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There are many particle physics experiments (SNO) which look for rare event phenomena. The search for neutrinoless double-beta decay using high
purity germanium detectors may be taken as an example. The detector
components and the shielding materials themselves are sources of
interfering radioactivity from the primordial isotopes of Th, U and K.
One way to search for low radioactivity materials for these experiments
is to do radiometric counting of the samples of interest. Even though it
profits from providing a knowledge of radioactive disequilibrium among
the decay chains of the primordial isotopes, this method requires access to
specialized low background counting equipment (Ref 1).
However, many of the particle physics experiments may not be concerned with such high sensitivity counting. The Sudbury Neutrino Observatory required many materials to be tested for building the neutrino detector for measuring the neutrinos from the Sun (Ref 2) . In most cases, a rapid selection of low radioactivity materials can be achieved by INAA of small samples. Uranium
can be readily determined by INAA using the 2.35d 239Np activity induced in
many materials by neutron activation. Similarly, Th can be easily determined
by INAA using the 27d 233Pa activity. In most cases, materials can be selected at the ppm (ug/g) range for Th and U values, without difficulty. In
favourable cases, Th and U values can be determined in the ppb (ng/g) range
(Ref 3).
In selected cases, values as low as in the ppt (pg/g) range were also determined
by INAA (Ref 4)
References:
1. "Measurements of Th, U, and K concentrations in a variety of materials"
P. Jagam and J.J. Simpson, Nuclear Instruments and Methods in Physics
Research, A 324 (1993) 389 - 398.
2. "Solving the Solar Neutrino Problem" A.B. McDonald, J.R. Klein, and D.L. Wark. Scientific American April 2003.
3. Table 3: Generic materials tested by NAA, p 396 in Ref 1
4. "Thorium and Uranium determinations in Acrylic by Neutron Activation Analysis" P. Jagam, J.-X.Wang, J.J. Simpson, Journal of Radioaanlytical and Nuclear Chemistry, Articles 171 (1993) 277 - 286.
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