Design of a fast multi-hit position sensitive detector based on a CCD camera

doi:10.1016/S0168-9002(01)01876-9

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 477, Issues 1–3, 21 January 2002, Pages 150-154

https://doi.org/10.1016/S0168-9002(01)01876-9 Get rights and content

Abstract

A new position sensitive detector has been designed for time-of-flight mass spectrometry. It combines a double micro-channel plate stage with a phosphor screen, the conductive coating of which is divided into an array of strip-like-shaped anodes. Time-of-flight signals are measured on the strip array with a $0.5 ns$ resolution, while a CCD camera records light-spots generated by ion impacts on the phosphor screen. With this particular imaging device, it is possible to accurately assign time-of-flight to positions recorded by the camera. This paper describes the main features of this new position sensitive detector and results obtained with a three-dimensional atom probe are presented.

Introduction

Three-dimensional atom probe techniques (3D-AP) makes it possible to analyse a metallic alloy in three dimensions on a nearly atomic scale [1], [2], [3]. The sample is prepared in the form of a very sharp tip (radius ≃20– $50 nm$ ) and cooled down to $15 K$ in a high vacuum environment. Single atoms are field evaporated from the tip surface by means of high-voltage pulses (rise time $≃1 ns$ , amplitude 1– $5 kV$ ) and radially projected towards a position sensitive detector (PSD) (Fig. 1). Their chemical nature is identified by time-of-flight mass spectrometry while the position from which they originate at the sample surface is derived from their impact positions by a reversal projection. After an analysis, the distribution of single atoms within the probed volume can be reconstructed in three dimensions.

Each evaporation pulse causes a few atoms (0–10) to be field evaporated. According to their chemical identity, these atoms can hit the detector at different times, at times very close together or even simultaneously. Because the atom-probe is used in alloy composition measurement, it requires the use of a PSD with multi-hit capabilities and a good timing accuracy $(1 ns)$ . Another requirement is that the detector has to be capable of catching ion waves every $500 μs$ since evaporation pulses have to be applied at a rate of $2 kHz$ . Despite numerous PSD having been designed over the last 30 years, those fitting 3D-AP requirements are rare [3], [4].

In this paper, a new PSD based on a CCD camera is presented. This PSD has multi-hit capabilities and timing performance close to that of micro-channel plates (MCP).

Section snippets

Position sensitive detector design

The detector [5] is basically an MCPs–phosphor screen assembly in which the conductive coating of the phosphor screen is divided into 16 strip-like-shaped anodes (Fig. 1). Electron clouds produced by ion impacts, generate light spots on the phosphor screen and signal output from strips that are used as stop signals for timing. By comparing the image recorded by the CCD camera and the distribution of time-of-flight measured on the anode array, it is possible to assign a time-of-flight to every

Imaging system

The light produced by ion impact is recorded by a CCD camera focused on the phosphor screen. The camera head features a fast gateable intensifier tube coupled to a DALSA CA-D1 256 sensor by means of a fiber-optic reducer. Focusing ensures that the active area of the detector fits in the $4.56 mm^{2}$ CCD sensor. The intensifier features an S20 input photocathode, a single MCP stage and a P46 phosphor output window. The gain ranges from 1 to 100.

In addition to intensifying the light level, the

Results and discussion

To be detected, an ion has to generate a timing and light signals over thresholds of timing and imaging channels. With a bias of $1650 V$ , MCP's stack operates in a well saturated mode ensuring that every atom generates a timing signal threshold [6]. Larger MCP's bias, that leads to broader charge distribution, have to be avoided. Indeed, ions generating large amounts of charge can cause several contiguous strips to be triggerred at the same time. These multi-strip signals require more complex

Conclusion

Results shown in this work demonstrate the multi-hit capabilities of this new position sensitive detector. This detector can be applied not only to a 3D atom probe but also to any other application where multi-hit capabilities and imaging facilities are required. Indeed, the transparent anode array can be easily adapted (number of anodes, shape, etc.) to new application requirements.

As compared to similar systems [3], [7], in which a photo-multiplier tube with a multianode output is used for

Acknowledgements

The authors would like to thank Xavier Sauvage and Jean Vu-Dinh for their helpful assistance.

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