Investigation of micro-channel plates as parallel-bore electron collimators for use in a proximity-focused ultra-fast streak tube

NUCLEAR

INSTRUMENTS

AND

METHODS

12 7

(i975) 87-92;

©

NORTH-HOLLAND

PUBLISHING

CO.

INVESTIGATION OF M I C R O - C H A N N E L PLATES AS PARALLEL-BORE E L E C T R O N C O L L I M A T O R S FOR USE IN A PROXIMITY-FOCUSED ULTRA-FAST STREAK TUBE* A L B E R T J. L1EBER, R O B E R T F. B E N J A M I N , H. D E A N S U T P H I N a n d C L I N T O N B. W E B B

Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87544, U.S.A. Received 14 April 1975 Micro-channel plates have been investigated for use as parallelbore electroia collimators for energies up to 10 keV. T h e collim a t i o n quality o f plates o f three aspect ratios has been m e a s u r e d using a dc beam. M e a s u r e d a n g u l a r acceptance profiles indicate

good collimation. This investigation was carried o u t as part o f a design s t u d y o f an ultra-fast proximity-focused streak tube using a micro-channel plate for transverse photoelectron velocity selection.

1. Introduction

slight modification, make surprisingly good collimators of electrons for energies up to l0 keV. Typical MCPs manufactured for electron-multiplier arrays are composed of lead glass, 50% PbO by weight, making them efficient absorbers of both charged and uncharged particles. Bore diameters are of the order of 8 pm on 12 #m centers in a hexagonal array. To prevent positive-ion feedback, the channels are sliced from the boule with a non-zero bias angle (angle between tubule axis and plate surface normal). For electron-multiplier applications a semiconductor is deposited inside the tubules and electrodes plated upon the plane surfaces of the MCP. Typical plate thicknesses are of the order of 0.55 ram. However, the fabrication state-of-the-art has advanced to the point where

When one first views a micro-channel plate (MCP) under a scanning electron microscope (fig. 1) he is struck with the uniformity of tubule diameters and the precision with which they are arrayed. In fact, it would appear at first glance that the least likely application of an MCP is an electron multiplier. The straightness of the bores and very large aspect ratios (length-to-bore diameter) should make them efficient parallel-bore collimators. It has been shown that the physical properties of MCPs make them effective collimators of soft X-rays1). In this paper we show that MCP's, with * W o r k p e r f o r m e d u n d e r the auspices o f the Energy Research & D e v e l o p m e n t Administration.

X 3000

X 6000

A

B

Fig. 1. Scanning-electron-microscope p h o t o g r a p h s o f a 0.30 m m thick MCP. T u b u l e bores are 7 . 9 # m diam. on 1 2 # m centers.

87

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A.J.

L I E B E R et al.

lengths of 1 cm are possible with excellent bore straightness2). An ideal electron collimator would be composed of tubules of a dense conductor to minimize edge scattering, bore leakage, and bleed off wall charge accumulated in dc operation in order to minimize charge effects. The typical MCP of 0.55 m m thickness offers potential geometric resolution of 14.5 mrad, while the minute bores offer very high spatial resolution. However, for very thick plates the ideal case of tubule walls being completely coated with conductor cannot be achieved, although it has been found that inconel can be coated through plates up to 0.55 m m thickness with fair certainty using special processing2-3). Therefore, to study wall-charge and other effects, three MCPs of varying aspect ratios were constructed and tested. These plates were made by etching out the tubules and were cut at zero-degree angles with respect to the boule axis. The experimental results are compared with theoretical predictions for energies up to 10 keV. The 10 keV incident electron energy represents a limitation of the present test setup rather than an upper limit on the M C P ' s collimation ability. As will be discussed later, the primary reason for this investigation was to test the possibility of using an MCP for transverse photoelectron velocity selection in ultra-fast visible and X-ray proximity-focused streak tubes~).

