Christopher J. Hall

Diffraction-enhanced imaging: improved contrast and lower dose x-ray imaging

Conventional x-ray imaging relies almost entirely on differences in the absorption of x-rays between tissues to produce contrast. While these differences are substantial between bone and soft tissue, they are very small between different soft tissue types resulting in poor visualization of soft tissues. Diffraction enhanced imaging (DEI) is currently in development by several groups as a new imaging modality that exploits information contained within the x- ray scattering distribution at low angles. We have used the SYRMEP beam line at the Elettra Synchrotron facility in Trieste, Italy to image a variety of tissue specimens, together with several phantoms. Mono-energetic photons in the range 17 keV to 25 keV were used with an analyzer crystal which diffracted the x-rays onto a detector. We have obtained some spectacular images which display remarkable contrast and resolution. The images can be processed to separate the pure absorption and pure refraction effects in a quantitative manner. These images demonstrate that DEI provides tissue morphology information not accessible with conventional radiographic imaging. The contrast caused primarily by refraction as the x-ray passes from one tissue type to another in the specimen is evident. Since x-ray refraction is much less energy dependent than absorption there is considerable potential for extremely low dose imaging. We believe that the potential of this technique is considerable and we present dat to illustrate the quality of the images.

X-ray calibration of the SODART flight telescopes

The on- and off-axis imaging properties and effective area of the two SODART flight telescopes have been measured using the expanded beam x-ray facility at the Daresbury synchrotron. From on-axis measurements the encircled power distribution and the point spread function at three energies 6.627 keV, 8.837 keV and 11.046 keV have been measured using a one-dimensional position sensitive detector. We found that the point spread function can be presented well by a function which is a sum of a Gaussian divided by the radius and two exponential terms where the first has a 1/e value close to 2 arcmin and the other a 1/e value of ca. 15 arcmin. The data have been used to calculate the half power diameter (HPD) for three different SODART focal plane detectors, the high energy proportional counter (HEPC) with a field of view (FOV) of 65 arcmin, the low energy proportional counter (LEPC) with a FOV of 33 arcmin and the 19 element solid state detector array (SIXA) with a FOV of 18 arcmin. We found that the HPD decreases with increasing energy due to poorer figure of the outermost mirrors. The HPD falls in the range from 2.4 to 3.8 arcmin depending on energy and FOV. Data have also been obtained on the on- and off-axis effective area at all three energies and compared to that obtained from a raytracing of an ideal telescope configuration. We found that the measured on-axis effective area integrated over a FOV of 105 arcmin is ca. 65% of the area predicted from an ideal geometry. Finally the one- dimensional detector data has been used to obtain the radial dependence of the on-axis HPD and the on-axis effective area and the data from the two-dimensional position sensitive detector has been used to obtain the azimuthal dependence of the on-axis HPD and the on-axis effective area.

Expanded beam x-ray optics calibration facility at the Daresbury Synchrotron

A facility for the calibration of X-ray Space Instrumentation has been established for the Daresbury Synchrotron. The facility provides a continuously tunable beam with (Delta) (lambda) /(lambda) <EQ 10-4 in the energy range from approximately 5 kev to more than 20 kev. At selected energies in the interval from 6 kev to 12 kev, the facility features a 1D sheet of X-rays, approximately 200 mm wide, obtained from an extremely asymmetric reflection in large perfect crystals of Si. The beam is collimated to < 20 arcsec. Data from tests using large (approximately 250 mm long) beam expander crystals in the energy range from 6 - 12 kev are presented. The planned calibration of the two X-ray telescopes (XSPECT/SODART and JET-X) will be described.

RAPID detector system: first user data

The RAPID detector system previously reported on in SPIE volume 2521, has successfully completed the final stage of its commissioning. During this period several of the Synchrotron Radiation Source User groups were invited to perform trial experiments with the new detector system. Over a period of one month in late 1998 several types of experiments were performed on the flagship small angle scattering station 16.1, ranging from polymer diffraction rheometry to time resolved muscle diffraction. The results and detector assessments from some of these experiments are presented. A discussion is made of refinements to the system which would further enhance its performance.

