The stability of CsI-coated microchannel plate X-ray detectors

Nuclear Instruments and Methods in Physics Research 224 (1984) 287-297 North-Holland, Amsterdam

THE STABILITY OF CsI-COATED MICROCHANNEL

287

PLATE X-RAY DETECTORS

M.J. W H I T E L E Y , J.F. P E A R S O N , G . W . F R A S E R a n d M.A. B A R S T O W x- ray Astronomy Group, Department of Physics, University of Leicester, Leicester LEI 7RH, England

Received 16 January 1984

Coating with caesium-iodide has been shown to increase the soft X-ray quantum detection efficiency of microchannel plate (MCP) electron multipliers. Measurements are now presented of the stability and reproducibility of such Csl deposition photocathodes. The effects of limited exposure to humid air and of storage in poor vacuum, high vacuum and dessicated air are described. The stability of Csl coatings under prolonged X-ray bombardment is discussed and the mechanical strength of thick (up to 14000 ,~) CsI layers briefly assessed.

1. Introduction The soft X-ray quantum detection efficiency of microchannel plate (MCP) electron multipliers may be very greatly increased by the deposition of a CsI photocathode at the MCP input [1,2]. CsI-coating may also confer a limited degree of intrinsic X-ray energy resolution if the MCP detector bias voltages are carefully chosen [2]. Such developments would appear to have importapt applications in imaging X-ray astronomy where MCPs are commonly used as focal plane detectors of high spatial resolution. For caesium-iodide coated MCPs to become acceptable, however, in satellite-borne X-ray astronomy or in related fields, the stability and reproducibility of their X-ray response must first be quantified. In this paper we examine the changes in the X-ray quantum detection efficiency, Q, of coated MCPs resulting from (1) deliberate exposures to laboratory air, (2) long-term storage in poor vacuum, high vacuum and dry air or dry nitrogen and (3) prolonged X-ray bombardment. The reproducibility of our CsI coating technique is indicated by comparison of efficiency data from six coated MCPs. Some studies of photocathode surface structure have been made using electron microscopy. We have also investigated the mechanical strength of our MCP photocathodes by subjecting CsI layers of equivalent thickness deposited on glass blanks to sinusoidal vibration. CsI is an optimum photocathode material for X-ray diode and streak camera applications [3,4]. Our results may therefore be of interest in these and other areas where X-ray photoemission is of technological importance.

0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2. MCP coating: substrate preparation, plate handling and reproducibility Our techniques for coating microchannel plates with CsI have been briefly described in a previous paper [1]. The deposition process is now described in rather more detail. The initial stage in plate processing consists of a vacuum bake at temperatures of 275-300 o C for a period of - 48 h. Such temperatures are fairly typical of those used industrially [5] and in laboratory experiments [6] to prepare sealed microchannel plate detectors; the duration of our bakeout is, however, rather greater than the 8-10 h typical of image tube processing. New microchannel plates copiously outgas water vapour. Bakeout of our MCPs was intended to reduce the water content of the glass in preparation for the deposition of CsI, a hygroscopic material. It appears that the channel electron multipliers (CEMs) used in the early photocathode survey of Smith and Pounds [7] were not prebaked; the quantum detection efficiency of their CsI-coated CEM fell rapidly over a period of several days storage [8]. We may speculate that poisoning of the deposition photocathode by outgassing of its substrate may have been partially responsible for this degradation. After cooling, the vacuum furnace is flushed with dry nitrogen and the MCPs removed, The plates are then transferred, sealed in dry N 2 filled polythene bags, to the coating rig for evaporation of the CsI photocathode. Optimisation of the coating geometry for particular applications has been described in a recent report [9]. Prior to evaporation the MCP is heated to a temperature of 100 o C. During evaporation it is slowly rotated to ensure uniformity of coating within individual chan-

