A count rate based contamination control standard for electron accelerators Page: 3 of 4
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Table 1. Radionuclide Categories
Category 1 Radionuclide Major Nuclide ALI/ Predicted DPM
Electron Capture, Gamma Decay Only Decay Characteristics 60Co ALI per 100 CPM
'Be 10.4% 477 keV gamma 667 30,000
s'Cr 9% 320 keV gamma 667 37,000
"Mn 100% 835 keV gamma 27 2,300
s7Co 87% 122 keV gamma 23 1,700
Category 2 Radionuclide Major Nuclide ALI/ Predicted DPM
Positron ((3) / Gamma Emitters Decay Characteristics 60Co ALI per 100 CPM
s'Co 20% 1490 keV Q+ 6.7 600
40% 511 keV gamma
100% 847 keV gamma
66% 1240 keV gamma
58Co 15% 474 keV (3+ 23 1,600
30% 511 keV gamma
99% 810 keV gamma
sZn 1.5% 327 keV (3 10 3,700
3.4% 511 keV gamma
49.0% 1115 keV gammarepresentative samples under controlled laboratory conditions
and checked in situ using gamma spectroscopy measurements,
to field frisker measurements of the same materials.
The response function is that 100 cpm over background (in
a relatively low photon background) corresponds to
approximately 30,000 dpm of 7Be on substrate tested at
Jefferson Lab. If one then applies the aforementioned factor
of 670 for radiotoxicity to the "Co based limit of 1000
dpm-dm2, the result would be a 'Be contamination control
limit of 670,000 dpm-dm2. Thus, using 100 counts per minute
over background as a 7Be limit would still result in
contamination control limits about 22 times (670,000/30,000)
more restrictive than the equivalent control limit for "Co.
Nevertheless, the RCG considers this conservatism to be
acceptable and the value 30,000 dpm-dm2 to represent a
reasonable control limit for Jefferson Lab operations. This
level is also convenient, as it can be used simultaneously in
the field for beta/gamma emitters as well as 'Be.
Other non-beta emitting nuclides similar to 'Be such as
cobalt-57, manganese-54 and chromium-51 (51Cr) can be
identified in radioactivity removable from surfaces at
Jefferson Lab. These nuclides are typically found in mixtures
that produce response functions during field frisking which
are generally as sensitive as that for 7Be. A review of the
energy versus efficiency response function normalized to
Cs-137 photons, provided by one manufacturer (3) for this
type of detector, indicates that the lowest response is between
approximately 200 keV and 400 keV. As a result, the
combination of low photon yield and low response for 51Cr
and for 'Be decay photon energies makes these nuclides
generally the most difficult to detect among these non-beta
emitting radionuclides.ANALYSIS
A comparison of the decay modes and radiotoxicity as
compared to "Co of radionuclides of concern (often detected
in various quantities and ratios as removable contamination)
is offered in Table 1. There are other radionuclides which are
not mentioned here because they are relatively easy to detect
due to their high photon energy and/or yield or high energy
charged particle emission. It is evident (and not unexpected),
that as radiotoxicity increases, detection by frisker generally
becomes more favorable; the detector has an increased
response due to charged particle emission and/or increased
photon emission energy and yield.
Table 2 compares derived limits (7) with activity expected
at the 100 cpm-dm2 counting level. As evident, the quantities
of radionuclides present are well below limits derived on the
basis of external exposure or ingestion (7).
There is no condition where there is an increase in risk
represented by the presence of 100 cpm of removable
contamination from these radionuclides compared to that
represented by 100 cpm of "Co. Consequently, the quantity
of radioactivity corresponding to 100 counts per minute over
background using field frisking equipment represents a
reasonable removable contamination control limit for the
radionuclides of concern at Jefferson Lab.
International documents reviewed by the RCG state that it
is "inappropriate to use the standard working limits for low
yield'photon only' emitting low radiotoxicity sources." These
documents indicate that licensees should ask their respective
regulators for relief from standard working limits for low
radiotoxicity radionuclides (4). Other references indicate that
standards should be based on acceptable working conditions
and risk (5). A 30,000 dpm-dm z limit is identified for
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May, R.T. & Schwahn, S.O. A count rate based contamination control standard for electron accelerators, article, December 31, 1996; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc691998/m1/3/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.