Nuclear Instruments and Methods in Physics Research A 348 (1994) 654-658 North-Holland

NUCLEAR INSTRUMENTS

& METHODS IN PHYSICS RESEARCH

Secbor~ A

Lens coupled scintillating screen-CCD X-ray area detector with a high detective quantum efficiency

Andreas Koch Detector Group, Experiments DiL'ision, European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, France

An X-ray area detector consisting essentially of a scintillating screen, a fast objective and a cooled 1242 x 1152 slow scan CCD camera is described. A detective quantum efficiency (DQE) of > 0.3 for a planar field of view of QII0 mm and photon energies between 10 and 20 keV is obtainable at integrated intensity levels above 108 eV/mm2/readout without any image intensification. At lower intensities the readout noise of the CCD camera (8 e /pixel) deteriorates the DQE. The spatial resolution of the detector is 100 ~xm (FWHM) and the dynamic range 104. The design easily allows the exchange of screens in order to optimize the detector at other photon energies. The performance of the detector will be discussed in terms of DQE, dynamic range, spatial resolution, spatial distortion and uniformity of response. The detector will be initially used for small angle scattering and diffraction experiments as part of the Microfocus Beamline at the European Synchrotron Radiation Facility (ESRF).

1. Introduction

Synchrot ron radia t ion facilities provide intense, highly monochromat ic and col l imated X-rays providing oppor tuni t ies for new exper iments in mater ia ls science and many o ther fields. This demands new 2-dimen- sional X-ray detectors with a high signal to noise ratio, spatial resolution, dynamic range and t ime resolution.

Of ten X-ray detec tors convert X-ray pho tons into "vis ib le" photons, i.e. into pho tons with an energy of a few elect ron volts, before they are recorded by an electronic detector . This in te rmedia te conversion can avoid radia t ion damage to electronic circuits. It easily allows an adapt ion to the dynamic range of the detec- tor, compared to direct X-ray detect ion, since the n u m b e r of finally de tec ted e lect rons per X-ray pho ton can be adjusted. For this conversion of X-rays into visible light a variety of phosphors are available which differ in efficiency of conversion, t ime response, ab- sorpt ion dep th and spectral emission.

Image intensif icat ion can be used by detec tors of this type, e.g. microchanne l plates or image intensi- fiers. Image intensif icat ion provides a high D Q E or the ability of single pho ton counting. The D Q E is def ined as the square of the signal to noise rat io (SNRou t) of the de tec ted signal to the square of the SNR~, of the incoming signal. The D Q E is a p a r a m e t e r which ex- presses the addi t ional noise in t roduced by the detec- tor. Image intensif icat ion may reduce the de tec tor per- formance, for example, the count ra te limit of mi- c rochanne l plates is of ten too low for synchrot ron radia t ion appl icat ions [1] and current ly available image intensif iers have curved input surfaces, which intro-

duce parallax and image distort ion. Fu r the rmore , im- age intensif iers are sensitive to magnet ic fields [2].

The de tec tor descr ibed here does not rely on any image intensif icat ion while preserving a high DQE. This is achieved by using a fast objective with a relative aper tu re of f / 0 . 8 7 and a scintillating screen with a high energy conversion efficiency. The objective images a p lanar field of Q l 1 0 mm from the scintillating screen on to a cooled CCD camera ( - 4 3 ° C ) with a demagnif i- cat ion rat io of 4 : 1. A p l anned improved de tec tor will opera te with a liquid n i t rogen cooled CCD camera in order to obta in a lower dark charge and to allow even longer exposure times. The CCD has to be thermally isolated from the o ther de tec tor components . A fibre optic t aper ins tead of a lens was therefore not consid- ered. The screen can be exchanged in order to opti- mize the de tec tor at di f ferent pho ton energies. At first, the de tec tor will be mainly used in the energy range from 10 keV to 20 keV for small angle scat ter ing and crystal diffraction at the Microfocus beaml ine at the ESRF.

