1. Materials Assay & ICPMS for DUSEL R&D

2. 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

3. 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

4. Easy Example: Cable
Desired budget for cable was a count/year (full spectrum)
Initial assay (above-ground, low-background gamma spectrometer) of 500-foot 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)

5. 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.

6. 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

7. View from another angle…

8. 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

9. 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

10. ICP-MS DETECTION RANGES Aqueous Standards
WEIGHT PREFIX U ATOMS/ml
10-3 (ppt) Milli x1018
10-6 (ppm) Micro x1015
10-9 (ppb) Nano x1012
10-12 (ppt) Pico x109
10-15 (ppq) Femto x106
10-18 (pp?) Atto
10-21 (pp??) Zepto
10-24 (pp???) Guaca
NORMAL
ICP-MS
RANGE
ULTRA TRACE

11. Direct Atto-gram/mL Detection
250 ag/mL Np-237
25000 MHz/ppm
Response (cps)
amu

12. 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

13. 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

14. 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

15. R&D Issues Newest instruments have plenty of raw sensitivity
Radiochemistry for specific systems is needed (and requires significant effort)
Dissolution
Tracers
Separation

16. 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