X-Ray Detector Laboratory

Soft X-ray Microscopy

X-ray Microscopy is used to study the elemental composition and structure of very thin slices of biological and materials samples. The basic features are shown in figure 1. Monchromatic, soft X-rays from a synchrotron are guided along a beam-line and impinge on a Fresnel zone plate. This has the same effect as an optical lens and brings the X-rays to a very sharp focus, less than 0.1 µm in diameter. The photons continue, diverging after the focus, and are counted by a high rate detector. The sample under study is placed exactly at the focal point, and raster scanned about the focal point. The X-ray count rate at each location of the sample yields an image with transmission information, reflecting differences in the elemental composition of the sample.

Figure 1. Diagram of main components of soft X-ray microscopy beam-line, X1A, at the NSLS

One of the key issues in these studies is maximizing the flux on the sample, in order to reduce the time taken to generate a statistically meaningful image. At beam-line X1A, typical count rates in the soft-x-ray region are 10⁶ to 10⁷ s^-1. The detector also requires high sensitivity to photons in the 250 to 750 eV energy range. To satisfy these challenges we have developed a multi-channel, gas-filled detector. The detector and electronics rationale is illustrated in figure 2. X-rays enter a multi-anode detector in the plane of the anode wires, and are absorbed exponentially in the gas. Eight anode wires are used in total, which are grouped into four channels in the following way: the first and second anode wires feed preamplifiers of their own, the third and fourth wires feed a common preamplifier, and wires five through eight feed a common preamplifier. This assures reasonably equal counting rates in each channel, taking account of the exponential X-ray absorption.

An open view of the wire detector is shown in figure 3, and a view of the completed detector, with electronics attached, is shown in figure 4. In the soft X-ray region, one of the major challenges is finding suitable materials that will seal the detector, yet still transmit the X-rays. Polypropylene and silicon nitride are two suitable candidates.

Figure 2. Block diagram of detector (left hand-side) and electronics (right hand side) developed for high rate, soft X-ray microscopy.

Figure 3. Front view of wire chamber. There are eight anode wires, grouped into 4 channels, with guard wire in between each anode wire. When closed, the assembly has a 100nm silicon nitride entrance window attached to the front.

Figure 5 shows the transmission efficiency for different thicknesses of the two materials. The three major x-ray energies of interest in X-ray microscopy are C_K (279 eV), N_K (393eV) and O_K (534eV). Because of the carbon K edge at 284eV, polypropylene has quite poor transmission performance for N_K X-rays. Hence silicon nitride is used in this work.

Figure 4. Completely assembled soft x-ray detector and electronics.

Figure 5. Soft X-ray transmission through silicon nitride and polypropylene.

A shaping amplifier with about 10ns peaking time is used in each channel. Figure 6 illustrates the response of one electronic channel to a step pulse, and a soft X-ray. The X-ray signal from each of the detector’s four channels feeds a discriminator/scaler unit (figure 2), permitting each channel to count in excess of 10⁶ s^-1.

Figure 6. Step response (left) and C_K X-ray response of detector/electronics system. Pole-zero adjustment within the shaping amplifier permits cancellation of the positive ion tail of the detector signal, and hence a nicely symmetric signal, without overshoot or undershoot, for X-rays.

Finally, Figure 7 shows an image taken with C_K X-rays of a soil sample, and also a resolution test taken with mask with line widths as narrow as 25nm at its center.

Figure 7. Left: Image from studies of of floculates in soil – part of a project to reduce erosion in soil loam.  Right: Test pattern, using C_K X-rays. Line width at center is 25 nm.

Reference:
Michael Feser, Mary Carlucci-Dayton, Chris Jacobsen, Janos Kirz, Ulrich Neuhausler, Graham Smith, and Bo Yu, "Applications and Instrumentation Advances with the Stony Brook Scanning Transmission X-ray Microscope," pres. SPIE Int'l. Symp. on Optical Science Engineering & Instrumentation, San Diego, CA, 19-24 July (1998); in X-ray Microfocusing: Applications and Techniques, I. McNulty, ed., Proc. SPIE 3449 (1998) 19-31.