ROSPEC - A Simple Reliable High Resolution Neutron Spectrometer

Abstract

first prototype of ROSPEC (Rotating Spectrometer) was constructed over 10 years ago with the aim of achieving a high resolution neutron spectrometer for radiation protection applications around sources of fission neutrons. Studies have confirmed that the main contribution to dose equivalent from such neutrons involve those over the energy range 50 keV to 4.5 MeV. Thus, a neutron spectrometer spanning this energy range would be adequate for the intended purpose. The selected sensors for ROSPEC were four spherical gas counters filled with different pressures of hydrogenous gas. The pressures were selected to cover four separate energy segments within 50 keV to 4.5 MeV with generous overlap between adjacent energy segments. The sensors rely on hydrogen recoil from elastic neutron scattering. Low energy recoil protons whose pulse heights were contaminated with gamma-induced signals were not used. Large signals from recoil protons whose ranges were comparable to the detector size were also rejected because of distortion of the theoretical response function. The first ROSPEC prototype was successfully used to determine the neutron spectra for several radiation fields of dosimetric interest. However, the analyses of the data required spectroscopic expertise. In recent years, the enormous advances in electronic miniaturisation and computer power has allowed the re-design of the prototype spectrometer into a compact, reliable, user-friendly instrument. Data acquisition and spectral unfolding can now be done with simple computer commands, transforming ROSPEC from a specialist tool to an everyday routine instrument for radiation protection. Typically, a reliable spectral measurement can be done over periods of hours, e.g. overnight. Over the past few years, several ROSPECs have been built for various groups who have used them to measure an enormous number of neutron spectra of interest to military and nuclear power applications. These spectra are providing a detailed understanding of important neutron interaction processes and a scientific basis for choosing personal neutron dosemeters for a variety of radiation environments. ROSPEC has already been accepted as the reference neutron spectrometer for NATO experimental studies. It is now rapidly being adopted as a reference secondary standard for neutron dosimetry in nuclear institutions.