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Description
Radioactive contamination of nuclear power plants must be precisely identified for safe decommissioning. Especially, hot spots should be evaluated for the safety of workers in high dose environments. Gamma cameras are widely used to investigate the hot spots of the radioactive materials. The output signals of the gamma cameras are saturated in high dose regions. Pinhole collimator is widely used to reduce the number of the gamma rays incidents in the detector to solve the problems. However, the noise is generated by interaction of the gamma rays with the collimator. Three major types play an important role in radiation measurements: photoelectric absorption, compton scattering, and pair production [1]. The characteristic X-ryas, the scattered gamma rays, and the annihilation gamma rays caused by these types should be minimized to reduce the noise. These noise are related to the components of the collimator. The purpose of this study was to design the noise reduction collimator for high dose environments. The Monte Carlo N-Particle Transport Code (version 6) was performed to investigate the characteristics of the noise of the collimator with various materials and the radioactive sources. The collimator consisted of an aperture and septa for shielding background radiation. The acceptance angle and source-to-collimator distance were around 37° and 100 cm, respectively. The diameter of hole of the aperture was 1 mm. The Signal-to-Noise Ratio (SNR) of the collimator was evaluated using aluminum, iron, copper, bismuth, lead, and tungsten. It also was evaluated with 241Am, 137Cs, 60Co, 22Na, and 152Eu sources. The experimental measurements were performed to verify the results of the simulations and the SNR also was investigated to evaluate the performances of the noise reduction collimator.
