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

2. The LENS-Sterile Proposal Proposal to insert a Mega-Curie 51 Cr source in the center of the LENS detector to observe multiple wavelengths of large Δm 2 oscillations in a few meters. 115 In + ν e → 115 Sn * + e - → 115 Sn + 2γ LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976). 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. 9/14/2012Jonathan Link

3. The LENS-Sterile Proposal Raju Raghavan 1937-2011 115 In + ν e → 115 Sn * + e - → 115 Sn + 2γ LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976). Proposal to insert a Mega-Curie 51 Cr source in the center of the LENS detector to observe multiple wavelengths of large Δm 2 oscillations in a few meters. 9/14/2012Jonathan Link

4. Gallex, Sage, 51 Cr and Sterile Neutrinos Giunti and Lavender (Mod. Phys. Lett. A22, 2499) noted that the low ratio of observed to expected ν + 71 Ge 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” 9/14/2012Jonathan Link

5. The Gallium Anomaly ν e disappearance in Gallex and SAGE from 51 Cr and 37 Ar source 9/14/2012Jonathan Link C. Giunti and M. Laveder, Phys. Rev. C83, 065504 (2011).

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. 51 Cr + s-shell e - → 51 V + ν e (+ 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 7 Be neutrinos have created new opportunities for groundbreaking neutrino physics using electron capture sources. 9/14/2012Jonathan Link

7. 90% of the time the capture goes directly to the ground state of 51 V and you get a 750 keV neutrino. 10% of the time it goes to an excited state of 51 V and you get a 320 keV photon plus a 430 keV neutrino. K shell capture L shell capture 51 Cr as a Mono-Energetic Neutrino Source 9/14/2012Jonathan Link

8. Advantages of 51 Cr Advantages of 51 Cr 1.Can be easily produced with thermal neutron capture ( 50 Cr has a ~17 barn neutron capture cross section). 2.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 3.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 4.Mega-Curie scale sources have been produced by both Gallex and SAGE. 9/14/2012Jonathan Link

9. The High Flux Isotope Reactor (HFIR) at ORNL HFIR operates at 85 MW with 23 operating days each fuel cycle. 9/14/2012Jonathan Link

10. The High Flux Isotope Reactor (HFIR) at ORNL Thermal neutron flux of 2.5×10 15 /cm 2 /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 9/14/2012Jonathan Link

11. Solar Neutrino Detectors & Source Sterile Searches What works for LENS may work for other low energy solar neutrino detectors. (ES rate ~10 2 × larger than 115 In 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) 9/14/2012Jonathan Link

12. Large Detectors and Centrally Located Sources (3+1) (3+2) 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. Models from the fit of Kopp, Maltoni & Schwetz arXiv:1103.4570 [hep-ph] Initial 2 MCi Source for a 70 day Run 9/14/2012Jonathan Link

13. Real Time Detectors Require Serious γ Shielding energy (keV) gammas/sec·10 keV Possible source and W-alloy shielding configuration… But what is the activity of W? The 320 keV gamma (10% of decays) is a non-issue compared to the internal bremsstrahlung in ~0.05% of decays. Spherical 9/14/2012Jonathan Link

14. 9/14/2012Jonathan Link Low-Background Counting of Tungsten Alloy 238 U < 5mBq/kg (from 234 Pa) 232 Th <40 mBq/kg (from 228 Ac) 40 K < 220 mBq/kg 214 Bi = 150±50 mBq/kg Signal Sample Background 214 Bi 214 Pb 208 Tl 40 K Measured at the Kimballton Underground Research Facility (KURF) Background Subtracted 234 Pa 228 Ac

15. SNO+ Source Deployment Case Study 9/14/2012Jonathan Link

16. Signal to Noise Ratio as a Function of Radius Assuming a uniform detector BG out to the fiducial radius. Detector BG grows as r 2 Source BG falls as e −r/λ 9/14/2012Jonathan Link

17. 3+1 contours from Kopp, Maltoni & Schwetz arXiv:1103.4570 [hep-ph] Sensitivity based on a χ 2 fit to signal and BG over the full energy range. 1.Not that sensitive to backgrounds 2.Source normalization and spatial resolution are critical to large Δm 2 resolution. 3.Statistics limited measurement. SNO+ Sensitivity 90% CL contours GLoBES 9/14/2012Jonathan Link

18. 9/14/2012Jonathan Link Borexino Sensitivity Source Under Detector 8.25 m (See Aldo Ianni’s Talk) 10 MCi Source Borexino is the only detector where we know this will work. One can use the vast solar phase to subtract backgrounds.

19. Conclusions 1.Mega-Curie scale sources of electron capture isotopes are a excellent source of low energy, mono-energetic neutrinos. 2. 51 Cr is likely the best source candidate for the future experimental program. 3.Sources as strong a 2 MCi could likely be produced at HFIR. 4.Such a source could be used for a sensitive search for eV sterile neutrinos with large, low-background scintillating detectors like Borexino and SNO+ 9/14/2012Jonathan Link

20. 9/14/2012Jonathan Link Question Slides

21. Electron Capture Neutrino Sources In 1973 Luis Alvarez proposed using a 65 Zn source to calibrate Ray Davis’ chlorine detector. Since then several such source have been proposed: Isotopeτ½τ½ E ν MaxProduction MechanismGammasNotes 65 Zn244 d1.3 MeVThermal neutron capture770 & 345 keV (50%)Proposed by Alvarez 51 Cr27.7 d750 keVThermal neutron capture320 keV (10%)Proposed by Raghavan, used by Gallex and SAGE 152 Eu13 y1.05 MeVUnknown121 keV -1.7 MeV (100%)Proposed by Cribier and Spiro 37 Ar34.9 d812 keVFast neutron 40 Ca(n,α) 37 ArInternal Brem. onlyProposed by Haxton, used by SAGE 9/14/2012Jonathan 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% 50 Cr) 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.) 1.67 MCi 1.89 MCi 9/14/2012Jonathan 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% 50 Cr) Fast neutron flux of 5×10 15 /(cm 2 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. 9/14/2012Jonathan Link

24. Source Production Scaling from Siloé to HFIR Using 1.the initial amount of 50 Cr, 2.the source strength after irradiation, and 3.the 51 Cr decay rate, The survival lifetime of 50 Cr (τ 50 ) in the Siloé Reactor is calculated to be about 13,500 days. Similarly, τ 50 for locations the HFIR core are calculated, accounting for the differences in core temperature and thermal neutron flux. This does not include 51 Cr production from non-thermal neutrons. 9/14/2012Jonathan Link

25. Check of the HIFR Production Model In the 1980’s ORNL studied 51 Cr 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. 53 Cr (9.5%) has a larger n capture x-section than 50 Cr (4.4%) 9/14/2012Jonathan Link

26. Chromium Self-Shielding 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. Chromium scans from large VXF 9/14/2012Jonathan Link

27. Sterile Neutrino White Paper For more information see the Light Sterile Neutrinos: A White Paper (arXiv:1204.5379 [hep-ph]) Outline: 1.Theory and Motivation (editors Barenboim & Rodejohann) 2.Astrophysical Evidence (Abazajian & Wong) 3.Evidence from Oscillation Experiments (Koop & Louis) 4.Global Picture (Lasserre & Schwetz) 5.Requirements for Future Experiments (Fleming & Formaggio) 6.Appendix: Possible Future Experiments (Huber & Link) Written from an international perspective for an audience including both the scientific community and funding agencies. Visit http://cnp.phys.vt.edu/white_paper/http://cnp.phys.vt.edu/white_paper/ 9/14/2012Jonathan Link