTF center for a total number of events of Data from DIFFUSE and VIAL QUARTZ 214 Bi The light yield, estimated from the position of the alpha peak, is compatible with the scintillator, indicating that the quenching effect is negligible.

Lino MiramontiNeutrino Geoscience Data have been fitted with a preliminary improved MonteCarlo code but some inputs have to be implemented due to the more complex geometry. Preliminary … in progress

Lino MiramontiNeutrino Geoscience Toward a measurement of 212 Bi spectrum: Pros and Cons of 212 Bi 212 Bi spectrum is much less complicated than 214 Bi; 212 Po has less than 10 excited states (to be compare with 82 of 214 Po). But unlike 214 Bi, the radon progenitor in the 232 Th chain, the 220 Rn, has an half-live of only 55.6 s. → We can’t start from 220 Rn has we did with 222 Rn for 214 Bi. We decided to start from natural thorium dissolved in Nitric Acid at 2%. Because Th is insoluble in PC we have used TriButyl Phosphate (TBP) to form stable hydrophobic complexes (these complexes are soluble in organic solvents) in order to “contaminate” the scintillator spilled from the CTF. The concentration of Th in TBP is measured by ICP-MS (≈ 100 ppb → ≈ 430 Bq/kg). The TBP concentration in the scintillator has to be as low as possible in order to avoid (minimize) variation in light yield and quenching effects. Fluorimetric measurements were performed in order to verify the light yield → no significant variation for a TBP concen. < 5%. 212 Po 214 Po

Lino MiramontiNeutrino Geoscience Because we want a sufficient statistic avoiding as much as possible light yield variations and quenching effects we have to increasing the source volume → cylindrical quartz vials. This vial has an external diameter of 5 cm and about 20 cm high for a total volume of about 340 cm 3. The total activity is ≈ 0.5 Bq. We have collected 1.82 × 10 5 events in 5.6 days. MC simulations are in progress in order to extracts feeding probabilities and specific signals of 212 Bi. Probably we need 2 different light yield; inside and outside the vial. α β

Lino MiramontiNeutrino Geoscience An interesting by-products: τ 1/2 measurement of 214 Po and 212 Po Table of Isotope reports: τ 1/2 ( 214 Po) = ± 2.0 μs(80% of references have been retrieved) τ 1/2 ( 212 Po) = 299 ± 2 ns (70% of references have been retrieved) 214 Po 212 Po From CTF data: τ 1/2 ( 214 Po) = ± 0.5 μs τ 1/2 ( 212 Po) = ± 1.1 ns Works are in progress to analyze in more detail systematic errors.