Standard Test Method for Determining Thermal Neutron Reaction Rates and Thermal Neutron Fluence Rates by Radioactivation Techniques

Importancia y uso:

4.1 This test method can be extended to use any material that has the necessary nuclear and activation properties that suit the experimenter's particular situation. No attempt has been made to fully describe the myriad problems of counting techniques, neutron fluence depression, and thick foil self-shielding. It is assumed that the experimenter will refer to existing literature on these subjects. This test method does offer a referee technique (the standard gold foil) to aid the experimenter when they are in doubt of their ability to perform the radiometric technique with sufficient accuracy.

4.2 The standard comparison technique uses a set of foils that are as nearly identical as possible in shape and mass. The foils are fabricated from any material that activates by an (n, γ) reaction, preferably having a cross section approximately inversely proportional to neutron speed in the thermal energy range. Some of the foils are irradiated in a known neutron field (at NIST or other standards laboratory). The foils are counted in a fixed geometry on a stable radiation-detecting instrument. The neutron-induced reaction rate of the foils is computed from the counting data, and the ratio of the known neutron fluence rate to the computed reaction rate is determined. For any given foil, neutron energy spectrum, and counting setup, this ratio is a constant. Other foils from the identical set can now be exposed to an unknown neutron field. The magnitude of the fluence rate in the unknown field can be obtained by comparing the reaction rates as determined from the counting data from the unknown and reference field, with proper corrections to account for spectral differences between the two fields (see Section 5). One important feature of this technique is that it eliminates the need for knowing the detector efficiency.

4.3 This test method follows the Stoughton and Halperin convention for reporting thermal neutron fluence. Other conventions are the Wescott convention (followed in Practice E481) and the Hogdahl convention. Practice E261 explains the three conventions and gives conversion formulae relating values determined by the different conventions. Reference (1)3 discusses the three thermal neutron conventions in detail.

Subcomité:

E10.05

Referida por:

E0720-23, E0721-22, E1297-18, E1854-19, E0704-19, E2006-22, E0481-23, E0705-18, E0261-16R21, E1005-21, E0944-19

Volúmen:

12.02

Número ICS:

17.240 (Radiation measurements), 27.120.30 (Fissile materials and nuclear fuel technology)