Low Background Physics at SNOLAB

Transcription

1 Low Background Physics at SNOLAB Ian Lawson Symposium 1

2 Outline Radioactive Background Information Description of the SNOLAB Low Background Counting System Low Background Counting, How it is done Future Low Background Counting Facilities: Includes the new Canberra Well and Vue des Alpes Detectors Now Operational Electrostatic (ESC) Counters Alpha/Beta Counting System Future Research and Development Ideas 2

3 Motivation Many of the experiments currently searching for dark matter, studying properties of neutrinos or searching for neutrinoless double-beta decay require very low levels of radioactive backgrounds both in their own construction materials and in the surrounding environment. These low background levels are required so that the experiments can achieve the required sensitivities for their searches. SNOLAB has several facilities which are used to directly measure these radioactive backgrounds. The backgrounds in question are on the order of 1 mbq or 1 ppb for 238 U, 232Th and 235U and 1 ppm for 40K, or better, measurements down to 1 ppt are required for many components. The problem backgrounds can include gammas, alphas and neutrons or resulting interaction products. The goal is to measure these backgrounds and then to reduce them to be as low as reasonably achievable. 3

4 Uranium Decay Chain 4

5 Thorium Decay Chain 5

6 Other Interesting Isotopes Usually Present: 40K kev 137 Cs kev K e g kev 137 Ba Ba 60 Co kev kev 235U Ar Cs 137m 60Co G kev kev kev kev kev g 1173 kev Ni g 1332 kev U a 231 Th 4 g s Occasionally Present: 54 Mn at kev Observed in Stainless Steel 7 Be at kev 138 La and 176Lu Observed in Carbon based materials, due to neutron activation, samples are particularly affected after long flights. Observed in rare earth samples such as Nd or Gd. 6

7 SNOLAB PGT HPGe Counter (The workhorse detector at SNOLAB) Additional lead used to dampen microseismic activity from blasting and rockbursts 7

8 SNOLAB PGT HPGe Detector Specifications Motivation Survey materials for new, existing and proposed experiments (to be) SNOLAB, such as SNO/SNO+, DEAP/CLEAN, PICASSO/COUPP/PICO, EXO,... Also survey materials for the DM-ICE and DRIFT experiments, and Canberra. SNOLAB in 2005, detector was in UG storage from 1997, continuous operations since 2005 Counter manufactured by PGT in 1992 Endcap diameter: 83 mm, Crystal volume: 210 cm3 Relative Efficiency is 55% wrt a 7.62 cm dia x 7.62 cm NaI(Tl) detector, Resolution 1.8 kev FWHM. Shielding 2 inches Cu + 8 inches Pb Nitrogen purge at 2L/min to keep radon out, as the lab radon levels are 150 Bq/m3. Detection Region Energy: kev 8

9 PGT HPGe Typical Detector Sensitivity (for a standard 1L or 1 kg sample counted for one week) Isotope Sensitivity for Standard Size Samples 238 U 0.15 mbq/kg 12 ppt 235 U 0.15 mbq/kg 264 ppt Th 0.13 mbq/kg 32 ppt K 1.70 mbq/kg 54 ppt Co 0.06 mbq/kg Sensitivity for Standard Size Samples Cs 0.17 mbq/kg Mn 0.06 mbq/kg 9

10 Unshielded and Shielded Spectra Counts (PGT Coax Detector) No Shielding Shielding In Place 10

11 Calibration Spectrum 57 Co 139 Ce 137 Cs 54 Mn 85 Sr 88 Y 65 Zn Ba Y 89 Kr 88 Y 11

12 Detector Efficiency From Mixed Calibration Sample Plot by James Loach The efficiency is scaled to individual samples using GEANT which takes into account the sample components, to account for the density difference between the calibration source and the sample, and the sample geometry. 12

13 Typical Stainless Steel Spectrum 214 Pb Tl Co 214 Bi 208 Tl 13

14 DAMIC Ceramic Spectrum 235 U 234m Pa 14

15 Analysis Techniques (235U Chain) 235 U at kev 235 U at kev 15

16 Analysis Techniques (235U Chain) If the decay chain is in equilibrium the results from each peak are combined to give the overall background for the parent isotope. Energy Np Nab Npb N Int (%) Eff (%) Rate (mbq) ± ± 4.8 0± ± ± ± ± ± ± 4.9 0± ± ± ± ± ± ± 4.0 0± ± ± ± ± ± ± 4.1 0± ± ± ± ± 13.0 Live Time: sec Sample Mass: 94.4 g Average Rate: ± 2.84 mbq ± mbq/kg ± ppb 16