This de tec tor concept can also be applied to smaller image fields where the D Q E can be increased. For larger image fields, it must be kept in mind that the light collection efficiency of an objective decreases quadrat ical ly with the demagnif ica t ion ratio, and the D Q E might deter iora te . For large fields image intensi- fication may become necessary.

2. Detector design

The exper imenta l conf igura t ion used is shown in fig. 1. The X-rays pene t r a t e a light t ight window,

0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 01 6 8 - 9 0 0 2 ( 9 4 ) 0 0 1 3 1 - P

A. Koch/Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 654-658 655

m

X-RAYS X-RAY WINDOW, LIGHT TIGHT

CINT1LLATIN(J SCREEN, (Jd202S:Tb FIELD OF VIEW O110 mm

SHUTTER

" ~ 1 ~ CCD-CAMERA j ~ 1242 x 1152

@ -43 "C 1 / r ~ wrrHVACUUM

WINDOW

OBJECTIVE, f/0.87 DEMAGNIFICATION 4: I

Fig. 1. Experimental configuration of the scintillating screen X-ray area detector.

t r anspa ren t down to low energies ( = 10% absorpt ion at 6.4 keV).

A scintil lating screen is used to convert the X-ray image into a visible image. Table 1 summarizes the characterist ics. The screen chosen is commonly used in mammography at X-ray energies of a round 25 keV and opt imized for high spatial resolut ion and high conver- sion efficiency. A slight drawback of this screen is its spectral emission at 550 nm which does not optimally match the q u a n t u m efficiency of the CCD. W h e n illu- mina ted f rom the back the spatial resolut ion measures 70 ~ m (FWHM). High resolut ion requires a th in phos- phor layer due to l ight scat ter ing within the mater ia l ; since a high absorp t ion is also requi red the pho ton energy used has to be adap ted to the layer thickness. For this screen an energy range be tween 10 keV and 20 keV is best suited. The high convers ion efficiency ~Tx/v of the phospho r is impor t an t for achieving a high DQE. The angular emission profile of the screen is nearly Lamber t i an . For the pe r fo rmance of high aper- ture lenses it is essential to account for the angular emission profile.

A mir ror reflects the visible light out of the direc- t ion of the X-ray b e a m in order to prevent rad ia t ion damage to the optical componen t s and to ease the shielding o f - the CCD camera. The mir ror has to be p laced sufficiently far away from the screen to avoid back ref lected light falling onto the screen.

A fast objective was specifically des igned for this camera in o rder to ob ta in a high collection efficiency Z/col, a large field of view and a high spatial resolut ion (Table 1). Due to the high ape r tu re of the lens, the dep th of focus is smaller than for convent ional objec- tives; the a l ignment is there fore much more critical. Fu r the rmore , the demagnif ica t ion rat io has to be ad- jus ted accurately, a 10% deviat ion already decreases the M T F at the Nyquist f requency of the system by

a round 50%. The lens demagnif ies an object by a factor of 4.

The CCD camera is Pel t ier cooled to allow long exposure t imes with a low dark charge. The q u a n t u m efficiency at 550 nm is ~Tv/e = 30%. The pixel size is 22.5 Izm x 22.5 t~m. This cor responds to an equivalent input pixel size in the p lane of the screen of 90 p~m x 90 p~m. All optical surfaces have ant i - ref lect ion coatings to minimize optical scattering.

