Decay Schemes

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

of LIS will have reduced concentration of these isotopes, resulting in lower intrinsic neutron

exposures. The LIS process can also result in significant reductions in gamma-ray

exposures for the product enriched in 239Pu. Much of the whole-body and most of the

extremity exposure is a result of surface contamination on the gloves and the interior of the

glovebox. The 241Am decay product, which results from the beta decay of 241Pu, is a major

contributor. Thus, the reduction of 241Pu can significantly reduce exposures to hands and

arms, as well as reduce the radiation streaming through glove ports in shielded gloveboxes.

2.2

NUCLEAR PROPERTIES

Of the 15 plutonium isotopes, the two that have proven most useful are masses 239 and 238.

Plutonium-239 is fissile, i.e., atoms of plutonium split upon exposure to thermal or fast neutrons.

Chemical reactions can release a few electron volts of energy per atom; however, when a plutonium

nucleus splits, it releases about 200 MeV of energy and two or three neutrons. This release of

energy makes 239Pu useful for nuclear weapons and reactor fuel. In fact, in light water reactors

(LWRs) much of the power originates from the fission of 239Pu, which is produced by neutron

capture in 238U. Because of its higher specific activity, 238Pu is used as long-lived heat sources for

powering planetary space missions where adequate solar energy is not available.

As mentioned before, all plutonium isotopes are radioactive. Isotopes with even mass numbers

(except mass number 246) are primarily alpha emitters. Isotopes of mass numbers 232, 233, 234,

235, and 237 also decay by electron capture; isotopes of mass numbers 241, 243, 245, and 246

decay by beta emission. Many of the alpha-emitting isotopes, such as 238Pu and 240Pu, also fission

spontaneously and emit neutrons. All of the particle emissions are accompanied by X-ray and

gamma-ray emissions over a wide range of energies.

A review of the nuclear properties of plutonium (e.g., cross-sections, nuclear levels, half-lives, and

fission yields) can be found in Volume 1 of the Plutonium Handbook: A Guide to the Technology

(Wick, 1967) and in American National Standards Institute (ANSI) Standard N317, Performance

Criteria for Instrumentation Used for In-Plant Plutonium Monitoring (ANSI, 1980a). Plutonium

decay schemes, neutron yields, and neutron energy spectra are described in the following sections.

2.2.1 Decay Schemes

The decay modes of some important plutonium and other isotopes and decay products are

shown in Table 2.3. For brevity, only the most abundant radiations have been included in

the table; more detailed information can be found in papers by Gunnink and Morrow (1967)

and Klein (1971), in ICRP Publication 38 (ICRP, 1983), and from the National Nuclear Data

Center. Most of the isotopes are strong alpha-emitters, making alpha heating a problem for

the storage and handling of large amounts of plutonium. The specific activities

2-5