288

M.J. Whitel£~' et al. / Stability of microchannel plate detectors

nels. Evaporation takes place at a belljar pressure of 10 -5 mbar. Evaporation rates are kept low ( < 50 A . s ~) in order to eliminate cracks from the deposited layers [11]. Substrate heating was adopted in an attempt to improve the durability of our CsI layers. It has been noted that in order to produce highly efficient thin film CsI(Na) and CsI(T1) scintillators heating the substrate to a temperature in the range 100-300 ° C is necessary [10,11]. More directly related to our application is the finding that substrate heating (160-200°C) increases the secondary electron yield of CsI layers and improves its stability under prolonged electron bombardment [12]. A very limited series of tests was carried out with CsI films deposited on pre-baked glass blanks in order to investigate the effects of substrate heating. Two sets of Csl layers, one set coated onto glass at 100 ° C, the other coated at room temperature, were examined for changes in visual appearance over a period of four months storage in dessicated air. Originally, almost all films were clear and blemish-free, with only the thickest (12000 and 14000 A) of those films coated "cold" exhibiting a slight opacity. Any differences in appearance between films coated "cold" and "hot", however, disappeared with time. All films had stabilised by the end of the test period, presenting a standard cloudy appearance, irrespective of coating temperature or thickness in the range 6000-14000 A. Presumably this opacity is due to superficial absorption of residual water vapour from within the dessicator vessel [13].

Scanning electron microscope studies failed to reveal any major differences in surface structure between films coated "hot" and "cold". A typical electron microscope image is shown in fig. 1. The amorphous pattern of this CsI layer, coated onto a glass substrate at 100°C, is very similar to that of a layer deposited by Antoniv et al. [12] at room temperature and shows none of the linear features observed by those authors after deposition at 200°C. It may be that a substrate temperature of 100 °C is too low to initiate the beneficial structural changes reported in ref. [12]. It is not practical to raise the MCP temperature in our current apparatus because of the proximity of the rotational drive to the MCP holder. Although these simple tests failed to find any clear advantage, mechanically or in terms of storage characteristics, a degree of substrate heating will be retained in future experiments, if only to improve MCP cleanliness at the time of Csl evaporation. Fig. 2 indicates the reproducibility of our coating technique. The 0.28 keV quantum detection efficiency of six CsI-coated MCPs, each operated as the front plate in a two-stage multiplier arrangement, is shown as a function of X-ray incidence angle to the channel axes. The data refers to freshly prepared photocathode surfaces. Table 1 lists the geometric (channel diameter D, length-to-diameter ratio L / D and channel bias angle #B), electrostatic (operating voltage V0 and mean electron collision energy eVc [9]) and photocathode parameters (coating angle &0 and thickness of CsI on the MCP

Fig. 1. Scanning electron microscope image of a 4000 ,k thick Csl film (overcoated with 100 ,~ aluminium). Magnification 5000 ×. Characteristic scale of surface roughness ~ 0.5 ~m. Csl layer deposited on glass substrate at a temperature of 100 o C.

289

M.J. Whiteley et al. / Stability of microchannel plate detectors Table 1 CsI-coaled microchann¢i plate a) characteristics MCP number

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Ltd., 4 New Road, Mitcham, Surrey CR4 4XY, England.

front surface, t F [9]) for all six test plates. According to our model [9] those MCPs with the thickest CsI coating on the channel walls (.corresponding to the combination &0 = 15 o, tF = 6000 A) and highest values of collision energy should exhibit the highest efficiencies at large angles of X-ray incidence. This is confirmed by the superiority of MCPs number 4 and 5 (see fig. 2 and table 1). Most importantly, however, the efficiencies of all six coated plates lie well above the equivalent curves for uncoated MCPs.

3. Effects of exposure to laboratory air

It was considered essential to quantify the effects of normal laboratory air on our CsI-coated microchannel o6

