The y-ray background from debris is expected to be strong. The strongest radioac­tive isotope in a MF of the TMI-2 accident was 137Cs, which ranged from 106 to 3 x 108 Bq/g [14]. The energies of the prominent у rays from nuclei listed in Table 2.1 is larger than the 661 keV y-rays from 137Cs, except for 10B. Accordingly, most of the measurements of the NRCA/PGA will not have interference with the y-rays from 137Cs. On the other hand, the Compton edge of the 661 keV y-rays surely overlaps with the 478-keV y-ray peak originating the 10B(n, ay)7Li reaction.

The y-ray spectrometer used for NRCA/PGA, therefore, requires properties of not only high-energy resolution and fast timing response, but also a high peak-to — Compton ratio.

To satisfy these requirements, a well-type spectrometer made of LaBr3 crystal has been proposed [2, 15]. In a study based on Monte Carlo simulations of a well — type LaBr3 spectrometer [15, 16], the Compton edge was successfully reduced by an order of magnitude. Such reduction enables us to roughly quantify 10B in a sample, even in the presence of high background y-rays from 137Cs.

A prototype LaBr3 y-ray spectrometer has been designed (Fig. 2.2). Because of the technical difficulty of producing a crystal with a well, the spectrometer is made of several detectors: a cylindrical detector and four square pillar detectors. The cylindrical crystal is 120 mm in diameter and 127 mm in length; each square pillar crystal is 50.8 x 50.8 x 76.2 mm. An arrangement of the detector pillars opens a square channel of 20 x 20 mm for the passage of collimated y-rays from the samples. This spectrometer will be tested soon.