The accuracy and precision of the experimental α-determination in the 1/E1+α epithermal reactor-neutron spectrum

Experimental determination and simulation of neutron and gamma flux parameters in horizontal channel for Prompt Gamma Neutron activation analysis implementation at Es-Salam research reactor

Since 2004, the k₀-standardization method has been applied and evaluated in three experimental channels at Es-Salam research reactor of the Algerian Centre de Recherche Nucléaire de Birine (CRNB). The NAA laboratory at CRNB is well involved in the implementation of the Prompt Gamma Neutron Activation Analysis to extend the field of analyses to some additional elements to the NAA method. In the frame work of the PGNAA research project, several works have been carried out in the present study including the experimental and simulation characterization of neutron and gamma flux parameters of the horizontal channel. The absolute and relative neutron fluxes of the studied channel have been determined by using a set of monitors including Al, Au, Cu, Fe and Zr. Furthermore, measurements of gamma and thermal neutron doses have been achieved using LiF: Mg, Ti (TLD 600 and TLD 700) detectors. The gamma dose rate at the experimental channel center position was found to be 0.6 Gy/min·MW. The experimental results obtained in this study have been used as input information for simulations and design of neutron facilities and radiation shielding, which are successfully established by SCALE code. The results were compared in terms of spectral detail and precision and a good agreement has been revealed. The value of the thermal neutron flux was found to be (6.29 ± 0.25) E+08 n/cm².s which allows the implementation of PGNAA at the Es-Salam research reactor.

Measurement of neutron energy spectrum at the radial channel No. 4 of the Dalat reactor

Introduction

Several compositions of neutron filters have been installed at the channel No. 4 of the Dalat research reactor to produce quasi-monoenergetic neutron beams. However, this neutron facility has been proposed to enhance the quality of the experimental instruments, and to characterize the neutron spectrum parameters for new filtered neutron beams of 2 keV, 24 keV, 59 keV and 133 keV.

Case description

In order to meet the demand of neutron spectrum information for calculation and design of filtered neutron facilities at the Dalat nuclear research reactor (DNRR), the experimental determinations of neutron flux and energy spectrum, up to 8 MeV, has been performed at the inner entrance of the horizontal channel No. 4 from the core of DNRR. The Westcott neutron fluxes as well as the α-parameter that represents the deviation of epithermal neutron distribution from the 1/E law were measured by applying the cadmium ratio and the multi-foils activation methods. The fast neutron spectrum was measured based on the iterative adjustment procedure with threshold reactions.

Discussion and evaluation

A set of pure metal thin foils with the diameter of 1.27 cm and thickness of 0.125 mm were used as threshold detectors to measure the integrated fluxes, and a calculation procedure on iterative adjustment was implemented to derive the differential neutron energy spectrum from the integrated values.

Conclusions

The neutron fluxes and spectrum parameters were characterized with the measured values of 4.80 × 10⁹, 1.98 × 10⁷, 5.06 × 10⁸ cm⁻² s⁻¹ and 0.0448 for the thermal, epithermal, fast neutron fluxes and the α-shape factor, respectively. The present result has been significantly applied to the input data for the Monte Carlo simulations in the developments of filtered mono-energetic neutron beam facility at the institute.

Photoneutrons from a beryllium reflector: a potential source of problems with Zr-Au flux monitors in k0 standardization based neutron activation analysis

The assumption that the shape of the epithermal neutron spectrum can be described, in any research reactor, by the 1/E^1+α function is a fundamental starting point of the k₀ standardization. This assumption may be questioned from a reactor physics viewpoint. The type of moderator, the existence of neutron reflectors, the additional production of (γ, n) neutrons and resonance capture by construction materials may be different for each reactor, with consequences for the shape of the neutron spectrum. This dependency may explain that various practitioners reported contradicting experiences with the use of Zr–Au flux monitors for the determination of the α-parameter. An objective view on the influence of the design of the reactor and irradiation facility on the shape of the neutron spectrum can be obtained by modeling. This has been applied in the Reactor Institute Delft for reactor configurations in which the irradiation facilities face the fuel elements with the presence of beryllium reflector elements. The Monte Carlo calculations indicate a distortion of the 1/E^1+α relationship at the higher energy edge of the epithermal neutron spectrum. This distortion is attributed to the formation and thermalisation of both photoneutrons and (n, 2n) produced fast neutrons in the beryllium, and has a direct impact on the resonance activation of ⁹⁵Zr, other than represented by the 1/E^1+α function. The obtained relationship between neutron flux and neutron energy was also used for estimating the f-value and compared with the value obtained by the Delft Cr–Mo–Au flux monitor.

