Multichannel Image Detectors Volume 2 - ACS Publications

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Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 8, 2015 | http://pubs.acs.org Publication Date: November 16, 1983 | doi: 10.1021/bk-1983-0236.ch013

Detection of Extreme UV and Soft X-Rays with Microchannel Plates: A Review O S W A L D H . W. S I E G M U N D and R O G E R F. M A L I N A Space Sciences Laboratory, University of California, Berkeley, CA 94720

Microchannel plates are becoming widely used in spectroscopy and two dimensional imaging at E U V (100 - 1000 Å) and soft X-ray wavelengths (10 - 100 Å) for astronomy and microscopy. Although bare microchannel plates have low sensitivity (5% - 10% quantum detection efficiency) in this spectral region, use of photocathodes can increase substantially (to 30%-40%) microchannel plate performance. This, combined with the high spatial resolution (60%), but is comparatively low (5%-10%) for soft X-rays and E U V radia tion. Electrons which are ejected from the channel walls are accelerated down the channel by the applied electric field, resulting in collisions with the wall, which produce further secondary electrons. This process leads to an electron avalanche travelling down the channel. The gain (electrons per detected photon) of single M C P ' s is typi cally 10 to 10 . Higher gains are normally achieved by cascading two or more M C P ' s in series, or using curved channel M C P ' s . 3

5

Quantum Detection Efficiency Under typical operating conditions (high negative H V on M C P front surface), the sensitive area of an M C P is limited to the open channel area (typically —60%), since electrons emitted by radiation hitting the interchannel web are not collected. One way of increasing the open area is to funnel (6, 1J_, 1_2) the input end of the channels, although practical limitations restrict the maximum open area to —80%. However, it is also possible to recover the electrons from the interchannel web by using a small posi tive potential (13) on the front of the M C P or by placing a grid at high negative poten tial 0 4 , .15) close to the front of the M C P to deflect electrons back into the channels. It is also worth noting that investigations 0 6 ) have shown that there is a dependence of the sensitivity on the polarization of the incident radiation. A summary of the data on the quantum detection efficiency (QDE) of bare M C P ' s is given in Figure 2, where we define the ' Q D E ' as the probability of an incident photon giving rise to an output electron pulse from the M C P . The studies of Fraser 0 7 ) and Parkes et al. (18) were made without a deflecting grid, while the detec tors of Martin et al. (15) and Taylor et al (14) included an electron deflecting grid which increased the efficiency by approximately 30% over a bare M C P . The angle of incidence of the detected photons relative to the channel walls is an important factor in determining the Q D E . This angle is indicated in Figure 2. Typical Q D E response curves as a function of photon incidence angle are given in Figure 3. The sharp drop in Q D E below — 5° —• 10° grazing angle is explained by the increase of channel wall reflectivity at low grazing angles 0 9 ) . Although the peak Q D E is reached at —10° to the channel wall for X U V photons, the peak angle gradually increases to —15° for E U V photons. A t progressively higher angles of incidence, the depth of penetration of the incoming radiation into the M C P material increases; thus, the probability of secondary electrons escaping reduces and results in lower Q D E .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


SIEGMUND AND MALINA

Detection

of Extreme

Figure I. Scheme of a microchannel

ν

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UV and Soft X-Rays

plate.

10

10

100

1000

Wavelength. (?)

0

10

20

30

Wavelength (A)

40

50

Figure 2. Quantum detection efficiency (%) of bare MCPs as a function of wavelength (A ). Key: ; 30° (14); *, 20° (17); +, 8° (15); Δ, S° (\%);andn, 20° (18).

Figure 3. Quantum detection efficiency (ψο) of bare MCPs as afunction ofgrazing angle of incidence (degrees). Key: ο,λ-44Λ ( 17); __ χ = 68Α (\S);—,\ = 44À (\S);and-~, λ = 584 A (97).


