Report on the FINDER Experiment at the HIgS Facility

Abstract

Homeland Security programs are developing systems that use nuclear resonance fluorescence (NRF) to isotopically map a container. One such system being developed at LLNL is FINDER (Fluorescence Imaging in the Nuclear Domain with Extreme Radiation). The proposed FINDER system works by impinging a tunable monoenergetic gamma ray beam onto a container under investigation. The photons pass through the container and a fraction of them scatter off of the interior components through various electromagnetic processes. One of these processes is NRF. At specific resonance energies, incident photons interact directly with the nuclei of special nuclear material (SNM) or other materials in the container. The incident beam is absorbed and scattered into all directions, depleting the spectrum at the resonant energy. The transmitted gamma ray beam accrues a notch a few eV in width after passing through the material of interest. This notched spectrum will impinge on a witness foil placed on the opposite side of the container relative to the gamma ray source. The witness foil will be made of material identical to the one being sought after. If there is a notch in the spectrum then there will be no NRF photons scattered from the witness foil. The corollary ismore » that if there is no notch in the transmitted spectrum then there will be NRF photons scattered from the witness foil. A simple arrangement of gamma-ray detectors focused on the witness foil, are used to measure the NRF photons. If the detectors see NRF scatter then there was no NRF scatter within the container; therefore, no material of the nature being sought after was in that container. Conversely, if there was no NRF scatter from the witness foil, then the NRF scatter took place from within the container; therefore, the material of interest is inside of the container. Recently, initial feasibility tests of FINDER was performed at the HIgS (High-Intensity Gamma Source) located at Duke University [1]. The preliminary results of these tests are discussed in this report. Our goals for these measurements were to demonstrate the concept of transmission detection and perform some initial validation of models of the FINDER concept. In particular, our models [2] indicated that backgrounds and nuisances are too small to obscure the high signal to noise of the FINDER technique. Therefore, FINDER offers extremely clear positive and negative signals for detecting SNM when measuring the high contrast attenuation of on-resonance gamma rays in transmission. Our initial demonstration of transmission detection provides a first check on our models--e.g. is there a physical process that we have forgotten to include? Also, previous work indicated that the notch could be obscured by small-angle scattering refilling the notch if the interrogating photon source is too broad in energy. While we did not expect notch refilling to be significant at HIgS which provides beam of gamma rays with {Delta}E/E {approx} 3%, we were able to set an experimental upper limit on the amount of notch refilling in our experimental set up. This report first describes the experimental setup for these measurements, gives a summary of the important results, and then concludes by discussing the extent to which we were able to validate our models [2] of the FINDER technique.« less