NASA Technical Reports Server (NTRS) 19970014147: Performance of Small Pore Microchannel Plates

10 Pages·0.65 MB·English

by  NASA Technical Reports Server (NTRS)

NASA-CR-203232
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Performance of small pore microchannel plates
O. H. W. Siegmund, M.A. Gummin,
T. Ravinett, S.R. Jelinsky, and M. Edgar
Space Sciences Laboratory, University of California, Berkeley, CA 94720
1.0 ABSTRACT
Small pore size microchannel plates (MCP's) are needed to satisfy the requirements for future
high resolution small and large format detectors for astronomy. MCP's with pore sizes in the rang_e 5gm
to 8gm are now being manufactured, but they are of limited availability and are of small size. We have
obtained sets of Galileo 8gm and 6.51.tin MCP's, and Philips 6gin and 7gm pore MCP's, and compared
them to our larger pore MCP Z stacks. We have tested back to back MCP stacks of four of these MCP's
and achieved gains >2x107 with pulse height distributions of <40% FWHM, and background rates of
<0.3 events sec -1 cm -2. Local counting rates up to =100 events/pore/sec have been attained with little
drop of the MCP gain. The bare MCP quantum efficiencies are somewhat lower than those expected,
however. Flat field images are characterized by an absence of MCP fixed pattern noise.
Keywords: microchannel plate, UV, spectroscopy
2.0 SMALL PORE MICROCHANNEL PLATES
High spatial resolution (<15gm) is essential for future high resolution imaging and spectroscopy
applications (FUSE)1. Currently, the size of the MCP pores is a significant limiting issue. Our current
delay line readouts have res,olutions of <,!5gm FWHM and <lgm electronic binning (Fig 1), and we can
observe 2 the pore position quantization effects for MCP s with the standard pore sizes of 10gin (12gm
center-center) or 12.5gm (15gin center-center).
A number of small pore (6gm to 8gm) MCP's were obtained from two manufacturers, and these are
described in Table I. In each case, except for the 8gm MCP's, the MCP's were used in a back to back
stack of 4 MCP's. This was done to ensure high saturated gain and tight pulse height distribution
necessary for high resolution imaging detector systems. The 81.tm MCP's were used in a Z stack
configuration.
Table I. Small 9ore MCP specifications
Pore size MCP size L:D ratio "Resistance Bias angle Manutacturer
6gm 25mm 55:1 =35M_ o Philips
33ram 50:1 =60M£2 12° Galileo
6.5gm
25mm 50:1 =100M_ o Philips
78[tttmm 25mm 80:1 =90Mr2 12° Galileo
The MCP's were mounted in a brazed metal-ceramic detector body with a circumferential spring
clamping arrangement for the MCP stack. The detector scheme is functionally similar to other devices
that we have implemented in various space programs. The incoming UV photons are detected by the
MCP channel walls, ejecting photoelectrons which impact the walls of MCP pores causing a charge
avalanche with an overall charge multiplication of =>107. The field of view (18mm for the 25ram
MCP's, and 25mm for the 33mm MCP's) is defined by an output aperture mask placed under the MCP
stack. The brazed detector body is attached to a vacuum flange, that also has electrical feedthroughs to
which the wedge and strip readout anode is mounted with a =10mm MCP to anode gap.
0-8794-2 T96-0/96/$6.00
98 15PIE Vol. 2808
60
50
40
O,o tt .S/^ /t -i-
1 50 300 350
Position (gm)
Figure 1. Histograms of three 10gm pinhole images taken with a95mm x 15mm helical
double delay line anode using aZstack of 10gm pore (12gin centers) MCFs
.... _.÷
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o
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Figure 2. Optical image of aGalileo 81.tinpore Figure 3. Optical image of aPhilips 7gm pore
MCP with a 1952 air force test mask incontact MCP with a 1952 air force test mask incontact
ii!_
Figure4.Optical image ofaGalileo 6.511mpore Figure 5. Optical image of aPhilps 6gm pore
MCP witha 1952air forcetestmask incontact MCP with a 1952 air force test mask in contact
SPIEVol. 2808 / 99
To evaluate the limiting resolution of small pores we have also microscopically examined the small
pore MCP's with a visible light back illumination through an Air Force resolution test mask (Figs. 2 to 5).
As the size of the MCP pores reduces, the finer a resolution pattern can be resolved. 81.tin MCP's can just
attem but the 6 m MCP's can resolve the 40 lp/mm pattern. Small pore MCP's
resolve the 28 lp/mm p , _t........... :........ t_ nrovidin_ better resolution
• 11 su oft a Wlae range oi iutu,_ L_,au,,-, ..... , r . --. •
wdl ho efully eventua, y . pP.................. ;_,__,_ "tical auuhcatlons such as high
perfoar_°_ce and reducing the ertects ot Mr.;t" nxea p_ttt_tt, ,_,,o,, ,,Jr cn ,-,-
resolution spectroscopy.
