Demonstration of tomographic imaging of isotope distribution by nuclear resonance fluorescence

Abstract

Computed Tomography (CT) using X-ray attenuation by atomic effects is now widely used for medical diagnosis and industrial non-destructive inspection. In this study, we performed a tomographic imaging of isotope (208Pb) distribution by the Nuclear Resonance Fluorescence (NRF), i.e. isotope specific resonant absorption and scattering of gamma rays, using Laser Compton Scattering (LCS) gamma rays. The NRF-CT image which includes both effects of atomic attenuation and nuclear resonant attenuation was obtained. By accounting for the atomic attenuation measured by a conventional method at the same time, a clear 208Pb isotope CT image was obtained. The contrast degradation due to notch refilling caused by small-angle Compton scattering is discussed. This study clearly demonstrates the capability of the isotope-specific CT imaging based on nuclear resonant attenuation which will be a realistic technique when the next generation of extremely intense LCS gamma-ray sources will be available. The expected image acquisition time using these intense LCS gamma rays was discussed.

Selective Isotope CT Imaging Based on Nuclear Resonance Fluorescence Transmission Method

“…We used a Tm-fiber laser system (TLR-50-AC-Y14, IPG Laser GmbH) with the maximum average power of 50 W, which was ten times as large as that of the previously employed laser system [16]. We operated it in a continuous Expected LCS gamma-ray spectrum obtained with a collimator hole diameter of 1-mm using EGS5 Monte Carlo simulations for an LCS gamma-ray flux of 1 × 10 8 photons/s.…”

“…When an isotope of interest is already present in the measured sample [computed tomography (CT) target shown in Fig. 2], the spectral flux of the transmitted photons at the resonant energy decreases due to resonant absorption, leading to a narrow and sharp dip (called a notch) in the transmitted gammaray energy spectrum [16]. When the transmitted gamma ray irradiates the witness target that includes the same isotope of interest, gamma rays are emitted from the isotope via the NRF process.…”

“…One of the remarkable features of NRF-CT is that the isotope of interest can be selectively identified from a sample containing several isotopes. Zen et al [16] demonstrated an image of a 208 Pb isotope in the natural lead, with a pixel size of 5 mm in the resolution. The CT target consisted of a natural lead rod implanted in an iron cylinder filled with aluminum (Al), with an iron rod and air (hole).…”

“…Because these two rods exhibit the same gamma-ray attenuation in atomic processes, it is impossible to distinguish the two rods using a standard CT technique based on the atomic attenuation of gamma rays. We employed an upgraded laser system that could generate a tenfold intensity of the LCS gamma-ray beam when compared to the previous system [16]. The energy of the gamma-ray beam was increased to enable excitation at 5.512 MeV, as the NRF cross section was two times larger than that at 5.292 MeV in previous work [16].…”

“…We employed an upgraded laser system that could generate a tenfold intensity of the LCS gamma-ray beam when compared to the previous system [16]. The energy of the gamma-ray beam was increased to enable excitation at 5.512 MeV, as the NRF cross section was two times larger than that at 5.292 MeV in previous work [16]. An increased number of data points were measured using an LCS beam with a smaller diameter of 1 mm and a scanning step of 2 mm to obtain a CT image with a higher spatial resolution than the previous work [16].…”