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Fig. 6-9 Neutron spectrum in a stainless steel shield for fusion reactors measured by the slowing down time method The combination of the slowing down time method (red symbols) and conventional techniques utilizing recoil protons (green and orange symbols) enables measurement of the neutron spectrum covering the entire energy range from 14 MeV down to thermal energy at the Fusion Neutronics Source facility (FNS).

The neutron energy spectrum is the most important fundamental quantity in various fields of atomic energy such as fission, fusion, and accelerator shielding. Many techniques have been applied to measure this spectrum. For the energy range between 1 eV and 1 keV however, accurate and widely applicable techniques for in situ neutron spectrum measurements have not been available, although many nuclear power plants are in operation. JAERI proposed the application of the Slowing Down Time (SDT) method, which is utilized for other purposes, as a new technique for neutron spectrum measurements in a medium in the energy ranges below 10 keV. Pulsed neutrons injected into a medium gradually slow down with decreases in their energies as time goes by. The slowing down time and the neutron energies are in one-to-one correspondence. Thus, when the time variation of a number of neutrons, is observed, the number of neutrons of each energy, i.e., the energy spectrum, can be determined. The SDT method was applied for a practical shielding experiment performed at the 14-MeV fusion neutronics source facility (FNS). Accurate neutronics spectra with experimental uncertainties around 10% were successfully measured in the energy range of 0.3 eV ~ 10 keV, although energy ranges in this part of the spectrum are difficult to measure with conventional methods.

Reference F. Maekawa et al., Measurement of Low Energy Neutron Spectrum below 10 keV with the Slowing Down Time Method, Nucl. Instrum. Methods, Section A 372, 262 (1996).

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