A Labview based FPGA data acquisition with integrated stage and beam transport control

Abstract

We report on a new FPGA based data acquisition system developed for the CSIRO Nuclear Microprobe (NMP) which is tightly integrated with both target positioning and beam transport. The data acquisition system called MicrodaQ is based on National Instruments Labview FPGA and numerous instrumentation modules spread over several PC’s. Beam transport uses a feedback control loop to optimise current on target for long unmanned experiments. These upgrades are discussed in detail and an example of the systems use for μ-Particle Induced X-ray Emission (PIXE) analysis on a Doriri apatite is briefly described.

Introduction

Nowadays, data collection systems for the Nuclear Microprobe (NMP) are having to cater for increasingly complex experimental configurations associated with Ion Beam Analysis (IBA) and the level of synchronization required for quantitative accuracy and cross-correlation [1]. Building specialised hardware to cater for these methods is expensive, time consuming and requires specialised electronics knowledge beyond the average ion beam experimentalist. However, the advent of Field Programmable Gate Arrays (FPGA) in commercial off the shelf systems such as those by National Instruments (NI) has revolutionised time-to-completion, reduced costs associated with building a Data Acquisition System (DAS). Systems based on an FPGA exhibit very little or no software latencies and can be customised for multiple experimental configurations. The adaptability of FPGA programming allows tight synchronization across techniques that would otherwise be performed independently. The systems of Bettiol et al. [1], Bogovac [2], [3] and Ryan et al. [4] are prime examples of the diverse configurations possible with FPGA’s. Importantly, both the Bogovac et al. and Ryan et al. systems replace any need for an ADC by pulse processing on the FPGA itself. For a review of data acquisition systems (DAS) used on microbeams the reader is referred to the review by Ryan [5].

Based at the University of Melbourne, the CSIRO NMP primarily caters for μ-PIXE and PIGE analysis of minerals [6] but has recently been used for Time-Resolved Ion Beam Induced Charge and hyperspectral-Ionoluminescence (IL) analysis of metal sulfides [7] and quartz [8], respectively. With an increasing need to accurately correlate these newer techniques with more traditional methods such as PIXE and Rutherford Backscattering Spectrometry (RBS), a highly flexible DAS platform is required. Moreover, recently there has been an increased impetus from CSIRO NMP users for large scan areas whilst maintaining the maximum spatial resolution and sensitivity. In part this push comes from competing technologies such as Synchrotron X-ray Fluorescence (SXRF) where centimetre sized scans are not uncommon [9]. Ryan et al. discusses this in detail elsewhere in these proceedings.

Whilst the ion optical configuration of the CSIRO NMP is arguably near optimal with respect to getting a maximum beam current density on target [6], limited accelerator brightness, beam damage and count rates too large to reliably process at higher currents means the NMP cannot compete with the scale of SXRF imaging [10] without the use of a pixelated X-ray detector such as Maia [9]. Having said that however, there are still several advantages of the NMP. These are: (1) sensitivity to a broader range of elements than typical observed with SXRF [9] and (2) beam time associated with a single experiment can be significantly longer with the NMP allowing moderately large scale imaging if the DAS is highly stable and the beam current on target can be used to close a feedback loop for beam transport over the entire course of a scan. The third and main advantage of the NMP is the sheer plethora of co-incident techniques available, some of which can be cross-correlated to improve certainty (RBS + PIXE), or provide complimentary chemical information (IL + PIXE) [11], [12], [13]. Whilst X-ray microprobes also have a range of methods at their disposal, most are performed independently on beamlines specialised to a particular method thereby requiring additional, hard to come by, beam time.

To really take advantage of the modern NMP with highly customised yet changeable experimental configurations, one requires a flexible stable DAS working in conjunction with beam transport. In this paper we report on the development of an FPGA based DAS called MicrodaQ which satisfies conditions (A) above and is able to provide real-time feedback for automating beam transport along the beam-line ensuring most of condition (B) is met. The system hardware and software design was chosen to be flexible enough to accommodate rapid prototyping of new experimental configurations. For example, shape scanning for Proton Beam Writing (PBW) has been implemented using bitmap images but has yet to be tested. Likewise, we also plan to accommodate high-speed communications with an Ocean optics Maya detector for hyperspectral IL [8].