Section 7.3.1. Introduction

In this chapter, we shall be concerned with the detection of neutrons having thermal and epithermal energies in the range 0.0002–10 eV (20–0.1 Å). Given the cost and the rarity of the neutron sources, it is clear that the recent trends in neutron diffractometry are more and more in the direction of designing new instruments around highly efficient and complex detection systems. These detection systems become more and more adapted to the particular requirements of the different experimental needs (counting rate, size, resolution, definition, shielding and background, TOF, etc.). It is therefore difficult to speak about neutron detectors and intensity measurements as such without reference to the complete spectrometers, and this should include the on-line computer.

Most neutron detectors for research experiments have been created and developed using fission reactors as neutron sources [i.e. with an upper limit of usable energy of 0.5 eV (0.4 Å)]. Given the relatively low intensity of reactor neutron beams, a very successful effort has been made to increase the detector efficiency and the detection area as much as possible. The recent construction of pulsed neutron sources extends the range of incident energy to at least 10 eV and generalizes the use of time-of-flight (TOF) techniques. A broad range of fully operational neutron detectors, well adapted to reactors as neutron sources, is commercially available, but this is not yet the case for pulsed sources. Probably due to the variation of intensity of the early neutron beams, it is a tradition in neutron research to monitor the incident flux with a low-efficiency detector, which in the best case has a stability of the order of 10⁻³, i.e. sufficient for most experiments.