One of the often neglected necessities of synchrotron radiation science is detector technology. Pixel Array Detectors (a.k.a. PADs), have been developed in the Gruner Group to meet the x-ray detection needs of the synchrotron community. At the most basic level, a PAD is an ASIC (application specific integrated circuit) that is coupled to a diode detector. The coupling of the diode to the ASIC is done by bump-bonding.

This detector configuration has several distinct advantages over other types of imaging detectors used for gathering x-ray data. When compared to CCD imagers, there is a greater potential for high speed imaging. This is because the ASIC takes advantage of commercial grade mixed-mode CMOS circuits. This offers a flexible platform for development. In addition, the diode itself offers superior performance to scintillator based imaging arrays that rely on detecting secondary photons produced by the scintillation material. X-rays that are absorbed by the diode are directly converted into electron hole pairs, which are swept into the ASIC for processing by an applied electric field. The fundamental signal to noise ratio of this sort of detection far exceeds that obtained with a scintillator. In addition, the ability to directly collect charge that is produced by an absorbed x-ray rather than optical photons produced in a scintillator film, leads to much improved spatial resolution.

Below is an illustration of a PAD. Although it is not to scale, it shows the basic components of the PAD. The top layer (blue) is the diode used for converting absorbed x-rays to charge. The diode is typically silicon, but other materials with shorter absorption lengths can be used (e.g. GaAs) for more efficient detection of harder x-rays. The bottom layer (green) is the CMOS ASIC. This is a custom, application specific integrated circuit made to the requirements of the (e.g. microsecond time resolved radiography, or high dynamic range x-ray detectors). The diode and the ASIC are mated using bump-bonds (represented by the balls between the the two). The bump bonds can be solder or indium technology. On the right hand side of the detector the I/O to the ASIC is seen.

The dimensions of the detector pixels vary widely depending on the application and the complexity of the electronics used. Typical values are between 100 and 200 microns on a side.

Projects

100 x 92 PAD for Microsecond Time-Resolved X-ray Imaging

This detector was developed for ultra-fast time resolved x-ray imaging. It allows a very high instantaneous frame rate by storing the signal of eight successive frames on an on-chip, per-pixel, array of holding capacitors. It has been extremely successful when used for time-resolved fuel spray radiography by allowing direct observation of the mass distribution of fuel injector jets and supersonic shockwave creation.

Mixed Mode PAD (a.k.a. Mixed Analog-Digital PAD = MAD PAD)

This detector is being developed jointly with industry in order to make a large dynamic-range x-ray detector array for macromolecular protein crystallography. The detector exploits the flexibility of mixed-mode CMOS processes by using a mixture of analog integration and digital counting techniques in every pixel.

LCLS

The LCLS detector is being developed for use with an X-ray Free Electron Laser (XFEL) to be brought on-line at SLAC (Linac Coherent Light Source (LCLS) ). The XFEL will produce femtosecond pulses of coherent x-rays with fluxes that are orders of magnitude above that produced with other man-made x-ray sources.

The PAD being developed is to be used with a single molecule scattering experiment that is only feasible with an extremely intense and short pulse of x-rays. To get meaningful scattering information off a single molecule, an extremely high flux of x-ray photons is required. The problem is, however, that the photons typically transfer enough energy to the molecule to destroy its structure. The femtosecond laser will provide such a short pulse of x-rays that the scattering information from the molecule is collected before it is "blown apart".

The PAD will be used to collect single molecule diffraction patterns (i.e. broad diffraction patterns from a nonperiodic structure) which will be analyzed to reconstruct the molecular structure.

Below is a possible tiling scheme for a PAD detector (not to scale!).

People

Sol M. Gruner Professor of Physics, smg26@cornell.edu
Mark W. Tate Senior Research Associate, mwt5@cornell.edu
Hugh Philipp Post-Doc, htp2@cornell.edu
Dan Schuette Graduate Student, drs39@cornell.edu
Lucas Koerner Graduate Student, ljk29@cornell.edu
Darren R. Southworth Graduate Student, drs59@cornell.edu

PADs in the Press

Shock Wave Movies

click here to see shock wave movies

Presentations and Selected Publications

pdf version of LCLS detector presentation
pdf version of CHESS presentation
pdf version of NSLS presentation

Xin Liu, Kyoung-Su Im, Yujie Wang, Jin Wang, David L.S. Hung, James R. Winkelman, Mark W. Tate, Alper Ercan, Lucas J. Koerner, Thomas Caswell, Darol Chamberlain, Daniel R. Schuette, Hugh Philipp, Detlef M. Smilgies, Sol M. Gruner. Quantitative Characterization of Near-Field Fuel Sprays by Multi-Orifice Direct Injection Using Ultrafast X-Tomography Technique. Society of Automotive Engineers (SAE) Technical Paper 2006-01-1041.