17 Measurements To Date For Each Experiment Experiment or Laboratory Total ( Today) SNO 11 SNO+ 125 SNOLAB 81 EXO 19 MiniCLEAN 56 DEAP 133 HALO 13 PICASSO 9 DM-ICE / DRIFT 23 COUPP / PICO 92 DAMIC 15 NEWS-SNOLAB 1 Total 578 Calibrations & Tests 118 Non-experimental measurements include water/snow samples from Fukushima fallout. See publication: Can. J. Phys. 90: (2012)

18 Canberra Well Detector at SNOLAB 18

19 Canberra Well Detector at SNOLAB Detector Volume: 300 cm3 Sample Well Typical Sample Bottle Volume is 3 ml 19

20 SNOLAB Canberra Well Detector Specifications Motivation Survey very small quantities of materials, concentrated samples or very expensive materials. Used by DAMIC, DEAP, PICO & SNO+ so far. Constructed by Canberra using low activity materials and shielding. Counter manufactured by Canberra in 2011 and refurbished in 2012, the cold finger was lengthened as it was too short to fit the shielding and the tail end and crystal holder were replaced to reduce radioactivity levels. Crystal volume: 300 cm3. Installed and operational in Shielding Cylindrical shielding of 2 inches Cu + 8 inches Pb Nitrogen purge at 2L/min to keep radon out, as the lab radon levels are 150 Bq/m3. Detection Region Energy: kev 20

21 Canberra Well Detector Sensitivity Isotope 238 U ( 226Ra) Sensitivity for Sensitivity for Standard Standard Size Samples Size Samples 0.05 mbq/kg 4 ppt 238 U ( 226Ra) 0.08 mbq/kg 6 ppt 228 Ac 0.2 mbq/kg 49 ppt 232 Th 0.4 mbq/kg 98 ppt U 0.02 mbq/kg 35 ppt Pb 0.15 mbq/kg 12 ppt

22 Unshielded and Shielded Spectra (Canberra Well Detector) 22

23 Vue des Alpes Detector at SNOLAB Detector was completed On August 30th. Background run in progress, expect to acquire ~30 days of background data to characterise the detector. Detector is expected to measure samples for nexo and for will be included for general counting. 23

24 Vue des Alpes Detector at SNOLAB 24

25 The Case For Expanded Low Background Counting Facilities One trait most of the current detectors/capabilities share, is that they are left over from previous experiments. Until now, experiments had to develop low background techniques within collaboration for each experiment, often re-inventing the wheel, at large cost. It is desirable to have these low background counting methods and detectors as part of the underground lab facilities. As experiments typically need these facilities most during detector design and construction, it requires a lot of funding and development time for experiments to take this on themselves. Then this developed resource can be under utilized once the supporting experiment is in operation. Beyond this, a global sharing and networking of low background counting facilities and resources would further benefit experiments and collaborations, allowing faster experiment design and identification of suitable materials. 25

26 SNOLAB Underground Facilities South Drift, was refuge and shower rooms for original SNO detector. Now to be refurbished for Low Background Lab. Cube Hall HALO Stub J-Drift Ramp SNO Utility SNO Drift SNO Cavern Cryopit South Drift Ladder Labs Personnel Facilities & Refuge Utility Area 26

27 Low Background Counters and Facilities A new dedicated space will be constructed at SNOLAB for a low background lab located in the South Drift. This drift is isolated from other drifts and is inaccessible to large equipment. This will help reduce micro-seismic noise which can effect Ge detectors. Increased air flow and perhaps other radon reduction techniques will be used. It is known that the compressed air from surface has substantially less radon than the lab air and can be used to reduce radon levels from Bq/m3 to 1-5 Bq/m3. South Drift Space can accommodate up to 5 Ge detectors, XRF, radon emanation chamber and have room for other types of counters which would benefit from low-cosmic ray background. Engineering design drawings are now in progress to accommodate the different detectors. 27

28 Options for SNOLAB Low Background Facilities We are currently surveying the community for input on capabilities that would best benefit future experiments at SNOLAB, and enhance SNOLAB s position as a leader in low background techniques. Items being investigated: Low radon air supply Low radon nitrogen supply Emanation chambers with Rn cryotraps XIA alpha screener (large area wire detector) Please contact Richard Ford (ford@snolab.ca) if you would like to contribute your ideas to SNOLAB's low background facilities. 28

29 Additional Low Background Counters Coming Soon SNOLAB Canberra 400 cm3 coaxial detector acquired in 2011 and refurbished into an ultra-low counter in 2013 to be installed, the shielding apparatus is currently being designed. 29