Table 1 Characteristics of the detector components

X-ray window MIB II, Quartz&Silice multilayer of polyimide and aluminium absorption: 10% at 6.4 keV

Scintillating screen MR Detail, Agfa material: Gd2OaS: Tb 0.3% Tb (weight) energy efficiency: r/x/v - 10% spectral emission: 550 nm, mainly density: 7.2 g /cm 3 deposition: 30 mg/cm 2 phosphor layer: 140 p~m plastic backing: 100 p~m phosphor protection coating: 8 txm spatial resolution: 70 tzm FWHM

Mirror aluminum coated, Ealing

Objective Super Farron, Optical Technology Devices relative aperture: f /0 .87 demagnification: 4:1 image field ~30 mm focal length: 76 mm collection efficiency: "qcol = 0.007 depth of focus: on image side: = 40 Ixm at 22 lp/mm, 50% MTF on object side: = 1 mm at 5.61 lp/mm, 50% MTF transmission of whole optical system: Y~v/t r ~ 0.7

CCD-camera TE/CCD-1242E&EM, Princeton Instruments CCD type: EEV 05-30 number of pixel: 1242 X 1152 pixel, size of pixel: 22.5 p~m x 22.5 Ixm readout noise: 8 e - rms at 200 kHz quantum efficiency at 550 nm: "qv/e = 0.3 full well capacity: > 500000 e - dynamic range: 16 bit gain: g = 2.5 e - / A D U non-uniformity: < 1.5% rms operating temperature: - 43°C dark charge: 6 e /p ixe l / s at -43°C readout time: 8 s

VI. X-RAY DETECTORS

656 A. Koch/Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 654-658

3. Detector performance

The detector as a whole was tested with respect to its DQE, dynamic range, sensitivity, spatial resolution, optical background, spatial distortion and uniformity of response. Table 2 summarizes the results.

3.1. Detective quantum efficiency

At high intensities the DQE depends only on shot noise and is intensity independent in this case. For an integrated intensity of smaller than 1000 k e V / p i x e l / readout ( = l0 s eV/mm2/ readou t ) , the readout noise reduces the DQE by more than 50%. If the detection process is shot noise limited the DQE is given by [3]:

D Q E ( SNR°ut )2 = ( 1 + r e l V a r ( n l ) + relVar(rl2)

rll

relVar(~73) ) 1 + + ' " , (1)

~ 1 ~ 2

where re lVar( rh)= 1 / r h for a Poisson process and relVar(r h) = (1 - rti)/rli for a Binominal process.

The relevant terms are given as efficiencies (Table 1) by: relVar(rl X / t r )

= (1 - ' rlx/tr)/ 'r/x/t r

relVarOTabs) = (1 - T l a b s ) / ? ' / a b s

energy dependent transmis- sion of X-rays through the X- ray window and the backing of the screen, energy dependent absorption in the phosphor,

Table 2 Overall performance of the detector

Energy range, optimized DQE

Equivalent input dark signal

Dynamic range Sensitivity Field of view Spatial resolution:

PSF at FWHM MTF at 3 lp/mm

Distortion-integral Distortion-differential PRNU Vignetting Readout time

10-20 keV = 30% (10-20 keV)

at integrated intensity > 1 × 10 s eV/mm2/readout

9× 106 eV/mmZ/s 104 15 keV/e Ql l0 mm

100 ~m 5O% 0.6% <1% 1.5% rms 30% 1 image/8 s

1

0 s .............................................. 7 / i [---:DQE ] ' " o DQE, measured

o . 6 .............................................. i ....................... i ....................... i ....................... i . . . . . . . . . . . . . . . . . . . . . cy : o . 4 ............................................... i ....................... i ...................... i ....................... i . . . . . . . . . . . . . . . . . . . . .

0.2 .............

0 ~ ~ ~

0 10 20 3b Xo Jo 60 PHOTON ENERGY [keV]

Fig. 2. Detective quantum efficiency at different X-ray ener- gies, the curve refers to the calculated DQE (Eq. (I)).

relVar([ E x / E v ]r/X/v) = 1 / [ E x / E v ] n X / v

E x / E v

relVar(~coj)

= (1 - nco0/nco~ relVar(-qv/tr)

= (1 - rlv/tr)/rlv/tr relVar(~v/e)

= 1 /~v/e

energy dependent conversion of X-ray photons into visible photons, ratio of X-ray photon energy to visible photon energy, collection of light by objective,

transmission of light in optical system, conversion of visible photons into electrons,