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plates. Vacuum breaks are inevitable in the development of windowless microchannel plate X-ray detectors. Even if detector assembly and modification are carried out in enclosures flushed with dry N 2 (as is our standard practice) contact of normally humid air with the hygroscopic photocathode surface cannot be totally excluded. A detector incorporating MCP number 4 of table 1 was subjected to a series of deliberate exposures to laboratory air [1]. The test vacuum system was repeatedly opened to the atmosphere, with the microchannel plate detector continuously in place, until the accumulated exposure time had reached eight hours. Fig. 3a shows the variation in open area quantum detection efficiency, measured at three different angles of X-ray incidence, with exposure time. These measurements were made on the CsI-coated half of MCP number 4. Within measurement error no overall decrease in efficiency was observed. Fig. 3b shows the corresponding variation of detector peak gain Gc and pulse height distribution fwhm value AGe~Go.There is a slight tendency for the peak gain to rise and the width of the output distribution to fall during the first half-hour of air exposures. All these air exposures were made while the relative humidity (RH) was in the range 44-56% and while the laboratory temperature was between 22 and 25 o C. It is interesting to compare our results with previously published accounts of the stability of CsI in air. According to Scott [14], CsI photocathodes are indefinitely stable in dry air but are ruined by exposures to air of RH >_ 50%. Premaratne et al. [3] have reported that X¢, the X-ray photocurrent, is unaffected by 30 min exposure to air of RH = 63%. Three hours under the same conditions, however, resulted in a significant decrease in Xc. On longer timescales, Saloman et al. [4] have reported X~ values unchanged after 18 h in "humid" air (RH unspecified) while Verma [15] has reported a halving of the CsI secondary electron yield coefficient after 24 h exposure to air of RH = 60%, temperature 27 °C. If any consensus view emerges from examination of all the data now available, it is surely that the sensitivity of CsI X-ray photocathodes and dynodes will not degrade as a result of the kind of brief contact with

M.J. Whitel£v et al. / Stabili O, of mlcrochannel plate detectors

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laboratory air encountered in the detector developments envisaged here, provided very high humidities are excluded. Fig. 3c shows the variation in detector noise count rate N. with accumulated air exposure time. The test

chamber used for these measurements incorporated a perspex viewing port of diameter - 20 cm. Noise counts were recorded both with this port covered by an opaque screen and with it open to the light. Fig. 3c shows that the noise count in both measurement configurations is steady at around 4 counts - s - 1 except for one measurement, made with the viewing port uncovered after the first 5 min long air exposure. That this increased noise count rate was associated with the CsI-coated half of the test M C P is clearly shown in fig. 4a. A noise image recorded on a storage oscilloscope after the first exposure and with the port uncovered reveals a sharp boundary between the CsI-coated (left hand side) and bare plate surfaces. There is no such differential brightening on any other noise image recorded during the

M.J. Whiteley et al. / Stability of rnicrochannel plate detectors sequence of air exposures. Fig. 4b was obtained immediately after fig. 4a, but with the perspex window covered over, i.e. with the detector in the dark. The most straightforward interpretation of this noise data is that the initial 5 min air exposure "sensitised" the CsI photocathode to visible blue light and that further exposures "killed" this sensitivity. The quantum detection efficiency of freshly-coated MCPs is known to be a steeply falling function of wavelength longward of 1800 ~,, with a value of 0.01% at a wavelength of 2000 ,~ [16]. Light related noise enhancements have been intermittently observed on other of our CsI-coated MCPs. A systematic study of this effect remains to be performed. We must further note that differentially brightened noise images have been observed with half-coated MCPs operated in a second vacuum chamber in which the available light paths to the detector are extremely tortuous. The observations are, perhaps, indicative of a second effect - the greater sensitivity to positive ions of the CsI coating (see section 4.3).

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4.1. Effects of storage in poor vacuum After initial testing, MCP number 3 was removed from its detector body and placed in an aluminium pressure vessel. This vessel was rough-pumped to a pressure of 10 -2 mbar and sealed. After 30 d storage the MCP was removed and the X-ray detector reassembled in its original configuration. A limited set of measurements with A1 K X-rays showed that the detection efficiency at large angles of X-ray incidence had fallen significantly (see fig. 5a). These measurements were curtailed by problems with the vacuum test chamber and with the detector's readout electronics. It was decided, therefore, to return the coated MCP to storage. A full set of efficiency measurements were taken after this second period of storage under rough vacuum, which lasted 10 d. As shown in figs. 5a,b, MCP open area efficiencies at large angles of X-ray incidence had then fallen to around one-half their original values. The convergence of the efficiency curves at small 0 values (0 < ~0, the central coating angle [9]) clearly shows that a degradation in the CsI coating is responsible for the observed fall in efficiency. For O << ~0 the response of the MCP lead glass largely determines the detector sensitivity. Monitoring of the pressure of the empty aluminium vessel showed an increase in pressure to around 3 mbar one week after sealing. It appears that outgassing from the (unbaked) walls of the storage vessel provided a more severe test of the CsI photocathode than had originally been intended. Water vapour may well have been a major component of the storage atmosphere.

