Speaker
Description
Nuclear resonance absorption/fluorescence (NRA/F) is the process by which a nucleus absorbs/emits electromagnetic radiation. Because the energy of this process is specific to the isotope, interrogating nuclear energy levels electromagnetically has been proposed for solutions in nuclear materials detection and pharmaceutical purity measurements [1]. Jentschel et al. recently demonstrated how a 1D transmission projection could be performed by using monoenergetic x-rays tuned to the 478 keV $^7$Li first excited state [2]. This paper outlines how NRA/F can be used to do isotope-specific imaging in higher dimensions, with particular emphasis on possible medical applications but can be extended to any imaging application.
The absorption cross section for these nuclear energy levels is exceedingly narrow, with Doppler-broadened ΔE/E widths as small as 10$^{-6}$ . Therefore, a source that has narrow energy bandwidth is a requirement to obtain appreciable signal from these transitions. Extremely brilliant Compton sources (EBCSs) are well-suited for probing these transitions as they are high in brilliance and are designed to have 10$^{-3}$ on-axis energy bandwidth with capability to reach 10$^{-5}$ after bandwidth filtering.
A 4.2 cm thick water phantom with 2- and 3-mm radii spheres of gadolinium was used. Geant4 was used to simulate the x-ray interactions with the G4NRF package for NRA/F physics. A simulated Compton source that is tuned to on-axis energy being the energy of the NRA line and a second image is obtained tuning the energy just below the NRA line. Experimental methods for obtaining these images will be discussed, as well as extensions to 3D images and use of different isotopes that are of medical relevance, like gadolinium.
[1] J. Pruet, et al., “Detecting clandestine material with nuclear resonance fluorescence,” Journal of Applied Physics, 99, 123102 (2006).
[2] M. Jentschel, et al., “Isotope-selective radiography and material assay using high-brilliance, quasi-monochromatic, high-energy photons,” Applied Optics, 61(6), C125 (2022).