The DQE is shown in Fig. 2 as calculated from Eq. (1). The marked data points refer to measurements made using an X-ray generator with a tungsten anode. The emitted bremsstrahlungs spectrum was filtered with foils of Cu, Zr, CsI, Ta giving peak emissions at 8.5 keV, 17.6 keV, 29.4 keV and 55 keV respectively. All energy dependent measurements in this article refer to this set-up. The bandwidth was measured with a scintillation counter (NaI photomultiplier) to be smaller than 30%. In order to calibrate the incident intensity on the detector, the number of incident pho- tons were counted. The result was corrected using the quantum efficiency of the scintillation counter, which is close to 1 for all energies. The figure shows that, between 10 keV and 20 keV, the DQE is approxi- mately 30%. This is the region for which the detector has been optimized. Below 10 keV the DQE decreases because the yield in visible photons per X-ray photon decreases, and the absorption of the plastic backing and the X-ray window increases. Above 20 keV the absorption of the phosphor decreases; however, at 50.2 keV gadolinium has its K-absorption edge which pro- vides an increase in DQE. An improvement in DQE is possible above energies of 15 keV (absorption: 70%) by using a thicker phosphor layer, however, an increase in layer thickness results in a loss of spatial resolution. An improvement in both, DQE and spatial resolution at higher energies, is obtainable if structured screens like

A. K o c h / N u c l . Instr. and Meth. in Phys. Res. A 348 (1994) 654-658 657

C s I : T l or C s I : N a screens are used [4]. CsI can be grown in needle like structures perpendicular to a screen surface which provides light guiding to a certain extent. The phosphor thickness can then be increased whilst preserving the spatial resolution.

3.2. D y n a m i c range a n d sensit iuity

The dynamic range D is given by the ratio of signal saturation to the intrinsic detector noise:

D = S s a t / N = 2 X 104, (2)

where S~ t = 216g and N = 8 e - rms readout noise (Table 1). The dynamic range can be still improved by a lower pixel readout rate (at 50 kHz, 4 e rms readout noise) or a higher gain g (Table 1). The CCD camera is cooled in order to maintain this high dynamic range at long exposure times. The limit is approximately 10 s for the Peltier cooled ( - 43°C) version used. The sensi- tivity, i.e. the energy which is necessary to produce one electron by this detector, was measured to be 15 k e V / e and can be calculated from the different efficiencies given in Table 1. Using the detector at X-ray energies below the energy given by the sensitiv- ity, i.c. 15 keV, degrades the D Q E considerably (cf. Eq. (1), Fig. 2).

3.3. Spat ia l resolut ion

The spatial resolution achieved with this detector is shown as the modulat ion transfer function (MTF) and the point spread function (PSF) in Figs. 3 and 4, respectively. The M T F was measured using a lead bar chart, and the PSF with a Q 30 ~xm platinum hole. Although the PSF is the Fourier transform of the MTF, both were measured to avoid inaccuracy. Mea- suring the MTF gives more accurate information for

100

80

60

40

20

-e-16.7 keV --o- 16.7 keV with optical filter

] -6- 29.4 keV i I- ~- -55 keV 1-.78 5 keV i o , i i i

o". i i i ............................ i ........................ °!u: ........... i ............................. i ...........................

i i o , i i

............................ i ............................. i ........................ ~i ............ i ........................

............................ i ......................................................... i ............................. i ........................

0 1 2 3 4 5 SPATIAL FREQUENCY [lp/mm]

Fig. 3. Modulation transfer function of the detector at differ- ent X-ray energies.