30 Additional Low Background Counters Coming Soon Soudan Gopher HPGe, to be relocated to SNOLAB: 2.0 kg of germanium, P-Type coaxial, made be Canberra. Dedicated to SuperCDMS Sensitivity of 1 mbq/kg for 3 week run Sample changes by staff. Queue and Analysis by UM Students 30

31 SNOLAB Backgrounds from the Local Rock Several rock, shotcrete and concrete samples have been assayed from the new laboratory using a Ge counter at U. of Guelph and ICP-MS methods. Each area of SNOLAB has several measurements. Ge Detector Results Material 232 Th 238 U 40 K (ppm) (ppm) (%) Average rock results 5.56 ± ± ± 0.12 Shotcrete ± ± ± 0.05 Concrete ± ± ±

32 Comparison of Ge Counting and ICP-MS Element Rock Sample 8 Ge Ge ICP MS K (%) 1.09 ± ± U (ppm) 1.24 ± ± Th (ppm) 5.44 ± ± Element Shotcrete Sample 15 Ge ICP-MS Rock Sample 11 ICP-MS Concrete Sample 14 Ge ICP MS K (%) 1.78 ± ± U (ppm) 2.46 ± ± Th (ppm) ± ±

33 Electrostatic Counting System Originally built for SNO, now used primarily by EXO. However, these counters are owned by SNOLAB so samples can be measured for other experiments. Measures 222Rn, 224Ra and 226Ra levels. Sensitivity Levels are: 222 Rn: gu/g 224 Ra: gth/g 226 Ra: gu/g 9 counters located at SNOLAB, 1 on loan to LBL (EXO), 1 on loan to U of A (DEAP). Work is ongoing to improve sensitivity even further. 33

34 Alpha Beta Counting System Currently located at the SNOLAB hot lab at LU so that radioactive spike sources can be measured for SNO+. Sensitivity for 238U and 232 Th is ~ 1 mbq assuming that the chains are in equilibrium. 34

35 Low Background Data Repository SNOLAB maintains a database in a spreadsheet format for each experiment. The table shows data from the standard gamma searches: 238 U, 235U, 232Th, 40K 137Cs, 60Co. While searching for the above gammas, we also search for any other peaks in the spectrum between 100 kev and 2800 kev, For example, 54Mn is usu. ally observed in steel. SNOLAB is a member of the Assay and Acquisition of Radiopure Materials (AARM) Collaboration, which has developed the Community Material Assay Database radiopurity.org. AARM website has information about individual detectors and contact details, an assay request form, MC tools, cleaning and handling procedures, etc... Going forward, SNOLAB will host the website and data base which is currently being developed. 35

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39 Future Improvements and R&D Improved neutron shields (detector response, spectrum) Improved material selection (more sensitive, better radiopurity e.g. PbWO4 with archaeological lead) Active shielding Going deeper underground Storage of freshly made construction materials underground Multisegmented crystals or multiple crystals Collaboration with producers (e.g. depleted Ge, crystal growing, Cu electroforming underground) Reorganisation and optimisation of existing screening facilities is necessary, because they are costly and measurement times can be rather lengthy. Harmonisation of how to report data and intercomparison programs for ultra low-level measurement techniques. 39

40 Applications Beyond Particle Physics Ultra low-level chemistry Particle astrophysics (material and techniques applicable to rare events experiments) Space science (e.g. micro meteorites, Mars samples, cosmic activation products, comet tail samples) Atmospheric samples (very) short lived isotopes, radionuclide composition, stratospheric samples) Ocean samples (e.g. deep ocean water - 60Fe) In general application of low background techniques to interdisciplinary fields: Low-level environmental radioactivity measurement and monitoring Radiodating (extension of determined ages towards the past) Geophysics (palaeoseismology, palaeogeology, sedimentation) 40

41 Summary SNOLAB low background counting program began in 2005 and on average 50 samples per year are counted. Counting queue is usually long. The counter is available for all SNOLAB experiments and can be made available to non-snolab experiments upon request (eg. DM-ICE, DRIFT). New gamma counters are being developed for the new low backgournd lab which is now under construction. The Canberra Well detector is now in full operation and 72 samples have now been counted. The Vue des Alpes nexo detector is now being characterised. The Canberra Coax detector is underground and engineering drawings of the shielding design are in progress. Specialized counting can be done using the Electrostatic Counters, Alpha-Beta Counters and materials can be emanated for Radon. New low background counting lab will be constructed at SNOLAB, final design drawings are now underway. 41