Neutron spectrum measurements

The efforts required to be put in for detailed reactor neutron spectrum measurements are too complex that even today, in some cases, theoretically calculated spectrum has to be used in lieu of measurement. This paper gives a brief description of some of the experimental methods used to obtain the neutron spectrum in reactors. By no means exhaustive, this is only an introduction to the vast subject of neutron spectrum measurements. The principle of the time-of-flight method is explained. The experimental arrangement for time-of-flight measurement with a pulsed source as well as with a neutron beam-chopper along with the design aspects, beam extraction, detectors and the expected resolution are given. The method of neutron flux measurement by reaction unfolding uses the following reactions: 1) elastic scattering of neutrons 2) neutron reaction with helium giving a proton and triton and 3) neutron reaction with Lithium giving an alpha particle and triton. The method of reaction unfolding is dealt with briefly. The various detectors used like the proportional counters, sandwich detectors, organic scintillation detector and nuclear emulsion technique are also discussed. Finally, the principle of obtaining neutron spectrum using the Bonner sphere technique is also mentioned. (author)