2. Experimental The experimental apparatus is shown in fig. 2. The vacuum system consisted of an all-stainless-steel bakeable chamber pumped by an ion pump. Typical GIMBAL MOUN~ FARADAY SHIELD ~

operating pressures were 10- 8 torr. This high-vacuum system was constructed to ensure adequate pumpout of the MCP tubules, and to avoid surface contamination. The gun assembly of a Tektronix type 561A cathoderay tube was used to obtain an electron beam with low angular divergence. The cathode was replaced with a Phillips Elmer cathode and activated in the chamber. After activation the gun was maintained in vacuum. This system provided excellent beam optics, stability, and controllability. The electron beam was passed through two fine mesh screens and into a field-free volume. In this volume the MCPs were mounted on a gimbal mount with two orthogonal axes of rotation which lay in the MCP plane. Rotation of the gimbal mount could be controlled remotely. After passing through the MCP the electron beam was registered on an aluminized P-20 phosphor screen assembly from a proximity-focused generation-II image intensifier. A 2 keV aluminum coating on the phosphor prevented electrons below this energy from reaching the phosphor. The spot intensity was measured by a Model 1980CDB Pritchard photometer. During the course of the experiment the gun current was monitored by reading the current on one of the grids in front of the MCP using a General Radio Corporation Model 1807 nanoammeter. The data-taking procedure typically consisted of maximizing the photometer reading by rotating about one axis and then sweeping about an orthogonal axis. Absolute MCP-transmission measurements were obtained by measuring the current on the MCP and on the phosphor cup with nanoammeter. Values obtained in this manner M.C.P

PHOSPHOR CUP

PRITCHARD PHOTOMETER

~ M I C R O M E TER DRIVE CABLES

Fig. 2. Schematic of MCP collimator test system. Vacuum-chamber details omitted for clarity.

1

MICRO-CHANNEL

P L A T E S AS P A R A L L E L - B O R E

may be optimistic, due to the lack of secondaryelectron suppression in this mode of operation. 3. Results

The measured angular acceptance shows that the MCP is an effective collimator of 5-10 keV electrons. Fig. 3 clearly shows the angular acceptance becomes narrower as the aspect ratio (collimator-length/tubulediameter) is increased from 39 to 106. One notes, for example, that the full width at half-maximum height (fwhm) for the 0.30 mm plate is about three times that of the 0.84 m m MCP. Fig. 4 shows the collimation ability of the 0.30 m m MCP for two incident energies. The absolute transmissions were measured to be 39 % for 10 keV and 26% for 5 keV. These values compare favorably with the geometric ratio (area of holes to front-surface area) of 41%. Measurements were made on all three plates for 650 x 1 0 - t 2 A and 3 x 1 0 - 9 A beams. The results at the higher currents are not shown, for they were essentially similar in shape to the lowercurrent measurements. Saturation effects in the phosphor precluded the possibility of higher-current investigations. The following analysis will show that the angular acceptance of MCPs can be quantitatively described by l

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the transmission of a perfect collimator, broadened by instrumental and background effects and by interactions of the electrons with the tubule walls. In the streakcamera application, the effect of wall interactions is estimated to be negligible because of the low density of electrons per tubule needed to record a picture. The data were analyzed by a least-squares fit to a calculated function, Fc,,¢(O), which is the folding of a perfect collimator with an instrumental profile:

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I = Io,

for 10[ -< ~i,

I = 0,

J0J > ctx.

(2)

The transmission of a perfect collimator can be readily calculated from geometric considerations, and is:

where ~¢ = microchannel diameter/length.

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(1)

where Tpc is the transmission of a perfect collimator having tubules of circular cross section with perfectly absorbing walls, and I(0) is the instrument and background function, approximated here as a rectangular distribution, for which:

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ELECTRON COLLIMATORS

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Fig. 4. Comparison of 0.30 m m thick MCP collimator for 10 keV and 5 keV incident electron beams.

