Evaluation of detection efficiency and neutron scattering in NAND detector array: FLUKA simulation and experimental validation

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Abstract

The light output response and efficiency of 5”×5” BC501A liquid scintillator neutron detectors composing the NAND detector array have been measured by time of flight technique using 252Cf source. Energy dependent detection efficiency was calculated using FLUKA Monte Carlo code and validated by experimental measurement. FLUKA simulation incorporated fission neutron correlation and kinematics of neutron emission from fission source determining the energy distribution of neutrons in the laboratory frame. The intrinsic features of the scintillator were studied by including particle dependent quenching and light output resolution. Comparison of FLUKA simulation of scintillation light produced by neutrons and gamma rays with measurements provided realistic results. The calculated neutron detection efficiency at 0.5 MeV threshold reproduces the experimental data reasonably well in the studied energy range, 0.5<En<12MeV. Neutron scattering from target chamber material in NAND was investigated using FLUKA concluding that the loss in neutron flux due to scattering from the chamber wall is 12%. The scattering amounts to background neutron counts in neighbouring detectors comparable to cross-talk events. The absolute efficiency of the NAND array was estimated to be 1.40% after taking into consideration the intrinsic efficiency of liquid cells, geometrical efficiency and flux loss due to scattering.

Introduction

Large volume liquid scintillator cells are widely used in neutron time-of-flight (TOF) measurements for nuclear physics experiments [1], [2], [3]. Fast neutrons detected in array of cells kept at large distance from the target allow high resolution TOF measurements covering wide range of neutron energies. Accurate estimation of neutron emission flux is of central importance in these measurements involving energy and angular distribution of neutrons. In most of these experiments, neutrons are detected in the presence of a large background of γ-rays and efficient discrimination between neutron and γ-rays is a necessary requirement for correct estimation of neutron flux. Unlike charged particles, the interaction of neutrons in the medium of the detector is relatively rare and therefore observed neutron counts require a correction for detection efficiency to determine the number of emitted neutrons. Due to their intrinsic properties like fast response time, high detection efficiency and excellent neutron-gamma (nγ) discrimination at relatively low energy threshold, organic liquid scintillator detectors have found wide spread use in fast neutron spectroscopy applications [4]. A large multi detector neutron TOF facility, National Array of Neutron Detectors (NAND) consisting of 100 liquid scintillator cells, has been built and used for nuclear physics experiments at Inter-University Accelerator Centre (IUAC) [5]. The cells mounted on a hemi-spherical dome structure are placed at 175 cm distance from target with total solid angle coverage 3.3% of 4π. A 50 cm radius spherical vacuum chamber fixed at the centre contains charged particle detectors and reaction targets.

The neutron detector array is currently being utilized to study the dynamics of nuclear fission induced by heavy ion reactions where neutron multiplicities are measured in coincidence with fission fragments [6], [7], [8], [9]. Neutrons with kinetic energy ranging from a few hundred keV to tens of MeV are usually emitted from these reactions and it is important to apply energy dependent efficiency correction to determine the neutron multiplicity from double differential energy spectra obtained from experiments. Another important aspect to be considered in the estimation of neutron emission flux is the scattering of neutrons by various materials in the flight path. Scattering causes loss of flux in the emission direction and in addition generates neutron background to other nearby detectors of the array. The cross-section for neutron scattering depends critically on characteristics of materials such as composition, density and thickness. Monte Carlo simulations can be utilized to incorporate these factors to estimate the percentage loss of flux and neutron background due to scattering from various materials. In order to determine the overall detection efficiency of the array and estimate loss of flux due to neutron scattering from the target chamber, we have performed Monte Carlo simulation and validated the results with measured data. The performance characteristics of the NAND array have already been reported recently [5]. In the present paper we discuss in detail the simulation study and the experimental technique used for measuring response function and detection efficiency of the array. Monte Carlo simulation has been performed using FLUKA particle transport code [10], [11] to generate light output and energy dependent neutron detection efficiency curve for BC501A detector used in the array. The results are validated with measurements using neutrons emitted from 252Cf source.

