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Development of a γ-Ray Spectrometer for NRCA/PGA

The γ-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 γ rays from nuclei listed in Table 2.1 is larger than the 661 keV γ-rays from 137Cs, except for 10B. Accordingly, most of the measurements of the NRCA/PGA will not have interference with the γ-rays from 137Cs. On the other hand, the Compton edge of the 661 keV γ-rays surely overlaps with the 478-keV γ-ray peak originating the 10B(n, αγ)7Li reaction.

Fig. 2.2 Design of a prototype LaBr3 γ-ray spectrometer, which consists of a cylindrical detector and four square pillar detectors. Collimated γ-rays pass through the square opening of the spectrometer to reach the cylindrical detector

The γ-ray spectrometer used for NRCA/PGA, therefore, requires properties of not only high-energy resolution and fast timing response, but also a high peak-toCompton 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 welltype 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 γ-rays from 137Cs.

A prototype LaBr3 γ-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 γ-rays from the samples. This spectrometer will be tested soon.

Experiments for NRD Developments

To evaluate systematic uncertainties in NRD, we have started experimental study at GELINA [5] under the collaboration between JAEA and EC-JRC-IRMM. The items to be studied are as follows: (1) particle size, (2) sample thickness, (3) presence of contaminated materials, (4) sample temperature, and (5) the response of the TOF spectrometer [3, 17]. Some experiments has been performed at GELINA [18–21]. A resonance shape analysis code, REFIT [22], has been adopted for the data analysis. Experiments on sample thickness were carried out at the 25-m TOF neutron beam line of GELINA. Cu plates with various thicknesses were measured with an NRTA method. Peaks at the 579 eV resonance of 63Cu were analyzed with the REFIT program. The evaluated areal densities are compared with the declared values, which were derived from measurements of the weight and the area of the

Fig. 2.3 Ratios of evaluated and declared areal densities. The 579 eV transmission peaks of 63Cu were analyzed with REFIT. Open circles indicate the results analyzed with the resonance parameter values taken from Mughabghab [23] (#6 in Fig. 2.4), and closed circles represent tentatively introduced values (#7 in Fig. 2.4), which reproduce the areal densities of Cu plates better. The lines are guides for the eye. Note: We also analyzed the transmission spectrum of a 2-cm-thick copper sample with the parameter #6. The obtained fitted curve, however, did not reproduce the peak shape at all. Thus, the misleading open circle data point was removed

Fig. 2.4 Experimentally obtained 579 eV 63Cu resonance parameters. Each data point is taken from different references (#1 [24], #2 [25], #3 [26], #4 [27], #5 [28], and #6 [23]). The data of #6 were utilized by REFIT originally; the data of #7 are tentatively introduced to reproduce the experimental transmission dips

samples. Figure 2.3 shows the results. The abscissa is the thickness of the Cu plates and the ordinate is the ratio of evaluated and declared areal densities. The open circles are the results analyzed with the resonance parameter values taken from Mughabghab [23] (#6 of Fig. 2.4); the closed circles are the results analyzed with the tentatively introduced values (#7 of Fig. 2.4), which reproduced the areal density of Cu plates better. Figure 2.4 shows measured 63Cu resonance parameters. The 579-eV resonance parameters of 63Cu may require being reevaluated. It should be emphasized that survey of the total cross sections of Pu and U isotopes is quite important to quantify NM.

Summary

We have proposed NRD for measurements of NM in particle-like debris of MF. The NRD system utilizes a compact neutron TOF system equipped with a neutron detector for NRTA and high-energy-resolution and high-S/N γ-ray detectors for NRCA/PGA. The rough design of a NRD facility is given. The capacity of NM measurements in the facility has been shown. Experiments on systematical uncertainties caused by sample properties, such as sample thickness and uniformity, are in progress under the collaboration between JAEA and EC-JRC-IRMM. The importance of confirmation of nuclear data has been shown in the case of Cu thickness measurements by NRTA.

Acknowledgments The research and development have been carried out under the agreement between JAEA and EURATOM in the field of nuclear materials safeguards research and development and are supported by the Japanese government, the Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT).

 
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