Imaging detectors for FUV and EUV wavelengths

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IMAGING DETECTORS FOR FUV AND EUV WAVELENGTHS G. W. Fraser X-ray Astronomy Group, Departmentof Physics andAstronomy, University of Leicester LE1 7RH, U.K.

ABSTRACT Microchannel plate (McP) detectors are currently the detectors of choice for astrophysical imaging and spectroscopy in the wavelength range 50—2000A. In this paper, we first describe alternative detectors for the EUV and FUV bands. In particular, we consider the extent to which back—illuminated Charge Coupled Devices (CCDs) may, in theory or in practice, replace MCPS in future satellite missions. we then report on a number of recent channel plate investigations of relevance to “open window” photon detection. These include : the study of fixed pattern noise sources ; the use of channel plates with square pores for simultaneously high spatial resolution and high quantum efficiency the measurement of linear polarisation sensitivity in MCPs and their alkali halide photocathodes and, finally, the modelling of channel plate gain response to intense, spatially non—uniform illumination. INTRODUCTION The manufacture and operation of microchannel plates, together with the enormous variety of channel plate readout methods, have all been extensively reviewed in the recent literature 1—31. In this paper, we shall instead concentrate on a small number of key detector developments for future satellite missions in EUV and FUV astronomy. The archetypal MCP detector of the future will be required to image, or to register grating—dispersed spectra, with good broad—band, spatially uniform quantum efficiency (>40%) and high spatial resolution (pixel sizes < 20 microns), perhaps over large areas (up to lOxlO square centimetres). Such a detector may also have to cope with very high local count rates (>10 counts per channel per second). In broad summary, the current goal of long—wavelength i’ICP detector development is “high resolution at high efficiency at high rate”. The basic elements of such a detector — specialised large format, small pore, low resistivity channel plates — are, in fact, already at hand. Manufacturers such as Philips Components 141 in the UK and Galileo Electro—Optics in the US 151 have already produced, for i 9truments such as the AXAF High Resolution Camera (HRC) 161 lOxlO cm channel plates with 12.5 or 10 micron pores. Small format NCP5 with 4 and 6 micron channels have been recently described by Laprade and Reinhardt 171 while Feller 181 has reported the development of actively cooled MCPS of only 500 kilohm resistance whit~t appea~ capable of ver~ hj1h count rate operation (up to 6 GHz.cm , or 10 counts.channel .s~ ). MCP5 with surface curvature have been produced for the ROSAT Wide Field Camera 191 and ALEXIS 1101 foci and to match the Rowland circle of a concave reflection grating lii. The development of radioisotope—free lead oxide glasses 121 has reduced the intrinsic channel plate background t~ 1~vels at or below the sea—level cosmic ray flux (~0.0l counts.cm .s ). The bulk of this paper considers three areas of continuing study (I’ICP image non—uniformity, high count rate response and linear polarisation sensitivity) and one new technical development — the use of channel plates with pores of square cross section to obtain high long wavelength quantum efficiencies without loss of spatial resolution. First, however, we consider the capabilities of alternative imaging detectors for the EUV and FUV.

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PROPORTIONAL COUNTERS

It is now perhaps surprising to recall that, in the earliest stages of the ROSAT Wide Field Camera project 1131, a tradeoff was performed beween an EUV channel plate detector and a thin lexan—windoweci imaging proportional counter (IPC), as developed by Bleeker et al. 1141 and as baseliried for the 6—250A band in the ESA EXUV mission study 1151. The large enhancement of MCP quantum efficiencies in the EUV band which has been achieved since 1982 (below) using alkali halide deposition photocathodes would today render such a comparison overwhelmingly one—sided. An EUV ( ), > 200A) ipc, requiring a thin (0.15 micron) pressure tight entrance window and a complex gas supply System, might at best (in the gas diffusion limit for normally incident radiation) achieve a fhwm spatial resolution of 300 microns at a wavelength of 250A 1141, compared to the wavelength—independent ~l00 microns fwhm easily achieved, for example, in the case of the Wide Field Camera NCP detectors 191 for a conical input beam with half—angle 30 degrees. It is in the l200—2200A FUV band that large area gas detectors continue to be proposed for astronomy 116,171. Here, the detection medium is a low—pressure mixture containing the photoionisable gas TMAE (Tetrakis (dimethylamine) ethylene), whose ionisation potential is only 5.36 eV. Multistep gas detectors have been intensively developed by the particle physics community for C~xerenkov Ring Imaging (RICH). Michau et al. 1171 anticipate a 50x50 cm TMAE chamber with a 3 mm LiF window with a quantum efficiency of 65% between 1000 and l700A and spatial resolution of order 100 microns. The vapour pressure, and hence, the uv absorption of gaseous TMAE is, however, a strong function of temperature. Temperature control of any TNAE counter in the satellite environment, together with stringent gas handling constraints (TNAE reacts with oxygen, for example 181) would seem to make this channel plate alternative rather unattractive from the engineering standpoint. Furthermore, the actual need for detectors of such very large area in FUV astronomy seems, at best, unclear. CHARGE COUPLED DEVICES of recent papers 119—211

