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Experimental Approaches to Sterile Neutrinos Using Low Energy Neutrinos Jonathan Link

Experimental Approaches to Sterile Neutrinos Using Low Energy Neutrinos Jonathan Link Center for Neutrino Physics Virginia Tech NOW 2012 9/14/12. 115 In + ν e → 115 Sn * + e - → 115 Sn + 2 γ. The LENS-Sterile Proposal. Scintillation Lattice:

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Experimental Approaches to Sterile Neutrinos Using Low Energy Neutrinos Jonathan Link

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  1. Experimental Approaches to Sterile Neutrinos Using Low Energy Neutrinos Jonathan Link Center for Neutrino Physics Virginia Tech NOW 2012 9/14/12

  2. 115In + νe → 115Sn* + e- →115Sn + 2γ The LENS-Sterile Proposal Scintillation Lattice: Voxels separated by clear films channel light down the coordinate axis by total internal reflection. Spatial resolution of order of cell size over root 12. Proposal to insert a Mega-Curie 51Cr source in the center of the LENS detector to observe multiple wavelengths of large Δm2 oscillations in a few meters. LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976). Jonathan Link

  3. 115In + νe → 115Sn* + e- →115Sn + 2γ The LENS-Sterile Proposal Raju Raghavan 1937-2011 Proposal to insert a Mega-Curie 51Cr source in the center of the LENS detector to observe multiple wavelengths of large Δm2 oscillations in a few meters. LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976). Jonathan Link

  4. Gallex, Sage, 51Cr and Sterile Neutrinos Giunti and Lavender (Mod. Phys. Lett. A22, 2499) noted that the low ratio of observed to expected ν +71Ge interactions in the Gallex and SAGE source experiments: R = 0.88 ± 0.05 may be due to sterile neutrino oscillations. Now commonly known as the “Gallium Anomaly” Jonathan Link

  5. The Gallium Anomaly νe disappearance in Gallex and SAGE from 51Cr and 37Ar source C. Giunti and M. Laveder, Phys. Rev. C83, 065504 (2011). Jonathan Link

  6. Electron Capture Neutrino Sources Electron capture isotopes decay to two bodies and as such produce a mono-energetic beam of neutrinos at low energies. 51Cr + s-shell e- → 51V + νe (+ X-ray) Sources such as this have played a critical role in the calibration of radiochemical experiments as a proxy source of solar neutrinos with a well known flux. Advances in detector technology such as: Borexino’s ability to do real time detection of 7Be neutrinos have created new opportunities for groundbreaking neutrino physics using electron capture sources. Jonathan Link

  7. 51Cr as a Mono-Energetic Neutrino Source K shell capture L shell capture 90% of the time the capture goes directly to the ground state of 51V and you get a 750 keV neutrino. 10% of the time it goes to an excited state of 51V and you get a 320 keV photon plus a 430 keV neutrino. Jonathan Link

  8. Advantages of 51Cr • Can be easily produced with thermal neutron capture (50Cr has a ~17 barn neutron capture cross section). • Has a long, but not too long, lifetime (39.9 days). • Longer lifetimes require more neutrons to get high rates • Shorter lifetimes lose too much rate in shipping and handling • Has one, relatively easy to shield, gamma that accompanies 10% of decays. • 5 cm of tungsten reduce 320 keV γ rate from 1 MCi to 1 Hz • 19 cm are needed to reduce 1 Ci of 1 MeV γ to 1 Hz • Mega-Curie scale sources have been produced by both Gallex and SAGE. Jonathan Link

  9. The High Flux Isotope Reactor (HFIR) at ORNL HFIR operates at 85 MW with 23 operating days each fuel cycle. Jonathan Link

  10. The High Flux Isotope Reactor (HFIR) at ORNL Thermal neutron flux of 2.5×1015/cm2/s in the target region. 40 times larger neutron flux than what was used by Gallex 4 times higher mean capture cross section than what was used by SAGE Jonathan Link

  11. Solar Neutrino Detectors & Source Sterile Searches • What works for LENS may work for other low energy solar neutrino detectors. (ES rate ~102× larger than 115In rate in LENS) • Mono-energetic neutrinos → known neutrino energy → • You don’t need a charged current process • You still need good spatial resolution to fix L/E • Candidate detectors include: • Large liquid noble gas scintillating detectors: Clean, XMASS, Xenon100 • Large LS detectors: Borexino, SNO+, Kamland • All these detectors would use electron elastic scattering • NC detection is another interesting idea • (Formaggio et al, Phys. Rev. D 85, 013009) Jonathan Link

  12. Large Detectors and Centrally Located Sources A centrally located source maximizes the interaction rate per MCi. With no oscillation the event rate is a flat function in radius. A source inside the detector would need to be well shielded. Initial 2 MCi Source for a 70 day Run (3+1) (3+2) Models from the fit of Kopp, Maltoni & Schwetz arXiv:1103.4570 [hep-ph] Jonathan Link

