Fig 1: A schematic of the GSS system with the necessary components. High-purity germanium (HPGe) detectors are used to detect the emitted gamma spectra with sufficient energy resolution and efficiency.

Gamma-stimulated Spectroscopy (GSS) quantifies elemental concentrations in-vivo in a novel way.  It uses a precisely tuned beam of high-energy gamma rays to quantify element concentration in the body to diagnose element-related disorders.

Gamma rays of a precise and specific energy are able to excite naturally occurring elements in the body. The excited element then spontaneously de-excites and emits a characteristic gamma spectrum. This spectrum can be used to quantitatively identify the elemental composition of the tissue or organ and diagnose diseases such as cancer.

Studies have indicated that changes in trace element concentrations in human tissue may be a precursor to malignancy in several organs such as the brain, prostate and breast. Our goal is to measure these concentration changes at a very early stage of tumor development to diagnose cancer and differentiate between malignant and benign tissue.

While the principle of the GSS technique is somewhat similar to another emerging quantitative imaging technique in our lab – NSECT, the physics interactions employed in GSS are different. GSS relies on nuclear resonant scattering between gamma rays and an elemental nucleus, which demonstrates resonant behavior that is a function of the incoming gamma energy. In order for a particular energy state in an element to be excited it is essential that the incoming gamma energy be within a narrow window around the target energy state of interest. Therefore, the technique requires a gamma source capable of providing a high flux of high-energy gamma rays with the capability of precise energy tuning.

		Figures 2 and 3 show the prototype system assembled at DFELL used in our first experiment. Figure 2 (right) is positioned looking into the beam, which exits through the PVC tube in the center. The sample is positioned at the center (visible as the white capped jar). HPGe detectors with lead shields are visible on either side of the sample. Figure 3 (above) shows and aerial view of the system with the same acquisition geometry. The system was used to scan a solution of iron to determine detection sensitivity. Results of the experiment are forthcoming.

The Duke Free Electron Laser Laboratory (DFELL) with its High-Intensity Gamma Source (HIGS) is ideal for this task. HIGS is a novel light source with intense photon flux, extremely small beam-related background, and a wide range of precisely tunable gamma-ray energies. It is capable of producing polarized gamma beams with energy up to 60MeV. The gamma ray beam is produced by Compton backscattering of the FEL photons with electrons in the storage ring, which facilitates the production of a gamma-ray beam that is mono-energetic, intense and highly polarized. The beam energy can be tuned precisely by controlling the energy of electrons and the magnetic field strength, and the energy spread can be controlled to within 1% through collimation. The beam can be pulsed at a 5 MHz repetition rate with 10 – 60 picoseconds pulse width. Beam pulsing allows the extraction of time-of-flight (TOF) information for synchronization with the detectors to acquire data within a narrow active time window corresponding to the expected arrival time of a gamma photon emitted from the sample.

Figure 1 shows a schematic of the GSS system with the necessary components. High-purity germanium (HPGe) detectors are used to detect the emitted gamma spectra with sufficient energy resolution and efficiency. Figures 2 and 3 show the prototype system assembled at DFELL used in our first experiment. Figure 2 is positioned looking into the beam, which exits through the PVC tube in the center. The sample is positioned at the center (visible as the white capped jar). HPGe detectors with lead shields are visible on either side of the sample. Figure 3 shows and aerial view of the system with the same acquisition geometry. The system was used to scan a solution of iron to determine detection sensitivity. Results of the experiment are forthcoming.