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Fig. 5. (a) Effects of storage under rough vacuum on MCP open area quantum detection efficiency Qc (0, 8.3 A). Filled circles - efficiencies before storage. Squares - efficiencies after the first, 30 d, period of storage. Open circles - efficiencies after a second, 10 d, period (accumulated storage time 40 d). The broken vertical line indicates the coating angle g0 = 15 o. (b) As fig. 5a, except that measurements were made with C K X-rays. 4. 2. Effects of storage in a dry atmosphere After completion of the air exposure tests described in section 3, the (largely unaffected) MCP number 4 of table 1 was used to study the long term effects of

292

MmJ. Whitelev et aim / Stability of microchannel plate detectors

storage in a dry atmosphere - dry nitrogen/dessicated air - at normal temperature and pressure. After the air exposure tests, the half-coated MCP, together with the rear plate of the two-stage multiplier, was placed in a dry N 2 filled polythene bag and stored in a dessicator jar charged with silica gel capsules, where the inferred RH was - 10%. At intervals of a few weeks the MCPs were retrieved, reassembled into their detector body and retested. Later in the measurement sequence, the method of storage was changed somewhat. The complete microchannel plate assembly, including electrodes, plate mountings etc., was placed intact in a sealed container (similar to that of section 4.1) filled with dry N 2 at slightly greater than atmospheric pres-

sure. Initial efficiency and gain measurements were made on June 29th 1982. The complete measurement sequence for MCP number 4 extended from that date for 450 d. Fig. 6a shows the variation in C K X-ray detection efficiency with time, for three angles of X-ray incidence. The air exposure measurements of section 3 occupy the first 10 d of the plate history. By the end of the tests, with final efficiencies, within error, identical to initial efficiencies, the coated MCP had been in some form of dry storage for 402 d. Efficiency measurements made with AI K (1.49 keV) X-rays reveal a similar degree of explicit stability. We may also infer an added immunity to plate handling from this data. Detector assembly and

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Representative error bars shown. (b) Plate history of CsI-coated MCP number 4. Detector peak gain Gc (open circles; left-hand scale) and output pulse height distribution fwhm zaG~/G c (filled circles; right-hand scale) vs time.

M.J. Whiteley et al. / Stability of microchannelplate detectors disassembly took place more than a dozen times during the measurement sequence; with each such plate manipulation, the opportunity arose for exposure to humid air and mechanical damage to the photocathode surface. Our safe storage time of 400 d for CsI-coated MCPs even exceeds the nine months dry N 2 lifetime reported by Saloman et al. [4] for planar CsI photocathodes. Fig. 6b shows the variation in detector peak gain Gc and fwhm AGc/G c with time over the same test period. The peak gain increased initially. After the first long (67 d) period of dry storage Gc increased over two-fold. These increases are attributed to gaseous absorption onto the MCP channel walls [18]. Apart from the peculiar reversion to low gain exhibited on Day 155, high gains in the 3.5-5.0 pC range were the norm for the remainder of the test period. The large scatter in Gc values in the period from Day 200 to Day 450 may in part be due to small variations in the MCP output count rate from observation to observation. The resistance of the rear plate of the two stage-MCP multiplier was so high (1688 MI2) that relatively small changes in count rate (60-200 counts, s -1) were later observed to cause significant changes in peak gain (6.8-4.0 pC). We conclude that coated plate storage in a normal pressure dry atmosphere is acceptable on long timescales, provided a recalibration of the detector gain is permissible afterwards. At the end of the measurement sequence the CsI - coated half of MCP number 4 was observed to have a definite "cloudy" appearance, similar to that of the films deposited onto glass blanks and stored for long periods (section 2). Changes in the opacity of Csl photocathodes do not, therefore, automatically indicate changes in X-ray sensitivity. 4.3. Effects of prolonged X-ray bombardment in high vacuum MCP number 1 of table 1, half-coated with 14000 ,~ CsI, was used in a series of experiments, culminating in an investigation of MCP energy resolution [2]. After completion of these measurements, the detector incorporating MCP no. 1 (described in detail in table 3 of ref. [2]) was stored intact in the test vacuum chamber in a normal pressure atmosphere of dry N 2 for a period of two months. The detector was then reactivated for use in an X-ray lifetest, involving prolonged bombardment at high count rates under high vacuum. The aims of this test were (1) to discover the effects of X-ray bombardment and time under vacuum on the quantum detection efficiency of a CsI-coated plate and (2) to investigate changes in gain and noise count rate arising from the same causes. Three test positions were identified on the front surface of MCP no. 1. Position A, on the CsI-coated plate half, was to be illuminated continuously by C K X-rays (44.7 ,~) from an electron bombardment source

