Refer to Practice E 261 for a general discussion of the determination of fast-neutron fluence rate with fission detectors.

²³⁸U is available as metal foil, wire, or oxide powder (see Guide E 844). It is usually encapsulated in a suitable container to prevent loss of, and contamination by, the ²³⁸U and its fission products.

One or more fission products can be assayed. Pertinent data for relevant fission products are given in Table 1 and Table 2.

¹³⁷Cs-^137mBa is chosen frequently for long irradiations. Radioactive products ¹³⁴Cs and ¹³⁶Cs may be present, which can interfere with the counting of the 0.662 MeV ¹³⁷Cs-¹³⁷, Ba gamma rays (see Test Methods E 320).

¹⁴⁰Ba-¹⁴⁰La is chosen frequently for short irradiations (see Test Method E 393).

⁹⁵Zr can be counted directly, following chemical separation, or with its daughter ⁹⁵Nb using a high-resolution gamma detector system.

¹⁴⁴Ce is a high-yield fission product applicable to 2- to 3-year irradiations.

It is necessary to surround the ²³⁸U monitor with a thermal neutron absorber to minimize fission product production from a quantity of ²³⁵U in the ²³⁸U target and from ²³⁹Pu from (n,γ) reactions in the ²³⁸U material. Assay of the ²³⁹Pu concentration when a significant contribution is expected.

Fission product production in a light-water reactor by neutron activation product ²³⁹Pu has been calculated to be insignificant (<2 %), compared to that from ²³⁸U(n,f), for an irradiation period of 12 years at a fast-neutron (E > 1 MeV) fluence rate of 1 × 10¹¹ cm⁻² · s⁻¹ provided the ²³⁸U is shielded from thermal neutrons (see Fig. 2 of Guide E 844).

Fission product production from photonuclear reactions, that is, (γ,f) reactions, while negligible near-power and research-reactor cores, can be large for deep-water penetrations (1).