CHELSI: Recent developments in the design and performance of a high-energy neutron spectrometer

Abstract The intrinsic pulse shape discrimination properties of CsI(Tl) form the basis of a high-energy neutron (>20 MeV) spectrometer (CHELSI) currently being developed at LANL that shows promise in satisfying the requirements of an ideal survey meter; lightweight, portable and real time display of dose. Charged particle spallation products generated in the scintillator via neutron interactions are identified on the basis of pulse shape using digital pulse processing. Conservative estimates of dose rate can be given in real time based on count rates and pulse height distributions. More accurate dose measurements can be done off-line using unfolding methods to analyze stored pulse shape versus energy data. As a precursor to the development of a portable instrument, data has been obtained using a 1″×1″ CsI-based probe and a digital spectrometer. This system has been used to collect data on the 90 m flight path at the LANSCE/WNR facility at an average neutron energy of 335 MeV. The spectrometer has the capability, in addition to storing individual waveforms for later analysis, of recording time-of-flight data and calculating a pulse shape parameter and pulse height for each scintillation event in real time. Combining these data with traditional multichannel analyzer data has yielded a set of empirical response functions with respect to neutron energy. Analysis of the charged particle spectra has yielded an overall average count rate of 0.12±0.02 cps/μSv h for a 1″×1″ CsI(Tl) scintillator in this neutron field.