Download Aalseth-icpms - Berkeley Cosmology Group

Materials Assay &
ICPMS for DUSEL R&D
Starting Points
•
238U
and 232Th chains are primary concerns
– Are not always in equilibrium with progeny
• Other backgrounds are also important
– Surface contamination, cosmogenics
• Next-generation experiments require a range of
materials purity levels, but the most stringent are
<1 uBq/kg
• Full range of assay techniques will be needed –
alpha, beta, gamma spectroscopy, mass
spectroscopy, radiochemistry, neutron-activation
analysis
Scope
• Look at examples favoring and disfavoring
gamma spectroscopy vs ICPMS
• Simple look at what favors
– Gamma spectroscopy
– ICPMS
• ICPMS Detection and Overview
• Challenges for direct measurement
• R&D implications
Easy Example: Cable
• Desired budget for cable was a
count/year (full spectrum)
• Initial assay (above-ground, lowbackground gamma spectrometer) of 500foot spool of cleaned cable gave limits of
<45 uBq/foot (<36 mBq/kg)
• Analysis of experiment efficiency showed
this would contribute <1.0 count/year
• Done! Start using cable…(IGEX)
Hard Example: Electroformed Cu
• Stringent limits of <0.1 uBq/kg desired
• Best gamma spectrometry limits <6-8 uBq/kg
– 90-day count, Homestake or LNGS, ~10-kg sample
• Developed dissolution and Th tracer chemistry for Cu
• Developed adsorbent (column) chemistry to partition Cu
from Th
• Used radiochemistry as front-end to ICPMS
• Electroformed copper sample result of 0.7 ± 0.6 uBq/kg
– (1-g sample, few-hour measurement, 7-fold replicate, 1 week
of setup for campaign)
– Many months of radiochemistry R&D to enable measurement
• Not there yet, work continues! But already better than
previous best gamma result. End in sight.
Unfinished Example: Resistor
• Desired radiopurity ~1 ppb 238U, 10 ppb 232Th
• Using LNGS screening detector (one of world’s
best) as example, this would require 1 kg of
material and a 100-day count
• Cost of 1 kg of chip resistors (about 1.7e6 units)
would be $1.7M!
• Conclusion: Turn toward clean chemistry for chip
resistors, FET ($27M/kg), etc. as front-end to
ICPMS
• ICPMS will require <1g of material for assay
View from another angle…
What Favors Gamma
Spectroscopy?
• Assembled commercial items (heterogeneous)
– Cables, electronic components, valves, etc.
• Used in small quantities = only moderate
radiopurity limits
– Means many can be assayed for better limits per item
• Cheap and available
– Can afford to buy many more than needed to support
assay of large quantities
• Modest volumes
– Needed to allow usable efficiency for reasonable
number of items
What Favors ICPMS?
• Easy dissolution chemistry
– Can “dilute-’n-shoot” when only moderate
limits needed
• Simple elements or compounds
– Best when radiochemistry is already
developed for the system
– Existence of appropriate isotopic tracers
– Complex systems can be analyzed, but
requires significant chemistry development
ICP-MS DETECTION RANGES
Aqueous Standards
238U
WEIGHT
PREFIX
ATOMS/ml
10-3 (ppt)
Milli
2.53x1018
10-6 (ppm)
Micro
2.53x1015
10-9 (ppb)
Nano
2.53x1012
10-12 (ppt)
Pico
2.53x109
10-15 (ppq)
Femto
2.53x106
10-18 (pp?)
Atto
2530
10-21 (pp??)
Zepto
2.53
10-24 (pp???)
Guaca
0.00253
NORMAL
ICP-MS
RANGE
ULTRA
TRACE
Direct Atto-gram/mL Detection
1E4
250 ag/mL Np-237
1 .8 E+ 0 5
1E3
25000 MHz/ppm
Response (cps)
1 .2 E+ 0 4
1E2
1 .2 E+ 0 3
1E1
2 .0 E+ 0 2
2 .5 E+ 0 2
1 .0 E+ 0 2
4 .0 E+ 0 1
7 .2 E+ 0 1
4 .6 E+ 0 1
2 .2 E+ 0 1
2 .9 E+ 0 1
1 .9 E+ 0 1
1E0
5 .6 E+ 0 1
2 .7 E+ 0 1
1E-1
1E-2
230
232
234
236
amu
238
240
242
244
ICPMS Generalities
• Elements/Isotopes in the environment that
are not naturally occurring, easy to detect at
instrument and method detection limits
– Pu239,240,242,242, Am241, Np237, Th230,229, Tc99, I129
• However elements like Th and U are
problematic
– Th and U at ppm levels in dirt
– Ultra-pure acids, reagents, lab supplies
– Sample introduction system of ICP/MS
Challenges for Direct Measurement
• Cosmogenics, e.g. 60Co in Cu, Ge
– Background limits more stringent than U, Th chains
– Each system has different challenges and opportunity
for purification, e.g. electrochemistry for Cu, zone
refinement for Ge
– May have to depend on measured production rates
and process knowledge
• Disequilibrium in Th, U chains
– Hard to measure at necessary levels
– May have to depend on higher-level validation of
equilibrium behavior for a particular system, then
process knowledge
Common Theme: Radiochemistry
• Opportunity to sample larger masses, get sensitive
results from smaller masses
– Ability to count atoms with MS
– Higher efficiencies for radiometric counting
• Alpha, beta measurement
• Requires
– Dissolution chemistry for system
– Tracer chemistry (radio or stable)
– Separation chemistry for system
• Challenge
– Clean chemistry
– Reproducible yielding
– Extremely high partition between analyte of interest and matrix
R&D Issues
• Newest instruments have plenty of raw
sensitivity
• Radiochemistry for specific systems is
needed (and requires significant effort)
– Dissolution
– Tracers
– Separation
Conclusions
• Gamma spectrometry when possible
– Inexpensive, non-destructive, nominal sample
preparation, detailed information when signal
is seen
• Radiochemistry when necessary (R&D
priority)
– Front-end to ICPMS
– Front-end to LSC or direct alpha/beta
– In combination with NAA