Past searches for kev neutrinos in beta-ray spectra

Transcription

1 Past searches for kev neutrinos in beta-ray spectra Otokar Dragoun Nuclear Physics Institute of the ASCR Rez near Prague supported by GAČR, P203/12/1896 The ν-dark 2015 Workshop TUM Institute for Advanced Study Garching, Germany, Dec 7-9, 2015

2 Topics 1. β-ray spectroscopy 2. Constraints on m ν of light neutrinos 3. Past searches for heavier neutrinos 4. Summary

3 1. β-ray spectroscopy History: powerful tool of experimental nuclear physics Present: model-independent constraint on the neutrino mass Magnetic β-ray spectrometer L. Meitner, O. Hahn, O von Bayern Natural radioactive substances β-spectrum is continuous J. Chadwick, 1914 RaB+C ( 214 Pb+ 214 Bi) E β up to 3.3 MeV Two detectors: current in ionization chamber Counts in Geiger counter Deutsches Museum in Munich O. Dragoun Review on conversion electron spectroscopy: Adv. Electronics and Electron Phys. 60 (1983) 1-94

4 Theory of β-decay assuming existence of both neutrino of W. Pauli and weak interaction (E. Fermi, 1934): dn de A F 2 2 E, Z 1 p E me E E E 0 0 m Kurie graph E. Fermi from measured β-spectra of that time: m ν << m e, probably m ν = 0

5 2. Constraints on m ν of light neutrinos Iron magnetic β-ray spectrometer ΔE instr = 1.5 kev at E = 167 kev Ω/4π = Cook et al. Phys. Rev. 73 (1948) Source Counter Kurie plot: m ν < 5 kev

6 From m ν < 5 kev (1948) to m ve < 2.3 ev (2005) MAC-E filter at Mainz Uni. (ΔE FW = 5 ev) Now: KATRIN monitor spectr. (ΔE FW = 1 ev) ν α > = Σ U αi ν i > α = e, μ, τ i = 1,2,3,(4?) ΔE instr versus m i m k Calculated β-spectrum of tritium m 1 = 0.2 ev m 2, m 3, U ei 2 from oscil. exp. m ve m e 3 1 U ei 2 m 2 i = ev

7 3. Searches for heavier neutrinos Phys. Lett. B R. E. Shrock (1980): 96 (1980) 159 search for neutrino mass states m i in β-spectra the kink at E 0 m i with amplitude U ei 2 U e4 2 < 0.1 for 0.1 kev m 4 3 MeV Kurie plots of tritium β-spectrum s exp E RE E s E, m de, th e Measured spectrum Predicted spectrum Spectrometer response function

8 3.1 The first claim for the 17 kev neutrino Simpson Phys. Rev.Lett. 54 (1985) 671 Tritium ions (10 15 MeV) implanted into Si(Li) detector. Depth ( mm) sufficient to stop all betas and bremsstrahlung photons Deviation at 1.5 kev massive ν component mass of 17.1 ± 0.2 kev admixture of 3 ± 1 %. Later, improved screening correction for low- energy beta admixture reduced to 1.1 ± 0.3 %. Hime and Simpson Phys. Rev. D 39 (1989) 1989

9 3.2 Further claim for the 17 kev neutrino (an example) Semiconductor spectrometer Hime and Jelley Phys. Lett. B257 (1991) 441 Si(Li) detector Anti-scatter baffle 35 S source (DATA/FIT 1) Detector aperture Source aperture

10 3.3 Evidence against the 17 kev neutrino (an example) Magnetic spectrometer with 30 independent detectors Ohshima et al. Phys. Rev. D 47 (1993) 4840 Conversion electrons from 109 Cd calibration source Response function of the whole setup

11 U e4 2 upper limits from 63 Ni β-spectrum Ohshima et al. Phys. Rev. D 47 (1993) kev ν admixture free: U e4 2 = (-1.1 ± 4.5) r 1.01 fixed : U e4 2 = r 1.45 U e4 2 at 95 % CL: < % for m 4 = 17 kev < 0.15 % for m 4 = 10.5 to 25 kev

