Journal of the Korean Physical Society, Vol. 61, No. 5, September 2012, pp. 702∼709 Monte-Carlo Simulation of the Prompt Gamma Neutron Activation Analysis System with a Femtosecond Laser Hyunha Shim, Byungsik Hong * and Kyong-Sei Lee Department of Physics, Korea University, Seoul 136-701, Korea Sungman Lee and Hyungki Cha Korea Atomic Energy Research Institute, Daejeon 305-353, Korea (Received 8 June 2012, in final form 27 July 2012) The prompt gamma neutron activation analysis (PGNAA) system is a useful tool to detect the concentrations of the various composite elements of a sample by measuring the prompt gammas that are activated by neutrons. The composition in terms of the constituent elements is essential information for the identification of the material species of any unknown object. A PGNAA system initiated by a high-power laser has been designed and optimized by using a Monte-Carlo simulation. In order to improve the signal-to-background ratio, we designed an improved neutron-shielding structure and imposed a proper time window in the analysis. In particular, the yield ratio of nitrogen to carbon in a TNT sample was investigated in detail. These simulation results demonstrate that the gamma rays from an explosive sample under a vast level of background can indeed be identified. PACS numbers: 07.05.Fb, 28.20.Gd, 29.40.-n Keywords: Prompt gamma-ray, Neutron beam, High-power laser, Monte-Carlo simulation, GEANT4 DOI: 10.3938/jkps.61.702 I. INTRODUCTION When neutrons are captured by atomic nuclei, charac- teristic prompt gammas are emitted by the excited nuclei within a time scale of the order of 10 -14 seconds. The prompt gamma neutron activation analysis (PGNAA) is a prominent method that uses these prompt gammas for the identification of hidden material. PGNAA can detect several elements in a compound simultaneously and non-destructively. This is in contrast with conven- tional neutron activation analysis (NAA), which is ap- plicable only when radioactive isotopes in the material emit gammas with adequate half-lives. This implies that conventional NAA requires some specific radioactive iso- topes that emit neutrons via fission decays whereas PG- NAA is free from this constraint and measures gammas in real time. Therefore, applications of PGNAA in vari- ous fields, such as on-line measurements in the coal min- ing industry, environmental research, and searching for landmines as part of homeland security, have been inves- tigated [1–4]. Basically, PGNAA utilizes the fact that explosives, il- licit drugs, and some innocuous materials contain light elements such as hydrogen, carbon, nitrogen, and oxy- gen, but with different composition ratios between them. * E-mail: bhong@korea.ac.kr As a result, the ratio of these elements provides a power- ful means by which to differentiate an unwanted material from an innocuous substance. In particular, a strong cor- relation is known to exist between the concentrations of O and N in explosives [5]. Therefore, the composition ratios of O/C and N/C have previously been adopted to identify hidden explosive materials [6,7]. A PGNAA system is mainly composed of two parts: the source generator of neutrons and the gamma detec- tion system. For neutron generation, the most popular and easy to handle method is to use radioactive isotope sources like 252 Cf. However, a new technique for neutron generation was recently investigated by using a femtosec- ond high-power laser at the Korea Atomic Energy Re- search Institute (Fig. 1). In this method, when intense laser beams bombard a deuterated polystyrene (C 8 D 8 ) target, neutrons are produced by deuteron-deuteron re- actions; therefore, pulsed neutron beams are provided at 2.4 MeV [8]. The maximum frequency of the neutron pulse generation is currently 10 Hz, but it is kept at 5 Hz for stable operation. The duration of each pulse is a few nanoseconds, and the number of neutrons per pulse is about 10 6 . However, an active development of the sys- tem is underway, and the goal is to generate a few times 10 8 neutrons per second by increasing the neutron pulse frequency and the number of neutrons per pulse simul- taneously. Unlike other methods, the pulsed neutron beam gener- -702-