Tsuyoshi Misawa¹, Kiyoshi Yoshikawa², Kai Masuda², Teruhisa Takamatsu³, Takeshi Fujimoto³, Kunihito Yamauchi⁴, Eiki Hotta⁴, Yoshiyuki Takahashi³, Takahiro Yagi³, Yasushi Yamamoto², Seiji Shiroya¹, Masami Ohnishi⁵, Hodaka Osawa⁵, Yoshikazu Kubo⁶, Makoto Tabei⁶, Yoshihiro Sugihara⁶ and Toshirou Doi⁶

Abstract:

A five year project on research and development of the humanitarian landmine detection system using a compact neutron source has been carried out.  The neutron source was a discharge-type compact fusion neutron source called IECF (Inertial Electrostatic Confinement fusion) of 300 mm inner diameter that had a 50 mm-thick water-jacket and a Mo hollow cathode.  The water jacket acted as a coolant of the system, neutron reflector and neutron shielding, and it produced more than 10⁷ n/sec DD neutrons (2.45 MeV) continuously and stably for long hours.  Neutrons from IECF were injected into the soil, with the landmine, and gamma rays emitted from the explosive material (mainly 10.83 MeV and 5.27 MeV capture gamma rays from nitrogen) were detected by combined scintillators with BGO and NaI(Tl).  When the amount of moisture in the soil was low, 2.22 MeV capture gamma ray emitted from hydrogen in landmine was also utilized to improve detection efficiency.

A new integrated system combined with IECF and three gamma ray detectors was fabricated. All operation can be conducted remotely and the position of detector alliance can also be changed to detect the buried position of landmine efficiently.  In the detection test of landmine, 30 g to 300 g of TNT and RDX dummy explosives (diluted for safe treatment) were used and buried depth (5 cm to 15 cm) and soil moisture (2% to 20%) were changed systematically.  To demonstrate the ability of the system objectively, blind tests were also carried out many times where operators had no information beforehand on the kind of detection target, including with or without landmine.  All measured gamma ray energy spectra from detectors were stored and processed automatically by personal computers.  When explosives were buried shallow, almost 100% detection rate for RDX was achieved within 10 min. measurement time (even for 30 g of RDX). However, the detection rate for 100 g TNT was down to less than 80% because of the low content of nitrogen atoms compared with RDX.  For more deeply buried landmines, the detection rate was reduced even for RDX, and those results largely depend on the soil moisture.  Through this research and development, it was found that the present system had a sufficient potential for humanitarian landmine detection.