A simple digital TDPAC spectrometer
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A simple digital TDPAC spectrometer T. A. Webb · Leo Nikkinen · Juan Gallego · D. H. Ryan
Published online: 12 October 2012 © Springer Science+Business Media Dordrecht 2012
Abstract We present a simplified digital time differential perturbed γ − γ angular correlation (TDPAC) spectrometer that demonstrates that such instruments can be built using primarily commercial components and with relatively modest coding effort. The system handles data rates of 70 kcps/detector with a timing resolution of better than 500 ps, and has been used with both 111 In and 181 Hf. Keywords TDPAC · Digital signal processing · Nuclear methods · Linux
1 Introduction Building a time differential perturbed γ − γ angular correlation (TDPAC) spectrometer can be challenging due to the combination of multi-step pulse processing with stringent timing resolution and stability requirements. Furthermore, since the two correlated γ emissions from a nuclear decay cascade have to be detected, only a small fraction of the detector pulses processed contribute to the final TDPAC spectrum, R(t), necessitating high event processing rates, typically on the order of 105 counts per second (cps) on each detector. Digital spectrometers, made possible by high-performance digitizers, elegantly minimize hardware, transferring the complexity associated with analogue signal processing to the task of developing custom software. The latter adds new considerations and freedom to the design process e.g. timestamp calculation [1], coincidence processing algorithms [1], data storage and the ability to reprocess data [2]. We based our spectrometer on several previously described designs [2–5], in which an Acqiris PCI digitizer card and PC is dedicated to each detector, reducing pulses to
T. A. Webb (B) · L. Nikkinen · J. Gallego · D. H. Ryan Department of Physics, McGill University, 3600 University Street, Montréal, QC Canada H3A 2T8 e-mail: [email protected]
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time–energy (t,E) pairs, with the time origin set by a common synchronization pulse sent to all the digitizers. Coincidence processing is then performed on a separate analysis PC, which also controls all of the acquisition cycles. In contrast to recent work, we have chosen to minimize automation in order to simplify the demands on the software. The capabilities of the U1071A digitizers used here, and previously described by Nagl et al. [3], make it possible to implement an effective and versatile spectrometer with a simple design. We describe the design, with particular emphasis on practical considerations in the software development.
2 Instrumentation The anode signals from four BaF2 detectors are digitized by Agilent Acqiris U1071A digitizers [6], which feature a “simultaneous multibuffer acquisition and readout” (SAR) mode [6, 7], allowing readout from one data bank while acquisition continues in another. As this model has two separate channels we use one each for the detector and synchronization pulses, eliminating the need to multiplex and subsequently separate the data and timin
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