3.0 MICROCHANNEL PLATE PERFORMANCE
3.1 Microchannel plate gain, pulse height distribution
The small pore 4 MCP stacks were generally tested under conditions where the entire field of view
uniformly illuminated with 2537A light (Hg vapor lamp). The results of these tests show variations
was rmance from stackup to stackup, and often these differences have been due to gaps between the
in peso MCP's a number of
MCP s caused by individual MCP warpage. We have in some cases restacked the
times to ensure that there are no gain variations across the field of view. The pulse height spectra are
gaussian in shape, demonstrating gain saturation. The gain and pulse height results for the Philips 71.tin
MCP's are shown in Figures 6 and 7. Gain of >2 x 107 is achievable, which is not surprising given a four
MCP stack. The pulse height distributions (PHD's) were quite good (--40% FWHM), as indicated by the
narrow distributions in Figures 6 and 7. The 4 MCP stacks of low L/D MCP's have fairly tight PHD's
even at low gains, compared with more commonly used Z stacks of 80:1 L/D MCP's. This is an
advantage for the discrimination of background events and reduction of the dynamic range necessary for
imaging electronics. Using a beam of 256A radiation covering about 15% of the field of view, PHD's of
=33% FWHM were obtained.
The Galileo 6.51xm 4 MCP stack was more difficult to stack to obtain a tight PHI). The gain is not
fully saturated at 2 x 107, and gains in excess of 3 x 107 were reached (Fig. 8) with pulse height
distributions of =85% FWHM (Fig. 9). There was always a slightly higher gain in the center of the field
of view, indicating slight MCP warpage. With all the small pore MCP stacks this is a potential problem
since the solid edges are slightly thicker than the active pore area. The Philips 61xm pore MCP stack did
not perform quite as well as the 7ttm MCP's. The gains reached were again very high (>3 x 107) (Fig.
10), and the PI-ID's were always between 55% and 50% FWHM (Fig. 11).
3.2 Background rate
The background rate and spatial distribution was assessed for each MCP type• Since the MCP field
of view was open to the vacuum tank for all these tests there is a slight contribution due to stray ions in
the tank. This is visible as a small "bump" in the background PHI) on the background vs threshold plots•
The spatial distribution for a 2200 sec integration of background events for the 71.tm pore Philips MCP
stack is shown in Figure 12. As expected from 40K [3decay in the glass, the distribution is essentially
uniform. We observe a negative exponential pulse height distribution for background events with <0.35
events cm "2 sec "1 for a threshold of =10% of the modal gain (Fig. 13). The overall background ra_'e is
=0.23 events cm -2 sec" 1 when the PHD peak due to tank ions is subtracted. This is lower then the rate
(=0.5 events cm -2 sec -1) for intrinsic _ decay MCP noise 3'4 for 40K in the Philips glass in Z stack 80:1
MCP configurations. Since the 71J.m MCP's are much thinner, there is less glass and hence less _ decay
MCP noise contributing to the intrinsic background rate. The 4 MCP 6_tm Philips stack has a similar
background rate behavior (Fig. 14). Again, if the tank noise peak is removed we get a rate of =0.27
-2 -1 which is similar to the 7_tm MCP performance. Thus one advantage of smaller pore
events cm sec • er ore MCP s The
P's is a reduced background rate at the same gain level, when comp_ed to l.arg .p ..... _,.,
MC .... , ..... J .... ,,¢ _ _a ,'vents cm -/" see "_ for a tlaresnola oi =z_v/o. me
Galileo 6..51xm MtSF S ha(1 a oacK£rounta ,[at_ ,,- -v .... hot sots for these
modal gain (Fig. 15). The Galileo_MCP s had a higher incidence of low amplitude P
IO015PIE Vol. 2808
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Figure 7. Pulse height & gain vs V for a4 MCP
Figure 6. Pulse height distributions for a4 MCP
stack of Philips 7_tm pore 50:1 IJD MCP's.
stack of philips 7ku'npore50: I L/D MCP's.
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Fi_mare 9. Pulse height & gain vs V for a 4MCP
Figure 8. Pulse height distributions for a4MCP
stack of Galileo 6.51xm pore 50:1 L/D MCP's.
stack of Galileo 6.5_tm pore 50:1 IJD MCP's.
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Figure 11. Pulse height & gain vs V for a 4 MCP
Figure 10. Pulse height distributions for a4 MCP
stack of Philips 6_tm pore 55:1 L/D MCP's.
stack of Philips 6_m pore 55:1 L/D MCP's.
