LINEAR DETECTOR FOR TIME-RESOLVED EXAFS IN DISPERSIVE MODE

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LINEAR DETECTOR FOR TIME-RESOLVED EXAFS IN DISPERSIVE MODE

H. Oyanagi, T. Matsushita, U. Kaminaga, H. Hashimoto

To cite this version:

H. Oyanagi, T. Matsushita, U. Kaminaga, H. Hashimoto. LINEAR DETECTOR FOR TIME-

RESOLVED EXAFS IN DISPERSIVE MODE. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-

139-C8-142. �10.1051/jphyscol:1986825�. �jpa-00226146�

LINEAR DETECTOR FOR TIME-RESOLVED EXAFS IN DISPERSIVE MODE

H. OYANAGI, T. MATSUSHITA*, U. KAMINAGA** and H. HASHIMOTO***

Electrotechnical Laboratory, Sakuramura, Niiharigun, Ibaraki 305, Japan

"National Laboratory for High Energy Physics, Ohomachi, Ibaraki 305, Japan

**Rigaku Corporation, Matsubaracho, Akishima, Tokyo 196, Japan

" * * Toray Research Center Inc., Sonoyama, Ohtsu, Shiga 520, Japan

ABSTRACT

A fast read-out system for a linear photodiode detector has been developed for a time-resolved EXAFS experiment in dispersive mode. A green light- emitting phospher (Gd2C>2:Tb) is deposited on a fiber optic faceplate, which is optically coupled with a linear photodiode array with 1024 sensor elements.

A 12-bit fast A/D converter is used to process 1024 channel data in 5 msec and store them in memory. Digital data can be routed and stored in a histogram memory having 16 frames with 20 bits. 16 spectra can be measured successively with a time resolution of 5 msec and a variable interval time.

An energy resolution of the total system is 5.6 eV at the Fe K-edge. for 1200 eV energy range. The application to a kinetic study demonstrates that time- resolved EXAFS or near-edge spectra can be obtained with a time resolution of 25 msec by use of the present detector system.

INTRODUCTION

The development of a fast linear detector with a high spatial resolution is essential in EXAFS experiments in dispersive mode [1]" where X-ray absorption spectra are measured as a spatial distribution of dispersed X-ray beam intensity. Since this arrangement is suitable for a time-resolved EXAFS experiment, efforts have been taken to utilize a linear photodiode array as a position-sensitive detector in order to achieve a rapid measurement of transmitted intensity [2]. Phizackerley et al. has used a thin phospher (YV04:Eu) to convert incident X-rays into visible photons to avoid X-ray damage to the silicon p-n junction [2],

Although a linear photodiode array has been recognized as a promissing position-sensitive detector with a high spatial resolution (50 urn), the time resolution has been limited by a slow data acquisition process of a read-out circuit which was originally developed for a low count-rate use in X-ray astronomy [3]. Therefore, a fast read-out circuit for a linear photodiode array has been developed for a time-resolved EXAFS experiment. In this paper, the design features and performance of a fast linear detector system are presented.

LINEAR DETECTOR SYSTEM

A linear photodiode array (RETICON RL1024SF) with 1024 sensor elements separated with each other by 25 um was used. Figure 1 f ows the schematic representation of a linear detector. A green light-emitting phospher (Gd202:Tb) was uniformly deposited on a fiber optic faceplate , which is optically coupled with sensor elements. The use of phospher has eliminated the two problems with a direct exposure of photodiode to X-rays, i.e., the rapidly

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986825

C8-140 JOURNAL DE PHYSIQUE

decreasing sensitivity in t h e high energy region ( ~ 5 keV) and a radiation damage 141. Although t h e radiation damage c a n b e reduced a t low temperatures ( -90-100"~) 151, t h e mechanism of radiation damage is not c l e a r yet.

Special c a r e has been taken t o obtain a fine powder of phosher, which was prepared by sedimentation. The thickness of phospher is estimated a s about 50-70 pm. The a c t i v e a r e a of photodiode a r r a y is 2.5 mm high and 25.6 mm wide. The spatial resolution of a photodiode a r r a y is about 50 Um (2 pixels) for a direct ilumination. However, considering t h e f a c t t h a t a diameter of optical fiber is 6 urn and t h e numerical a p e r t u r e is 1, o r incoming visible photons a r e accepted within a n a p e r t u r e of 45 degrees, a n optimum thickness of phospher i s estimated t o b e about 75 urn.