This article is organized in the following way. In Section 2, we present the experimental method used for neutron time of flight measurement and determination of intrinsic efficiency of 5×5 BC501A. Section 3 discusses the model used to describe neutron emission spectrum of 252Cf for efficiency calculation. In the subsequent section, FLUKA simulations of light response and detection efficiency are discussed. Section 5 describes the role of neutron scattering from target chamber material on neutron flux measurements. The estimation of neutron emission flux from measured neutron counts is described in Section 6. Section 7 summarizes the results of simulation and measurements.

Section snippets

Experimental method for efficiency measurement

There are various methods reported in literature for measurement of fast neutron flux using scintillators [12]. Among them, associated particle method is considered to be the most accurate technique in which charged particle(s) emitted along with neutrons are detected in coincidence to estimate incident neutron flux. To measure the intrinsic efficiency (defined as ratio of number of neutrons detected to the number of neutrons incident on detector volume) of BC501A detector, associated particle

Modelling of 252Cf neutron source for FLUKA simulation

The efficiency measurement using 252Cf source is a model dependent technique where the neutron emission spectrum is modelled based on experimental observables [23], [24]. The neutron evaporation from an excited fission fragment is considered as a cascade process. Le Couteur and Lang expressed the cascade evaporation spectrum of neutrons from an excited system with the formula [15], f(En)dEn=const.Enλexp(En/Teff)dEnwhere λ is close to 511 and Teff is 1112 of nuclear temperature [15]. The

FLUKA simulation of BC501A characteristics

The response function and detection efficiency of BC501A for monoenergetic neutrons and γ-rays have been simulated using FLUKA package, version 4.0, a well-established Monte Carlo code for simulating particle interaction and transport in scintillator media. FLUKA is used along with an advanced graphical interface, Flair [28], making it a versatile tool for simulation of particle interactions in different materials and detector geometries. GEANT4 [29] models are also commonly applied in detector

Scattering of neutrons by materials

In big detector arrays, one dominant factor which limits accurate estimation of neutron flux from a given reaction is the scattering of neutron by various materials on its flight path. Even though careful consideration was given in choosing minimum amount of scattering materials between target and detectors in NAND facility, there are inevitable physical structures such as vacuum chamber wall, detector coverings, etc., that remain as sources of neutron interaction causing neutron attenuation

Estimation of neutron emission flux

With accurate knowledge of intrinsic efficiency of detectors, flux loss due to scattering and limited solid angle subtended by the detectors (geometrical efficiency), the emission flux can be determined from the measured neutron counts. The efficiency curve derived from Eq. (12), integrated over neutron energy range yields overall efficiency of a single detector. For 252Cf source kept at the centre of NAND array, ηintegral=(1FNMndΩ4π)EPthEnmax1f(En)(dNPdEn)(En)dEnwhere Enmax is the maximum

Summary and conclusions

We have investigated the light output response and intrinsic efficiency of BC501A liquid scintillator of NAND array using monoenergetic γ-ray sources and fast neutrons from 252Cf fission source. Monte Carlo computation using FLUKA reproduced measured light output of the detector for γ-rays and neutrons after smearing simulated data with resolution function using a least square minimization method. The detector resolution parameters, α, β, and γ have been derived for 5×5 BC501A detector with

CRediT authorship contribution statement

N. Saneesh: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing – original draft. Divya Arora: Methodology, Investigation, Validation. A. Chatterjee: Software, Review & editing. K.S. Golda: Methodology, Investigation. Mohit Kumar: Resources, Investigation. A.M. Vinodkumar: Resources, Review & editing. P. Sugathan: Investigation, Methodology, Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Authors acknowledge Dr A. Jhingan for fruitful discussions and help during experimental setup on detectors and electronics. The work was supported by the SERB grant no IR/S2/PF-02/2007 by Department of Science and Technology (DST), Govt. of India. Author Divya Arora acknowledges the research fellowship support provided by University Grants Commission (UGC), Govt. of India .

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