Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
INSTRUMENTATION Jack W. Frazer Howard V. Mafmstadt William F. Ulrich
A New Detector for Spectrometry The use of this device in the vacuum ultraviolet is shown. However, the technique can feasibly be applied in other spectral regions and in mass spectrometry by A. Boksenberg Department of Physics, University College London
his article discusses a new techTnique for reading spectral information in the form of a spatial distribution of charge, a t present under development in the Physics Department of University College London [A. Boksenberg, R. L. F. Boyd, J. C. Jones, Nature, 220, 556 (1968)], that promises to be useful in ultraviolet spectrometry, mass spectrometry, and possibly other applications too. -4 charge image is first recorded on the surface of an insulating layer by direct photoemission, photoconduction, or other means, according to the character of the information to be detected. For example, to record a spectrum in the vacuum ultraviolet, a n insulating photoemitter can be conveniently used In an otherwise conventional spectrograph : then the unbalanced positive charges that remain after the photoninduced surface emission of electrons, constitute the recorded image and this is a direct analog of the spectral pattern to which the layer is exposed. To extract the information thus recorded, a reading process is employed that is basically similar t o the charge measuring technique used in the vibrating reed (dynamic capacitor) electrometer, but here added mechanical sophistication is required for the measurement of charge images. I n the vibrating reed electrometer, a quantity of charge is determined by mechanically varying the value of an associated capncitor and measuring the resulting alternating current with a
sensitive amplifier. I n the new application described here, a charge image residing on the surface of a thin insulating layer backed by a grounded conducting plate is read out, element by element, by means of a fine, closely scanning, vibrating, conducting probe connected to a current amplifier, the direction of vibration being normal to the surface. During the entire reading operation the surface of the layer is never touched by the probe, so the image remains intact and may repeatedly be read without detriment. But a more important consequence of nondestructive reading is that the probe can remain over each effective image element sufficiently long to enable the inherent charge level to be measured as accurately as desired by use of the narrow-band lock-in amplifier technique. In other words, the image can be read out without substantially degrading it with noise introduced in the reading process. This is in direct contrast t o the conditions obtaining in a conventional television camera tube-which uses electron beam readout-where the image is necessarily destroyed as it is read; a normal wide-band amplifier must then be used so the signal-to-noise advantage of a lock-in amplifier is lost. Nevertheless, this loss can be effectively reduced by intensifying the image before recording, although this is usually a t the cost of considerable vomplexity and a severe limitation in dynamic range.
However, the real value of the new image detecting technique follows from the total avoidance of electron optics both in the reading and recording processes. This allows the image area to assume almost any desired extent and contour-as is true for photographic film-and makes the system particularly appropriate for use in spectrographic instruments where focal surfaces are often long and curved. I n comparison, the need for accurately defined electric and magnetic fields for electron acceleration and focussing in image intensifiers and television camera tubes severely constrain the permissible geometrical extent of their sensitive areas. Figure 1 shows, in schematic form, the various practical steps of the detection procedure for use in the vacuum ultraviolet. First, the image is recorded by exposing a highly insulating photoemitting layer through an open mesh held a t a high positive potential relative to the conducting backplate. This mesh collects the emitted photoelectrons while recording and is subsequently drawn aside, or otherwise removed, for reading. I n general, the storage layer is not homogeneous but consists of a dielectric spacer layer with a thin surface film of photoemitter, as indicated. Such a surface layer is not difficult to find for detection in the vacuum ultraviolet since many insulators are also efficient photoemitters in this region, notably the alkali halides. Probably the best of these is cesium iodide, having near unit quanVOL. 41, NO. 7 , JUNE 1969
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I NST R UMENTAT I 0 N
Photon
Electron collecting mesh Electron Y P h o t o e m m i t t i n g layer .CC Insulating CI layer c-c Conducting backplate
Pre-amplifier
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Reference ac signal
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Vibrating conducting orobe scanned
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tum efficiency (the ratio of the numbers of emitted electrons to incident photons) over the range 10-1000 A. In comparison, the appropriate photographic emulsions have efficiencies between one and two orders of magnitude lower. Next, the reading process: since spectra essentially contain information in one dimension only, the scanning action can be correspondingly onedimensional and the probe knife-edged in form and held normal to the dispersion direction. T o get a better understanding of the reading process, we define the image density in the vicinity of the probe a t any instant as (+ and the respective capacitances of the probe and the conducting backplate to an area a of surface beneath the probe C,(t) and C, the former being a function of time because of the impressed vibration. For simplicity, we neglect fringe effects and assume the potential difference between probe and backplate is zero. Then, if the instantaneous charge on Cl(t) is q l ( t ) and the induced signal current is i, ( t ):
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Feedback
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Electron gun scanned over surfaee
Figure 1. Schematic showing the practical steps in the detection procedure in the vacuum ultraviolet. Top: Recording the image: Middle: Reading the image; Bottom: Erasing the image
