High spatial and temporal resolution photon/electron counting detector for synchrotron radiation research

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Abstract

This paper reports on the development of a high resolution electron/photon/ion imaging system which detects events with a timing accuracy of <160 ps FWHM and a two-dimensional spatial accuracy of ∼50 μm FWHM. The event counting detector uses microchannel plates for signal amplification and can sustain counting rates exceeding 1.5 MHz for evenly distributed events (0.4 MHz with 10% dead time for randomly distributed events). The detector combined with a time-of-flight angular resolved photoelectron energy analyzer was tested at a synchrotron beamline. The results of these measurements illustrate the unique capabilities of the analytical system, allowing simultaneous imaging of photoelectrons in momentum space and measurement of the energy spectrum, as well as filtering the data in user defined temporal and/or spatial windows.

Introduction

In addition to high brightness, excellent spectral resolution, broad tunability and polarization control, synchrotron sources provide short time (<100 ps) pulsed radiation, enabling a number of unique experimental techniques. The ability to register simultaneously not only timing but also spatial information for synchrotron excited photons, electrons, or ions allows new studies to be done that were not previously feasible. One of the key elements of such an experimental system is a high temporal (sub-nanosecond) and spatial resolution detector. Recently, there has been rapid progress in development of position sensitive event counting devices based on microchannel plate (MCP) technology, primarily developed for astrophysical applications [1], [2], [3]. All the advances of this new generation of detectors can now be successfully implemented in synchrotron beamline instrumentation. The two-dimensional spatial resolution of these detectors can be as low as <15 μm FWHM [4] with a timing accuracy of photon/electron/ion detection less than 150 ps. Another attractive feature of MCP detectors is their very low dark count rate (<0.1 counts cm−2 s−1). These devices can be built with active areas exceeding 6 cm in diameter and can have high detection efficiencies to X-ray and UV photons and especially to charged particles. The detection can be configured to be insensitive to positive or negative particles by the application of retarding meshes, allowing signal differentiation at the plane of the detector. One of the previous limitations of MCP technology was relatively low counting rate capabilities, not exceeding 100 kHz. The rapid developments in event encoding electronics have led to substantial improvement in the speed of signal processing, allowing event detection with counting rates exceeding 1 MHz.

In this paper we demonstrate the unique experimental capabilities of an MCP detection system used in a Time-of-Flight (TOF) based Angle Resolved Photoemission Spectrometer which is being built at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL).

Section snippets

Position and time sensitive event counting detector

The position sensitive detector used in the experiments has a 25 mm diameter active area and includes an MCP stack consisting of three 60:1 (L/D), 33 mm diameter MCPs with 12 μm pores and 13° pore bias, Fig. 1, Fig. 2. The event positional encoding is performed by a Cross Delay Line (XDL) anode capacitively coupled to the processing electronics (Fig. 1), allowing independent control of all the voltages on the detector. The latter fact provides the possibility to vary the potential on the detector

Results and discussion

The results of the experiments described below should only be considered as an illustration of the capabilities of the detection system rather than the ultimate performance of the ARPES system under construction and should not be used for the interpretation of physical phenomena.

As mentioned earlier, the detection system is capable of differentiation between photon and charged particle (electrons in our case) events by either event timing or by potentials applied to the detector. The two images

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There are more references available in the full text version of this article.

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