have reported on the use of MOS CCDs for the EUV and FUV bands. The short absorption lengths of silicon and silicon dioxide in these bands, and up into the blue region of the visible spectrum, are shown in figure 1 1211. The expected quantum efficiency of a CCD directly illuminated through the back surface, with no electrode absorption losses, is at least 55% throughout the 50—2000A band 1221. Figure 2 shows a compilation 1191 of measured back—illuminated CCD efficiencies in the lOO—l700A band, incorporating the real effects of native Si0 2 surface dead layers and/or incomplete charge collection and/or surface contaminants. A number

In a 1987 review, vallerga and Lampton

1191 compared and contrasted

l’ICP and CCD detectors for “open window” space astronomy. These authors

found in favour of channel plate devices on the following grounds (1) fundamentally superior suppression of visible light (MCPs have an essentially “null” response beyond the photoelectric threshold of their photocathode, while CCD5 are, of course, efficient detectors in the visible) (ii) greater flexibility of format (iii) a photon counting capability at all wavelengths shortward of the photoelectric cutoff (iv) less susceptibility to contamination (v) the ability to operate without cryogenic cooling or large passive radiators In the interim period, the development of CCDs with l electron equivalent noise charges 1231 has moved the prospective wavelength threshold for single photon counting using CCDs out to around l100I~ (for three sigma significance and one electron noise), although the longest wavelength for which a peaked0CCD pulse height distribution has actually been reported remains 44.7A. In the same period, however, the susceptibility of MOS CCD5 to radiation (proton) damage has also begun to be quantified 124,251. High energy protons produce displacement damage in the silicon lattice. The resultant electron traps reduce the charge transfer efficiency (CTE) of the device. Thus, the probability of producing a space borne CCD detector with sustainable energy resolution in the EUV —the ultimate “small signal” CCD application, since a 200A photon would generate only ‘~l7 electrons in Si— would appear to be vanishingly small. In summary, to hold.

the conclusions of vallerga and Lampton 1191 appear still

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Figure 1. Absorption depths of photons silicon dioxide (broken curve) j2l(.

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2. Compilation of measured back—illuminated CCD quantum efficiencies 1191. FIXED PATTERN NOISE SOURCES IN MCP DETECTORS Recent studies of MCP image readout have shown that there are subtleties in the development of the output charge cloud, in the fabrication of the plates themselves and, in certain cases, in the Moire~’ relationship of the channel array to the readout pixel array, all of which can contribute to the fixed pattern noise (flat—field non—uniformity) of the detector. For any spectroscopic mission, such as NASA’s Lyman/FUSE 1261, the detection of weak absorption features (say) depends critically on the uniformity of the detector response. Many MCP readout elements operate by the sharing of charge between patterns of conductors. In order to optimise the performance of such devices, it is therefore vital to understand the manner in which the output charge cloud develops between the ouput face of the MCP “stack” and the anode plane. A number of groups 127—291 have recently studied the development of the charge cloud using mosaiced Wedge and Strip (WS) anode arrays or partitioned crossed—grid arrays. The most important conclusion of these studies is that the radial profile of the charge cloud is a function of the detector gain, as shown in figure 3. This effect can render the event centroid dependent on pulse height at the periphery of a wS anode 1271 ; it may also be implicated in the pronounced “chickenwire” patterns discovered by Vallerga et al. 1301 during the calibration of Wedge and Strip detectors for the EUVE mission (see figure 4). While the imaging studies of refs.(27—29) measured changes in charge cloud profile with detector gain by altering the bias voltages of the constituent NCPs, it is the case that gain is also an inherent function of position across a microchanriel array for any fixed set of bias voltages. Using a prototype Wide Field Camera detector with continuous resistive anode readout, we have shown at Leicester 1311 that it is the NCP gain (figure 5), and not the local open area fraction, which varies with the periodicity (~O,8 mm) of the multifibres which make up any