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Taking into account the angular factor, the Q D E as a function of wavelength shows a slow increase in Q D E as photon energies decrease from soft X-rays to E U V followed by a drop-off in Q D E above 1200 A . This is caused by the decreasing absorption depth of incident photons (19) as the wavelength increases, resulting in higher secondary electron ejection probability, followed by a cutoff which is determined by the work function of the M C P material. Regarding the nature of the active surface composition of the bare M C P ' s , detailed investigations have been made by Panitz et al. (13) and Siddiqui (20). These show that, although the M C P faces are coated with an electrode material (Ni, Cr, nichrome), there is a thin (100 A ) top surface layer rich in potassium (20) that has been transported up from the underlying glass. The inner surfaces of the channels also have a surface layer which is rich in alkali (primarily K) metals (oxides), Si, and S1O2 (13, 20). These surface layers, in addition to the composition of the bulk glass (PbO + S i 0 , predominantly), determine the Q D E (19). Improvements in the Q D E may, however, be obtained by the use of photocathode materials deposited on the M C P surface, which will be discussed later in this paper. 2

M C P G a i n and Pulse Height Distribution The gain and pulse height distribution of M C P ' s have been studied by many authors (21-27). Single straight channel M C P ' s may be operated up to gains of 10 -10 (12, 21-23) and give negative exponential output pulse height distributions (1_2, 21, 22, 25). [A number of authors have developed models for the gain characteristics of M C P ' s (12, 26, 27).] The M C P gain increases rapidly with applied voltage (12, 23, 26, 27) up to 10 -10 , and then begins to level off. This 'saturation' of the gain (at high gains) occurs because the charge density in the output end of the channel is high enough to initiate space charge effects. The resulting charge build-up on the channel walls lowers the accelerating field and reduces the kinetic energy of the electrons. This, in turn, causes a reduction in the secondary electron yield and hence, limits the gain. The maximum gain achievable at fixed applied voltage and channel length over diameter ratio (Ljd) has been shown (21, 27) to be proportional to the channel diameter and is in the range 10 -10 for common M C P geometries. Normally, however, straight channel M C P ' s cannot be operated at these high gains because of ion feedback effects. Positive ions are produced by collisions of electrons with residual gas molecules (at pressures >10~ torr) in the channels and gas molecules released from the channel walls during multiplication (22, 23, 28). The ions travel back up the channel and may strike the channel wall near the M C P input face, releasing electrons, which results in a secondary electron avalanche and afterpulsing. 4

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The problem of ion feedback has been circumvented in a number of ways. First, if the M C P ' s are baked in vacuum, 'scrubbed' by running them at high current, and not reexposed to air, the problem of desorbed gas from the channel walls is greatly reduced. G o o d results are achieved in this way for sealed tube devices. A t high gains, however, the baking and scrubbing are insufficient to suppress ion feedback, so various geometrical modifications of M C P ' s have been devised. One method is to use M C P ' s with curved channels ((L/d) typically in the range 80:1 —• 140:1), so that ions strike the channel walls closer to their point of origin than in a straight channel M C P . This reduces the size of the secondary electron pulses associated with ion feedback to acceptable levels, allowing gains of up to 2 x 10 to be used. A t these high gains, the pulse height distribution is peaked with a Gaussian type shape and a F W H M on the order of 30% - 60% (30-32, 34). The narrow pulse height 6


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UV and Soft X-Rays

distribution is a consequence of the limiting of the gain when 'saturated' mode, as described previously.

operated in the

Ion feedback may also be reduced by using a tandem (V) or triple (Z) stacking arrangement of M C P ' s . These configurations are oriented such that there is an angle of about 15° —20° between the channels at the stack interfaces. Ions generated at the stack output face cannot proceed past this bend, and thus produce only small secondary electron pulses. Tandem and Ζ arrangements have operational gains as high as 10 -10 ( L 28> 33), with a gain/voltage curve shape similar to that of single M C P ' s (Figure 4). A pair of 40:1 L/d M C P ' s separated by 25 μ m —• 150 μ m is a commonly used tandem configuration (23, 28).


