The Princeton NaI Experiment (SABRE) Frank Calaprice Princeton University Cosmic Frontier Workshop SLAC March

Collaborators Princeton University – Jay Benziger (Chemical Engineering) – Frank Calaprice (Physics) – Cristiano Galbiati (Physics) – Alex Wright (Post doc. now at Queen’s Univ. Canada) – Emily Shields (Grad Stud.) – Jingke Xu (Grad Stud.-> Post doc.) Aldrich-APL (now Sigma Aldrich): NaI powder Seastar Chemicals – M.V. Laboratories: NaI powder Radiation Monitoring Devices (RMD): Crystal growing Hamamatsu: PMTs

DAMA/LIBRA Data Is this dark matter? A new NaI experiment with lower background is needed to clarify this situation. Bernabei, et. al. Eur. Phs. C (2008)

DAMA/LIBRA Singles Spectrum Bernabei, et. al. Eur. Phs. C (2008) Modulation of ~0.01 cpd/kg/keV (dru) sits on a flat background of ~ 1 dru and a peak of 0.5 dru at 3 keV

The Road to Lower Background NaI 1.Low background NaI scintillation crystals. – Develop low radioactivity NaI powder for crystals Standard commercial powder : [K ] ~ 300 ppb DAMA/LIBRA[K] ~ 20 ppb; [U], [Th] ~ 10 ppt Desired material:[K] ~ 1 ppb; [U], [Th] < 1 ppt – Crystal growing process shown to improve radio-purity. 2.Low-background Photomultiplier Tubes – Hamamatsu 3” metal bulb PMTs: much lower background. – Low temperature capability. 3.Active liquid scintillator and water shields – Prompt cosmogenic backgrounds rendered negligible. – Deep site such as SNOLab to reduce delayed cosmogenics. – Internal crystal and PMT gamma background (e.g, 40 K) reduced by liquid scintillator veto.

Sodium Iodide Powder Typical commercial grade NaI powder: [K] ~ 300 ppb Collaboration Princeton- Aldrich APL started in 2010 – First results with [K] = 35 +/- 10 ppb (months of  counting) – Result demonstrated method for higher purity NaI powder. – Company sold with full ownership to Sigma Aldrich in 2011 R&D temporarily halted. – Restarted production of NaI with [K] < 20 ppb? (chem. meth.) Seastar Chemicals- MV Industries – Developed sensitive analytical methods to measure [K] – Produced NaI samples with [K] < 20 ppb (chemical meth.) – Ongoing research to improve radio-purity of NaI. Validation of chemical analysis by counting underway. – Singles  counting is slow with [K] ~few ppb. – Coincidence counting method being developed at Princeton.

Crystal Growing Saint Gobain: – Locked up In agreement to supply high purity crystals only to DAMA. Radiation Monitoring Devices (near Boston) – Expertise in crystals for radiation detectors. New owners of Hilger in UK. – Produced high quality NaI(Tl) crystals in test program. – Investigated other methods to remove K. Significant reduction of impurities found. – Preparing to grow radio-pure crystals. Radio-purity of NaI(Tl) crystals to be tested by Princeton

A Major Breakthrough for NaI powder The recent development of high purity NaI powder is a major breakthrough for low background NaI(Tl) detectors. In combination with improved crystal growing techniques, a new generation of DM experiments is possible with much lower background. The lock-up by DAMA of the Saint Gobain technology for low background NaI crystals inadvertently aided this development.

New Photomultiplier Tubes New PMTs developed by Hamamatsu for cryogenic use in liquid xenon and liquid argon offer lower backgrounds than earlier glass tubes. PMT’s like the 3” R11065 feature a metal bulb with synthetic quartz window, and low background feedthrough plate for wires. – Radioactivity is more than 10 times lower than PMTs with “low radioactivity” glass. – Larger PMTs (5”) are being considered. Low temperature operation may be useful for NaI detectors for higher light yield.

Simulated Detector Backgrounds PMT radioactivity measured in Hamamatsu R Metal Bulb PMT. Crystal radioactivity is assumed same as in DAMA crystals. DAMA has flat backgrounf of ~ 1 dru (x3 improvement) ~1.5 dru at 3 keV (x 5 improvement) Reducing crystal radioactivity below DAMA is very possible but has yet to be proven.

Liquid Scintillator – Water Shields Water Tank 11m x 10m With PMTs Borated Liquid Scintillator in 4 m diam. sphere Darkside 50 Lar cryostat Access tubes for NaI detector assemblies

Inside the Liquid Scintillator Vessel The inside surface is covered with a Lumirror reflector to enhance light collection. Scintillation is detected by 110 8” Hamamatsu R5912 Photomultipler tubes. Expected light yield is about 700 pe/MeV.

Measuring K, U, and Th in NaI Crystals K, U and Th to be measured by operating NaI-PMT assembly in small liquid scintillator detector. Coincidence detection: – K: 3 keV x-ray MeV gamma ray – U, Th: Delayed Bi-Po coincidences. Expected Sensitivity: – U, Th few ppt – K, few ppb

Conclusions A breakthrough in NaI radio-purity occurred in the last year. New PMT technology offers lower background for the complete detector assembly. Active liquid scintillator/water shields provide excellent rejection of prompt cosmogenic background and additional rejection of NaI- PMT background. With lower backgrounds possible, the time is right for a new NaI experiment.