A position-senstive detector system for electrons using a charge-coupled device

Nuclear Instruments and Methods 195 (1982) 323-325 North-Holland Publishing Company

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A POSITION-SENSITIVE DETECTOR SYSTEM FOR ELECTRONS USING A C H A R G E - C O U P L E D DEVICE Peter J. HICKS *, Suzannah DAVIEL **, Barry W A L L B A N K and John C O M E R Physics Department, The Universi(v, Manchester MI3 9PL, U.K.

A position-sensitivedetector system for electrons has been developed which employs microchannel plates, a phosphor and a charge coupled imaging device. This has been incorporated into a hemispherical electron analyser and results show an improvement of a factor of 100 in sensitivity over the best existing spectrometers.

1. Introduction Electron impact and photoelectron spectrometers c o m m o n l y use hemispherical electron analysers. These analysers disperse the electrons in energy, the dispersed image being scanned across the exit slit to record the electron energy spectrum. However, such electron analysers do produce at their exit an extended image representing an electron energy spectrum which is linear in energy, to a good approximation [1]. Employing an exit slit therefore rejects a large fraction of the available signal. To overcome this, we have developed a system which can detect and process the electrons arriving at any point within the extended image. Position-sensitive detectors have been used before and there are two main types. One technique used, for example, by Lampton and Paresce [2], involves the use of microchannd plate electron multiplier followed by a resistive anode. The charge produced on the anode by the arrival of an electron event divides, and its arrival position is determined by measuring the pulses produced at each end of the anode. The second method uses an array of photosensitive dements, readout from which is accomplished by a scanning technique. One example has been described by Gelius et al. [3] and uses a vidicon tube to view the image produced on a phosphor screen. * Present address: Department of Electrical Engineering and Electronics, UMIST, Manchester, UK. ** Present address: Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver BC, Canada.

A disadvantage of the resistive anode system is that it only detects events one at a time. The vidicon system, however, can detect events simultaneously, but only if they occur at different points on the detector during one scan. This means that both of these suffer from a limitation in the overall count rate that can be processed. The present system, which is based on a scanned array, can detect multiple events at each point during one scan, so it can be used for much higher intensities.

2. The multideteetor system The present multidetector system [4] is used in an electron energy loss spectrometer but it could equally be employed in a photoelectron spectrometer. A schematic diagram of the present instrument is shown in fig. 1. A monochromatic beam of electrons is crossed with a gas beam and electrons scattered from the target are focused at the entrance to a hemispherical deflector. The scattered electrons are dispersed in energy at the exit, and the final image, which is 25 mm long, impinges on a microchannel plate (Mullard G25/25). This device multiplies the electrons and the intensified image is accelerated onto a phosphor screen (produced by Rank Precision Industries, Sidcup, Kent). This gives an optical image of the electron energy loss spectrum which exists at the exit to the hemispherical deflector. The optical image is then focused onto a charge-coupled device (Fairchild CCD 11 IF). The charge coupled device has a linear array of 256 photosensitive elements and the optical image generates a charge pattern in this

0029-554X/82/0000-0000/$02.75 © 1982 North-Holland

vI. PHOTON DETECTORS ELECTRON SPECTROMETERS

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P.J. Hicks et al.

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A position-sensitive detector spstem

array. After integr&tion for a short period, typically 2 ms, this charge is transferred to a storage array on the device. While the next set of data is accumulating, the charge in the storage array is read out, digitized and accumulated in the data system. At the input to the analyser, correcting rings are used to provide the required field and potential. At the output, however, more care must be taken to preserve the resolution over the extended image. Here, two resistive plates.are inserted, one at each side of the image, and a current is passed through them to generate the correct potential and field at all points within the image. The detector extends over an energy range which is about 12% of the mean energy of the electrons passing through the analyser. This spectrum can be scanned across the detector by changing the potential difference between the electron analyser system and the target. A spectrum covering a wider energy range is therefore obtained by scanning the potential in steps equivalent to one channel. For each step the data from the 256 elements of the detector are accumulated in the appropriate part of the final spectrum stored in the data acquisition system. Every channel of the detector is therefore used to accumulate each point of the final spectrum for an equal time. This technique has the advantage that it completely eliminates any non-uniformity in response between channels resulting from the microchannel plates, the phosphor or the CCD. The data system for the position sensitive detector is based on an LSI 11 microcom-

puter. This is buffered by a specially designed high speed microprocessor system which provided the control described above.

4. Results

Fig. 2 illustrates the improvement in sensitivity that can be obtained with the multidetector. The upper spectrum was recorded on the present instrument and the lower one probably represents the best low energy electron impact spectrum of nitrogen published to data [5]. The resolution and sensitivity are similar in the two spectra, but the total collection time is very much shorter with the multidetector. Differences in relative intensity for the peaks can be attributed to the different scattering angle. Allowing for this difference in scattering angle and improvements in spectrometer design, these results indicate a striking improvement of at least a factor of 100 in sensitivity over the best existing spectrometers. Although there have been improvements in the resolution of electron spectrometers, it has often not been possible to take advantage of these improvements because of the accompanying loss of signal. However, the increased sensitivity given

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ENERGY LOSS (EV) Fig. 3. Electron enery loss spectrum of carbon monoxide taken with an incident electron energy of 80 eV and a scattering angle of 2% The energy interval per channel is 3.4 meV and the observed width of the narrowest peaks is 9 meV. The states marked are as classified by Huber and Herzberg [7] and a full analysis of this data is given in Daviel et al. [8].

b y the m u l t i d e t e c t o r a l l o w s h i g h q u a l i t y results to b e o b t a i n e d w i t h t h e b e s t r e s o l u t i o n c u r r e n t l y att a i n a b l e . T h i s is i l l u s t r a t e d in fig. 3 w h i c h s h o w s a n e l e c t r o n e n e r g y loss s p e c t r u m t a k e n in c a r b o n m o n o x i d e . T h e m e a s u r e d full w i d t h at h a l f m a x i m u m for t h e n a r r o w e s t p e a k s o n the s p e c t r u m is 0.009 eV w h i c h m e a n s t h a t t h e r e s o l u t i o n o f the electron selector and analyser were better than 0.006 eV. I n a d d i t i o n , t h e signal to n o i s e r a t i o is s u p e r i o r to t h a t o f the b e s t p r e v i o u s l y p u b l i s h e d spectrum which was taken with a resolution of

0.06 eV [6].

References

[1] E.M. Purcell, Phys. Rev. 54 (1938) 818. [2] M. Lampton and F. Paresce, Rev. Sci. Instr. 45 (1974) 1098. [3] U. Gelius, E. Basilier, S. Svensson, T. Bergmark and K. Siegbahn, Uppsala University, Institute of Physics Report no. 817 (1973). [4] P.J. Hicks, S. Daviel, B. Wallbank and J. Comer, J. Phys. E: 13 (1980) 713. [5] D.G. Wilden, P.J. Hicks and J. Comer, J. Phys. B: 12 (1979) 1579. [6] A. Skerbele and E.N. Lassettre, J. Chem. Phys. 55 (1971) 424. [7] K.P. Huber and G. Herzberg, Molecular spectra and molecular structure IV: constants of diatomic molecules (Van Nostrand, New York, 1979). [8] S. David, B. Wallbank, J. Comer and P.J. Hicks, to be published.

VI. PHOTON DETECTORS ELECTRON SPECTROMETERS