Brian G. Rodricks

The APS X-ray undulator photon beam position monitor and tests at CHESS and NSLS

Abstract The advent of third generation synchrotron radiation sources, like the Advanced Photon Source (APS), will provide significant increases in brilliance over existing synchrotron sources. The APS X-ray undulators will increase the brilliance in the 3–40 keV range by several orders of magnitude. Thus, the design of the photon beam position monitor is a challenging engineering task. The beam position monitors must withstand the high thermal load, be able to achieve submicron spatial resolution while maintaining their stability, and be compatible with both undulators and wigglers. A preliminary APS prototype photon beam position monitor consisting of a CVD-diamond-based, tungsten-coated blade was tested on the APS/CHESS undulator at the Cornell High Energy Synchrotron Radiation Source (CHESS) and on the NSLS X-13 undulator beamline. Results from these tests, as well as the design of this prototype APS photon beam position monitor, will be discussed in this paper.

Development of a fast pixel array detector for use in microsecond time-resolved x-ray diffraction

A large-area pixel x-ray detector is being developed to collect eight successive frames of wide dynamic 2D images at 200kHz rates. Such a detector, to conjunction with a synchrotron radiation x-ray source, will enable time-resolved x-ray studies of proteins and other materials on time scales which have previously been inaccessible. The detector will consist of an array of fully-depleted 150 micron square diodes connected to a CMOS integrated electronics layer with solder bump-bonding. During each framing period, the current resulting from the x-rays stopped in the diodes is integrated in the electronics layer, and then strored in one of eight storage capacitors underneath the pixel. After the last frame, the capacitors are read out at standard data transmission rates. The detector has been designed for well-depth of at least 10,000 x-rays (at 12 keV), and a noise level of one x-ray. Ultimately, we intend to construct a detector with over one million pixels (1024 by 1024). We present the result of our development effort and various features of the design. The electronics design is discussed, with special attention to the performance requirements. The choice and design of the detective diodes, as they relate to x-ray stopping power and charge collection, are presented. An analysis of various methods of bump bonding is also presented. Finally, we discuss the possible need for a radiation-blocking layer, to be placed between the electronics and the detective layer, and various methods we have pursued in the construction of such a layer.

NEW CDTE PHOTOCONDUCTOR ARRAY DETECTOR FOR X-RAY APPLICATIONS

A CdTe photoconductor array x‐ray detector was grown using molecular beam epitaxy (MBE) on a Si(100) substrate. The temporal response of the photoconductor arrays is as fast as 21 ps rise time and 38 ps full width half‐maximum (FWHM). The spatial resolution of the photoconductor was good enough to provide 75 μm FWHM using a 50 μm synchrotron x‐ray beam. A substantial number of x‐ray photons are absorbed effectively within the MBE CdTe layer as observed from the linear response up to 15 keV. These results demonstrate that MBE grown CdTe is a suitable choice of the detector materials to meet the requirements for x‐ray detectors.

A statistical technique for characterizing X-ray position-sensitive detectors

Abstract We present a technique for characterizing X-ray sensitive photodiode arrays and charge-coupled device (CCD) arrays. The technique uses simple statistical estimators (means, variances and correlation functions) to determine the response, noise, resolution and detective quantum efficiency of a position-sensitive detector. We apply this technique by characterizing a linear diode array and a CCD array exposed to direct illumination by X-rays. Correlations between neighboring pixels were important, and they are included in the calculation of the detective quantum efficiency and noise of the detector.

Prototype photon position monitors for undulator beams at the Advanced Light Source

Design criteria are described, and test results are presented, for prototype undulator beam position monitors at the Advanced Light Source (ALS). The design is based on monitors presently in use at the National Synchrotron Light Source, Brookhaven National Laboratory, with modifications to account for the widely varying and large K values of the undulators to be installed at the ALS. In particular, we have modified the design to simplify the thermal engineering and we have explored techniques to suppress the response of the monitors to soft photons, below 100 eV, so that the beam position can be determined by measuring the higher energy photons which are better collimated.

