Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report Page: 78 of 156
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Previous tritium sorption studies were dependent on mass balance relationships to
determine partition coefficients (e.g., Tanaka and Yamamoto, 1976; Ono et al., 1980;
Bindal et al., 1984). However, there are sizable discrepancies in the results of these
different studies. An alternative approach is to measure tritium sorption in the solid
matrix using secondary ion mass spectrometry (SIMS, or ion microprobe). An advantage
of this method over bulk analytical methods is the ability to detect all elements and their
isotopes at trace concentrations with high spatial resolution (e.g., Wilson et al., 1989). In
principle, SIMS is well suited for tritium determination because of the high sensitivity of
the instrument for hydrogen analyses, and because interferences at mass-3 are related
only to molecular hydrogen species (Ottolini et al., 1995). Moreover, the ability to
produce high-resolution depth profiles using the ion microprobe is particularly beneficial
for investigating diffusional isotopic exchange processes (e.g., Chacko et al., 1999).
Quantitative SIMS analysis of tritium sorption and exchange requires the development of
a reference standard containing a known amount of tritium implanted in a matrix with a
major element composition similar to the tritium-bearing "unknowns". This report
describes a novel method for producing tritium-implanted reference standards, and
discusses the methods used to develop and test these standards for use on the ion
microprobe. To our knowledge, reference standards of this type did not exist prior to the
current study.
Method of Reference Standard Development
Tritium reference standards were prepared by irradiating high purity Si and Si02 targets
with thermal neutrons in a 3He atmosphere. Tritium is produced by the reaction
3He (n,p) 3H
and virtually all of the resultant 3H is implanted within the target, or in the walls of the
sample container. By using pre-polished wafers of Si and SiO2, the 3H implanted
materials were essentially ready-to-use following the irradiation. Details of the
preparation method are as follows.
High purity wafers of semiconductor-grade silicon or low-OH Si02 (Suprasil 300,
Heraeus-Amersil) were individually flame-sealed inside silica tubes (0.9 cm i.d., 0.1 cm
wall thickness, -7 cm length) containing 555 torr (0.73 atm) of 3He gas. A total of five
individual target capsules were prepared, two containing Si, and three containing Si02.
The capsules were sent to the Oregon State University Radiation Center where they were
irradiated in a Triga Research Reactor for 3 hours 40 minutes at 1 MW power to deliver a
total thermal neutron fluence of -1015 n/cm2 to the samples.66
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Davisson, M. L.; Eaton, G. F.; Hakemi, N. L.; Hudson, G. B.; Hutcheon, I. D.; Lau, C. A. et al. Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report, report, July 1, 2001; California. (https://digital.library.unt.edu/ark:/67531/metadc1411019/m1/78/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.