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~.20~0Ô’10

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x (mm) Figur~ 3. Difference betwe,n MC? charge cloud profiles for low gain (lxl0 ) and high gain (2xlO ) operation. The low gain distribution has more charge in the central core, the high gain profile has more charge in the spatial wings 1271. “two—draw” channel plate structure. The same general conclusion has been drawn from the study of “chickeriwire” patterns in MCP—based image intensifiers 1321. The patterns of figure 4 may therefore arise from the slight mislocation of events emanating from points around the MC? multifibre boundaries, because of their distinct (gain dependent) charge profiles. The chickenwire effect, of course, may be removed in practice by dividing each image by a reference image obtained under flat—field illumination 1301. For future large format, high resolution detectors, such as the Multi—~node ~crochannel Array (MAMA) 1331, the implication is that 10 or 10 event reference accumulations will be require to reduce differential non—linearities to the 1% level. Of perhaps more fundamental concern than the “print through” of channel plate irregularities is the problem of Moire’ fringing in the case of detectors, such as the MAMA system, with spatially “fixed” pixels. A recent analysis by Lawrence 1341 has examined the “beating” of a hexagonal channel array and a square pixel array. This analysis concludes that amplitudes of several percent in the flat field Moire pattern may persist even for small inter—channel pitch—to—pixel size ratios. Interestingly, one of the comments made by Lawrence 1341 is that use of a square MC? structure aligned with the image pixels would produce a relatively benign Moire pattern. MCPs with pores of square cross—section are, in fact, the subject of the next section of the present paper. MICROCHANNEL PLATES WITH SQUARE PORES Since 1982, when the first reports appeared of the high soft X—ray, EUV and FUV quantum efficiencies of CsI—coated microchannel plates 35,36(, the use of (primarily) alkali halide deposition photocathodes has been very thoroughly investigated. Researchers at the Space Sciences Laboratory of the University of California at Berkeley, in particular, have made intensive efforts to establish the optimum cathode material for particular bands in the EUV and FUV, reporting results from C5I— 1371, KBr— 1381, MgF.,— 1391 and KC1—coated 40( microchannel plates. In so far as the wavelength—dependent responses of these materials are determined by their band gaps and linear absorption coefficients, the Berkeley MCP survey is in a sense a technological extension of the complete (20 compound) alkali halide photoemission study of Metzger 1411. Nevertheless, very high efficiencies, relative to those of “bare” microchannel plates, have now been stably achieved. KBr, currently among the most favoured FUV photocathodes, yields, for example, around 40% at the peak of th~ efficiency—versus—angle response of a coated MC? under l000A illumination. The comparable figure for a bare channel plate is less than 10%.

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Figure 4. High resolution WS image of 1 cm diameter spot illuminated by l7lA photons, showing contrast—enhanced hexagonal structure (“chickenwire”) correlated with the internal nultifibre structure of the microcharinel plates 1301.

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Figure 5. Gain map, obtained under 44.7A illumination, of 5 mm diameter test spot. Wide Field Camera (WFC) detector F3, using resistive anode readout. The chickenwire seen in such gain maps is much less marked in direct images obtained during the WFC test programme I 31 I Figure 6. Wide Field Camera spatial resolution measured by the method of wire shadows. (a) Electron collecting field between repeller grid and front MC? — 25 V/mm. Indicated fwhm resolution — 87 microns. (b) Zero field between repeller grid and front MC?. Indicated fwhm resolution — 63 microns. The improvement in spatial resolution is due to the elimination of the “image halo” of photoelectrons originating on the interchannel web 1311.

