Quantitative comparison between experimental and simulated gamma-ray spectra induced by 14 MeV tagged neutrons

Sea container inspection with tagged neutrons

Neutron inspection of sea-going cargo containers has been widely studied in the past 20 yr to non-intrusively detect terrorist threats, like explosives or Special Nuclear Materials (SNM), and illicit goods, like narcotics or smuggling materials. Fast 14 MeV neutrons are produced by a portable generator with the t(d, n)α fusion reaction, and tagged in both direction and time thanks to the alpha particle detection. This Associated Particle Technique (APT) allows focusing inspection on specific areas of interest in the containers, previously identified as containing suspicious items with X-ray radiographic scanners or radiation portal monitors. We describe the principle of APT for non-nuclear material identification, and for nuclear material detection, then we provide illustrations of the performances for 10 min inspections with significant quantities (kilograms) of explosives, illicit drugs, or SNM, in different cargo cover loads (e.g. metallic, organic, or ceramic matrices).

Improved inertial confinement fusion gamma reaction history 12C gamma-ray signal by direct subtraction

The Gamma Reaction History (GRH) diagnostic located at the National Ignition Facility (NIF) measures time resolved gamma rays released from inertial confinement fusion experiments by converting the emitted gamma rays into Cherenkov light. Imploded capsules have a bright 4.4 MeV gamma ray from fusion neutrons inelastically scattering with carbon atoms in the remaining ablator. The strength of the 4.4 MeV gamma ray line is proportional to the capsule's carbon ablator areal density and can be used to understand the dynamics and energy budget of a carbon-based ablator capsule implosion. Historically, the GRH's four gas cells use the energy thresholding from the Cherenkov process to forward fit an estimation of the experiment's complete gamma ray spectrum by modeling the surrounding environment in order to estimate the 4.4 MeV neutron induced carbon gamma ray signal. However, the high number of variables, local minima, and uncertainties in detector sensitivities and relative timing had prevented the routine use of the forward fit to generate carbon areal density measurements. A new, more straightforward process of direct subtraction of deconvolved signals was developed to simplify the extraction of the carbon areal density. Beryllium capsules are used as a calibration to measure the capsule environment with no carbon signal. The proposed method is then used to appropriately subtract and isolate the carbon signal on shots with carbon ablators. The subtraction algorithm achieves good results across all major capsule campaigns, achieving similar results to the forward fit. This method is now routinely used to measure carbon areal density for NIF shots.

Organic scintillators response function modeling for Monte Carlo simulation of Time-of-Flight measurements

In neutron Time-of-Flight (TOF) measurements performed with fast organic scintillation detectors, both pulse arrival time and amplitude are relevant. Monte Carlo simulation can be used to calculate the time-energy dependant neutron flux at the detector position. To convert the flux into a pulse height spectrum, one must calculate the detector response function for mono-energetic neutrons. MCNP can be used to design TOF systems, but standard MCNP versions cannot reliably calculate the energy deposited by fast neutrons in the detector since multiple scattering effects must be taken into account in an analog way, the individual recoil particles energy deposit being summed with the appropriate scintillation efficiency. In this paper, the energy response function of 2″×2″ and 5″×5″ liquid scintillation BC-501 A (Bicron) detectors to fast neutrons ranging from 20 keV to 5.0 MeV is computed with GEANT4 to be coupled with MCNPX through the "MCNP Output Data Analysis" software developed under ROOT (Carasco, 2010).