Monday, April 14, 2014

Digital Neutron-Gamma discrimination with Liquid Scintillators

In recent years CAEN has developed a complete family of digitizers that consists of several models differing in sampling frequency, resolution, form factor and other features. Besides the use of the digitizers as waveform recorders (oscilloscope mode), CAEN offers the possibility to upload special versions of the FPGA firmware that implement algorithms for the Digital Pulse Processing (DPP); when the digitizer runs in DPP mode, it becomes a new instrument that represents a complete digital replacement of most traditional modules such as Multi-Channel Analyzers (MCAs), QDCs, TDCs, Discriminators and many others. In this application note, CAEN describes the capability of the series x720 (12 bit, 250MSps) to perform neutron-gamma discrimination based upon the digital pulse shape analysis. The development of this FPGA firmware was based upon liquid scintillating detectors of type BC501-A.
All detector and neutron/gamma source based tests were performed at the Triangle Universities Nuclear Laboratory (TUNL) on the campus of the Duke University in collaboration with Prof. Mohammad W. Ahmed.

Neutron-Gamma discrimination with liquid scintillators
Liquid scintillating detectors are widely used to achieve neutron-gamma discrimination due to their effective Pulse Shape Discrimination (PSD) properties.
The light emission of liquid scintillating detectors comprises of a fast decay component, as well as a substantial slow decay component. These components arise from de-excitations of different atomic states in the scintillator. The relative population of these states depends on the energy loss (dE/dx) of the particle. In organic liquid scintillators, these components strongly depend on the energy loss.
The gamma rays interact in the scintillator mostly via atomic Compton scattering or pair production mechanisms. The neutrons are detected by scattering them with the protons. These two different processes for gamma-rays and neutrons give rise to significant difference in the slow decay component of the light emission. This difference becomes the basis of pulse shape discrimination in the liquid scintillating detectors.
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