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Dose to Lens of Eye - doe-std-1128-98_ch10181
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Tissue Equivalent Proportional Counter - doe-std-1128-98_ch10183


DOE-STD-1128-98
6.3.6.1 Multisphere Spectrometer System
The multisphere or Bonner sphere spectrometer (Bramblett et al., 1960)
is the neutron spectrometer system most often used by health physicists
for neutron energy spectrum measurements, perhaps because it is simple
to operate. Multisphere spectrometers are typically used for measuring
neutron energy spectra over a wide energy range from thermal energies
to over 20 MeV although detailed energy spectra are not obtained. With
the use of an appropriate spectrum unfolding code, the multisphere
system will determine the average neutron energy, dose equivalent rate,
total flux, kerma, and graphical plots of differential flux versus energy
and dose equivalent distribution versus energy.
The multisphere spectrometer consists of a set of polyethylene spheres of
different diameters, typically 3 in. to 12 in. A thermal neutron detector,
such as a 3He proportional counter or a 6LiI scintillator is positioned at
the center of each sphere, and the count rate measured. The neutron
energy spectrum can be determined from the ratio of counts from
different detectors. However, the spectral unfolding algorithms do not
provide mathematically unique solutions. The most appropriate solutions
are obtained by making an initial guess that the spectrum consists of a
fission spectrum with a 1/E "tail." Multisphere spectrometers have
demonstrated accuracies of 15% when exposed to 252Cf sources with
calibrations directly traceable to NIST (Brackenbush et al., 1991).
Figure 6.5 demonstrates the type of neutron energy spectra measured by
the multisphere spectrometer. The plot shows the logarithmic plots of
four multisphere spectrometer measurements made at a distance of 50 cm
from 1 kg of plutonium for "bare" plutonium fluoride (i.e., no
intervening shielding), plutonium fluoride shielded with 10 cm (4 in.) of
acrylic plastic, "bare" plutonium oxide, and "bare" plutonium metal. The
plutonium fluoride has the highest neutron emission rate and corresponds
to the highest peak in the graph. The lowest peak corresponds to the
moderated plutonium fluoride spectrum with 4 in. of acrylic plastic
shielding. These measurements are typical of the neutron energy spectra
in plutonium processing areas containing glove boxes.
The spectra contain a significant fraction of low-energy scattered
neutrons from the glove boxes and the thick concrete floor and walls of
the facility. The spectra are distinctly different from neutron emission
spectra (see Section 6.2), which do not contain scattered or background
neutrons.
6-30


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