Nuclear Inst. and Methods in Physics Research, A 939 (2019) 55–60 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima Development and validation of a Geant4 application to calculate the Response Matrix of a set of Superheated Drop Detectors under various external pressures Shahryar Badiei a , Gholamreaza Raisali a,∗ , Mohammadreza Kardan b , Amir Moslehi a , Peiman Rezaeian a a Radiation Applications Research School, Nuclear Science and Technology Research Institute, AEOI, PO Box 11365-3486, Tehran, Iran b Reactor and Nuclear Safety Research School, Nuclear Science and Technology Research Institute, AEOI, PO Box 143995-1113, Tehran, Iran ARTICLE INFO Keywords: Superheated Drop Detector Response matrix Fast neutron Spectrometry Geant4 ABSTRACT In this paper, a Geant4 user application program was developed and validated to calculate the response of Superheated Drop Detectors (SDDs) by Monte Carlo simulation. As the most important quantity to be incorporated in the program, the thermodynamic efficiency ‘‘’’ of the superheated droplets was determined through comparison of the simulation results with the experimental data including the results reported in the literature as well as the measured values obtained in this study. For validation of the simulation program, the determined values of ‘‘’’ for each of the three types of detectors made from R-12 (CCl 2 F 2 ), R-114 (C 2 Cl 2 F 4 ) and isobutene (C 4 H 10 ) were incorporated in the simulation program to predict the response of each type of these detectors for other neutron fields or under other external pressures. The simulation program predicted the response for different neutron energies and external pressures, with less than 15% relative difference from the measured values. Then by using the evaluated simulation program, the response matrix of a set of indigenously fabricated R-12 SDDs was calculated under external pressures of 0.86, 1.32, 1.87, 2.40 and 3.01 atm for 18 neutron energy bins from 1.415 MeV to 514 MeV. The calculated response matrix will be used for spectrum unfolding of the data obtained from these detectors. In conclusion, it was found that the developed Monte Carlo based simulation application program, validated through comparisons with experimental data, can be applied as a suitable tool for calculating the response of SDDs, made from various superheated materials at given temperature and pressure, as well as the response matrix of several detectors under various external pressures, to spectrum unfolding of a set of experimental measured values. 1. Introduction The determination of neutron energy spectrum is a vital issue due to the strong energy dependency of several types of neutron detec- tors and dosimeters [1]. Many techniques and devices like Bonner spheres [2–4], time of flight method [5,6], proton recoil detectors [7], semiconductor solid state detectors [8] and capture gated neutron spectrometers [9] are widely used for determining neutron spectra. A frequently-used method to determine the neutron energy spectrum, is the unfolding of the response of several detectors with different thresh- old energies. Superheated Drop Detectors (SDDs) which consists of tiny superheated drops distributed uniformly in a host gel medium [10– 12], can be utilized as detectors where their response functions and threshold energies vary by changing the physical conditions such as temperature or pressure. Easy and rapid fabrication, as well as their ∗ Corresponding author. E-mail address: graisali@aeoi.org.ir (G. Raisali). simplicity in application and also their low cost are the main ad- vantages of these detectors. For these detectors, there is no need for complicated detector design, associated electronics, pulse processing techniques and lengthy measurement procedures. While SDDs are exposed to a neutron field, charged particles are produced as a result of neutron interaction with constituent nuclei of superheated drops. These charged particles move inside the drops, depositing their energies in the medium. If the energy deposited along a path twice a distance called the ‘‘critical radius’’ [13], exceeds a threshold value, the superheated drop evaporates and consequently forms a bubble in the host medium [10]. The number of bubbles is proportional to the incident neutron fluence [14,15] as well as the total volume of the droplets suspended in the host medium. In addition, the number of formed bubbles is also dependent on the superheated liquid https://doi.org/10.1016/j.nima.2019.05.060 Received 9 January 2019; Received in revised form 11 May 2019; Accepted 19 May 2019 Available online 21 May 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.