90

A.J. LIEBER et al. was defocused to illuminate the entire plate from its usual diameter of approximately 1 m m 2. Significantly, the area that had been under study transmitted less electron intensity than the surrounding area which had not been exposed to the focused beam. We believe the accumulated charge in the long tubules that were subject to the focused beam electrostatically shut off these channels. The tubules recovered with a time constant indicative of charge bleed off. Although we had hoped to obtain MCPs which were plated through with conductor, other evidence indicates that during the MCP manufacture the coatings were not applied properly. In fact, even on the 0.30 m m MCP the front and rear surfaces were not electrically connected. Therefore, these tests in reality represent a lower performance limit. We expect even better collimation with chopped electron beams, which will more nearly simulate tube operation.

Fig. 5 compares experimental results and the "broadened perfect collimator" model discussed above for the 0.30 m m MCP, while fig. 6 makes the same comparison for the 0.84 m m MCP. The figures show good agreement between experiment and theory. The fit between peak maximum and 5-10 % of peak height is especially important for the streak-tube application. The model's inability to fit the "wings" of the experimental curve does not affect this application, because the signal here would likely be below detection threshold. Quantitatively, there is some difference between the best-fit parameters and a geometric collimator model. In both the 0.30 m m and the 0.84 m m case we cannot fit the data using the purely geometric aspect ratio and the measured electron-beam divergence. We hypothesize that the excessive broadening in the angular acceptance is due to charge buildup on the tubule walls which electrostatically deflects some electrons. Other possible broadening mechanisms include wall penetration and X-ray generation. The former is dubious, because the range of a 10 keV electron is less than 1 pm in the wall material, but the latter represents a distinct possibility, since it may also contribute to the "wings" of the curves. However, it should be noted that X-ray generation would not seriously affect the streak-tube operation. Wall charge accumulation is evidenced in several ways, but its effect on streak-tube operation may be considerably less than that measured with dc beams. A particularly graphic demonstration of the effect was made following a data run with the 0.84 m m MCP which had lasted several minutes. The electron beam ill

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4. Application

As noted above, this investigation was undertaken to determine if a MCP has suitable properties for application as a velocity selection system in a proximityfocused streak tube. This tube is under design study as a possible means of overcoming shortcomings of present ultra-fast streak-camera systems. Virtually all present fast streak cameras are based upon the RCA type C-73435 shutter intensifier. This intensifier was designed over a score of years ago and originally intended as a fast-rastering camera rather than for faststreak usage. Improvements to this tube in this country and Europe have been rather heroic efforts to overcome the tube's inherent weaknesses, but the resultant )

THEORETICAt

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EXPERIMENTAt POINTS FOR i 0.30 Ram.THICK MC P _ (/:= 5 mr

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Fig. 6. Theoretical

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curve

-2 0 2 M ILLIRADIAN$

fit to

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experimental points

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for 0.84 mm

thick MCP collimator data for 10 keV incident beam. Straight lines connect calculated points.

M I C R O - C H A N N E L PLATES AS P A R A L L E L - B O R E ELECTRON C O L L I M A T O R S MCP

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COLLIMATOR DEFLECTION P LATES

PHOTOCATHODE

ALUMINIZED

FI BEROPTICS

n-

3cm

-- I

Fig. 7. Schematic of proximity-focused streak tube. The photocathode geometry shown is for the visible application.