7

8

The pulse height distributions for a tandem of two 40:1 L/d M C P ' s in this arrangement are poor (100%-200% F W H M ) (23, 28, 31) compared to curved channel M C P ' s . However, by increasing the L/d ratio of each of the M C P ' s to 80:1, or greater, and using each M C P in a near saturated mode, pulse height distributions of 30% — 50% F W H M have been achieved (1_, 25, 27, 33). Figure 5 demonstrates the progressive narrowing of the pulse height distribution as the M C P gain is increased for a tandem of two 80:1 (5) M C P ' s placed back to back (no gap). Recent results (27) also indicate that reducing the channel diameter improves the pulse height distribution. The configuration of the interface between the two M C P ' s of a tandem is an important factor in determining the gain/pulse height performance. For a back to back tandem pair of M C P ' s , electrons from the first M C P can enter only a maximum of 3 channels (35) in the second M C P , assuming flat M C P surfaces. If a small gap between the M C P ' s is introduced, the electrons from the first M C P can spread into many more channels of the second plate. This results in an increase of the gain, since more chan nels contribute; however, the pulse height distribution is degraded because the channels of the second plate are not as highly saturated. If a field is applied across the gap (23, 2 7 , 36) between the M C P ' s , the gain drops and pulse height F W H M is found to improve. This results because the electrons from the first plate are accelerated into fewer channels of the second M C P . The configuration used in any application depends on the desired gain/pulse height characteristics. It should be noted that the gain of M C P ' s is not constant with accumulated dosage. Lifetests 0 , 12, 25, 29) of M C P ' s have shown that there is an initial sharp drop in the gain, associated with outgassing of the channel walls, followed by a plateau region of stable operation. The behavior of M C P parameters after re-exposure to air (37, 38) indicates reversible increases in gain attributable to adsorption of gas in the channels. For this reason, it is advisable to 'scrub' (or 'burn in') M C P ' s until the pla teau region is reached before use. Another frequently-used technique is to bake out (29) M C P ' s in vacuum; however, this can cause an additional permanent drop of the M C P gain. It has been suggested (13) that, in addition to driving off adsorbed gases and water vapour, the bake causes desorption of the alkali metals which are responsible for the secondary emission properties of the M C P . Desorption of alkali metals under electron bombardment may also play a part in determining the overall lifetime of M C P ' s , although there is, at present, insufficient information to characterize M C P life time properties completely. Unlike conventional photomultiplier tubes, M C P ' s can be subjected to high mag netic fields (>100 G ) without significant changes in gain and pulse height distribution (39). This is attributable to the small size and length of the M C P channels and the high electric fields within the channels. Magnetic fields are, however, a more serious concern in the readout system located behind the M C P .




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1400

1600 Voltage Applied (Each MCP)

Figure 4. Gain / voltage curve for a tandem pair of mi L/d MCPs with d = 12.5 μτη.

A

a.

Figure 5. Pulse height distribution of a tan dem pair of MCPs each having L/d = 80.1 anda = 12.5 μm. Conditions (top): gain, 10 ; and FWHM, 58ψ . Conditions (middle): gain, 1.9 χ 10 ; and FWHM, 41%. Con ditions (bottom): gain, 4.6 10 ; and FWHM, 32%). 6

0

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Number of Events

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259

Noise characteristics The number of background noise events Q 8 , 23, 29) from both single and tan dem M C P ' s is generally very low (< 1 count/cm /sec), but this can still be a significant limitation in applications such as X-ray and E U V astronomy, where very low count rates are experienced. Lower thresholds must be carefully chosen to exclude the low gain exponential tail responsible for a large fraction of the background counts. Most manufacturers now produce M C P ' s which have, on average, dark count rates of < 0.5 counts/sec/cm , and we have found that selected plates can have rates as low as 0.1 - 0.2 counts/sec/cm . Investigation Q ) of the sources of background events sug gests that, at sea level, cosmic ray events, intrinsic radioactivity, and thermionic emis sion contribute less than 5% to the overall background rate. It has been suggested CO that defects in the M C P channel walls are responsible for the background events through field emission. This correlates with the non-saturated pulse height distribu tions obtained for background events using a tandem, indicating that background events are generated all along the channel walls (1). 2

2


2

Other noise phenomena which may impair the performance of M C P ' s include 'switched on' channels, 'hot spots', 'dead' channels, and field emission from mounting hardware. Due to improved selection and production techniques, the incidence of 'dead' channels and 'switched o n ' channels is now relatively low. However, hot spots, producing relatively large numbers of spurious events, are a more common occurrence. Currently this effect is thought to be associated with dust or damage on the front sur face of M C P ' s . Certainly our experiences suggest that, if care is taken with cleanliness and handling of M C P ' s , the occurrence of hot spots can be kept very low. T i m e Response and Count Rate Performance A single event electron pulse emerging from a M C P can be extremely short (< 1 ns) in duration (23, 30, 40-42). Due to the high fields and short distances, the transit time of the electrons through a M C P is calculated to be on the order of 250 ps (40:1 L/d M C P ) and the transit time spread to be about 50 ps (42). Experimental results with M C P photomultiplier tubes (PMT's) indicate (23, 30, 40, 41) that pulse widths of 200 - 300 ps can be achieved with single M C P ' s . Fast M C P P M T ' s with Ζ M C P configurations have longer (23, 40, 41) pulse widths (300 - 500 ps), due to the extra length of the channels. Slowing-down effects, caused by the reduction of electron ener gies in the saturated mode also contribute to this. The recovery time of a single channel after an event is determined by the rate at which the extracted charge can be replaced. For a typical 1 m m thick M C P of resis tance 300 m f t , the channel recovery time t is of the order 20 ms (21, 23), where r — RC (R and C are the effective channel resistance and capacitance, respec tively). So, the maximum event rate per channel is —50 counts/sec, and if this rate is exceeded, the channel does not recover fully between events and the gain drops as a consequence. Since an M C P has on the order of 10 - 10 channels, the overall count ing rate can be very high ( > 1 0 counts/sec), provided no channel experiences more than —50 counts/sec. Some manufacturers (5, 6) provide M C P ' s with low channel resistance to reduce the channel time constant in applications which require high local ized counting rates. c

