Background radioactivity level estimation and passive shield optimization using adjoint Monte Carlo method

The current study proposes a procedure to estimate the activity concentration of natural radionuclides and to optimize passive shielding solutions for HPGe detectors using adjoint Monte Carlo (MC) simulation technique of Geant4 for the first time. The background spectrum is acquired for 1.56 × 10⁶ s using an HPGe detector model (GC3020), set inside a shielding solution, during 2021-2022 to estimate the activity concentration of natural radionuclides inside the shielding. While, a background spectrum for 65,000 s is acquired with shielding removed to estimate the concentration of natural radionuclides in the building materials of the laboratory. The detector design used in the simulations is validated by comparing computed and measured Full Energy Peak Efficiency (FEPE) for point sources ²⁴¹Am, ¹⁵²Eu, ¹³⁷Cs, ¹³³Ba, and ⁶⁰Co. Adjoint MC simulations are used to compute the activity concentration of natural radionuclides assuming an isotropic distribution. The activity concentration of ⁴⁰K, ²²⁶Ra and ²³²Th in the building material is found to be 524 ± 140, 83 ± 20 and 65 ± 18 Bqkg^-1, respectively. The computed values are found in good agreement with the published data. The natural radioactivity levels of ⁴⁰K, ²²⁶Ra and ²³²Th measured in lead shielding are 155.7 ± 0.1 mBqkg^-1, 24 ± 13 mBqkg^-1 and 33 ± 17 mBqkg^-1 respectively. The radiological risks arising due to natural radioactivity is assessed by calculating radium equivalent activity (Raeq), indoor radiation hazard index (Hin) and annual effective dose equivalent. All the radiological parameters are found below their permissible limits and building materials may be considered radiologically safe. The optimal lead shield thickness for the detector is determined to be 12 cm, resulting in reduction of background signal by two orders of magnitude compared to an unshielded detector. The adjoint MC simulations in Geant4 are 10³-10⁴ times more rapid as compared to normal simulations for shield optimization of HPGe detectors and therefore, are identified as viable computing solution to calculate the activity of the background radiation.