Mark W. Tate, Darol Chamberlain, Sol M. Gruner. Area x-ray detector based on a lens-coupled charge-coupled device. Rev. of Sci. Instr. 76, 2005, 081301.

A.G. Angello, F. Augustine, A. Ercan, S. Gruner, R. Hamlin, T. Hontz, M. Renzi, D. Schuette, M. Tate, W. Vernon. Development of a mixed-mode pixel array detector for macromolecular crystallography. Nuclear Science Symposium Conference Record, 2004 IEEE, 7, 4667 - 4671

X. Liu, J. Lie, X. li, S.-K. Cheong, D. Shu, J. Wang, M. W. Tate, A. Ercan, D. R. Schuette, M. J. Renzi, A. Woll, S. M. Gruner. Development of Ultrafast Computed Tomography of Highly Transient Fuel Sprays. Developments in X-ray Tomography IV, ed. U. Bonse, Proc. of SPIE, 5535, 2004, 21-28.

Sol M. Gruner, Mark W. Tate, Eric F. Eikenberry. Charge-coupled device area x-ray detectors. Rev Scientific Instruments, 73, 2002, 2815-2842

Andrew G. MacPhee, Mark W. Tate, Christopher F. Powell, Yong Yue, Matthew J. Renzi, Alper Ercan, Suresh Narayanan, Ernest Fontes, Jochen Walther, Johannes Schaller, Sol M. Gruner, and Jin Wang, X-ray Imaging of Shock Waves Generated by High-Pressure Fuel Sprays. Science,Vol 255, Number 5558, p1261, (2002).

M.J. Renzi, M.W. Tate, A. Ercan, S.M. Gruner, E. Fontes, C.F. Powell, A.G. MacPhee, S. Narayanan, J. Wang, Y. Yue and R Cuenca, Pixel array detectors for time resolved radiography (invited). Review of Scientific Instruments, Volume 73, number 3, March 2002, 1621-1624

G. Rossi, M.J. Renzi, E.F. Eikenberry, M.W. Tate, D. Bilderback, E. Fontes, R. Wixted, S. Barna, S.M. GrunerDevelopment of a pixel array detector for time resolved X-ray imaging. AIP Conf. Proc, (2000).

P.J. Sellin, G. Rossi, M.J. Renzi, A.P. Knights, E.F. Eikenberry, M.W. Tate, S.L. Barna, R.L. Wixted, and S.M. Gruner, Performance of semi-insulating gallium arsenide X-ray pixel detectors with current-integrating readout. Nuclear Instruments & Methods in Physics Research, Section A, Volume 460, Issue 1, p 207, (2001).

G. Rossi, M. Renzi, E. F. Eikenberry, M. W. Tate, D. H. Bilderback, E. Fontes, R. Wixted, S. Barna, and S. M. Gruner, Tests of a prototype pixel array detector for microsecond time-resolved X-ray diffraction. J. Synchrotron Rad. 6, p 1096 (1999).

E. F. Eikenberry, S. L. Barna, M. W. Tate, G. Rossi, R. L. Wixted, P. J. Sellin, and S. M. Gruner, A pixel-array detector for time-resolved X-ray diffraction . J. Synchrotron Rad. 5, p 252 (1998).

S. L. Barna, J. A. Shepherd, M. W. Tate, R. L. Wixted, E. F. Eikenberry, and S. M. Gruner, Characterization of a prototype pixel array detector (PAD) for use in microsecond framing time-resolved X-ray diffraction studies . IEEE Trans. Nucl. Sci. 44, p 950 (1997).

S. L. Barna, J. A. Shepherd, R. L. Wixted, M. W. Tate, B. G. Rodricks, and S. M. Gruner, Development of a fast pixel array detector for use in microsecond time-resolved X-ray diffraction . Proc. SPIE 2521, p 301 (1995).