The thermal noise of photodiode c a n be reduced by cooling. The d e t e c t o r is mounted on a copper block which c a n b e cooled down t o -55°C by a thermoelectric cooler but t h e noise level a t -20°C is already comparable t o t h a t of preamplifier noise. The d e t e c t o r housing with a Kapton window is e l t h e r evacuated by a mechanical pump o r filled with dry nitrogen gas.

Figure 2 shows t h e block diagram of a fast read-out system. The four-phase clock t o o p e r a t e a linear photodiode array, preamplifiers, and analog multiplexer circuits a r e installed within t h e d e t e c t o r housing. The four video lines sampling t h e output of every fourth pixel a r e fed into charge- sensitive amplifiers which produce t h e pulse proportional t o t h e charge depleted in the photodiode p-n junction. Since t h e overall d a t a acquisition t i m e is o f t e n determined by t h e AID conversion rate, a fast 12-bit AID converter (5 u s e c conversion) is used t o process 1024 channel d a t a in 5 msec.

The output signal is digitized and s t o r e d in a histogram memory.

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Fig. 3 F e K-EXAFS oscillations of iron foil.

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Fig. 4 Fourier transform of F e K-EXAFS.

Digital d a t a c a n b e routed i n t o 16 frames. Each f r a m e memory consists of 1024 channels with 20 bits. The exposure time and integration number as well a s interval t i m e between t h e frames can b e chosen between 1 msec and 999 sec.

The e n t i r e read-out circuit is controled by a hardware logic interfaced with a microcomputer (LSI-11/23). The microcomputer system is equipped with a hard disk (20 MB) a s well as two floppy disks (2MB) and a main memory of 64 KB with a floating processor. As peripherals, a graphic display with a hardcopy unit and a printer a r e connected t o the microprocessor. The d a t a acquisition software is written in FORTRAN language under RT-I1 operating system.

Experimental parameters a r e easily s e t up from e i t h e r t h e hardware control logic or t h e microcomputer. Digitized d a t a a r e displayed by both an oscilloscope and a graphic display terminal. For a n experiment which requires a precise timing, a s t a r t timing c a n b e accepted.

PERFORMANCE

The performance of this linear d e t e c t o r was evaluated by comparing EXAFS and near-edge spectra for various materials obtained by a dispersive and point- by-point methods. A linearity of photodiode output was measured by monitoring the transmitted X-ray beam intensity with various thicknesses of absorber (aluminum foil). The dynamic range of photodiode a r r a y for t h e X-rays a t 9 keV (Cu K-edge) is estimated as lo3. Absorption s p e c t r a a r e obtained by measuring t h e intensity of incident and transmitted X-rays and taking a logarithm of (io/i). All measurements w e r e carried out when t h e s t o r a g e ring was operated a t 2.5 GeV. Higher harmonics a r e negligible due t o a platinum coated mirror placed between t h e polychromator and a linear detector.

T h e energy resolution of t h e spectrometer is given as a convolution of t h e spatial resolution of a linear d e t e c t o r and a geometrical resolution [I]. The total resolution was measured by placing a narrow slit (20 ~ m ) a t 50 mm from a photodiode array. The FWHM (full width a t half maximum) of a sharp peak arising from X-rays escaped from the slit was 5.6 eV a t t h e F e K-edge (7.1 keV) when an energy range of 1200 eV is covered. With R=2460 mm, a geometrical energy resolution is estimated a s 1.73 eV a t the F e K-edge. Since t h e slit width corresponds t o 0.98 eV in energy, t h e spatial resolution of a linear d e t e c t o r is a decisive f a c t o r of t h e total resolution. A b e t t e r energy resolution (2 eV a t 9 keV) is achieved either by a direct exposure of photodiode o r by use of silicon (311) crystal.

