Relative intensities of prompt γ-rays from the 35Cl(n,γ)36Cl reaction with thermal neutrons as secondary γ-ray intensity standards
Highlights
► The relative intensities of prompt γ-rays from the 35Cl(n,γ)36Cl reaction were measured with three HPGe detectors. ► The escape peak ratios were simulated by GEANT4 and compare the experiments. ► Useful secondary γ-ray intensity standards were proposed.
Introduction
The γ-ray spectrometry to the high-energy region of 10 MeV is widely used in nuclear engineering and nuclear science, e.g., as a cross section measurement for the transmutation of long-lived fission products (LLFP) or prompt γ-ray activation analysis (PGAA). Currently, large-volume Germanium crystals can be fabricated, and clover- or cluster-type Ge detectors are utilized in many facilities to measure prompt γ-rays. To determine the required quantities in each field, the detection efficiencies must be precisely determined. Therefore, γ-rays from the 14N(n,γ)15N reaction are usually employed as a primary standard because this reaction emits γ rays of up to 10 MeV with proper energy spacing. However, facilities that can provide thermal neutrons, such as nuclear reactors, are needed, and even if they are available, the cross section for thermal neutrons is small (80 mb); obtaining statistics is thus time consuming. Therefore, the 35Cl(n,γ)36Cl reaction with thermal neutrons can be used as a secondary standard because it has a large cross section (33 b) and it emits γ-rays of up to 9 MeV despite the difficulties in the spectrum analysis caused by the complicated level scheme. Many experimental or evaluated values have been proposed, e.g., in experimental, Révay et al. (2004), Raman et al. (2000), Venturini and Pecequilo (1997), Coceva et al. (1996), Krusche et al. (1982), Kennett et al. (1981) and Loper and Thomas (1972), in evaluation, NNDC (2010) and IAEA (1991). However, there are discrepancies between the reported values. It is important to note that the intensities are principally deduced using the detection efficiencies, which are based on γ-rays from the 14N(n,γ)15N reaction.
Recently, precise intensities of γ-rays from the 14N(n,γ)15N reaction were proposed by our group, Miyazaki et al. (2008), using a liquid Nitrogen target (Sakane et al., 2005). There are discrepancies between other proposed values and the values by Miyazaki et al. (2008). It is important to determine the intensities of the prompt γ-rays from the 35Cl(n,γ)36Cl reaction based on the detection efficiency determined using the 14N(n,γ)15N reaction by Miyazaki et al. (2008).
In this study, prompt γ-rays from the 35Cl(n,γ)36Cl reaction were measured using three HPGe detectors that have different crystal volumes, 22%, 60% and 90%, and well-determined relative peak efficiencies. Based on appropriate analyses of multiple peaks, and on a correction for the mixing of single- and double-escape peaks combined with a Monte Carlo simulation (GEANT4) (Agostinelli et al., 2003), the relative intensities of 16 prompt γ-rays are proposed as secondary γ-ray intensity standards. We focused on intensive γ-rays, which can be adopted to determine the efficiency of HPGe detectors in the high-energy region.
Section snippets
Experiments
Three 22%, 60% and 90% HPGe detectors were used to measure prompt γ-rays from the 35Cl(n,γ)36Cl reaction. The relative detection efficiency at a sample-to-detector-distance of 20 cm for every detector was predetermined using the radioactive sources 56Co and 152Eu and prompt γ-rays from the 14N(n,γ)15N with the Monte Carlo simulation code GEANT4, as described in Miyazaki et al. (2008). The total efficiencies, which were used to correct the coincidence summing effects, were experimentally
Analysis of the full energy peak
A typical prompt γ-ray spectrum from the 35Cl(n,γ)36Cl reaction measured with the 22% HPGe detector is shown in Fig. 3. Most of the peaks are full-energy peaks (FEPs) originating from the 35Cl(n,γ)36Cl reaction; there are also single escape peaks (SEPs) and double escape peaks (DEPs). The other background prompt γ-rays from 1H, 6Li, 12C, 14N, 16O and 207Pb are difficult to observe in this spectrum.
The γ rays of interest are 786.3+788.4, 1131.3, 1162.7+1164.9+1171.0, 1327.4, 1601.1,
Results and Discussion
The relative intensities normalized by the 7414.0 keV γ-ray are listed in Table 1. The origins of the uncertainties are summarized in Table 2. The present results and their uncertainties were deduced. The uncertainties were categorized as follows: (i) σstat. that originated from the peak count of prompt γ-ray statistics themselves, including other corrections, and (ii) σsys. that originated from the full energy peak efficiencies as described above. The weighted mean values of the relative
Conclusions
The relative intensities of prompt γ-rays from the 35Cl(n,γ)36Cl reaction with thermal neutrons were determined using three HPGe detectors with relative detection efficiencies of 22%, 60% and 90%. By processing the appropriate corrections, especially the evaluation of escape peak ratios, the deduced values were consistent each other within an uncertainties range of 1%. The accuracies are much better than previously reported values. The previous data are almost in agreement with the present
Acknowledgments
This work was performed as a part of the Research Collaboration Program of the Research Reactor Institute, Kyoto University. The authors would like to express their appreciation to Dr. Hideo HARADA for the use of a 90% HPGe detector. The authors are also grateful to Prof. Emeritus Kiyoshi KAWADE for his valuable discussion and encouragement.
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