293

with a SiC-coated anode. Position B, also on the CsIcoated half, was to serve as a reference spot. Changes here in the efficiency of the CsI layer could be attributed to time spent under vacuum, since position B was to be illuminated only for brief (30 s) periods at intervals of several hours. A third X-ray beam position, position C, served as a reference spot on the uncoated half of the MCP. The areas of the MCP chosen had not been heavily bombarded prior to commencement of the lifetest. Each test spot was 6 mm in diameter. Initially, absolute efficiency measurements were made at all three test positions using an argon-methane filled single wire proportional counter which could be lowered into the X-ray beam [2]. These measurements, made at an X-ray angle of incidence of 15 0, are reported in table 2. After their completion the proportional counter was flushed with nitrogen. This was done in order to reduce the total pressure in the detector chamber due to leakage through the thin proportional counter window and, more importantly, to eliminate carbon contamination (in the form of methane) from the MCP operating environment. The X-ray lifetest of the MCPs was conducted at pressures within the stainless steel vacuum chamber [2] of 1-2 × 10 -6 mbar. The test was conducted with an output count rate from test spot A of 2100 + 400 counts, s -1 and continued until some 4 × 109 counts, cm -2 had been abstracted. Such a dose corresponds to an average count rate of - 1 0 0 cm - 2 . s -1 for one year, a flux and duration typical of satellite borne X-ray astronomy experiments. Bombardment of the test spot was not continuous. The source anode had to be periodically recoated in order to maintain the X-ray count rate. The design of the vacuum chamber, however, allowed the X-ray source to be modified without breaking the detector chamber vacuum. Measurements were therefore made over a period of 23 d with the MCP high voltages continuously applied in high vacuum. The relative efficiencies of test spots A, B, C were measured throughout by traversing the X-ray beam appropriately and recording MCP output count rates. The open area efficiency ratios: RI=

count rate at test position B count rate at test position A '

R2=

count rate at test position C count rate at test position A '

are plotted as functions of accumulated count at position A in figs. 7a,b. Both ratios change very little over the period of the lifetest. That this constancy can be interpreted in terms of the stability of the CsI coating under X-ray bombardment was confirmed by post-lifetest measurements of the absolute efficiencies at positions A and C (see table 2). Inspection of the "before" and "after" efficiencies measured at position B also

294

M.J. Whiteley et aL / Stability of microchannel plate detectors Fig. 8 shows the variation in M C P dark noise count rate, N,, over the period of the lifetest. The dark noise decreased by more than a factor two from beginning to end. Since the bombardment spot constituted such a small fraction of the detector active area (0.28 cm 2 in 7 cm 2) such changes cannot be attributed to 'burning in" of the detector but must rather be attributable to time spent by the detector under vacuum. Many prescriptions for low-noise operation of MCP detectors have been given in the literature [6,19]. Our data would appear to clarify somewhat the relative importance of "scrubbing" the MCPs with input radiation and simple outgassing in such noise reduction formulae. We note that no noise hotspot was ever observed at the position of intense X-ray bombardment in these tests and that at their conclusion there was no difference in noise count rate between CsI-coated and

Table 2 MCP open area quantum efficiencies Qc (15 o 44.7 A) at three test positions before and after X-ray bombardment of MCP number 1 Position

Efficiency (%)

A (CsI) B (CsI) C (bare MCP)

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28.0 + 1.4 26.7 _+1.3 8.1 + 0.4

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indicates stability of the CsI coating under high vacuum. Such stability, on timescales up to sixty days, has been demonstrated for planar CsI photocathodes by Premaratne et al. [3].

