Data acquisition system development for the detection of X-ray photons in multi-wire gas proportional counters

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Abstract

A new data acquisition system coupled to a backgammon-type gas proportional counter capable of single-photon counting over a wide range of count rates has been developed and replaces a CAMAC-based system. The new apparatus possesses improved architecture, interface technology, speed and diagnostic capability. System efficiency and throughput is significantly improved, especially in addressing earlier problems of hardware buffer downloads containing zero or repeat data and inefficient gating control. The new system is a PXI-based data acquisition apparatus including additional electronics, controlled by a graphical programming environment. It allows development of superior diagnostic tools for system optimisation and more stable performance. System efficiency is improved by 10% over a wide range of count rates (0.5 Hz–50 kHz). For the Backgammon Detector type, this represents a significant improvement in performance and applicability over previous systems. Characteristic and few-electron spectra collected on the new acquisition system are illustrated.

Introduction

Coupled to the wide variety of sensors and detectors, analogue and digital signal processing and data acquisition has played an essential role in experimental physics. Recent development of digital processing systems where raw signals are directly digitised and processing is software based [1], [2], [3] are beginning to show an impact, but current trends complete most of the signal processing prior to digitisation. These data acquisition systems are still crucial and continue to play an important role in experimental physics.

The X-ray Optics group at the University of Melbourne have described a Johann-type curved crystal X-ray spectrometer that was employed for critically testing Quantum Electrodynamics in highly charged medium-Z systems [4], [5]. A backgammon-type multi-wire gas proportional counter (MWPC) was used to detect X-ray photons in two dimensions using a combination of resistive and capacitive charge division. We here detail a new low-cost, high performance data acquisition system overcoming speed and buffer limitations for Backgammon Detector applications.

Section snippets

Earlier operation

The system acquires four signals from the detector (for every photon detected) to achieve two-dimensional encoding. The electronics and data acquisition system are designed to cope with a wide range of count rates (<0.5 Hz and >50 kHz) and discriminates against signals originating from low or high-energy photons.

An expanded electronics and data acquisition system acquires inputs from the experimental spectrometer in addition to the detector acquisition system [4]. The additional instrumentation

Issues for development

The previous system (Fig. 1) performed adequately, however under certain circumstances deficiencies were apparent. Due to asynchronous digitisation, inclinometer and temperature signal acquisition was dramatically compromised by high detector fluxes. During spectrometer calibration, detector count rates range from 500 Hz to 10 kHz. Multiple detector data transfers across the CAMAC-PC interface commonly occur each second, resulting in interruption to inclinometer data acquisition.

Coupled with this

New apparatus

The new data acquisition system includes four additional linear gate and stretchers, an extra gate and delay generator and a new data acquisition chassis (Fig. 2). The chassis is a PXI-based system (National Instruments model NI PXI-1031) with two digitisers. The first is a 3 MHz four channel simultaneously sampling 14-bit digitiser (NI PXI-6132) used for detector data acquisition. The second, for inclinometer and temperature data acquisition, is a 1.25 MHz 16 bit 16 channel scanning digitiser

Results and discussion

Good statistics are vital for highly precise energy assignments. After optimisation of the signal electronics, the initial assessment of the data acquisition system was successful, and a number of details became apparent that were previously undetected due to the limited diagnostic capability in the CAMAC-based system. Robust diagnostic tools are need to analyse signal output for system optimisation and problem solving. The new control software addresses the system's ‘real-time’ diagnostic

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