c

C

C

c

c

5

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For a tandem M C P configuration, a number of channels of the second M C P are excited by a single event in the front M C P . Therefore, neighboring channels of the first M C P will, in effect, cause some of the same channels of the second M C P to be


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excited. So, although the recovery time for a single event is similar to that for a single MCP,

the overall counting rate for the tandem is lower, by a factor equivalent to the

number of channels which a single event excites in the second M C P . Photocathode Materials


Photocathode materials are used routinely to enhance the detection efficiency of M C P ' s at soft X-ray and E U V wavelengths. Unlike detectors with optical photocathodes, most M C P detectors for this wavelength range operate in a 'windowless' mode, because there are no suitable window materials sufficiently strong to hold back 1 atm. Since either production of photocathodes in situ or transport of unsealed devices under ultra high vacuum is usually impractical in space instrumentation, photocathodes which are stable under exposure to air are normally selected. Since the photon energies of interest are in excess of 10 eV, photocathode materials with relatively high work functions (a few eV) can be used. Widely used materials include M g F , L i F and C s l . Many other alkali halides are suitable, although some precautions need to be taken, since many of these compounds are hygroscopic. 2

Photocathodes may be used in two configurations, opaque (or reflection) photocathodes and transmission photocathodes. Detailed calculations of the performance of both types of photocathode may be found in the articles of Henke (43-45). Opaque photocathodes usually are deposited directly onto the input face of the M C P such that the photocathode material penetrates a short distance into the channels. Thus incident radiation which enters the channels strikes the photocathode material, resulting in enhanced detection efficiency. A s in the case of a bare M C P , radiation striking the interchannel web is not normally detected, so electron deflection grids in front of the M C P are sometimes used to further the enhancement of detection efficiency. Transmission photocathodes for E U V and soft X-ray applications can be deposited on very thin substrates such as parylene, polypropylene, lexan, aluminum, or even fine mesh grids. This photocathode assembly is normally used in a proximity-focussed configuration mounted close to the M C P input (300 μ,ηι —• 1000 μ,ιτι). Electrons emit ted from the photocathode are then accelerated to the M C P by a high electric field (500 — 1000 V/mm). One of the limitations of this configuration, however, is that the pho toelectrons spread laterally in the photocathode/MCP gap, resulting in a practical limita tion of —50 μπγ F W H M for the position resolution (33, 46). In general, transmission photocathodes also show lower quantum yields (electrons/incident photon) than opaque photocathode configurations. Cesium Iodide Cesium iodide has received much attention recently as a potential photocathode material (17, 47-55) by virtue of its high quantum efficiency in the soft X-ray and E U V regions. A compilation of C s l efficiency data in the range 10 A - 2000 A for a number of different configurations is given in Figure 6. (Grazing angle of incidence is indi cated). The measurements of Fraser 0 7 ) , Martin et ai 0 5 ) , and McClintock (47) were taken with opaque C s l coatings on the surface of M C P ' s . However, the Q D E ' s meas ured by Fraser refer to efficiency of C s l in the channels alone, since the electrons emit ted from the interchannel web were not collected. O n the other hand, in the measure ments of McClintock and Martin et al. electrons from the interchannel web were col lected with an electric field. In the latter case 0 4 ) , the channels were found to contri bute little (—20%) to the overall quantum efficiency. This was because the C s l coating inside the channels was too thin (

Multichannel Image Detectors Volume 2 - ACS Publications

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