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M.J. Whiteley et al. / Stability of microchannel plate detectors

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296

M.J. Whiteley et aL / Stabilit_v of microchannel plate detectors

bare plate halves. Early in the history of MCP number 1 an excess of noise (of the second kind described in section 3) had been associated with the CsI-coated half of the plate. The noise count rates of fig. 8 refer to a charge acceptance window of 0.1-10 pC. Increasing the lower level discriminator to around 1 pC would result in noise levels of less than 0.2 counts, c m - 2. s- l without significantly affecting the registration of X-ray events. Fig. 9 shows the variation in detector peak gain over the period of the lifetest. The ordinate here is abstracted charge per unit area of the bombarded spot. The gain decreased uniformly for most of the test with a narrow plateau of stable gain extending between charge levels 3-5 × 10 -3 C .cm -2. The fwhm of the output pulse height distribution varied between 110% and 150%; these large values are attributable to the lack of an accelerating inter-plate voltage in the test detector configuration. The peak gain can be restored to its original levels by modest increases in MCP bias voltages. Gain decay can therefore be compensated for in any long-term satellite experiment by use of an adjustable high voltage power supply. A fuller analysis of gain decay (which is a property of the glass of the rear MCP of the detector pair and not of the MCP photocathode), together with the results of three further MCP lifetests conducted in our laboratory, will be given in a future report. The metallisation of CsI photocathodes under prolonged electron bombardment, involving the liberation of iodine and the production of an easily visible Cs rich layer, has been reported by several authors [12,15]. Visual inspection of MCP number 1 after completion of our lifetest revealed no changes in appearance which could be associated with the X-ray bombarded spot.

and over) in five 1.5 mm wide strips. Each strip was, of course, scanned before vibration. Each blank tested was subjected to four sinusoidal vibration sequences. The first three covered the frequency range 50-2500 Hz at vibration levels of 5, 15 and 21 g; the fourth consisted of 800-2500 Hz at 30 g. The CsI layers examined were those of 6000, 10000 and 14000 A thickness coated "hot" and "cold" (see section 3). After correcting for a background level of - 4 particles per strip examined (established by vibrating the glass blank holder empty) only about 3 particles per strip were attributable to each Csl film. No significant differences were noted between layers coated hot or cold, nor between 6000 A and 14000 A thicknesses. No large areas of any film broke free from its substrate.

6. Conclusions Our results indicate that CsI photocathodes can stably and reproducibly increase the soft X-ray quantum detection efficiency of microchannel plate electron multipliers. Of the processes to which we have subjected our coated MCPs, only storage under rough vacuum produced a serious degradation in sensitivity. Most of our storage and air exposure testing on coated MCPs was performed with the originally developed 6000 A front surface coatings [1]. We may, however, infer from our work with glass blanks that the later, thicker MCP coatings [2] will prove equally robust in terms of handling and storage. This work was financially supported by the SERC. The authors wish to thank G. McTurk for providing the electron microscope images.

5. Mechanical strength of Csl layers It is important that the photocathode layer deposited on the front surface of an MCP is mechanically stable. If normal handling, or, in the case of a satellite or sounding rocket experiment, launch stresses were able to dislodge fragments of the > 1/~m thick front surface layer noise "hot-spots" could conceivably arise from entry of these particles into the microchannels. At the end of the storage tests described in section 2, six CsI layers deposited on glass blanks were vibrated on a table used to qualify sounding rocket instrumentation and counts of dislodged particles made to assess their gross mechanical stability. The blanks were mounted inverted in a purpose built sealed jig which could be filled with dry nitrogen. Particles vibrated loose were collected on a glass plate coated with an oil film. For inspection the lower half of the jig was installed on an X - Y table and a zoom microscope employed to count particles (size - 5 ~m

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