12 3.4 Evidence against the 17 kev neutrino (an example) Si(Li) spectrometer with magnetic guiding Mortara et al. Phys. Rev. Let. 70 (1993) 394 Solid angle Ω = 2π sr weak source (kbq) smaller E loss smaller pileup No baffles smaller e scattering Adjustable impinge angle smaller e backscattering 80⁰ on source, 30⁰ on detector Magnetic mirror effect e backscattered with θ > 30⁰ are returned to detector within ns reduced backscattering tail

13 Comparison of measured β-spectra with theory (Mortara et al.) Pure 35 S source Q β - m 4 = 150 kev Composite 35 S + 14 C source Added 1.35% of 14 C U e4 2 = 0.85% 2 r 2.8 U e4 2 = 0 2 r 0.88 Fitted: 35 S + (1.4±0.1)% 14 C 2 r S only 2 r 3.59 No shape corrections were needed Experiment had sufficient sensitivity Fitting admixed β-spectrum is more difficult than searching for a kink

14 U e4 2 upper limits from 35 S β-spectrum Mortara et al. Phys. Rev. Let. 70 (1993) 394 U e4 2 m 4 (kev) The 17 kev ν admixture: not 1 % but < 0.18 %

15 3.5 Our experience from searches of kev neutrinos a) False 0.3% admixture of 17 kev ν in 35 S β-spectrum Si(Li) spectrometer Mass separated source Müller et al. Z. Naturforsch. 49a (1994) 874 s exp E RE E s E, m de, th e MC simulation of individual elastic and inelastic collisions R(E,E ) calculated without any adjustable parameter Neglected scattering on diaphragm wrong R(E, E ) false 0.3 % admixture of the 17 kev neutrino

16 b) Precision description of measured 241 Pu β-spectrum Q β = 20.8 kev, T 1/2 = 14 y Dragoun et al. J. Phys. G 25 (1999) 1839 Vacuum evaporated source γ-spectroscopy: activity, purity, homogeneity α-spectroscopy: thickness Our electrostatic electron spectrometer Adjustable ΔE instr Known spectrometer response function R(E, E )

17 MC simulation of electron energy losses within radioactive samples individual elastic and inelastic electron collisions Verification of Pu energy loss function Calculated shapes Lorentzian for natural width Gaussian for resolution function MC simulated energy losses Measured at JINR Dubna Application to 241 Pu β-ray source Energy losses due to backscattering within Pu layer within contamination overlayer

18 Electron spectra of 241 Pu and 241 Am Source: 241 Pu 6.5 MBq, atoms 1.7 nm thick Electrostatic spectrometer: region of 0.2 to 9.2 kev 5 ev step, 1 s exposure per point and sweep sweeps, 5700 h, 10 8 β-particles Upper limits on U e4 2 from three β-emitters

19 2 r c) Test of measurement stability Comparison of partial spectra taken in successive time intervals Universal method: no theoretical spectrum is needed no test run is needed 1 n jm i j, m w i k jm m n 1 1 Ni N j 2 m =1,2,..,M j = 2,3, M j m Dragoun et al. Nucl. Instr. Meth. 116 (1974) 459 See also Végh et al. Nucl. Instr. Meth. A281 (1989) 605

20 3.6 The best upper limits on U e4 2 Derived from measured β-ray spectra see e.g. arxiv: H, MAC-E-Filter 2 3 H, MAC-E-Filter Re, low-temp. calorim. 4 3 H, magn. spectr. 5 3 H, implant. in Si(Li) spectr Ni, magn. spectr Ni, magn. spectr S, Si(Li) sp. + magn. colim Cu, magn. spectr F, magn. spectr.

21 4. Summary Past β-spectroscopy : U e4 2 < for m 4 = 2 40 kev < for m 4 = kev 17 kev neutrino: U e4 2 1% found in 7 different experiments at 4 different institutions using 5 different isotopes NOW DISSPROVED see review by Wietfeldt and Norman Phys Rep 273, 149 Lesson for future experiments: - Calibrations should bracket the studied interval - Examine long-term stability of the apparatus - χ 2 test is not enough, prove sufficient sensitivity - MC simulations are educational, but examination of systematic uncertainties is unavoidable Good luck to our followers!