Standard Test Method for Measuring Reaction Rates by Radioactivation of Neptunium-237

Abstract

5.1 Refer to Practice E261 for a general discussion of the determination of fast-neutron fluence rate with fission detectors. 5.2 237Np is available as metal foil, wire, or oxide powder. For further information, see Guide E844. It is usually encapsulated in a suitable container to prevent loss of, and contamination by, the 237Np and its fission products.5 5.3 One or more fission products can be assayed. Pertinent data for relevant fission products are given in Tables 1 and 2. (A) The lightface numbers in parentheses are the magnitude of plus or minus uncertainties in the last digit(s) listed.(B) Decay gamma from 137Cs beta decay, which produces 137mBa, τ(137mBa) = 2.5545 (13) min, is in transient equilibrium with 137Cs, τ(137Cs) = 30.018 (22) year. The 137mBa decay radiation provides an alternate dosimetry metric for some applications.(C) Photon decay probability of subsequent daughter 140La decay.(D) With 140La, τ(140La) = 1.67858 (21) d, in transient equilibrium with 140Ba, τ(140Ba) = 12.753 (5) d, the ratio of 140La decays to 140Ba decays is given by τ(140Ba)/[τ(140Ba)–τ(140La)] = 1.151573. When 140La is the activity measurement, the 140Ba decay probability is equal to this ratio multiplied by the 140La probability of gamma emission in decay. The 140Ba decay probability is then used, in conjunction with the fission yields in Table 2, to provide a dosimetry metric for the 237Np fission rate.(E) Primary reference for half-life, gamma energy, and gamma emission probability is Ref (2) when data are available. For 103Ru, data were not found in BIPM and the NuDAT/ENSDF data were used (3, 4).(F) The sum of the half-lives for the mass A = 140 beta decay chain nuclides leading to 140Ba is ~83.5 s. The 140Ba half-life is 12.8 d. Thus, assuming that the cumulative fission product yield for 140Ba can be considered to be attained within ten half-lives for this chain, the cumulative fission product yield for 140Ba can be considered to represent the number of 140Ba atoms that feed the transient equilibrium for 140Ba-140La. A similar statement can be made for the 137Cs-137mBa transient equilibrium where the sum of the half-lives for the mass A = 137 beta decay chain nuclides leading to 137Cs is ~4.6 min, the 137mBa half-life is 2.5545 min, and the 137Cs half-life is 30.018 years. (A) The IRDFF-II recommended fission yields are the recommended source. These values were adopted from the JEFF-3.3 radioactive decay data and fission yields sub-libraries (5).(B) All yield data given as a %; RC represents a cumulative yield; RI represents an independent yield.(C) The neutron energy represents a generic “fast neutron” spectrum and has been characterized/documented in the JEFF 3.1.1 fission yield library as representing results for an average incident neutron energy of ~0.4 MeV. 5.3.1 137Cs-137mBa is chosen frequently for long irradiations. Radioactive products 134Cs and 136Cs may be present, which can interfere with the counting of the 0.661655 MeV 137Cs-137mBa gamma ray (see Test Method E320). 5.3.2 140Ba-140La is chosen frequently for short irradiations (see Test Method E393). 5.3.3 95Zr can be counted directly, following chemical separation, or with its daughter 95Nb, using a high-resolution gamma detector system. 5.3.4 144Ce is a high-yield fission product applicable to two to three-year irradiations. 5.4 It is necessary to surround the 237Np monitor with a thermal neutron absorber to minimize fission product production from trace quantities of fissionable nuclides in the 237Np target and from 238Np and 238Pu from (n,γ) reactions in the 237Np material. Assay of 238Pu and 239Pu concentration is recommended when a significant contribution is expected. 5.4.1 Fission product production in a light-water reactor by neutron activation products 238Np and 238Pu has been calculated to be insignificant (1.2 %), compared to that from 237Np(n,f), for an irradiation period of twelve years at a fast neutron (E > 1 MeV) fluence rate of 1 × 1011 cm−2·s−1, provided the 237Np is shielded from thermal neutrons (see Fig. 2 of Guide E844). 5.4.2 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 (6). 5.5 This dosimetry reaction is important in the area of reactor retrospective dosimetry (7, 8). Good agreement between neutron fluence measured by 237Np fission and the 54Fe(n,p)54Mn reaction has been demonstrated (9, 10). The reaction, 237Np(n,f) F.P., is useful since it is responsive to a broader range of neutron energies than most threshold detectors. 5.5.1 Fig. 1 shows the energy-dependent cross section for this dosimetry reaction. The figure shows that, while it is not strictly a threshold detector, because of its sensitivity in the greater than 0.1 MeV neutron energy range it can function as a detector with good sensitivity in the fast neutron region. In the fast fission 252Cf spontaneous fission benchmark field, ~1 % of the 237Np fission dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a fast burst 235U reactor, ~5 % of the 237Np fission dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a well-moderated pool-type research reactor, ~50 % of the fission response from the 237Np(n,f) reaction comes from neutron energies less than 0.1 MeV. The importance of this low neutron energy sensitivity should be determined based on the application. 5.6 The 237Np fission neutron spectrum-averaged cross section in several benchmark neutron fields are given in Table 3 of Practice E261. Sources for the latest recommended cross sections are given in Guide E1018. In the case of the 237Np(n,f)F.P. reaction, the recommended cross section source is the International Reactor Dosimetry and Fusion File (IRDFF-II) nuclear data library (11). Fig. 1 compares the recommended IRDFF-II cross section with the EXFOR experimental data (12, 13).