Introducing high efficiency image detector to X-ray imaging tomography

A high efficiency X-ray image detector was introduced in an x-ray imaging micro-tomography system. The detector is a visible light conversion type which consists of phosphor screen, straight-fiber (1:1) optics and scientific CMOS device. The obtained CT images show high reproducibility of X-ray linear absorption coefficient of test material (Cu/Al concentric pattern). The x-ray intensity could be reduced about 10 % of previous system with lens-coupled image detector system.


Introduction
An imaging (full-field) X-ray microscope using Fresnel zone plate (FZP) has been developed in some facilities [1][2][3]. They achieve the spatial resolution of better than 1 micrometer. Increasing the efficiency of objective zone plate and image detector is essential to get better quality of images in xray imaging tomography. Those affect scan time, radiation damage on the sample. Concerning the image detector, the signal-to-noise ratio and frame rate are rapidly modified by cutting edge technology such as scientific CMOS (sCMOS) device. The image detector which commonly used in high energy X-ray region is visible light conversion type. The detector consists of phosphor screen, visible light optics and image device. The most used visible light optics (coupling techniques) are lensand fiber-couplings (fiber optics plate, FOP). The advantage of FOP system is high efficiency (conversion gain) with high numerical aperture optics. In this study, we have introduced FOP-sCMOS detector in the imaging X-ray microscope optics.

System
The experiments were carried out at BL47XU in SPring-8. A schematic illustration of the experimental setup is shown in Figure 1. The developed system consists of an "in-vacuum type" undulator, a Si(111) double crystal monochromator cooled with liquid nitrogen, a double Rh-coated total-reflection mirror system to suppress the higher harmonics, X-ray microscope optics with sector condenser zone plate (sCZP), objective Fresnel zone plate (FZP), Zernike phase plate and an X-ray image detector. The zone material of the sCZP, the FZP, and the Zernike phase plate is tantalum. The parameters are summarized in Table 1. The inner diameter, outer diameter and pattern thickness of the Zernike phase plate are 151 m, 159 m and 0.96 m, respectively. High precision stages are made by Kohzu precision Co., Ltd. All automatic translation stages have accuracy better than 1m. The sample rotation stage (RD55-01) has been specially developed for high load capacity. The effective pixel size at 8 keV is about 90 nm/pixel. The detail of the X-ray microscope optics was described in elsewhere [4][5][6].

X-ray image detector
We have tested some detectors to fit the X-ray full field microscope optics. The specifications of detectors for imaging microscope were measured at experimental hutch 1 of BL20B2 in SPring-8 using 10keV X-ray, which is summarized in Table 2. All of the experiments were done under the "photon limited condition", therefore the S/N of the images was determined by the number of X-ray photon. Type (a), (b) and (c) has same sCMOS chip (FL-400, Hamamatsu Photonics) inside. Phosphor screen and sCMOS chip are connected by 1:1 fiber optics plate in type (a) and (b). Type (c) consists of a phosphor screen (P43 10um-thickness), a tandem lens system of two camera lens (two AI AF Nikkor 50mm f/1.4D) and sCMOS camera (C11440-22C, Hamamatsu Photonics). All of them have the magnification factor of one. Therefore, the effective pixel sizes are the same as pixel size of sCMOS chip. The type (a) has highest conversion gain and the highest modulation transfer function (MTF) (Figure 2). Therefore the type (a) detector (Hamamatsu Photonics, C12849-1015S) was used in this study. 6.5 x 6.5 6.5 x 6.5 6.5 x 6.5  Table 2. The MTF was measured at BL20B2 of SPring-8 using 10keV energy X-ray.

Evaluation of the system
For the evaluation of the spatial resolution and quantitative properties of the X-ray linear absorption coefficient (LAC) measured by tomography, the artificial test patterns of Aluminium (Al) and copper (Cu) multilayer were fabricated at AIST Kansai [7,8]. Cu and Al were deposited onto rotating Al wire substrate by DC magnetron sputtering apparatus. 300 nm pitches of Al and Cu (Each line width is 150 nm.) are clearly resolved. It shows that the spatial resolution is higher than 300 nm. Potentially it would be around 200 nm because the spatial resolution (5% MTF) of the image detector is 2.2 pixels (197 nm in this case) and wobble of the rotation stage (Kohzu precision RD55-01) is less than 100nm.
As shown in Figure 2, FOP system has better MTF than lens system with the same phosphor screen. The line profiles of CT images also show the same result (Figure 3 (c)).

Demonstration
As a demonstration, a proturan head was imaged with the developed system. The proturan was fixed and dehydrate with Bouin Solution and ethanol, respectively. Although the scan time was only 15