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Spectrometers and data acquisition session summary
Ultramicroscopy 0 North-Holland
3 (1979) 373-374 Publishing Company
SPECTROMETERSAND
DATAACQUISITIONSESSIONSUMMARY the addition of a compensatingac deflection drawn from the line voltage with suitable adjustmentsfor magnitudeand phase.The last solution only works, however, when the magnitudeand phaseof the disturbance is constant over long periodsof time. Batson argued that the effects of such fields on the resolution in Wien filters are smallerfor the slower electrons and that therefore the effect in suchanalyzers is likely to be more noticeable for a high operating potential than a lower one. It waspointed out that for low resolution (-2 eV) magneticsector spectrometersthe magnetic properties of a mild steel are asgood asthe high quality magneticmaterials.A few minor magneticinhomogeneitiescould even be tolerated in the pole piece material of such a system. Stability of the magnet power suppliesdid not appearto be a major problem. Provided they are not switched off, commercial power suppliesof high stability exist. Severalsourcesof spectrometerinstrumental background were discussed.Scattering from the slits, bolt holesand bolt projections near the slit assemblywere identified asproblem areas.Other sourcesof instrumental background include secondaryelectrons emitted by stray electronsstriking the inside of the spectrometer and afterglow of the scintihator detector. It wasreported that coating with carbon black cuts down the secondaryelectron emissionsubstantially. Finally, the problem of matching spectrometersto the exit of the electron microscopewasdiscussed. This is a problem for STEM operation sincethe high incident beamconvergencenecessaryfor high spatial resolution STEM needsto be matched to the lower spectrometeracceptanceangle. Thus, post specimen lens optics to achievethis matching-arenecessaryto achieve a suitable collection efficiency. In addition, someversatility is necessaryto adapt the spectrometer to operation of the microscopein various modes, e.g. microscopy or diffraction. The chromatic aberration of theselenseswasidentified asa problem and in particular, Rosenoted that the third order chromatic aberration can be significant and would affect critical-
Although spectrometerscan be madein a variety of designs,much of the discussionwasconcerned with the design,construction and optimisation of the simple magneticsector analyser. For a given spectrometer resolution can only be improved by decreasing the spectrometeracceptanceangle,thus decreasing intensity. Eventually the signal/noisewill decreaseto an unacceptablelevel. Improvement beyond these limits necessitatescorrections of aberrationsand considerablediscussionof spectrometeraberrationsensued. Curvature of the entrance and exit faces of the spectrometerand entrance and exit anglescan all be adjustedto provide control over the secondorder aberrationsof uniform field sector magnets.Shuman reported the discovery of conditions whereby the appropriate secondorder aberrationscould be made to vanishsimultaneouslysuchthat a straight line imagewould be produced in addition to having a focal plane perpendicularto the central ray. The first condition would allow for high resolution with largeacceptance angles,the secondcondition is necessaryfor high quality parallel recording. The condition for production of a straight line imagewasconfirmed by Zeitler, reporting work due to Engel. Zeitler alsodiscussedthe performance of a spectrometer built meeting this design.Such spectrometersare valuable becausethey match the output of the spectrometer to the parallel detectors now beginningto becomeavailable. A semanticplea wasvoiced by Zeitler who pointed out that instrumentsthat usesuchoutputs should be termed spectrographs.Some discussionof the use of line focussing(with a line detector aperture as opposedto double focussing)led to the conclusion that an improvement in resolution (perhapsby 2 or 3) might occur. Somediscussionof practical problemsarisingin spectrometersthat either degradethe resolution or introduce instrumental background developed.Stray ac electromagneticfields were of considerableconcern and the curesreported include adequatemagnetic shielding,proper attention to electrical groundsand 373
374
Summatyof
ly the design placement of the post specimen lenses. A similar matching problem also exists in some cases between the spectrometer output and the detector. . Much interest was shown in semiconductor based parallel detector systems with the possibility of direct electronic readout. In addition to the efforts described in the keynote paper by Johnson, exploration of the usefulness of a charge-coupled diode Array was reported by Chapman. So far the experience with these systems has been somewhat disappointing because of limited dynamic range and relatively high noise level. Attempts to improve device performance by cooling them to liquid nitrogen temperatures have resulted in non-uniform and non-reproducible changes in dark current and gains of individual diodes, thus providing another source of noise. Radiation damage .was identified as another problem, although it was pointed out that the cost of devices like these reduces very rapidly and it was not unreasonable to expect the cost to come down to the point that this may not be a serious limitation. The general expectation appeared to be that while these systems were somewhat disappointing at the present time, continuing development of these devices may improve them to the point where they will be useful as parallel detectors. Other parallel detection systems based on conversion to photons were also discussed. At present, the standard method of obtaining spectra is to sweep the dispersed electrons across a slit and the scintillator/photomultiplier detector combination appears to be the preferred detection method. It was pointed out the maximum count rate for this system is in the region of 10 MHz. Electronics capability up to -200 MHz does exist but the limiting feature is probably the photomultiplier tube. The virtues of pulse counting as opposed to analogue counting were debated, and it was suggested that the need for a large dynamic range could be avoided by the use of analogue recording for the highintensity low-loss peak and digital recording for the low-intensity high-energy loss peaks. Difficulties in adjusting the photomultiplier tube were identified in this respect. Two techniques for handling the statistics problems were mentioned. Spence mentioned the use
session 2
of variable energy windows and Batson mentioned the technique of recording counts until a fixed count is reached and recording the time needed to reach that level. Reasons for recording the zero-loss data were requested. One answer was that relative concentrations do not require zero-loss data but that absolute concentrations do require them. A second reason was that any attempt to invoke multiple scattering compensation or Fourier analysis of the data would need such data. Variable channel recording (above) would necessitate variable channel Fourier analysis programs. Discussion of the count statistics elicited two observations that suggested that Poisson statistics were not being obeyed. A suggestion that data processing techniques commonly used in Auger electron spectroscopy (AES) might be valuable in ELS studies brought a report from Newbury that a special session on spectral techniques in AES, ELS and EDS was held at the Microbeam Analysis Society in Ann Arbor (20-24 June, 1978). The use of differentiated electron spectra (N’Q vs. 5”) is declining in AES in cases where meaningful background subtraction is required. Application of NQ vs. E is preferred for the application of deconvolution based on physical models. Application of differentiation degrades S/N and therefore sensitivity. General discussion led to the conclusion that recording the spectrum directly led to the least degradation and is to be preferred for numerical work. The derivative technique is valuable in recognizing peak positions but can often be handled quickly by numerical differentiation of the recorded data if suitable computational facilities are available. Moreover, differentiation is only useful if the spectrum is not S/N limited. Finally, control aspects of computers were discussed. The advent of cheap microprocessors is rapidly broadening the capability of sophisticated control, local storage and higher level on-line processing. Current control uses also provide for compensation for drift in angle, changes in emission current and possible spatial drift. One drawback to microprocessors at present is that assembly language is necessary to program most currently available systems.