MBE grown CdTe photoconductor array detector for x‐ray measurements

A photoconductor array was made using molecular‐beam epitaxy (MBE) grown CdTe. CdTe has been found to be an excellent material for high‐energy photon detection. The objective is to develop an array detector with high efficiency and fast response toward x rays. There is considerable interest in the development of new x‐ray detectors for use in the new synchrotron‐radiation sources. Photoconductor arrays with gaps ranging from 5 to 50 μm between elements and 100 μm pitch size have been fabricated. The temporal response of the detectors was measured using 100 fs Ti:sapphire laser pulses. The temporal response of the photoconductor arrays is as fast as 21 ps rise time and 38 ps full width half maximum (FWHM). Spatial and energy responses were obtained using x rays from rotating anode (ANL) and synchrotron‐radiation sources (NSLS, beam line X‐18 B). The spatial resolution of the photoconductor obtained was 75 μm FWHM, for a 50 μm beam size. The best results were obtained for those arrays with the best crystal ...

Ultrafast molecular beam epitaxy (MBE) CdTe photoconductor array for synchrotron radiation

MBE (molecular beam epitaxy) grown CdTe layers were processed to fabricate a photoconductor array for the diagnosis of short x-ray pulses from synchrotron radiation sources. The MBE (111)B CdTe layers were grown on (100)Si substrates. Photoconductor arrays were fabricated with gaps of 5 - 50 micrometers using conventional photolithography. Electroless Au or sputtered Au/Ni was used as a contact metal. The temporal response of the resulting CdTe photoconductor was measured with mode-locked 100 fsec Ti:Sapphire laser pulses. The FWHM of single crystalline CdTe photoconductor response pulse is as short as 37 psec with a 20 psec risetime. The photoconductor responds linearly to the x-ray tube photon flux with fixed accelerating voltage up to 40 kV. A significant response increase to the x-ray beam is observed for a layer with good crystalline quality. Spatial response of the CdTe photoconductor array was measured using rotating anode and synchrotron x rays for different beam sizes. Excellent spatial resolution was obtained from narrow angular radiation synchrotron x rays. The CdTe photoconductor was exposed to synchrotron x rays for 60 hours without any noticeable degradation.

Fast photoconductor CdTe detectors for synchrotron X-ray studies

Abstract The Advanced Photon Source will be the brightest source of synchrotron X-rays when it becomes operational in 1996. During normal operation, the ring will be filled with 20 bunches of positrons with an interbunch spacing of 184 ns and a bunch width of 72 ps. To perform experiments with X-rays generated by positrons on these time scales, one needs extremely high speed detectors. To achieve the necessary high speed, we are developing MBE-grown CdTe-based photoconductive position sensitive array detectors. The arrays fabricated have 64 pixels with a gap of 100 μm between the pixels. The high speed response of the devices was tested using a short pulse laser. X-ray static measurements were performed using an X-ray tube and synchrotron radiation to study the devices response to flux and wavelength changes. In this paper, we shall present the response of the devices to some of these tests and also discuss different physics aspects that need to be considered when designing high speed detectors.

A large area detector for x-ray applications

Abstract A large area detector for X-ray synchrotron applications has been developed. The front end of this device consists of a scintillator coupled to a fiber-optic taper. The fiber-optic taper is comprised of 4 smaller (70 mm × 70 mm) tapers fused together in a square matrix giving an active area of 140 mm × 140 mm. Each taper has a demagnification of 5.5 resulting in four small ends that are 12 mm diagonally across. The small ends of each taper are coupled to four microchannel-plate-based image intensifiers. The output from each image intensifier is focused onto a Charge Coupled Device (CCD) detector. The four CCDs are read out in parallel and are independently controlled. The image intensifiers also act as fast (20 ns) electronic shutters. The system is capable of displaying images in real time. Additionally, with independent control on the readout of each row of data from the CCD, the system is capable of performing high speed imaging through novel readout manipulation.

In situ X-ray diffraction of the early stages of the crystallization of Fe80B20

A new technique has been used for the early stages of crystallization of amorphous materials, like metallic alloys. In situ X-ray diffraction has been performed during the early stages of crystallization of Fe80B20. The samples are resistively heated to 600°C in a customized vacuum chamber. A programmable charge-coupled device detector records simultaneously the evolution of the three phases: α-Fe, Fe3B and Fe2B in the minute scale. This is the first in situ X-ray diffraction study of this system in these temperature and time scales. Interesting behaviours have been seen: appearance and disappearance of phases, α-Fe supersaturation solution in boron (found for the first time in this compound), and migration of B out of the α-Fe matrix. The two-dimensional diffraction pictures show topography irregularities indicating crystallite inhomogeneties.