Neutron Yields and Spectra - STD-1128-98master0034

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

(See Section 6.3.3 for more information.) Also, 241Am can contribute to neutron dose.

Americium-241 contributes to increased alpha emission which affects the neutron dose as

well as radiolysis and helium retention and release. Because of its importance to

radiation exposure, the fractional amount of 241Am produced by beta decay from 241Pu is

given as a function of time since chemical separation (see Figure 2.2).

2.2.2

Neutron Yields and Spectra

Plutonium and plutonium compounds also emit neutrons from spontaneous fission and

from alpha-neutron reactions with light elements. The spontaneous fission half-life and

the neutron yields from spontaneous fission and alpha-neutron reactions for plutonium

metal and plutonium compounds are provided in Section 6.0 of this TS. The approximate

neutron yield from a substance with a known isotopic composition can be determined by

adding the contributions from each component. This procedure and its limitations are

described in detail in Section 6.0, which also discusses neutron dose equivalent rates.

Energy spectra from Pu-Be and Pu-B neutron sources are shown in Figure 2.3 Because

of licensing restrictions on plutonium, these sources have been replaced with sources

fabricated from americium. Metallic plutonium emits neutrons having a Maxwellian

energy distribution, with an average energy of about 1.9 MeV. Plutonium compounds

and alloys also emit neutrons from alpha-neutron reactions, and these neutrons have

significantly different energies:

PuF4, about 1.3 MeV

10% plutonium-aluminum alloys, 1.6 MeV

PuO2, slightly more than 2 MeV

PuBe13, 4.3 MeV.

Plutonium compounds or alloys containing sodium, magnesium, silicon, chlorine, carbon,

or oxygen have significant alpha-neutron yields, but little information is available about

their neutron energy spectra.

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