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Such high detection efficiencies are not, however, achieved without cost. Typically one—third of the optimised detector response comes from the inter—channel web, from which electrons are collected by use of a so—called “repeller grid”. Photoelectrons returned to their plane of emission in this way may enter channels far from the original point of photon interaction. It has long been predicted 142,431, but only recently quantified for coated MCP5 (figure 6 1311), that there must exist a tradeoff between quantum efficiency and spatial resolution whenever the repeller grid technique is used. We now ask : is there anything further to be gained, in terms of microchannel plate quantum detection efficiencies in the EUV and FUV ? (One recent report 1441 suggests that there is certainly no advantage in using ion assisted deposition or sputtering, rather than straightforward thermal evaporation, to produce CsI photocathodes for the FUV). In our laboratory we have recently begun to investigate the performance of microchannel plates with pores of square cross section (figure 7). The original motivation for this work was to produce a compact form of the Photoemission Polarimeter (PEP) (below) 1451. We have procured a number of small (25 mm diameter) square—pore plates from Philips Components 141 who, in common with other MC? manufacturers 1461 had experimented with square—pore production during the late nineteen—seventies. The square side length is 85 microns and the pitch of the square array, 100 microns, giving an open area fraction of 72%. A square channel array of course gives a higher open area than a hexagonally packed array, for the same (minimum) septal thickness 1461. The channel “length—to—diameter” ratio, L/D, for the test plates is approximately 50:1. A square—pore MC? was operated as the input plate of a two—stage chevron multiplier, with a conventional (L/D 80:1, D 10 micron) channel plate as the output plate. Charge gains of 10 pC/pulse were achieved for a front plate bias of 1150 Volts, coupled to a limiting pulse height full—width—at—half—maximum of only 100%. A full account of the imaging, gain and noise characteristics of this detector will be given in a later report. Figures 8 and 9 now show, however, that very high absolute quantum detection efficiencies are observed when the uncoated square—pore MC? is illuminated with either soft X—ray (C K) or EUV (Hell) radiation. No repeller grid was incorporated in the detector for either measurement. No attempt was made to align the axes of the channel array to the incident beam direction. At the shorter wavelength the peak detection efficiency is equal, within counting error, to the open area fraction of the channel structure and much higher than has ever been achieved with conventional MCP5 and any combination of photocathodes and electron—collecting fields. Currently, we are investigating the response of the square—pore detector throughout the EUV and FUV bands, using the calibration facilities developed for the ROSAT Wide Field Camera. We expect to observe very high efficiencies indeed when the square pore MC? is suitably coated with C5I, without recourse to any (image—blurring) repeller grid. The physical reason for the superiority of the square—pore format appears clear, and clearly related to its claimed superiority in X—ray focusing I 47 I . At the angles and wavelengths represented in figures 8 and 9, photon reflection is an important process. Illuminating a circular channel with a parallel beam at an angle 8 to the channel axis produces a variation in grazing angle to the channel wall around the channel perimeter. The grazing angle varies smoothly from 8 dow~ to zero 1431. Thus, for example, a significant fraction of 44.7A radiation incident at an angle 0 10 degrees to a conventional microchannel would suffer reflection, even although the critical angle for reflection from lead oxide glass at that wavelength is only 6.6 degrees. Reflected radiation, interacting “far” down a channel, initiates electron avalanches which can develop through only part of the channel potential difference and which, being of small magnitude, may fail to be counted above any lower level charge discriminator. Illuminating a square channel, by contrast, produces at most two discrete values of the grazing angle, which interchange, for a given ray, at each reflection. We have constructed a detailed model of the square—pore response along the lines already described for conventional channels in ref.(43). This model, together with our further EUV and FUV efficiency measurements will be described in a future report.

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For the moment, we tentatively submit that the detector goal of “high resolution at high efficiency” (above) will best be served by the use of channel plates with pores of square cross—section, when maximisation of the open area fraction and minimisation of the square side length (figures of 80% and 10 microns appear entirely feasible) have taken place.

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Figure 8. Comparison of square—pore and conventional MC? quantum detection efficiencies as functions of angle of incidence. Incident wavelength — 44.7A (C K x—rays). Open circles — conventional MCP, uncoated, open area 63%, no contribution from 0 inter—channel web. Filled circles — conventional MC? bearing l4000A C5I photocathode, deposited at 4 degrees to the channel axes, including contribution from inter—channel web. Squares — prototype square—pore NC?, uncoated, open area 72%, no contribution from inter—channel web. EO I

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B (degrees) Figure 9. Comparison of square—pore and conventional MC? quantum detection efficiencies as functions of angle of incidence. Incident wavelength — 304A (Hell line). Squares — square—pore MC?, uncoated, no contribution from inter—channel web. Curve a — C5I—coated conventional MC?, including contribution from inter—channel web 1481. Curve b — uncoated conventional MCP, open area 55%, no contribution from repeller grid I~9I.