C8-142 JOURNAL DE PHYSIQUE

Figure 3 shows the F e K-EXAFS oscillations of iron foil measured by (a) a point-by-point method, (b) energy-dispersive mode with 3.5 s e c integration, and (c) energy-dispersive mode with 35 msec integration. A cylindrically bent triangular silicon ( I l l ) crystal was used a s a polychromator in a dispersive experiment while a silicon (311) channel-cut monochromator was used i n a point- by-point measurement. The energy s c a l e in a dispersive mode was calibrated by an interpolation between t h e F e K-edge (7.114 keV) of iron foil and Ni K-edge (8.334 keV) of copper foil which w e r e measured simultaneously.

The EXAFS oscillations obtained by t h e t w o methods a g r e e well each o t h e r both in position and magnitude. The magnitude,of oscillation in (b) is smaller than (a) by 20 % in t h e low k range (kc6.0 A-l) whereas t h e dispersive mode gives t h e s a m e magnitude of oscillation with t h a t of a point-by-point method in the high k range (k>6.0

A-').

These results suggest t h a t a difference in t h e magnitude is due t o t h e lower energ resolution of dispersive mode which averages out sharp structures a t k=4-5

1-1.

Figure 4 shows t h e results of Fourier transform pf t h e d a t a shown in Fig. 3.

The positions of a prominent peak located a t 2.2 A due t o t h e contribution of

$he nearest neighbors a g r e e with e a c h other within an experimental error (0.01 A). The positions of m o r e distant shells located a t 3.6

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in (b) and (c) also agree those of a point-by-point method. The magnitude of Fourier transform for t h e EXAFS d a t a in a dispersive mode is smaller than t h a t of point-by-point mode by 10-15 %. It should be noted t h a t t h e d a t a taken i n 35 msec gives essentially t h e s a m e features in t h e radial function with t h e d a t a integrated over 100 times. These results suggest t h a t t h e noise in EXAFS d a t a taken in 35 msec by a n energy dispersive spectrometer is statistical since a high frequency random noise is canceled out during t h e Fourier transform.

A new type of photodiode array (Hamamatsu S2301-512Q) with 512 sensor elements separated by 50 urn has been tested in a direct exposure mode. This photodiode array has superior signal-to-noise ratio because of a low-noise three-phase switching register constructed by a Plasma-Coupled Device (PCD).

Preliminary experiments indicated t h a t the EXAFS spectra extending over 300 e\!

with a n energy resolution of 2 eV c a n be obtained in less than 10 msec.

T h e present linear d e t e c t o r system has been applied t o kinetic studies of chemical reacfion by Matsushita e t at. [6], which demonstrates that t h e near- edge s p e c t r a c a n b e recorded with a time resolution of 25 msec using a stopped- flow apparatus. The time resolution is now limited by t h e signal-to-noise ratio a t t h e preamplifier partly due t o a low quantum efficiency of phospher.

Further improvement of time resolution is possible if the incident photon flux can be increased by two orders of magnitude. The use of a secondary electron multiplier such a s a microchannel plate (MCP) a s an image intensifier is expected t o realize a time resolution of a few msec. A new intensified photodiode d e t e c t o r has been built and t h e details will appear elsewhere.

ACKNOWLEDGEMENT

The authors express their sincere thanks t o Dr. S. Suzuki for kindly providing them t h e phospher powder. They would like t o thank Prof. I(. Kohra for his encouragements during t h e course of experiment.

REFERENCES

1 T. Matsushita and R.P. Phizackerley, Jpn. J. Appl. Phys. 20 (1981) 2223.

2 R.P. Phizackerley, Z.U. Rek, G.3. Stephenson, S.D. Conradson, K.O. Hodgson, T. blatsushita and H. Oyanagi, J. Appl. Cryst. 16 (1983) 220.

3 L.N. Koppel, Rev. Sci. Instrum. 47 (1976) 1109.

4 R.C. amble, J.D. Baldeschwieler and C.E. Giffin, Rev. Sci. Instrum.

50 (1979) 1416.

5 E. Dartyge, A. Fontaine, A. Jucha, and D. Sayers, "EXAFS and Near Edge S t r u c t u r e 111" ed. by K.O. Hodgson, B. Hedman and J.E. Penner-Hahn (Springer-Verlag, Berlin, 1984) p. 476.

6 ,T. ~viatsushita, H. Oyanagi, S. Saigo, U. Kaminaga, H. Hashimoto, H. Kihara,

N. Yoshida and &I. Fujimoto, t o be published in Jpn. J. Appl. Phys. Lett.