Development of Algorithms and Their Hardware Implementation for Gamma Radiation Spectrometry

Development of Algorithms and Their Hardware Implementation for Gamma Radiation Spectrometry

Imbaby Ismail Mahmoud (Egyptian Atomic Energy Authority, Egypt) and Mohamed S. El Tokhy (Egyptian Atomic Energy Authority, Egypt)
DOI: 10.4018/978-1-5225-0299-9.ch002
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Abstract

The chapter describes how to develop algorithms for Gamma ray spectrometry portable instruments and then to implement them on FPGA devices as hardware platform. At first we consider the development of more accurate spectrum evaluation programs including pileup detection and recovery, dead time correction, coincidence summing and resolution enhancement algorithms which are implemented in MATLAB. The input signals are obtained through experimental setup or simulated model. Four different approaches are studied and evaluated for peak pileup problem within a spectroscopy system. In addition, x-ray spectrum evaluation enhancements are carried out and several algorithms are developed and compared. Hardware implementation using Xilinx DSP Boards as well as further improvements and modifications are discussed.
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Introduction

Spectroscopic gamma-ray systems are used for many research, industrial, medical and security applications (Knoll, 2000). The advantages of a digital system for gamma ray spectroscopy in comparison with a classical analog system are reflected in the possibilities of implementation of complex algorithms and simple and rapid modification of algorithms used for signal processing. Using these systems allow the highest quality of measurements to be achieved at both low and high counting rates with various radiation detectors (Bolic & Drndarevic, 2002). Thallium-doped sodium iodide (NaI(Tl)) scintillation crystals coupled to photomultiplier tubes provide medium resolution spectral data about the surrounding environment. Pulse pileup distortion is a common problem for radiation spectroscopy measurements at high counting rates (Bolic & Drndarevic, 2002; Gardner & Lee, 1999; Guo, Gardner & Li, 2005). Moreover, pileup is one of the most delicate problems of any spectrometric method that is related to the extraction of the correct information out of the experimental spectra. In many applications as much as 80% of information can be lost due to the effects of dead time and pulse pileup (El_Tokhy, Mahmoud & Konber, 2010). The effects of pulse pileup in applications of nuclear techniques include the following issues. Imposing a fundamental limit on detector throughput (and therefore source intensity), decreased spectral accuracy and resolution, as peaks in the energy spectrum spread, reduced peak-to-valley ratios due to false detection of pulses, and causing significant detector dead time in the system (El_Tokhy et al., 2010). Therefore without a correction on the response function of the detector system incorrect physical data are obtained from an analysis of measured spectra (Morhac & Matousek, 2009). The deconvolution methods are widely applied in various fields of data processing and various approaches can be employed (Morhac et al., 2009). On one hand it must decompose completely the overlapping peaks while preserving as much as possible their heights, positions, areas and widths.

Coincidence summing can occur when a radionuclide emits two or more photons simultaneously in cascade (Radu, Stanga & Sima, 2009; Han & Choi, 2010). These emitted photons are within the resolving time of the spectrometer. Consequently, correction of the full-energy peak areas of the emissions in the spectra is required. However, a greater concern is that a full energy event from one transition will sum with a Compton event in another transition, thereby effectively decreasing the efficiency for the detection of an isotope (Decman & Namboodiri, 1995). The aim of gamma ray spectrometric analysis is to determine the activity concentration of gamma ray emitting radionuclide (Dovlete & Povinec, 2004) and the associated coincidence summing of the results. A realistic evaluation of the coincidence summing effects is a difficult task, especially in the case of nuclides with complex decay schemes. It implies an intricate combination of decay scheme parameters with peak and total efficiencies specific to the measurement conditions (Radu, Stanga & Sima, 2009).

Furthermore, the requirements for using higher resolutions in the field of nuclear experiments and data analysis are continuously increasing. High resolution gamma ray spectroscopy at relativistic beam energies is an experimental challenge (Wollersheim et al., 2005). Signals coming from various sources are overlapped due to the limited resolution of the equipment. For instance gamma ray spectra often contain overlapping photo peaks which make it difficult to estimate correctly their positions, areas and the corresponding radionuclide activities. The accuracy and reliability of the analysis depends on the treatment in order to resolve strong overlapping peaks (Morhac & Matousek, 2009).

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