10 °

10 -1

I 0 -2 k ~

10-3

10 4

10 .5

0 200 400 600 800 1000

DISTANCE FROM CENTRE [gm]

Fig. 4. Point spread function of the detector at different X-ray energies, at the centre and at the edge of the image.

images containing low spatial frequencies (e.g. small angle scattering in some cases), whereby the PSF gives more accurate information for images containing sharp details (e.g. crystal diffraction). The tails of the PSF are important in order to resolve small and intense diffraction peaks close to each other. The F W H M of the PSF was measured to be around 100 ixm. Measure- ments at the edge or centre of the image field, or at different energies, do not show significant differences, nor do sagittal or tangential measurements. The PSF curves in Fig. 4 were determined by averaging over several slices. The PSF was measured down to relative signal levels of 5 × 10 -4, limited by dark charge noise. The optical "un i fo rm" background caused by one illu- minated pixel due to scattered light inside the camera was estimated to be 10 -s , from measurements made where half of the image was il luminated uniformly with X-rays. In the unexposed region the intensity back- ground is about 1%. In the case of the MTF measure- ments, optical filtering was used in order to suppress other weaker emission lines of the phosphor because for these emission lines the imaging quality of the objective is worse. However, filtering did not improve the resolution significantly (see Fig. 3). The Nyquist frequency of the CCD corresponds to an equivalent input frequency of 5.6 l p / m m .

3.4. Spat ia l dis tort ion a n d u n i f o r m i t y o f response

Spatial distortion has been measured by placing a hole mask in the plane of the screen. The holes had a 5 mm pitch and a diameter of 500 ixm over the whole field of view. The integral distortion is 0.6%. The differential distortion is smaller than 1%. It may be numerically corrected.

The uniformity of response was investigated at in- tensity levels near saturation. One can distinguish be- tween variations with small spatial frequencies in- volved, like vignetting, and pixel to pixel variations

VI. X-RAY DETECTORS

658 A. Koch/NucL Instr. and Meth. in Phys. Res. A 348 (1994) 654-658

(pixel response non-uniformity - PRNU). Vignetting is about 30% at the borders of the image field. The P R N U of the whole system is 1.5% rms averaged over a uniformly exposed area. Single pixel defects of the CCD occur additionally. All additive (dark charge) and signal proport ional non-information carrying signals (PRNU) can be removed by background substraction and flatfielding, respectively.

view and a dynamic range of 10 4. This detector has to be considered as an economical solution for certain requirements within a wide range of applications at the ESRF. If single photon counting, a larger D Q E or larger field of view are required other X-ray detectors based on image intensifiers can be provided by the Detector Group at the ESRF, but with a poorer spatial resolution [6].

3.5. Time response

The time response, i.e. the time decay of the sys- tem, especially of the phosphor, was sufficient with respect to the time between successive exposures of about 8 s. No image detail > 10 -4 could be observed after this time. This is not the case for all types of phosphors used in radiography applications [5].

Acknowledgements

I would like to thank Alain Beynet for his work on the mechanical design of the detector and Steven Gib- ney for building and testing detector components, es- pecially a wide range of scintillating screens.

The test of this detector at the beamlines of the E S R F will start in September 1993. More information about linearity of response, radiation damage and sta- bility in terms of temperature and mechanics is ex- pected from these measurements.

4. Conclusions

An area detector based on a scintillating screen and a CCD camera has been demonstrated. The detector requires no image intensification, achieves a D Q E > 30% between 10 keV and 20 keV, has a planar field of

References

[1] C.F.G. Delany and E.C. Finch, Radiation Detectors (Clarendon, Oxford, 1992).

[2] I.P. Csorba, Image Tubes (Sams, Indianapolis, 1985). [3] U. Arndt and D.J. Gilmore, J. Appl. Cryst. 12 (1979) 1. [4] K. Oba, M. Ito, M. Yamaguchi and M. Tanaka, Adv.

Electron. Electron Phys. 74 (1988) 247. [5] J.P. Moy, A. Koch and M.B. Nielsen, Nucl. Instr. and

Meth. A 326 (1993) 581. [6] J. Morse and J.P. Moy, presented at this Conference (3rd

London Conf. on Position Sensitive Detectors, London, UK, 1993).