Radiation Measurements 132 (2020) 106248 Available online 23 January 2020 1350-4487/© 2020 Elsevier Ltd. All rights reserved. Design of a multi-shell portable neutron spectrometry system based on indium foil detectors Alvie Asuncion-Astronomo a, * , Frederick C. Hila b , Cheri Anne M. Dingle b , Charlotte V. Balderas b , Rafael Miguel M. Dela Cruz a , Neil Raymund D. Guillermo b a Nuclear Reactor Operations Section, Philippine Nuclear Research Institute – DOST, Diliman, Quezon City, 1101, Philippines b Applied Physics Research Section, Philippine Nuclear Research Institute – DOST, Diliman, Quezon City, 1101, Philippines A R T I C L E INFO Keywords: Response function MCNP Neutron spectrometry Bonner spheres ABSTRACT A portable neutron spectrometry system was designed based on thermal neutron detectors embedded in concentric polyethylene spherical shells. The system is fexible and can accommodate the use of either active or passive neutron detectors in different confgurations. In this work, the response matrix of the system with In-115 foil detectors was calculated with MCNP5 v.1.6. Activation foils were chosen as an ideal detector for the planned use of the system in medical accelerator environments. Calculations were performed using ENDF/B VII.0 and ENDF/B VIII.0 data libraries. The response functions calculated with the two libraries differ by as much as 11.6% in the thermal energy region for the largest moderator. A sensitivity analysis was also performed to evaluate the effect of main design parameters on the response matrix. 1. Introduction Neutrons exhibit unique properties that make them ideal for numerous applications in felds like environment and agricultural research, biomedical research, nanotechnology, material science, and nuclear physics (Kardjilov et al., 2018), (Fragneto et al., 2018). Under- standing these particles are also essential in the operation of fssion re- actors and in the development of fusion reactors and new fssion reactors (Gori� canec et al., 2018; H€ außler et al., 2018; P� erez et al., 2019). In the medical sector, linear accelerators (LINACs) and positron emission to- mography (PET) cyclotrons produce neutrons as a byproduct (Karimi et al., 2019; Khabaz, 2018; Vichi et al., 2019). These applications require the characterization of neutron felds to evaluate the potential neutron dose to radiation workers and the public. However, neutron fuence-to-dose conversion coeffcients are largely dependent on neutron energy. It is thus essential to determine the neutron spectrum to ensure that dose from neutrons are properly evaluated. The most widely used neutron spectrometer is the Bonner sphere spectrometer (BSS), which consists of a thermal neutron detector embedded in the center of polyethylene (PE) spheres with different di- ameters (Bramblett et al., 1960). Neutron moderation in the PE spheres depends on the incident neutron energy and the size of the sphere. Therefore, several different-diameter moderating spheres are required for a BSS system to resolve the energy distribution of neutrons in a given location. Due to the number of required spheres and its high density, conventional Bonner spheres tend to be bulky, heavy, and challenging to use in feld measurements. However, BSS remains to be the standard device used in neutron spectrometry due to its isotropic response and sensitivity to neutrons over a broad range (Thomas and Alevra, 2002). Several studies proposed alternative confgurations of moderator sets and thermal neutron detector. These include the use of cylindrical PE moderators (Ghal–Eh et al., 2017; G� omez-Ros et al., 2015; Liamsuwan et al., 2018) that are ideal for collimated neutron felds. Nested moderator confgurations (Liamsuwan et al., 2018), (Dubeau et al., 2012) and multiple detectors embedded in a moderator (G� omez-Ros et al., 2010), (G� omez-Ros et al., 2012) were likewise designed to provide a more compact alternative to BSS. Various options are also available for the thermal neutron detectors to be embedded in the moderators. Active neutron detectors such as 10 BF 3 and 3 He proportional counters, and 6 LiI (Eu) scintillators have been used that can perform real-time measure- ments (Thomas and Alevra, 2002). However, these detectors are vulnerable to dead-time losses, pulse pile-up and electromagnetic in- terferences that are typical in LINACs, PET cyclotrons, and other intense radiation felds (Vega-Carrillo et al., 2014). Passive neutron detectors provide a better alternative in these harsh environments. Passive de- tectors also have the additional advantage of reduced cost, low * Corresponding author. E-mail address: ajasuncion@pnri.dost.gov.ph (A. Asuncion-Astronomo). Contents lists available at ScienceDirect Radiation Measurements journal homepage: http://www.elsevier.com/locate/radmeas https://doi.org/10.1016/j.radmeas.2020.106248 Received 29 June 2019; Received in revised form 19 November 2019; Accepted 22 January 2020