SPIE Vol. 2808 / 107
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articular MCP's, which increased the rate at small thresholds. The overall rate of =0.53 events cm -2 sec-
is about a factor of two higher than the standard Philips glass because 87Rb is present rather than 40K.
The background rates for standard Philips (12l.tm) and Galileo (10[tm) 80:1 Z stacks is shown in Fig.16.
These are compared with an early batch of low noise Galileo 101.tm MCP's with and without lead
shielding. A recent result with new Galileo low noise glass is also included. These are 95 x 20mm curved
surface MCP's in a Z stack of 3 x 80:1 MCP's for the FUSE explorer satellite. This result is very
encouraging, with rates of <0.04 events em -2 see "1 at =20% modal gain threshold (without lead
shielding). If small pore MCP's can be made from this material we expect very low intrinsic background
rates (<0.02 events cm -2 sec-1).
3.3 Counting rate performance
At high local event rates there is a progressive drop in the MCP stack gain and a degradation of the
pulse amplitude distribution 5,6,7 due to the finite response time of MCP channel(s). The counting rate
limit is proportional to the resistance of the MCP's, and is also dependent on the area illuminated. Only
the Philips 6ktm MCP's were low in resistance, so these were chosen for rate dependence tests. The 6l.tm
MCP's have resistance in the rang8e 170MDJcm 2 (=35M£2 per MCP), which is slightly less than the
MCP's used for the SOHO satellite .Tests were made to evaluate the response to high local counting rate
conditions such as those in some astronomical spectroscopy and imaging experiments. The high local
7
counting rates on the MCP were generated using a variable flux from a pen ray lamp to illuminate a
pinhole mask with 101xm 25]xm, 50l.tm and 751.tin pinholes. The gain as a function of local counting rate
for the Philips 6_tm 4 MCP stack is shown in Figure 17. The results show that the MCP gain remains
stable up to =100 events pixel "1 sec-lfor the 10t.tm and 25ttm pinholes. So for this size input function the
high local rates cause no loss of events or resolution degradation. For the 50_tm holes the gain drop is
=30% at =100 events pore "1 sec "1, and the gain drop is =70% at =100 events pore -1 sec-lfor the 75ktm
pinholes. The PHD behavior (Fig. 18) for the 10[tm and 25_m pinholes is essentially independent of the
counting rate, with only a hint of broadening at =100 events pore -1 see -1. Both the 501.tm holes and the
75ktm pinholes show significant PHD broadening at the higher counting rates. The 751.tm pinhole PHD's
begin to broaden at even a few counts pore-1 sec-1. These effects are essentially in accord with earlier
observations of the dependence on the illuminated area 7. One must also appreciate that a large number of
pores are affected at the output MCP of the stack, and it is here where the charge extraction is largest and
limitation occurs. It is also important to note that, for example, the counting rate in the 251.tm pinhole at
100 events pore -1 sec -1 is over 1000 events sec"1, which is a significantly high rate.
3.4 Quantum detection efficiency
The quantum detection efficiency (QDE) of the bare 6wn, 6.51.tm, 7_tm and 8_tm MCP's were
measured as part of the overall characterization. The QDE vs wavelength properties were measured in an
ultra high vacuum chamber at =10 -6 torr with the MCP detector mounted such that the incident angle
was =13 ° to the MCP pore axis. Monochromatic radiation was provided by a gas discharge hollow
cathode source in combination with a lm grazing incidence monochromator. After corrections are made
for the contribution of the background radiation the absolute QDE's are derived from flux measurement
comparisons with a reference standard. The choice of reference standard depends on wavelength
measured; a NIST ultraviolet windowless photodiode, or a NIST far ultraviolet windowed photodiode
were employed. The accuracy of the absolute QDE measured is determined largely by the accuracy of
the reference standards. The NIST EUV photodiode has a most probable error of 10% from 68A to
1216A, and the NIST far ultraviolet photodiode has a most probable error of 10% from 1216A, to 2537A,.
The results presented in Figure 19 show that the efficiencies for the Philips MCI:'s are slightly lower
than expected compared to other bare MCP data9 which we consider more "normal". The same kind of
problem was also apparent (Fig. 20), although less severe, for the Galileo MCI>s. The open area ratio of
5PIE Vol. 2808 / 103
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Rate (/pore/sec) Rate(/pore/sec)
Figure 17.Gain vscounting rate & spot area Figure 18.Gain vs counting rate & spot area
for a 4 MCP stack of Philips 6_tm pore MCP's for a 4 MCP stack of Philips 61.tm pore MCP's
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Figure 20. QDE of 6.5gin and 8gin pore Galileo
Figure 19. QDE of 6_tm and 7_tm pore Philips
MCP's as a function of wavelength. MCP's asafunction ofwavelength.
