[en] Active interrogation (AI) has been used as a technique for the detection and identification of Special Nuclear Material (SNM) for both proposed and field-tested systems. Idaho National Laboratory (INL) has been studying this technique for systems ranging from small systems employing portable electronic neutron generators (ENGs) 1 to larger systems employing linear accelerators as high-energy photon sources for assessment of vehicles and cargo2. In order to assess the feasibility of new systems, INL has undertaken a campaign of Monte Carlo simulations of the response of a variety of masses of SNM in multiple shielding configurations to a pulsed neutron source using the MCNPX3 code, with emphasis on the neutron and photon response of the system as a function of time after the initial neutron pulse. We present here some preliminary results from these calculations. 1. D.L. Chichester and E.H. Seabury, 'Using Electronic Neutron Generators in Active Interrogation to Detect Shielded Nuclear Material,' IEEE Transactions on Nuclear Science 56 (2009) pp 441-447. 2. J.L. Jones et al., 'Photonuclear-based, nuclear material detection system for cargo containers,' Nuclear Instruments and Methods in Physics Research B 241 (2005) pp 770-776. 3. D.B. Pelowitz, 'MCNPXTM User's Manual version 2.6.0,' Los Alamos National Laboratory Report LA-CP-07-1473 (2008).