Multisphere Spectrometer System - doe-std-1128-98

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.

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