Multisphere Spectrometer System - STD-1128-98master0194

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

-- liquid scintillator spectrometers

-- proton recoil spectrometers.

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

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