Applications of Monte Carlo Code for a Gamma Resonance System Analysis L. Wielopolski, A. Hanson, I. Dioszegi, M. Todosow, Brookhaven National Laboratory, Environmental Sciences Department, Upton, NY POC – Lucian Wielopolski, Objective To implement Monte Carlo calculations using MCNP code for the analysis and optimization of a Gamma Nuclear Resonance Absorption (GNRA) and Gamma Fission (GF) systems. These systems are planned for detection of explosives and nuclear materials concealed in either small packages or large shipping cargo containers. GNRA is based on element specific nuclear resonance absorption of high energy gamma radiation, whereas GF is based on a photofission reaction that occurs in fissionable material at above ~ 6MeV threshold energy. GF results in prompt and delayed neutrons that can be measured using neutron detectors. The usefulness of the proposed approaches has been demonstrated for GNRA using 9.17 MeV gamma radiation for detection of nitrogen present in the explosives and, for GF, using a 6 MeV Bremsstrahlung radiation for detection of delayed neutrons in fissionable material. The main hurdle to overcome in implementing MCNP for GNRA is the lack of photon cross sections libraries that include photon nuclear resonance interactions. Thus these cross sections need to be prepared on individual basis for each element of interest and then incorporated into the existing standard MCNP cross section libraries. One MCNP library has been modified for the 9.17 MeV level in the nitrogen, however, in its current configuration it does not include the angular correlation given by (1-0.44P 2 ). Although this is not critical for transmission calculations it will be important to consider for nuclear fluorescence calculations. Summary It is critically important for gamma nuclear resonance absorption measurements to modify the cross section libraries for MCNP to include the nuclear resonance cross sections for the elements of interest. At present only the nitrogen library has been modified. For nuclear fluorescence it is equally important to ascertain that cross sections have been modified for all nuclear levels of interest and that angular correlation of gamma emission is included. At present these two factors are not included in the cross sections. Use of additional regular detectors to measure nuclear fluorescence simultaneously with the transmitted radiation will improve the signal. Nuclear fluorescence spectra are preferably collected in a backward configuration reducing the Compton scattered to the detector and shifting its energy below 500 keV. Controlling factor in a stand off configuration is the inverse square distance from the nuclear fluorescence to the detector. The attenuation in air is manageable even at large distances. The results will depend on positioning of the detectors. Initial simulations of the transmission through an explosive using a monoenergetic gamma beam and of transmission through air using energy distributed source demonstrate the extra resonance attenuation. A Gamma Resonance System A GNRA system consists of a proton accelerator equipped with a suitable target material upon which impinging protons produce a resonance gamma beam via a proton resonance (p,  ) reaction. The resonance gamma radiation attenuation by the nitrogen present in the explosive is measured in the transmission mode using nitrogen resonance detectors. The nitrogen signature is indicative of presence of explosive. Alternatively elements can also be measured in nuclear resonance fluorescence mode by placing regular detectors around the inspected container. Each element in the system as well as the configuration of the entire system require engineering-optimization that can be accomplished using MCNP calculations. Two aspects of nuclear fluorescence yield and gamma energy distribution together with stand off calculations are presented below.  abi =  2 g Nitrogen Cross Sections The angular correlation is maximum at 90° that need to be considered when optimizing a stand off system based on nuclear fluorescence. Resonance cross section is given by the Breit-Wigner formula:  abi =  2 g  a  bi (E-E R ) 2 +  2 /4 where g is a statistical factor given by: g = 2J + 1 (2s + 1)(2i + 1) Nuclear Absorption versus Nuclear Fluorescence A hypothetical spherical detector (in gray) surrounds an explosive with a mass m that is irradiated with a gamma beam emanating from a proton accelerator. The incident gamma beam is partially attenuated by the atomic interactions and partially by nuclear resonance interactions with nitrogen. The energy distributions of the gamma radiation incident on the detector surface at 0, 45, 90,135, and 180 degrees are shown in the spectra above. These results show that; 1) it is conceivable to measure gamma radiation resulting from the nuclear fluorescence and 2) the backward angles are preferable over forward angles due to reduced Compton background. Increased mass of the explosive increases the yield of nuclear fluorescence in the backward angles. This results from competition between penetration of the incident resonance radiation and escape of the fluorescence radiation that is out of resonance toward the detector. Stand – off Calculations Source emission :270  /s/cm 2 /mA at distance of 1 m Cotton density:0.3 g/cm 3, (C 6 H 10 O 5 ) n,  /  = cm 2 /g HMX:1.9 g/cm 3, (C 4 H 8 N 8 O 8 ),  /  = cm 2 /g on resonance  /  = cm 2 /g Detector:BaF 2, 4.89 g/cm 3 Air (weight fraction): 14 N 0.755, 16 O 0.232, Air Density: g/cm 3 Air Attenuation:off resonance  /  = cm 2 /g, N Attenuation on resonance  /  = cm 2 /g, (exp) 36611%23%46% %38%60% %62%77% %79%88% %95%97%10 TotalOn Resonance Off Resonance Pixel Size (cm) Transmission In AirDistance (m) Geomet. Factor 1/4  r 2 Stand Off Considerations Transmission profile of HMX explosive (5cm radius, 4 cm thick ) embedded in an LD3 container filled up with cotton, detected by twenty one BaF 2 detectors. Initial System Modeling Photon energy distribution in- and out- of resonance impinging upon a single detector after traversing 50 m of air, using 10 eV wide scoring beans the additional attenuation due to resonance cross section is clearly visible Cluster # BaF 2 Detectors Filled Container d Explosive r Point Source E= MeV