1. GeMPI-type low background counting system for SURF Kara Keeter 15 September 2014

2. S URVEY OF OTHER HPG E SCREENERS S ENSITIVITIES, DEPTH, AVAILABILITY W HAT WE PROPOSE, AND HOW IT FITS IN WITH THE OTHERS D ESIGN BASED ON G E MPI U SE ELECTROFORMED C U, BENEFITTING FROM MJD EXPERIENCE AND INFRASTRUCTURE C APACITY TO ACID - ETCH ALL C U AND P B SHIELDING TO REMOVE SURFACE CONTAMINATION V ARIETY OF O PPORTUNITIES FOR STUDENTS Outline

3. According to a survey of screening needs conducted as part of the 2013 Snowmass on the Mississippi Community Planning*, G2 dark matter experiments will need to screen approximately: –160 samples using HPGe screeners of > 1 mBq/kg, –400 samples using HPGe screeners of 0.05 – 1 mBq/kg, –and 65 samples using HPGe screeners of < 0.05 mBq/kg. This doesn’t include the next generation of neutrino experiments, etc. Each sample requiring ultralow background screening requires lengthy measurement times (~several weeks). Many of the existing screeners are already committed to certain experiments. *From 2013 Snowmass Community Summer Study White Paper, Low Background Materials and Assay, A Supplement to the Cosmic Frontier CF1 Summary, J. Cooley, P. Cushman, E.W. Hoppe, J.L. Orrell, R.W. Schnee; http://www.slac.stanford.edu/econf/C1307292/docs/submittedArxivFiles/1311.3311.pdf Assay Needs

4. SURF: 2 (4300 mwe: CUBED, Oroville) Berkeley: 2 (surface; 1 w/muon veto) PNNL: 6 (surface; shielded); 2 arrays (surface; unshielded); multiple others (30 mwe) KURF: 2 (1450 mwe) Soudan: 2 (2100 mwe) SNOLAB: 2 (surface); 1 (6010 mwe) –0.009 ppb 238 U, 0.02 ppb 232 Th232, 87 ppb 40 K Compare to LNGS GeMPI: (3800 mwe) –0.001 ppb 238 U/ 232 Th; 1 ppb 40 K HPGe Screening Facilities in North America From 2013 Snowmass White Paper, http://www.slac.stanford.edu/econf/C1307292/docs/submittedArxivFiles/1311.3311.pdf

5. Best in the world as a model The GeMPI detectors at Gran Sasso provide a model for ultra-low background germanium detectors used to screen materials for radiopurities at the picogram level. There are no similar detectors in the U.S. Those in Europe are in constant use. Cross-sectional view of crystal (2.2 kg) and cryostat design of GeMPI (I).* *Heusser G., Laubenstein M., Neder H., “Low-level germanium gamma-ray spectrometry at the μBq/kg level and future developments towards higher sensitivity” (2006) Radioactivity in the Environment, 8 (C), pp. 495-510.

6. Cross-sections and outside views of the GeMPI (I) detector and shield. *Heusser G., Laubenstein M., Neder H., “Low-level germanium gamma-ray spectrometry at the μBq/kg level and future developments towards higher sensitivity” (2006) Radioactivity in the Environment, 8 (C), pp. 495-510. Best in the world as a model

7. GeMPI III detector and shield* 2.3 kg p-type coaxial GeMPI III and IV both suffer from 207Bi contamination, transferred during crystal mounting via contaminated tweezers They have yet to reach the sensitivity of GeMPI I. Best in the world as a model * from “Highly Sensitive Gamma-Spectrometers of GERDA for Material Screening: Part 2” D. Budj ́aˇs, et al., 2007; arXiv:0812.0768arXiv:0812.0768

8. * from “Gator: a low-background counting facility at the Gran Sasso Underground Laboratory”, L. Baudis et al.; AR X IV :1103.2125 V 2. Best in the world as a model Gator: 2.2 kg p-type coax Soudan -> LNGS

9. Best in the world as a model Gator vs. GeMPI * from “Gator: a low-background counting facility at the Gran Sasso Underground Laboratory”, L. Baudis et al.; AR X IV :1103.2125 V 2.

10. Best in the world From Marie Odile Lampert’s (Canberra) talk this morning: Custom designed ULB BEGe5030 detector: –Latest background measurement: 223 counts per day (50-3000 keV) by courtesy of Pia Loaiza –This is close to the GeMPI detectors: 60 counts per day [100-2730 keV] (Heusser et al., Radioactivity in the Environment, Vol. 8, 2006, pp. 495-510) –Less than 0.09 cpm (15- 1500 keV): 10 mn for 1 count –Measured at Modane Underground Lab (LSM), 4500 mwe, FWHM 850 eV @ 122 keV

11. We are proposing a HPGe screener to be located in the BHSU Underground Campus on the 4850L of SURF (4300 mwe) –Will complement existing CUBED and Oroville screeners –P-type Coaxial HPGe Low-Background Gamma-Ray Detector with carbon fiber endcap and Remote Preamplifier and HV Filter –Design based on GeMPI –Use electroformed Cu, benefitting from MJD’s experience and infrastructure –BHSU has capacity to acid-etch all Cu and Pb shielding to remove surface contamination Proposed detector

12. Purchase from either Canberra or Ortec a P-type low-background HPGe detector with the largest available (~3 kg, approximately 9 cm in diameter and 9 cm in length) Ge crystal. We will accept delivery of a complete crystal and cryostat system so that the vendor can establish the energy resolution and efficiency to specifications before delivery, and so that we can confirm a working system upon receipt and use it to assay the materials to be used in the custom modifications. The detector will be transported to Homestake and located in the BHSU Underground Campus at the 4850L as soon as possible after manufacture. The initial shielding configuration will be built, and this HPGe will be installed and made available for screening in its as-delivered configuration as soon as possible. Initial Plan

13. Next configuration: replace cryostat and cold finger system with electroformed copper parts and other well characterized materials. –Expected deposition rate of copper is 0.001-0.004” per day. –It will take ~100 days of bath time to complete the copper needed for the cryostat pieces. –The entire electroforming process, including machining the mandrels, quality assurance and restarts for poor growth rates, and machining of the end product parts would come to about a year. Use ICP-MS and 1 st configuration HPGe screener to screen parts for next configuration. OFHC copper will be purchased as soon as possible and brought underground for storage to mitigate cosmogenic activation. All assembly will take place in a dedicated glove box with closed loop gas circulation and an airlock system in the proposed underground clean room in order to mitigate radon daughters contamination. Improvements to implement

14. Y EAR O NE –Procurement contract with vendor –Design shield, mechanical system, airlock/glove box cover, DAQ, etc. –Vendor begins detector fabrication –Begin procurement of OFHC copper and lead and other materials –Begin processing (acid-etching) of shield materials –Begin Cu electroforming Year Two –Accept delivery of HPGe screener at BHSU Underground Campus –Install shield, systems –Take background readings and screen materials for next configuration –Complete Cu electroforming, and machine cryostat parts –Optimize shield design –Replace commercial cryostat with custom electroformed cryostat, and rebuild shield –Sensitivity studies and optimization Timeline

15. And onward…