10-MHz photon counting detector system for time-resolved x-ray diffraction

We present the first results from two highly parallel detector systems designed for fast time resolved x-ray diffraction experiments. The readout systems have been designed to give throughputs well in excess of 107 events per second. The detector systems have been designed to allow high flux diffraction patterns to be collected with very much reduced rate effects when compared with previous designs. This has been achieved using wire microgap proportional counters coupled to multi-channel data acquisition systems. The efficiency and low noise of the detector coupled to the speed of the readout has produced a detector system capable of more fully exploiting the time resolved diffraction stations on the UK Synchrotron Radiation Source at the Daresbury Laboratory. Modifications to the design will be presented which will allow the system to cope with even higher count rates in the future.

Contrast enhanced refraction imaging

An attempt has been made, for the first time, to extend the capabilities of diffraction enhanced imaging (DEI) using low concentrations of a contrast agent. A phantom has been constructed to accommodate a systematic series of diluted bromine deoxyuridase (BrDU) samples in liquid form. This was imaged using a conventional DEI arrangement and at a range of energies traversing the Br K-edge. The images were analyzed to provide a quantitative measure of contrast as a function of X-ray energy and (BrDU) concentration. The results indicate that the particular experimental arrangement was not optimized to exploit the potential of this contrast enhancement and several suggestions are discussed to improve this further.

Preliminary results from a high-pressure imaging spectroscopic proportional counter

The concept, operational principle, and test results are presented for a new type of high-pressure high-spatial resolution proportional counter with enhanced spectroscopic capability. The detector in its baseline configuration is to be filled with a xenon/quench gas mixture at 5 bar and is to be sensitive over the 5-150 eV energy range. The position resolution will range from 0.5 mm at the lower energies to around 1 mm at the upper end of the energy range. The very high timing resolution of this new detector allows high count rate capacity and enables the application of the escape gating technique to achieve a high spectral resolution at energies above the xenon K edge.

Synchrotron radiation facility for x-ray astronomy

A proposal for an x-ray optics test facility based at a synchrotron radiation source is presented. The facility would incorporate a clean preparation area, and a large evacuable test area. The advantages of using a synchrotron as the source of the test radiation are discussed. These include the ability to produce a highly parallel beam of monochromatic x rays ranging from 200 eV to around 70 keV.

Simulation and response of the MART-LIME x-ray detector

The MART-LIME is a large area x-ray experiment planned to be launched on board the Russian satellite Spectrum X-Gamma, as the high energy imager of a complement of broad band co- aligned x-ray telescopes. The energy range covered is 5 - 150 keV with an angular resolution of 8.6 arcminutes. The final detector configuration is now in its testing phase and includes the high pressure window comprising the 6 by 6 degree collimator, and the multiwire proportional counter (MWPC). The response to x-ray sources was investigated during the tests carried out at the Daresbury Laboratory (Warrington, UK) facilities The MWPC was filled up by a xenon-argon-isobutane gas mixture in order to evaluate the efficiency of the detector and in particular its linearity over the whole approximately 2,000 cm2 sensitive area. At the same time the various parts of the apparatus have been simulated by using a Monte Carlo program. Results on the detector response and simulations are presented.

X-ray study of a test quadrant of the SODART telescopes using the expanded beam x-ray optics facility at the Daresbury synchrotron

The imaging properties of a test model of the SODART telescopes have been studied using an expanded beam X-ray facility at the Daresbury synchrotron. The encircled power and the point spread function at three energies 6.627 keV, 8.837 keV and 11.046 keV have been measured using 1D and 2D position sensitive detectors. The data have been used to calculate the Half Power Diameter (HPD) for three different SODART focal plane detectors. The High Energy Proportional Counter (HEPC), the Low Energy Proportional Counter (LEPC) and the 19 element solid state array detector (SIXA). At 6.627 keV and 8.837 keV the HPD is 2.5 - 3.0 arcmin for all detectors whereas it is somewhat larger at 11.046 keV for HEPC and LEPC but essentially unchanged for SIXA. Finally, the data are used to point to improvements that can be introduced during the manufacture of the flight telescopes.