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THE LINEAR POLARISATION SENSITIVITY OF MCPS Dispersed spectra ito the EUV and FUV will carry, if not the polarisation signature of the celestial source, then certainly the polarisation signature of all the reflecting elements (mirrors, dispersers and gratings) in the optical chain. The first assessment of spectrometer polarisation has recently been reported for case of the EUVE Spectrometer 1501. It has now been known for some time 1511 that the quantum efficiency of lead oxide giass MCP5 is sensitive to the linear polarisation state of 584—l2l6A radiation (see figure 10). Figure 11 now shows that CsI, a commonly—used NC? photocathode material, is also polarisation sensitive in the FUV, suggesting that the alignment of future alkali—halide—coated channel plate detectors to beams emanating from grating spectrometers (such as those proposed for Lyman 1261) may be quite critical if maximum throughput is to be achieved. It is also the case, following on from our previous discussion, that a proper statement of any MC? quantum detection efficiency in the bands of interest here must consist of a percentage yield, plus the accompanying spatial resolution, plus the polarisation state of the test beam with which the measurements were made. A full account of our investigations of polarisation sensitivity in soft X—ray and UV photoemission is given in ref.(52). Very recent (June 1990) EUV measurements made at the SERC Daresbury synchrotron radiation source indicate that polarisation sensitivity is a universal property of all (insulator or metal) photocathodes. 12 I

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Figure 10. Variation of MC? detector modulation contrast factor 52 with photon angle of incidence for l2l6A (H2), 950—l000A (N,) and 584 (He) radiation. Measurements from ref.(5l). A preferential ~ensitivity to p—polarised light is indicated. Figure 11. Variation of relative photoyield with angle of grazing incidence to a l500~thick CsI photocathode 1521. Modal wavelength of UV test bean — l550A. The individual symbols represent measurements made after different exposures of the cathode to humid air. The curves are the predictions of various models described in detail in ref.(52). (a) s—polarised radiation (b) p—polarised beam. Higher count rates are observed with p—polarised light.

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THE HIGH COUNT RATE OPERATION OF MCPS As noted in the introduction, future NC? detectors, especially those reading out gratings with high wavelength resolution 1261, will be required to cope with extremely non—uniform, locally intense illumination. Gain suppression at high count rate is, of course, a very well—known characteristic of channel plate detectors which, until recently, seemed well understood in terms of a simple “rule of thumb” namely, that the pulse current (output charge times count rate) which any MC? can deliver is limited to 10—30% of the standing current (bias voltage divided by MC? resistance) flowing in the illuminated area. New measurements in our laboratory 1531 and elsewhere have shown that the real situation is rather more complex. The rate of change of (modal or average) gain with count rate (per unit area) depends on the size of the area illuminated. The highest count rates are achieved when the ratio of quiescent channels to active channels is maximised. That is, when the illuminated area occupies only a small fraction of the MCP active area. 0 0

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we have recently completed a thorough experimental and theoretical study of count rate effects. Measurements have been made on some thirteen differemt detector configurations, incorporating MCP5 with resistances in the range 27—2450 Megohms, using both UV and soft X—ray illumimation and spot sizes ranging from 100 microns to 17 mm in diameter. Figure 12 shows a compilation of some of our data obtained using high resistance Philips Components 141 NCPs in a chevron configuration. When the illuminated area constitutes a large fraction of 6the total MCP area, and the number of active channels is of order 10 , the pulse current—to—standing current ratio indeed saturates at about 0.4, in line with “rule of thumb” expectation. When, however, the number of active channels is small (of order 80), the pulse current exceeds the nominal local standing current (computed from the local bias voltage — resistance quotient) by a very large factor. The limiting value of the current ratio is 30, not 0.3. We b~lieve that by repeating each area illumination with both UV (2540A) and soft X—rays, we have ruled out “enlargement effects” due to UV reflection from the collimating hole used to define the spot size. The results of figure 12 in fact represent the manifestation in a photon counting context of an effect which is well—known in image intensifier studies — the physical increase of the conduction (standing) current under intense illumination 1541. The effective resistance of a channel plate changes (falls) in quasi—continuous operation because, according to Guest 1541, the cascades of electrons form parallel resistive paths. An output current which exceeds the

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G.W.Fraser

nominal conduction current can then be drawn from the multiplier. Other conductivity—enhancing processes — the effects of electron bombardment on the channels walls, for example, may also be implicated. In the present context, the results of figure 12 are very good mews for detector designers. Th~ local dynamic range of microchannel plates even of very high (10 ohm) re~sist~ance is unexpectedly good, encompassing the 10 coumts.chamnel . s rate set as a goal in the introduction to this paper. A full account of our count rate study will be given in a separate paper. ACKNOWLEDGEMENTS GWF wishes to thank his colleagues Jim Pearson, John Lees, Mike Pain and Martin Barstow for providing experimental data used in this review. Thanks also to G. NcTurk for his assistance in obtaining the electron micrograph shown in figure 7. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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22.

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