Nuclear Instruments and Methods in Physics Research A 459 (2001) 577}580 E!ects of soil moisture on the detection of buried explosives by radiative neutron capture S. Pesente, M. Cinausero*, D. Fabris, E. Fioretto, M. Lunardon, G. Nebbia, G. Prete, G. Viesti Laboratori Nazionale di Legnaro, INFN, via Romea 4, I-35020 Legnaro (PD), Italy Dipartimento di Fisica dell 'Universita & and INFN Sezione di Padova, I-35131 Padova, Italy Received 21 February 2000; received in revised form 11 August 2000; accepted 31 August 2000 Abstract The e!ect of soil moisture on the sensitivity for detecting buried landmines by neutron capture reactions has been experimentally investigated. An explosive simulant melamine sample has been placed at di!erent depths in soil having di!erent water contents. In dry soil it is found that the decrease of counting rate by increasing the depth follows the decrease of the average solid angles. In the case of soil with higher water content, an extra yield of prompt gamma-rays following thermal neutron capture in nitrogen is evidenced due to the extra moderation provided by the presence of additional hydrogen nuclei.  2001 Elsevier Science B.V. All rights reserved. The search for hidden explosives is one of the most interesting and worthwhile applications of nuclear techniques. In particular, neutron-induced reactions have been employed in the past to pro- duce characteristic gamma-rays from the deexcita- tion of nitrogen nuclei, which mark the presence of explosive materials [1]. Speci"cally, thermal neutron capture reactions have been used, detecting the prompt 10.8 MeV photon following radiative neutron capture by N nuclei. Thermal neutrons can be produced by slowing down the fast neutrons emitted from a radioactive source (typically a Cf "ssion source) in a properly designed moderator. This technique has already been applied for the detection of * Corresponding author. E-mail address: cinausero@lnl.infn.it (M. Cinausero). landmines [2] and is currently under study in the research program EXPLODET (EXPLOsive DE- Tection)ofINFNaimedatprovidingnewtoolsfor humanitarian demining operations [3]. In a previous work [4], we have described the measurements performed in order to determine the optimumdimensionsofaneutronmoderatortobe used for the detection of buried objects. Those measurements also provided some hints of the moderation properties of a real terrain with 20% water content, which are brie#y reiterated below:  the soil has the property of defocussing the #ux ofthermalneutronsproducedbythesourceplus moderator assembly;  thesoilactsbothasare#ector and as additional neutron moderator;  the number of capture events (corrected for the variation of the solid angle) increases when the sample is moved from the surface to a certain 0168-9002/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0168-9002(00)01058-5