Neutron Emission Yields - STD-1128-98master0178

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

H = 0.0097 S/r2

(6.4)

where H = dose equivalent rate, mrem/h

r = distance from the center of the source, cm

S = neutron emission rate from the plutonium source.

The total neutron emission rate, S, is the product of the mass of plutonium (in

grams) times Y, the total neutron yield per gram of plutonium

(neutrons/second/gram) from spontaneous fission, (",n) reactions with low atomic

number elements in contact with the plutonium, and fission-induced neutrons.

Generally, fission-induced neutrons are required because stringent criticality safety

rules prevent accumulation of enough moderator and plutonium to significantly

increase neutron emission rates. But kilogram quantities of metals or compressed

oxides can have significant multiplication and increased emission rates. For

example, neutron emissions from 1 kg plutonium metal "buttons" are generally

measured to be 30% higher than the calculated neutron emission rate.

6.2.3.2 Neutron Emission Yields

The neutrons produced by spontaneous fission and ",n reactions can be estimated

from the following information.

Most neutrons from spontaneous fission originate from the even plutonium

isotopes: 238Pu, 240Pu, and 242Pu. Because it is the most abundant, the isotope 240Pu

is the most important source of spontaneous fission neutrons. Decay progeny of

plutonium have very low spontaneous neutron emissions. Table 6.7 contains

spontaneous fission yields for plutonium and other isotopes that may be found in

plutonium facilities within the DOE complex. These data are taken from

NUREG/CR-5550 (Reilly et al., 1991) and are believed to be more current then the

previously published PNL values (Faust et al. 1977, Brackenbush et al., 1988). As

a rule of thumb, nuclides with even numbers of protons and neutrons have the

highest spontaneous fission neutron emission rates. The spontaneous fission rate

for odd-even nuclides is about 1000 times less, and the rate for odd-odd nuclides is

about 100,000 less. Spontaneous fission neutrons are emitted with a Maxwellian

energy distribution given by the equation:

(/ E ) Exp ( E / 1.43 MeV )

(6.5)

N(E)

where N(E) is the number of neutrons as a function of the energy E in MeV.

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