Neutron Emission Yields - doe-std-1128-98

DOE-STD-1128-98

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, ( α ,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 α ,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:

N ( E ) = ( E ) Exp ( E / 1.43 MeV )

(6.5)

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

MeV.

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