Liquid Scintillation with the Triple-to-Double Coincidence Ratio (TDCR) Method

In order to advance the use of the TDCR at NIST as a primary standardization technique, a number of improvements to the existing system have been made.

A new optical chamber design has been implemented that allows the photomultiplier tubes (PMTs) to be closer to the liquid scintillation vial, thereby increasing the light collection efficiency. Tests using ⁶³Ni indicated about a 20 % improvement in doubles counting efficiency as a result of these modifications.

Another important modification to the system is the construction of a new acquisition system based on Field Programmable Gate Array (FPGA) technology. The aim of this project is to use commercially available FPGA hardware to perform the functions currently being carried out with several Nuclear Instrumentation Module (NIM) electronics modules. This includes pulse discrimination and logic level translation, event handling, coincidence logic processing, and event counting.

The system currently installed consists of a NIMBox NDL8 (Wiener, Plein und Baus) unit that serves as a pulse discriminator and level translator, and a National Instruments 7830R PCI FPGA card that handles all event decisions, coincidence logic processing, livetime monitoring, and event counting. All configuration and control is done using LabView with the LabView FPGA module.

Testing is currently underway to benchmark the performance of the new FPGA-based system against the original NIST acquisition system that utilizes traditional NIM electronics, including the MAC3 coincidence logic and livetime unit. Once the FPGA system is shown to give the same results as the old acquisition system, the next step will be to include an additional counting channel for a high-efficiency gamma-ray detector. This will allow the detector system to be used as a traditional TDCR spectrometer, as well as a quasi-4pb-g coincidence/anticoincidence spectrometer. It will also permit its use as a Compton spectrometer to measure the Compton spectrum of electrons produced in the scintillator from a standard external source, allowing the independent determination of the form of the quench function for that cocktail and the implementation of the Compton Efficiency Tracing Method proposed by Cassette [1].

References

1. Cassette, P. and Do, P., "The Compton source efficiency tracing method in liquid scintillation counting: A new standardization method using a TDCR counter with a Compton spectrometer," Appl. Radiat. Isot., 66, 1026-1032 (2008).