Characterization of a 9-MV Linear Accelerator Using Spectroscopy and Dosimetry

Shielded special nuclear material (SNM) detection is challenging because passive radiation emissions provide relatively little signal. Photon interrogation sources can induce (γ,n) and (γ,f) in fissionable material, providing a strong radiation signal for detection. In this work, we use an ion chamber and a sodium iodide detector to characterize the photons produced by a 9 MeV linear accelerator (linac) in our lab. An Exradin A12 ion chamber and steel attenuators are used to determine a half-value layer (HVL) for the linac to compare to the vendor data sheet. A 15.24-cm diameter, 15.24-cm height sodium iodide detector is then used to perform spectroscopy of the linac beam. To reduce pile-up, lead collimation is used to reduce the incident photon current and a thick aluminum attenuator was added to filter photons less than 4 MeV. Using the ion chamber, we measure a HVL of 2.76 cm with 95% confidence interval of [2.75, 2.79], which is in close agreement with our simulated HVL of 2.69 cm with 95% confidence interval of [2.67, 2.74]. The vendor data sheet documents 3.00 cm; the disagreement most likely stems from differences in the experimental setup. The measured linac spectrum appears to have an endpoint between 9 and 10 MeV, but the complex measured spectrum requires additional experiments and simulations to validate. Future work will investigate the neutron capture gamma-ray contribution from linac shielding to the detected spectrum, implement time-gating, and investigate the use of additional detector types to confirm the sodium iodide result.