  13. Real Time Detectors Require Serious γ Shielding The 320 keV gamma (10% of decays) is a non-issue compared to the internal bremsstrahlung in ~0.05% of decays. gammas/sec·10 keV Spherical energy (keV) Possible source and W-alloy shielding configuration… But what is the activity of W? Jonathan Link

  14. Low-Background Counting of Tungsten Alloy Measured at the Kimballton Underground Research Facility (KURF) Signal Sample Background 214Pb 234Pa 238U < 5mBq/kg (from 234Pa) 232Th <40 mBq/kg (from 228Ac) 40K < 220 mBq/kg 214Bi = 150±50 mBq/kg 214Bi 228Ac 40K 214Bi 214Bi 208Tl Background Subtracted Jonathan Link

  15. SNO+ Source Deployment Case Study Jonathan Link

  16. Signal to Noise Ratio as a Function of Radius Detector BG grows as r2 Source BG falls as e−r/λ Assuming a uniform detector BG out to the fiducial radius. Jonathan Link

  17. SNO+ Sensitivity Sensitivity based on a χ2 fit to signal and BG over the full energy range. Not that sensitive to backgrounds Source normalization and spatial resolution are critical to large Δm2 resolution. Statistics limited measurement. 90% CL contours GLoBES 3+1 contours from Kopp, Maltoni & Schwetz arXiv:1103.4570 [hep-ph] Jonathan Link

  18. Borexino Sensitivity (See Aldo Ianni’s Talk) Borexino is the only detector where we know this will work. One can use the vast solar phase to subtract backgrounds. 10 MCi Source 8.25 m Source Under Detector Jonathan Link

  19. Conclusions Mega-Curie scale sources of electron capture isotopes are a excellent source of low energy, mono-energetic neutrinos. 51Cr is likely the best source candidate for the future experimental program. Sources as strong a 2 MCi could likely be produced at HFIR. Such a source could be used for a sensitive search for eV sterile neutrinos with large, low-background scintillating detectors like Borexino and SNO+ Jonathan Link

  20. Question Slides Jonathan Link

  21. Electron Capture Neutrino Sources In 1973 Luis Alvarez proposed using a 65Zn source to calibrate Ray Davis’ chlorine detector. Since then several such source have been proposed: Jonathan Link

  22. The Gallex Sources • Made in the Siloé reactor in Gernoble, France (35 MW) • Two sources produced from the same enriched Cr (38.6% 50Cr) 1.67 MCi 1.89 MCi The average temperature across the Cr was ~525 K, which gives a flux averaged cross section of ~16 barns. (My production estimates are scaled from these numbers assuming that their entire neutron flux is thermal.) Jonathan Link

  23. The Sage Source • Made in the BN-350 fast breeder reactor at Aktau, Kazakhstan. • Irradiated 512.7 g of Cr (enriched to 92.4% 50Cr) • Fast neutron flux of 5×1015/(cm2 s) was locally moderated near the Cr to give an average cross section of about 4 barns. • Longer exposure: 90 days at 520 MW and 16 days at 620 MW. Source strength of 516 kCi. Jonathan Link

  24. Source Production Scaling from Siloé to HFIR • Using • the initial amount of 50Cr, • the source strength after irradiation, and • the 51Cr decay rate, • The survival lifetime of 50Cr (τ50) in the Siloé Reactor is calculated to be about 13,500 days. • Similarly, τ50 for locations the HFIR core are calculated, accounting for the differences in core temperature and thermal neutron flux. This does not include 51Cr production from non-thermal neutrons. Jonathan Link

  25. Check of the HIFR Production Model In the 1980’s ORNL studied 51Cr production in HFIR using rods of natural chromium in the Small VXF and Large VXF locations. With a 5.7 cm diameter rod in the Large VXF location they got 0.0673 MCi. While the 3.1 cm rod in the Small VXF yielded 0.0707 MCi. The thermal neutron attenuation length in natural chromium is 4.5 cm, so the difference from expectation at the large VXF may be due in part to self-shielding. 53Cr (9.5%) has a larger n capture x-section than 50Cr (4.4%) Jonathan Link

  26. Chromium Self-Shielding Chromium scans from large VXF Self-shielding was also likely an effect at Siloé. The neutron attenuation length in Siloé was about 4.1 cm. The chromium was in two parallel boxes 50 cm high by 12.6 cm long by 1.4 cm wide. Jonathan Link

  27. Sterile Neutrino White Paper • For more information see the Light Sterile Neutrinos: A White Paper (arXiv:1204.5379 [hep-ph]) • Outline: • Theory and Motivation (editors Barenboim & Rodejohann) • Astrophysical Evidence (Abazajian & Wong) • Evidence from Oscillation Experiments (Koop & Louis) • Global Picture (Lasserre & Schwetz) • Requirements for Future Experiments(Fleming & Formaggio) • Appendix: Possible Future Experiments (Huber & Link) • Written from an international perspective for an audience including both the scientific community and funding agencies. • Visithttp://cnp.phys.vt.edu/white_paper/ Jonathan Link

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