BS ISO 8529-1:2021 pdf download – Neutron reference radiations fields Part 1: Characteristics and methods of production
4 Broad spectrum neutron reference radiation fields produced with radionuclide sources
4.1 Overview In this clause, neutron reference fields produced with radionuclide sources are specified, which are particularly suited for the calibration of neutron‑measuring devices (see Reference ). Thermal neutron reference radiation fields are achievable by moderating radionuclide sources, but are covered by Clause 6.
4.2 Types of calibration sources The radionuclide sources given in Table 1 shall be used to produce neutron reference radiation fields. The numerical values given in Table 1 are to be taken only as a guide to the prominent features of the sources, since the properties of a specific source vary with the construction of the source, because of scattering and absorption of neutron and gamma radiation, and with the isotopic impurities of the radioactive material used. Hence details of the source encapsulation are specified (see 4.3), and the method for determining the anisotropy of the neutron emission is specified (see Annex E). 252 Cf has a high specific neutron emission rate and 252 Cf sources are therefore comparatively small. Because of their short half‑life of 2,647 years, they need regular replacement. The D 2 O‑moderated 252 Cf source is ideally composed of a point 252 Cf source located in the centre of a 300 mm diameter heavy‑water sphere, surrounded by
a) a 0,8 mm thick iron shell, and
b) a1 mm thick cadmium shell.
In practice, a number of designs have been developed in reference laboratories, being slightly different in terms of construction details, such as the guide used to locate the source in the sphere centre, the material used to contain the heavy water, and the structure used to suspend or hold the moderating sphere. In addition, every moderating assembly has specific D 2 O purity and 252 Cf source capsule. The experience of reference laboratories suggests that variability in the construction of D 2 O‑moderated 252 Cf sources results in non‑negligible differences in the energy distribution of the neutron fluence  . Laboratories should characterize their D 2 O‑moderated 252 Cf sources by simulations and spectral measurements. The energy distribution of the neutron emission rate and spectrum‑averaged quantities of these fields should be checked through comparisons. A representative spectrum of the D 2 O‑ moderated 252 Cf source was derived, for the purposes of this document, by Monte Carlo simulations. In this model, 11,4 % of the source neutrons are absorbed in the moderating assembly.
See Annex C for details. 241 Am‑Be (α,n) neutron sources include appropriate alloys, mixtures or compounds of americium, such as a compressed mixture of americium oxide and beryllium as appropriate. See Annex D for details.
In addition to the sources listed in Table 1, sources such as 241 Am‑B(α,n)  , Pu‑Li(α,n)  , Pu‑Be(α,n)  , 241 Am‑F(α,n)  , 241 Am‑Li(α,n)  and 244 Cm  are also used but are not addressed specifically in this document 1) .
4.3 Source shape and encapsulation The shape of the source would ideally be spherical, but most practical sources are cylindrical. In the latter case, it is preferable that the diameter and length are approximately the same. The thickness of the encapsulation should be uniform and small compared to the external diameter. For a 241 Am‑Be(α,n) source, the spectral distribution, mainly in the energy range below approximately 2 MeV, depends, to some extent, on the size and the composition of the source  . See Annex D for more details. Sources should comply with ISO 2919 encapsulation requirements  .
4.4 Photon component of the neutron field For 252 Cf, the ratio of photon to neutron ambient dose equivalent rate is dependent upon the age of the source because of the build‑up of gamma‑emitting fission products, as well as upon source encapsulation. The 5 % value reported in Table 1 refers to new sources. During the first 30 years, this is likely to remain below 10 %  .BS ISO 8529-1 pdf download.