Thus, the signal current is periodic with the probe vibration and its amplitude is proportional to the surface charge density and t o the rate of change of Cl(t)-i.e., t o the frequency of vibration. Also, it can be shown that optimum signal generation occurs when C, and the mean value of C l ( t ) over one cycle are approximately equal. The current is passed t o a sensitive current pre-amplifier, then to a lock-in amplifier where phasing reference is provided by the vibrator oscillator, and finally t o a recorder. The sensitivity of the system is such that a charge equivalent to a few tens of electrons can be detected with certainty using a vibration frequency of 10 kHz and a reading rate of ten image elements a second. I n practice, the probe is vibrated by means of a piezo-electric crystal operating near resonance. Scanning may be achieved by mounting the probe unit on a movable carriage which is borne against a reference surface accurately related to the surface of the storage layer. Alternatively, the probe unit may be fixed and the storage layer moved. The spatial resolution of the system is naturally dependent both on the geometrical configuration of the probe and its mean separation from the surCircle No. SO on Readers’ Service Card-
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ANALYTICAL CHEMISTRY
Resolve 10 Nanosecond Signals Buried In Noise Complex repetitive waveforms are accurately resolved to 10 nanoseconds and recovered from noise in the new PARTM High Resolution Boxcar Integrator. The Model 160 achieves signal recovery by time averaging a small portion of a coherent waveform over a large number of repetitions. Because the mean value of the noise apaveraged output results only e waveform. To recover the and averaged is either man-
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INSTR uMENTATI o N
face. The probe width and mean separation both need to be in the region of 10 microns to achieve a resolution comparable to that obtainable photographically. Furthermore, the second harmonic component of the signal gives a better resolution than the first, since it is generated predominantly when the probe is closest to the surface during each vibration cycle. For optimum resolution performance, the probe must also be electrically blinkered by means of carefully designed guard electrodes-not shown in the diagram-in order to better confine its “view” to the region of the storage surface in its immediate vicinity. To maintain the resolution-and therefore, the probe mean separationconstant over the whole of the scan for a nominal separation of only a few microns is a difficult mechanical undertaking but can be adequately achieved if aided by servo control. This also has the effect of stabilizing sensitivity, a feature which likewise becomes increasingly necessary with decreasing separation. Such servo action may be realized simply by applying a correcting bias to the piezoelectric vibrator to stabilize the mean separation and, if necessary, also controlling the vibrator drive oscillator to stabilize amplitude. One way of measuring the two parameters to be controlled is by the use of a conducting strip deposited along the edge of the storage surface and an overlaying subsidiary probe mounted on the vibrator. By maintaining a constant potential difference between them and measuring the first and second harmonic components of the resulting signal current, the mean separation and vibration amplitude can be uniquely determined. However, in applications where the passive mechanical performance of the system is adequate for resolution but not for maintaining a sufficiently constant sensitivity, the latter may be acceptably stabilized by the usual method of negative feedback, as shown in the diagram. After reading, the image is erased for a fresh exposure by scanning it with a low velocity electron beam, a procedure that effectively replaces the electrons that were emitted from the surface during recording. As a first commercial application of the image detecting system, work is proceeding to apply it in place of the photographic plateholder in 5 Hilger and Watts two-metre grazing incidence vacuum spectrograph (normally 90 A
ANALYTICAL CHEMISTRY
Typical record
Figure 2. Vacuum spectrograph with image detector in place of photographic plateholder
working in the range 5-950 A ) , in collaboration with the U.K. Atomic Energy Authority, Hilger and Watts Ltd., and the U.K. National Research Development Corporation. The main advantages here to be gained over the use of film are: a high detecting efficiency, linearity of response, one to two orders of magnitude increase in dynamic range and an electronic readout that obviates the need to change exposed plates-obviously a great operational convenience for a vacuum instrument. The basic components of the experimental system are illustrated in Figure 2. -4s described above, the sequence of operations consists of (a) conversion of the ultraviolet spectrum image to a static charge image, (b) electromechanical readout and (c) erasure. In the current trials, the collecting mesh, reading and erasing heads are fixed and the storage layer, which is coated on to a large curved glass block, is moved to the collecting mesh for recording and then slowly scanned past the reading and erase heads a t the completion of the exposure. The wavelength range covered in this trial instrument is 5-250 A. A portion of a typical record obtained in preliminary and as yet unoptimized experiments is included in the figure and shows spectral features in the region 100-200 A obtained with an air-filled flash tube source. The quality of the storage layer is such that it is possible to retain spectra as charge images for many days after exposure. As an additional function, the exposure can be electronically controlled by appropriately gating the potential of the collecting mesh relative to the backplate to alternately inhibit and en-
courage photoemission as required (the backplate is a metallic film deposited on the glass support block before applying the storage layer). Although this in itself is probably a n unnecessary complication, it does lead to an interesting extension of the basic detection technique, namely, in time-resolving transient spectral phenomena. This can be achieved by constructing a collecting mesh in the form of a number of longitudinal sections insulated from one another. During exposure, the applied potential is gated from negative to positive and back to negative for each in turn, thus recording the spectral information as a stack of adjacent time-staggered spectrum strips. T o read the composite image, it is envisaged that the piezo-electric vibrator would contain a separate probe system for each spectrum strip. The use of the reading technique for detection in other spectral regions is inherently more difficult than in the vacuum ultraviolet, largely because of problems associated with the use of suitable layers. I n the near ultraviolet and optical regions, a photoconductive rather than photoemissive approach is currently being studied, the former being preferred since it avoids the inconvenience of a vacuum envelope not now needed optically. Mass spectrometry is another possible area of interest for application of the technique. Here, ions conventionally separated and then received on a n insulating layer could be directly read out without further processing. Alternatively, charge multiplication by secondary emission at the storage surface could provide a useful degree of intensification t o aid in the detection of very low levels of impurities.