Experimental evaluation of neutron performance in boron-doped low activation concrete

Reaction rate distribution in concrete with/without boron dopant up to a thickness of 60 cm was measured using Yayoi fast reactor located at University of Tokyo. The 7 reaction rates such as (197)Au(n,gamma), (59)Co(n,gamma), (115)In(n,n'), (55)Mn(n,gamma), (23)Na(n,gamma), (94)Zr(n,gamma) and (96)Zr(n,gamma) were measured at 12 different depths, and the reduction of the reaction rate as a result of boron doping was quantitatively analysed. These reaction rates were also used to determine epithermal neutron spectrum shape parameter. Monte Carlo simulations of the experimental setup were performed using the MCNP-5 code. Simulated depth profiles of reaction rates and the epithermal neutron spectrum shape parameter agreed with the experimental results with fair accuracy. This experimental results provide useful data to benchmark the accuracy of neutron transport codes in the prediction of transmission and neutron spectrum distortion in boron-doped concrete.

Modified method of α determination in the 1/E1+α epithermal neutron spectrum of reactor

A modified method has been developed for the α determination in the 1/E^1+α epithermal neutron spectrum of reactor. It is based on the Cd-covered and without Cd-covered irradiations of two monitors. This method was applied to determine the α value in the channels of Dalat reactor and the results were compared with those obtained by the other methods. It appeared that the results of the modified method were in quite good agreement with those of other methods. It also showed that the modified method was simple in practical uses and a good application in the experiment of α determination in the reactor irradiation channels.

Determination of k0-values for the reactions 94Zr (n, gamma) 95Zr and 96Zr (n, gamma) 97Zr-97mNb by irradiation in highly thermalized neutron flux

Determination of k(0)-factors for zirconium isotopes was performed by co-irradiation of Zr and Au-Al. Due to the highly thermalized irradiation position at FRM-II, interferences from epithermal neutrons were found largely decreased for (96)Zr (n, gamma) (97)Zr-(97m)Nb, the reaction with the highest Q(0)-value in all (n, gamma) reactions. Results showed that, the (95)Zr k(0)-values from this work were the same as the recommended ones. For (97)Zr-(97m)Nb 743.4 keV gamma-line, the new k(0)-value was 2.8% higher compared to the recommended value, which is not a significant difference. These results are helpful in clarifying the suspicion about the Zr k(0)-factors.

Analysis of k0 neutron activation analysis at the University of Missouri Research Reactor

k(0) Instrumental Neutron Activation Analysis (k(0) INAA) was evaluated at the University of Missouri Research Reactor (MURR). The MURR, unlike many other research reactors that employ k(0) INAA, is refueled on a weekly basis. To determine if the neutron spectrum is stable enough for routine k(0) INAA analysis, the neutron spectrum parameters alpha and f were measured over a 9 month period. The average values of f and alpha measured using the Cd ratio multi monitor technique were 57.4+/-4.5 and 0.039+/-0.012, respectively. It was determined that k(0) INAA could be used at the MURR by employing the average value of alpha and f. However, significant measurement bias could occur for elements with high Q(0) values if the true spectral parameters deviate by more than 10% from the average value.

Installation and calibration of Kayzero-assisted NAA in three Central European countries via a Copernicus project

An account is given of the installation and calibration of k0-based NAA--assisted by the DSM Kayzero/Solcoi software package--at the KFKI-AEKI, Budapest, the NPI, Rez and the IJS, Ljubljana. Not only the calibration of the Ge-detectors and the irradiation facilities are discussed, but also other important topics such as gamma-spectrometric hard- and software, QC/QA of the IRMM-530 Al-Au flux monitor and the upgrade of the Kayzero/Solcoi code. The work was performed in the framework of a European Copernicus JRP, coordinated by the Laboratory of Analytical Chemistry, Gent, with DSM Research, Geleen, as the industrial partner.