104 /5PIE Vol. 2808
Figure 21. Fiat field image for a4 MCP stack Figure 22. Flat field image for a4 MCP stack
of Philips 6_m pore MCP's. of Philips 7_m poreMCP's.
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Figure 23. Flat field image histogram slice for a Figure 24. Flat field image histogram slice for a
4 MCP stack of Philips 6_tmpore MCFs. 4 MCP stack of Philips 71.tmpore MCP's.
1600 1200 ,. ........ _......... , .........
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Figure 25. Flat field image statistics for a Figure 26. Flat field image statistics for a
4 MCP stack of Philips 61ampore MCP's. 4 MCP stack of Philips 7_tm pore MCP's.
SPIE Vol. 2806 / 105
the small pore MCFs is 60% to 65% for all the MCFs, compared with =65% for normal 10l.tm and
12.5txm pore MCP's. Therefore there are no obvious changes in the open area ratio and this does not
seem to be an issue. Over the past few years we have noticed on a number of occasions that the QDE of
some batches of MCP's that the QDE was significantly below that expected. After a considerable
amount of study we have established that the actual QDE of the photoemitting surfaces is probably not
degraded. We surmise that problem is possibly due to inefficient detection of the emitted photoelectrons
by the MCP (electron detection probability in Ref 10). We are currently attempting to resolve this
problem with the manufacturers and will report on this at a future date.
3.5 Flat field characteristics
Deep flat field images (>1000 events/pixel) for the 6l.tm and 7gm MCP stacks were obtained under
illumination with 2537A light (Hg vapor lamp). The contrast enhanced flat field response of the 61.tinand
7_tm MCP stacks are shown in Figures 21 and 22 respectively. Flat field images were acquired at rates
of ---10,000 events/see, and binned into a 128x128, 125btm bin format. The only visible flat field
modulation effects are the edge intensity enhancements due to reflections of light by the detector clamps.
Typically the dominating modulation seen in flat field images is that of the MCP multifibers. MCP
modulation at the multifiber boundaries is usually of the order 10%-15%11. It is unusual that these
MCP's do not show the MCP multifiber structure. Earlier results have linked the MCP multifiber
modulation to physical distortions of the MCP pores at the boundaries of the multifibers 11
Histograms of the intensity maps for the images in Figures 21 and 22 are shown in Figs. 23 and 24
respectively. The intensity variations in the central areas are small and have no scale length pattern
correlations. The statistical variations of the flat field intensity in the central 5mm of the images are
shown in Figures 25 and 26. The expected variations are 6.5% and 6.9% FWHM for the 6l.tm and 71am
flat fields respectively. The measured values are very close to those expected, which also confirms that
there is little multifiber modulation.
4.0 ACKNOWLEDGMENTS
Many grateful thanks are extended to the many unmentioned people who contributed to this effort at
U.C. Berkeley, this work was supported by NASA grants NAGW-2640 and NAGW-1290.
5.0 REFERENCES
1. Sahnow, D. C. Bowers, O.H.W. Siegmund, J. Stock, and M.A. Gummin, Proc. SPIE, 1945, 390-397,
(1993)
2. O.H.W. Siegmund, M.A. Gummin, J. Stock, D. Marsh, R. Raffanti, and J. Hull, Proc. SPIE, 2006,
176-187 (1993).
3. Siegmund, O.H.W., Vallerga, J., and Wargelin, B., IEEE Trans. Nucl. Sci., NS-35,524 (1988).
4. Fraser, G.W., Pearson, J.F., and Lees, J.E., Nucl. Instrum. & Meth., A254, 447 (1987).
5. Slater, D.C., and J.G. Timothy, Proc. SPIE, 1549, 68 (1991).
6. Siegmund. O.H.W., and J.Stock, Proc. SPIE, 1549, 81-89 (1991).
7. Fraser, G. W., Pain, M.T., Lees, J. E., Pearson, J. F., Nucl. Instrum. & Meth. A, preprint (1991). _"
8. Siegmund, O. H. W. M. A. Gummin, T. Sasseen, P. Jelinsky, G. A. Gaines, J. Hull,. M. Stock, M.
Edgar, B. Welsh, S. Jelinsky, and J. Vallerga, Proc. SPIE, 2518, 344-355, (1995)
9. Siegmund, O.H.W., K. Coburn, and R.F. Malina, IEEE Trans. Nucl. Sci. NS-32, 443-447 (1985).
10. Siegmund, O.H.W.E. Everman, J. Vallerga, J. Sokolowski, and M. Lampton, Applied Optics, 26(17),
3607 - 3614 (1987).
11. Vallerga, J.V., Siegmund, O.H.W., Vedder, P.W., Gibson, J., Nucl. Instrum. & Meth. A310, 317-322
(1991).
106 / 5PIEVol. 2808