2.2.1 The Concept of NRD

Neutron resonance densitometry is a method of a combination of neutron resonance transmission analysis (NRTA) and neutron resonance capture analysis (NRCA) or prompt gamma-ray analysis (PGA). The fundamental principles of NRTA and NRCA are described by Postma and Schillebeeckx [6].

In NRTA, neutron transmission is measured as a function of neutron energy with a TOF technique. Characteristic neutron transmission dips of Pu and U isotopes are observed in the neutron energy in the range of 1-50 eV [7, 8]. Measurements of these transmission spectra can be carried out with a short-flight path TOF system [9,10].

Although strong y-ray radiation from MF samples does not interfere with NRTA measurements, reduction of neutron flux caused by nuclei with large total cross section (such as H, B, Cl, Fe) makes accurate NM quantification difficult. Nevertheless, the quantities of these contained nuclei could not be determined by NRTA only, because these nuclei do not resonantly interact with neutrons in this energy range. To identify and to quantity the composing isotopes, the NRCA/PGA method is required. Characteristic prompt у rays ware utilized. Table 2.1 shows prompt y-rays emitted from nuclei after neutron capture reaction. Most of these discrete prompt Y-rays have significant intensities. The information obtained by NRCA/PGA enables us to determine the appropriate sample thickness and mea­surement time. This information also supports NRTA analysis.

Fig. 2.1 A rough draft of an NRD facility.

The neutron flight path length for NRTA is 5 m and that for NRCA/PGA is 2 m