system has been expensive and complex at bestS'6). Among the tube's weaknesses are: its sector-focused geometry requires low conductance to reduce spacecharge distortion and maintain resolution. In ultra-fast streak work the resultant loss in tube gain must be compensated for by the addition of an expensive image-intensifier follow-on unit. Since it has been known for some time that ultra-high extraction fields are necessary at the photocathode to minimize photoelectron velocity dispersion, a carefully engineered grid structure must be added to the nominally lowfield sector-focused geometry to establish this fieldV). The presence of this structure and field causes electronoptics problems in the tube. Finally, the overall length of the tube of approximately 22 cm allows the longitudinal photoelectron velocity dispersion to reflect as intolerable time dispersion in the X-ray application of the tube and hence limits resolution. Another type of intensifier, the proximity-focused intensifier, displays none of the shortcomings mentioned above. These tubes are based upon parallel-plategeometry and operate at electric fields approaching breakdown to map photoelectrons to the phosphor. Such tubes are capable of very high current conduction in the pulse mode while maintaining high spatial resolution 8). In reviewing the design criteria for a picosecond Xray streak camera and a sub-picosecond visible camera it is soon apparent that, beside tube resolution and cost, another figure of merit is sensitivity. This is especially important when dealing with non-laser applications of fast streak tubes. Any system to control longitudinal photoelectron velocity dispersion results

in an increase in overall tube length, which in turn requires a higher-velocity selection and loss in tube sensitivity. An alternative is to keep the tube short as in a proximity-focused system. Usage of a MCP as a transverse velocity selector limits this spread for the minimum additional length. MCPs are precise structures with very flat polished surfaces. Their usage therefore allows higher extraction fields at the photocathode than for grid structures. In fact this region of the tube can be pulsed to momentary overvoltage for the highest-possible extraction field. Fig. 7 shows schematically the proximity-focused streak tube currently under development and test. The tube follows the traditional geometry of the generationII proximity-focused intensifier. The dc data presented above actually are encouraging lower limits for the performance of an MCP in this geometry. If we assume r o u g h l y 10 4 electrons are necessary to record a streal~, these electrons are distributed over approximately 2000 MCP tubules lying along the slit. Thus the average of 5 electrons per tubule is hardly enough tO produce enough space charge to degrade seriously MCP collimator ability. However, as with any new device, many questions remain to be answered. Among these are how much scattering actually does take place in long-tubule MCPs, and how badly this degrades the emerging velocity spread of the electrons. Furthermore, MCP transmission reflects directly on tube sensitivity and must be measured under actual conditions. These and other salient questions must be answered through prototype evaluation. Although theoretically the limits on streak resolution for visible and X-ray applications

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A . J . LIEBER et al.

are very encouraging, actual resolution o f the tube remains to be d e m o n s t r a t e d . W e feel t h a t the simplicity o f the device justifies c o n t i n u e d investigation, even if figures o f merit p r o v e to be equivalent to present systems. The a u t h o r s wish gratefully to a c k n o w l e d g e the efforts o f Steven G i t o m e r a n d Stanley H a l l on c o m p u t e r modelling o f the p r o x i m i t y - f o c u s e d streak tube. W e w o u l d like to t h a n k R o b e r t S m i t h for the scanningelectron-microscope p h o t o g r a p h s . W e w o u l d like especially to t h a n k G e n e M c C a l l for his c o n t i n u e d enthusiastic s u p p o r t o f the project a n d his m a n y helpful suggestions. Finally, the s u p p o r t o f the L-4 staff is also gratefully a c k n o w l e d g e d .

References 1) A. Lieber, R. Benjamin, P. Lyons and C. Webb, Nucl. Instr. and Meth. 125 (1975) 553. 2) D. Ruggieri, L. S. E. Division Varian Associates, Palo Alto, California (September 24, 1974). 3) j. Cuny, Electro-optical Division, International Telephone and Telegraph Corp., Fort Wayne, Indiana (March 13, 1975). 4) AEC Patent Disclosure, Use of a channelplate electron multiplier assembly passively in a proximity focused streak tube as an electron collimator (August 2, 1974). 5) D. Bradley and G. New, IEEE 62 (1974) 3. 6) R. Engstrom and R. Fitts, SPIE 42 (August 1973). 7) E. Zavoiskii and S. Fanchenko, transl, from Dokl. Akad. Nauk. SSSR 108 (May-June 1956) 218-221. 8) A. Lieber, Rev. Sci. Instr. 43 (1972) 1.