On-line analysis of coal by neutron induced gamma spectrometry

Abstract

Thermal neutron prompt gamma activation analysis (PGNAA) has proven to be a useful tool for the multi-elemental characterization of coal. The use of isotopic neutron sources allows the construction of relatively small irradiation facilities for the on-line analysis of large volume samples. As a first step to an on-line analysis system for process control in a coal blending plant we have developed a facility using a Cf-252 neutron source with an actual strength of 1 mg. This source, which is shielded by layers of high purity lead and paraffin, can be moved to the center of cylindrical sample holders containing up to 150 kg of coal. To avoid background radiation the sample holders are made from polyethylene. The prompt gamma rays emitted from the sample are measured with a 22% high purity germanium detector housed in a special horizontally extended cryostate assembly. The detector is positioned on top of the sample; it is surrounded by a multi-layer decreasing Z-absorber to reduce low energy gamma background. Paraffin loaded with 95% enriched lithium-6-carbonate is used between sample and detector as a gamma window with high absorption for thermal neutrons. Moreover the detector is shielded against background radiation by lead and natural lithiumcarbonate. The data processing system consists of high count-rate analogue electronics including pileup-suppression and a fast data acquisition system, which permits on-line analysis of the collected data. Analysis of several samples of simulated coals as well as Western Germany coals shows a good spectral response of our system for the elements H, C, Al, Si, S, Cl, Ca, Ti and Fe, whereas the sensitivity for N, K and Na is somewhat lower. This means, that the quantitative determination of the above elements will be possible within 20 minutes. Optimization of the system is continued with a stress on analysis time and detection sensitivity.