1 Impact and Mitigation of Naturally Occurring 32SiMary Bliss*, Ray Bunker, Zachary S. Finch, isaac Arnquist, John L. Orrell Pacific Northwest National Laboratory Low Radioactivity Techniques 2017, Seoul Korea Session 8, Purification Techniques for Solids

2 Impact and Mitigation of Naturally Occurring 32SiIntroduction DAMIC unwanted background radiation 32Si in the environment The commercial silicon industry Dirt to detectors Analytical Chemistry The difficulty of 32Si assay Separation of 32Si and 32P from commercial Si sources From metallic Si From gaseous Si compounds 32Si mitigation in the detector supply chain Session 8, Purification Techniques for Solids

3 Session 8, Purification Techniques for SolidsDAMIC measurement Measurement of radioactive contamination in high-resistivity Si CCDs Spatially correlated betas from successive 32Si and 32P decays Result: 32Si activity = decays/(kg of Si)/day C.L.) Candidate Event 32Si beta t½ ≈ 140 yr 32P beta t½ = 14 days JINST 10 (2015) P08014 arXiv: v2 Session 8, Purification Techniques for Solids 3

4 DAMIC silicon WIMP detector~80 events/day/kg unwanted background from 32Si* Impacts current sensitivity and future scale up to larger systems ~1 in 1018 atoms are 32Si Future detectors need 103 to 104 less 32Si 32Si half life is 153 years How does “young” 32Si appear in a single crystal detector? How can it be detected before crystal growth? How can it be removed before crystal growth? *Measurement of radioactive contamination in the CCD’s of the DAMIC experiment, Journal of Physics: Conference Series 718 (2016) , doi: / /718/4/042057 Session 8, Purification Techniques for Solids

5 Environmental 32Si productionProduced via cosmic-ray spallation 40Ar(n,4p5n)32Si Precipitated by rain and snow as Si(OH)4 (methane structure) Very chemically active in water Surface deposition: 1–20 mBq/m2/yr 32SiO2 analysis used in precipitation studies, ocean mixing, and rock weathering Discovery Papers D. Lal and D.R. Schink, Low background thin-wall flow counters for measuring beta activity of solids. Rev. Sci. Instrum. 31 (1960) 395 D. Lal, E.D. Goldberg, M. Koide, Cosmic-ray-produced 32Si in nature. Science 131 (1960) 332 Session 8, Purification Techniques for Solids 5

6 DAMIC is two orders of magnitude lower in 32Si32Si in Seawater vs. DAMIC DAMIC 80 decays/day/kg Si 6 X Si atoms per kg Si 3 X Si atom concentration 3 parts in 1018 (atom) The Atlantic Ocean* 6.5 X103 decays/day/kg SiO2 5.2 X Si atoms/kg ocean Si 1.9 parts in 1016 (atom) DAMIC is two orders of magnitude lower in 32Si than seawater silicon. *H. Craig et al., Earth and Planetary Science Letters 175 (2000) Session 8, Purification Techniques for Solids

7 Silicon Metal Production

8 Example Mining Operations – Surface MinesSpruce Pine Mining District, North Carolina, USA The Quartz Corporation, and the Unimin Corporation Links to Industry with videos Session 8, Purification Techniques for Solids

9 Water is used throughout ore processingSilicosis – untreatable respiratory tract damage caused by fine rock dust A primary occupational health concern Mandatory dust abatement for worker health and environment Mining Construction Glass and Cement plants Water uses Fine Mists to control dust Mixed with ore for pumping Ores are washed Acidic and basic solutions to remove contamination Froth flotation to remove low density material Environmental regulations keep industrial water on site Settling ponds Water can introduce 32Si Session 8, Purification Techniques for Solids

10 Quartz/Sand mines are bigPea Creek Sand Mine Lighthouse mine Australia 911Metallurgist.com Session 8, Purification Techniques for Solids

11 Session 8, Purification Techniques for SolidsFrom ore to metal Surface Mines – quartzite High purity – glass industry including crucibles for Czralchski growth of Si Smelters – carbon reduction High iron – Ferrosilicon Low iron, phosphorus, boron – Chemical and Semiconductor Metal Grades Alloy 98% Metallurgical grade 99.9% High purity metallurgical grade – 99.99% Polymers High grade alloys Semiconductor feedstock Session 8, Purification Techniques for Solids

12 Semiconductor siliconMetallurgical grade → gas phase → high purity metal SiCl3H – HCl treatment to form trichlorosilane gas Fluidized Bed Reactors SiCl3H is decomposed to the metal on suspended particles Source of most solar grade silicon today 6-7 N purity Siemens Process SiCl3H is decomposed to the metal on resistively heated rods More expensive to operate than fluidized bed Source of most high resistivity silicon 9-10 N purity Session 8, Purification Techniques for Solids

13 Single Crystal SiliconCzochralski Growth (pulling) Largest commercial production Uses glassy SiO2 crucibles to dope the crystals with oxygen Grown p or n doped Feedstock is “chunks” Most commercial semiconductor wafers are from this process 100 Ohm-cm (proportional to purity) Floating Zone Growth Specialty – Detectors, TeraHz communications, IR optics Uses “sticks” from the Siemens Process Containerless process Usually induction heating Further purifies the material >10,000 Ohm-cm Session 8, Purification Techniques for Solids

