Impact and Mitigation of
Naturally Occurring 32Si
MARY 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
1
Impact and Mitigation of Naturally
Occurring 32Si
Introduction
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
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DAMIC measurement
Measurement of radioactive contamination in high-resistivity Si CCDs
Spatially correlated betas from successive 32Si and 32P decays
Result: 32Si activity = 80+110-65 decays/(kg of Si)/day (@95% C.L.)
Candidate Event
32Si
beta
t½ ≈ 140 yr
JINST 10 (2015) P08014
arXiv:1506.02562v2
32P
beta
t½ = 14 days
Session 8, Purification Techniques for Solids
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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) 042057, doi:10.1088/1742-6596/718/4/042057
Session 8, Purification Techniques for Solids
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Environmental 32Si production
Produced 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
32SiO
2
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
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32Si
in Seawater vs. DAMIC
DAMIC
80 decays/day/kg Si
6 X 106 32Si atoms per kg Si
3 X 10-19 32Si atom concentration
3 parts in 1018 (atom)
The Atlantic Ocean*
6.5 X103 decays/day/kg SiO2
5.2 X 10-17 32Si 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) 297-308
Session 8, Purification Techniques for Solids
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Silicon Metal Production
7
Example Mining Operations – Surface
Mines
Spruce Pine Mining District, North
Carolina, USA
The Quartz Corporation, and the
Unimin Corporation
Links to Industry with videos
http://www.thequartzcorp.com/en/
http://www.unimin.com/
Session 8, Purification Techniques for Solids
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Water is used throughout ore processing
Silicosis – 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
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Quartz/Sand mines are big
Pea Creek Sand Mine
Lighthouse mine Australia
911Metallurgist.com
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From 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
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Semiconductor silicon
Metallurgical 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
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Single Crystal Silicon
Czochralski 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
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Detection of 32Si in commodity silicon
Decay chain
32Si →32P + b32P →32S + b-
(t1/2 = 150 y; Q = 227 keV)
(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
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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
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32P
from biogenic SiO2 - Low Background
Beta Counting
Lots of reagents and steps…
Finch et al. Journal of Radioanalytical and Nuclear Chemistry (2016) 307:2313-2319
Finch et al. Journal of Radioanalytical and Nuclear Chemistry (2016) 307:2451-2458
Session 8, Purification Techniques for Solids
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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
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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
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Detection is not removal
Separation 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 99.998% 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, 49-66 (2010)
Session 8, Purification Techniques for Solids
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Conclusions
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
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Extra Slides
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Challenge – Impact on SuperCDMS
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SuperCDMS
SNOLAB
with zero
Si-32
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arXiv:1610.00006
SuperCDMS
SNOLAB
with DAMIC
Si-32 levels
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SuperCDMS SNOLAB sensitivity reach with and without Si-32
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22
Direct Si metal dissolution
The reaction during
dissolution produces two
main gases:
NOx and SiF4
Upper: Si with
only HF
Lower: Si HF
mixture within
seconds of
adding HNO3
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), 1867-1871.
doi:10.1021/ac60120a015
2
23
Direct Si metal dissolution
Reclamation Solution:
Utilize hydrolysis of SiF4 with water to produce a SiO2 gel, similar to
what was encountered with the sedimentation chemistry
SiF4 + 2H2O→ SiO2 + 2H2SiF6
A rudimentary bubbler-type system was constructed to proof concept:
Left: Reaction
vessel with gases
forced through
water on right
Right: Formation
of SiO2 gel on side
of bubbler vessel
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