WP 7: Digital Coincidence Counting (DCC) for radionuclide metrology
Background
A strong evolution in nuclear instrumentation is required to equip the next generation nuclear power plants as the regulations coming from nuclear authorities are more and more demanding, especially in terms of nuclide specific activity determination and detection limits etc. Power plant operators will require new equipment, performing more efficiently and capable of delivering more information with better precision to improve the production process control and the safety of operation with no additional cost. These requirements will result in new demands directly from energy producers or nuclear instrument manufacturers, on national metrology laboratories to perform accordingly.
By taking advantage of the new possibilities offered by digital signal processors (DSP), FPGA based systems (field programmable gate arrays) and the performances in terms of sampling frequencies at relatively low cost, manufacturers of nuclear instrumentation are or will be developing more and more counting systems based on digital treatment instead of analogue ones.
However, national metrology laboratories still mainly use conventional analogue systems. It is now a necessity for them to obtain experience and expertise in digital treatment to be able to conceive and develop such systems for metrological applications that meet the metrological needs. They must take the lead and gain recognised expertise to accompany the evolution in the nuclear industry and be able to assess the potential performance of such systems. Furthermore, it is of interest to the new generation nuclear power plants to take advantage of these developments in DSP, which this JRP will apply to a primary standardisation method developed in WP6 for the purpose of an on-site measurement systems of high metrological quality.
State of the art
Digital acquisition systems developed at NMIs for the purpose of digital coincidence counting have up until now utilised signals from radiation detectors and associated shaping amplifiers to determine estimates of the pulse arrival times and peak amplitude of the signal (Keightley et al., 2007). The limited sampling rate of existing ADCs has not allowed fast pulses to be sampled accurately; hence the pulse shape is determined largely by the amplifier integration of output pulses.
Those systems that have attempted to sample and store the entire pulse shapes have exhibited a limited dynamic range of source activities (due to the large data throughput rates), and those which have attempted to store only estimates of the pulse arrival times and amplitudes necessarily reject relevant information.
Recent advances in the field of high-speed digital sampling and digital signal processing along with the advent of user-configurable FPGA devices can significantly improve digital coincidence counting in the provision of primary standards of radioactivity. State-of-the-art technology nowadays provides 12-bit cards working at Giga-samples-per-second rates, with on-board FPGA devices, which greatly enhances the application of digital signal processing for the implementation of digital coincidence counting.
Aims of the work package
This work package will develop a high-sampling-speed digital system for the purpose of performing activity measurements by coincidence counting in order to:
Enable an in-situ detector system (analogue electronics would not apply to being portable). A portable system was proposed in WP6.
Create a cost-effective system that is less labour intensive – coincidence techniques for activity determination normally requires manual variation of parameters while a digital system could be automatic.
Allow for pulse shape discrimination which in turn can be used to discriminate between different types of radiation (e.g. neutrons and gamma-rays)
Allow for higher count-rates of activity, which would give a more dynamic range of radioactive samples that can be measured.
Description of work
The JRP-Partners will develop a high performance 2-channel (DCC) and 3-channel (TDCR) acquisition system with associated pulse characterisation and software that will be tested for both spectrometric and coincidence measurements with regard to:
count-rate dependence pulse-height resolution pulse timing resolution, off-line processing of coincidence counting with variable resolving times.
Optimal design of a DCC system requires previous knowledge of the pulse characteristics. Therefore, pulses from at least two types of proportional counters (atmospheric and pressurized) will be digitized with a high speed sampling card at a rate of at least 1 Gs-1. The digitized shapes will be analysed by a set of numerical algorithms to establish the parameters that best characterise the pulses depending on their origin (alpha particles, beta particles, neutron and gamma-rays) and to identify piled-up pulses at high count rates. A numerical database of digitized pulses will be made available to JRP-Partners to help in the design of the algorithms to be implemented in the final hardware and DSP system.
This digital system will be developed in co-ordination with WP6, but designed so that it can be applied to other detectors of ionising radiation and other detector configurations with up to three channels. The work also involves validation measurements of the systems by comparison with analogue system and by inter laboratory comparison of radioactive sample. The final report will include the description of the DSP system along with recommendations regarding the feasibility of digital sampling of pre-amplifier pulses, pile up discrimination and the benefits of off-line versus on-line digital systems.
Scientific tasks
7.1 Development of DCC suitable for Radionuclide metrology (NPL, CEA, CIEMAT, ENEA)
7.2 Validation of DCC (NPL, CEA, CIEMAT, ENEA)
7.3 Knowledge transfer relating to DSP/DCC for radionuclide metrology (NPL, CEA, CIEMAT, ENEA)
Selected references
[1] J. Keightley and T. S. Park, "Digital coincidence counting for radionuclide standardization", Metrologia 44 (2007) S32.
[2] Los Arcos, J. M.; García-Toraño, E. (1994). A new digital pulse height analysis method for radiation spectroscopy. Nuclear Instruments and Methods in Physics Research Section A, Volume 339, Issue 1-2, p. 99-101.
[3] Los Arcos, J. M.; García-Toraño, E.; Olmos, P.; Marín, J. (1994). Radiation spectroscopy by digital pulse height analysis. Nuclear Instruments and Methods in Physics Research Section A, Volume 353, Issue 1-3, p. 251-253.
[4] García-Toraño, E. and Los Arcos, J. M. (1996). Liquid scintillation counting by digital pulse height analysis. In Liquid Scintillation spectrometry 1994. RADIOCARBON 1996, p.25-29.