14 Detection of 32Si in commodity siliconDecay chain 32Si →32P + b- (t1/2 = 150 y; Q = 227 keV) 32P →32S + b- (t1/2 = 14 d; Q = 1.71MeV) 32Si is the same mass as 32S – Elemental Mass Spectrometry does not work. Accelerator Mass Spectrometry can work but there is still a need for sample concentration. Session 8, Purification Techniques for Solids

15 32Si – Detect 106 atoms in 1kg? Issues in adapting Earth Science methods Environmental samples are soluble in water Dissolve SiO2 in basic solution, purify Precipitate SiO2 with acid Milk purified SiO2 for 32P by dissolving in basic solution Repeat as necessary From the metal Acid digestion – Difficult to control, dangerous products 3Si + 4HNO3 → 3SiO2 + 4NO + 2H2O SiO2 + 6HF → H2SiF6 + 2H2O 3Si + 4HNO3+ 18HF → 3H2SiF6 + 4NO + 8H2O (combined) Silicon products are gases. Will 32P stay in liquid phase? Session 8, Purification Techniques for Solids

16 32P from biogenic SiO2 - Low Background Beta CountingLots of reagents and steps… Finch et al. Journal of Radioanalytical and Nuclear Chemistry (2016) 307: Finch et al. Journal of Radioanalytical and Nuclear Chemistry (2016) 307: Session 8, Purification Techniques for Solids

17 xSiF4(g) → xPF3(g) + xF2(g) → xPF5(l,g)32Si from gases? SiF4 – Boiling point of -86C, melting point of -90C Does 32P build up in stored gases? 1-2 kg of metal in gas cylinder PF3 or PF5? Good thermodynamic data on SiF4 but not PF3 or PF5 PF3 – Boiling point of -101C, Melting point of -151C PF5 - Boiling point of -84C, Melting point of -94C xSiF4(g) → xPF3(g) + xF2(g) → xPF5(l,g) Can gaseous 32P be removed from liquid SiF4? What are the chemical reactions in the cylinder? Session 8, Purification Techniques for Solids

18 32Si from gases? SiCl3H – Boiling point of 32C, melting point -127C Does 32P build up in stored gases? 1-2 kg of metal in gas cylinder PCl3 Boiling point of 76C Evaporate SiCl3H? PCl5 Boiling point of 167C What about competing reactions involving HCl and HF? Can gaseous SiCl3H be removed to leave 32P compounds behind? What are the chemical reactions in the cylinder? Session 8, Purification Techniques for Solids

19 Detection is not removalSeparation of 32P compounds is required for detection even by radioassay methods Development of separation methods may lead to “cleaner” Si supplies but does not remove 32Si The Avogadro Project* Isotopically enriched 28Si to redefine the kilogram SiF4 separated via gaseous centrifuge Reported enrichment levels may meet our needs Based on reduction of 29Si and 30Si Price estimate of $140/g for 0.5 to 5 kg for % 28Si Significant price increase for higher 28Si concentrations Need a reliable supply Only available as SiF4 - still need to convert to metal Industrial expertise *Becker et al. Phys. Status Solidi, No.1, (2010) Session 8, Purification Techniques for Solids

20 Session 8, Purification Techniques for SolidsConclusions We expect 32Si to be present in all commodity silicon metal Regardless of grade Water is the source of 32Si in commodity silicon It is not feasible to find a “dry” source of silicon Daughter 32P assay can be used to qualify silicon source material Complex chemistry Dangerous chemicals Detection first Quantification requires milking The Avogadro Project A possible alternative source of purified silicon Is this an affordable route to reduce 32Si? Not easy or cheap to convert to metal – industry help needed Still need crystal growth and semiconductor fabrication Session 8, Purification Techniques for Solids

21 Extra Slides

22 Challenge – Impact on SuperCDMSSuperCDMS SNOLAB sensitivity reach with and without Si-32 SuperCDMS SNOLAB with DAMIC Si-32 levels SuperCDMS SNOLAB with zero Si-32 arXiv: 22

23 Direct Si metal dissolutionUpper: Si with only HF Lower: Si HF mixture within seconds of adding HNO3 The reaction during dissolution produces two main gases: NOx and SiF4 As SiF4 is produced, Si mass in solution decreases Take advantage of this behavior as the phosphate daughter should remain in solution 1 step chemical separation adapted from NAA analysis of Si wafers NAA Paper: Kant, A., Cali, J. P., & Thompson, H. D. (1956). Determination of Impurities in Silicon by Neutron Activation Analysis. Analytical Chemistry, 28(12), doi: /ac60120a015 2323

24 Direct Si metal dissolutionReclamation Solution: Utilize hydrolysis of SiF4 with water to produce a SiO2 gel, similar to what was encountered with the sedimentation chemistry A rudimentary bubbler-type system was constructed to proof concept: SiF4 + 2H2O→ SiO2 + 2H2SiF6 Left: Reaction vessel with gases forced through water on right Right: Formation of SiO2 gel on side of bubbler vessel