Sum peak in gamma-energy spectra of NaI(T1) detectors

Preliminary study on the detection efficiency and estimation of minimum detectable activity for a NaI(Tl)-based seawater monitoring system

To monitor radioactivity levels in seawater Korea Institute of Nuclear Safety has installed and been operating 18 NaI(Tl)-based gamma detectors around the Korean peninsula. This study was conducted to estimate the detector efficiency and MDA of ¹³⁷Cs in seawater for measurement situations. For this purpose, experiments in the air and a water tank, and Monte Carlo simulations were performed using a seawater radioactivity monitor system with 3 in. × 3 in. NaI(Tl) scintillation detector. In the geometry reliability assessment using certified reference materials in a disc source, the validity of simulations was obtained by comparing measurement and Monte Carlo simulation results. The FWHM of the seawater radioactivity monitor were obtained from the results of the water tank measurement for applying a Gaussian Energy Broadening (GEB) option to Monte Carlo N-Particle (MCNP) radiation transport code. In addition, the detection efficiency of ⁴⁰K in the water tank was measured and compared with the Monte Carlo simulation results in order to estimate the MDA and the detection efficiency of the seawater radioactivity monitoring system. For the based condition of water tank, ⁴⁰K concentration in water tank was controlled to 10.13±0.18 Bq/L, similar to that of real marine. In laboratory water tank experiments, the detection efficiency of the radioactivity monitor for ⁴⁰K was measured at 0.184±0.005 cps/(Bq/L), the Monte Carlo simulations showed the similar result of 0.182±0.002 cps/(Bq/L), and the detection efficiency of ¹³⁷Cs was estimated to be 0.224±0.009 cps/(Bq/L) from the simulations. For 3h measurement in the water tank based condition, the MDA of ¹³⁷Cs was estimated to be 0.077±0.003 Bq/L. Future research will include detailed studies for detector sizes and seawater salinities.

Efficiency study of a big volume well type NaI(Tl) detector by point and voluminous sources and Monte-Carlo simulation

The activity of environmental samples is usually measured by high resolution HPGe gamma spectrometers. In this work a set-up with a 9in.x9in. NaI well-detector with 3in. thickness and a 3in.×3in. plug detector in a 15-cm-thick lead shielding is considered as an alternative (Hansman, 2014). In spite of its much poorer resolution, it requires shorter measurement times and may possibly give better detection limits. In order to determine the U-238, Th-232, and K-40 content in the samples by this NaI(Tl) detector, the corresponding photopeak efficiencies must be known. These efficiencies can be found for certain source matrix and geometry by Geant4 simulation. We found discrepancy between simulated and experimental efficiencies of 5-50%, which can be mainly due to effects of light collection within the detector volume, an effect which was not taken into account by simulations. The influence of random coincidence summing on detection efficiency for radionuclide activities in the range 130-4000Bq, was negligible. This paper describes also, how the efficiency in the detector depends on the position of the radioactive point source. To avoid large dead time, relatively weak Mn-54, Co-60 and Na-22 point sources of a few kBq were used. Results for single gamma lines and also for coincidence summing gamma lines are presented.