3 (1979) 373-374 Publishing Company
SPECTROMETERSAND
DATAACQUISITIONSESSIONSUMMARY the addition of a compensatingac deflection drawn from the line voltage with suitable adjustmentsfor magnitudeand phase.The last solution only works, however, when the magnitudeand phaseof the disturbance is constant over long periodsof time. Batson argued that the effects of such fields on the resolution in Wien filters are smallerfor the slower electrons and that therefore the effect in suchanalyzers is likely to be more noticeable for a high operating potential than a lower one. It waspointed out that for low resolution (-2 eV) magneticsector spectrometersthe magnetic properties of a mild steel are asgood asthe high quality magneticmaterials.A few minor magneticinhomogeneitiescould even be tolerated in the pole piece material of such a system. Stability of the magnet power suppliesdid not appearto be a major problem. Provided they are not switched off, commercial power suppliesof high stability exist. Severalsourcesof spectrometerinstrumental background were discussed.Scattering from the slits, bolt holesand bolt projections near the slit assemblywere identified asproblem areas.Other sourcesof instrumental background include secondaryelectrons emitted by stray electronsstriking the inside of the spectrometer and afterglow of the scintihator detector. It wasreported that coating with carbon black cuts down the secondaryelectron emissionsubstantially. Finally, the problem of matching spectrometersto the exit of the electron microscopewasdiscussed. This is a problem for STEM operation sincethe high incident beamconvergencenecessaryfor high spatial resolution STEM needsto be matched to the lower spectrometeracceptanceangle. Thus, post specimen lens optics to achievethis matching-arenecessaryto achieve a suitable collection efficiency. In addition, someversatility is necessaryto adapt the spectrometer to operation of the microscopein various modes, e.g. microscopy or diffraction. The chromatic aberration of theselenseswasidentified asa problem and in particular, Rosenoted that the third order chromatic aberration can be significant and would affect critical-
Although spectrometerscan be madein a variety of designs,much of the discussionwasconcerned with the design,construction and optimisation of the simple magneticsector analyser. For a given spectrometer resolution can only be improved by decreasing the spectrometeracceptanceangle,thus decreasing intensity. Eventually the signal/noisewill decreaseto an unacceptablelevel. Improvement beyond these limits necessitatescorrections of aberrationsand considerablediscussionof spectrometeraberrationsensued. Curvature of the entrance and exit faces of the spectrometerand entrance and exit anglescan all be adjustedto provide control over the secondorder aberrationsof uniform field sector magnets.Shuman reported the discovery of conditions whereby the appropriate secondorder aberrationscould be made to vanishsimultaneouslysuchthat a straight line imagewould be produced in addition to having a focal plane perpendicularto the central ray. The first condition would allow for high resolution with largeacceptance angles,the secondcondition is necessaryfor high quality parallel recording. The condition for production of a straight line imagewasconfirmed by Zeitler, reporting work due to Engel. Zeitler alsodiscussedthe performance of a spectrometer built meeting this design.Such spectrometersare valuable becausethey match the output of the spectrometer to the parallel detectors now beginningto becomeavailable. A semanticplea wasvoiced by Zeitler who pointed out that instrumentsthat usesuchoutputs should be termed spectrographs.Some discussionof the use of line focussing(with a line detector aperture as opposedto double focussing)led to the conclusion that an improvement in resolution (perhapsby 2 or 3) might occur. Somediscussionof practical problemsarisingin spectrometersthat either degradethe resolution or introduce instrumental background developed.Stray ac electromagneticfields were of considerableconcern and the curesreported include adequatemagnetic shielding,proper attention to electrical groundsand 373
374
Summatyof
ly the design placement of the post specimen lenses. A similar matching problem also exists in some cases between the spectrometer output and the detector. . Much interest was shown in semiconductor based parallel detector systems with the possibility of direct electronic readout. In addition to the efforts described in the keynote paper by Johnson, exploration of the usefulness of a charge-coupled diode Array was reported by Chapman. So far the experience with these systems has been somewhat disappointing because of limited dynamic range and relatively high noise level. Attempts to improve device performance by cooling them to liquid nitrogen temperatures have resulted in non-uniform and non-reproducible changes in dark current and gains of individual diodes, thus providing another source of noise. Radiation damage .was identified as another problem, although it was pointed out that the cost of devices like these reduces very rapidly and it was not unreasonable to expect the cost to come down to the point that this may not be a serious limitation. The general expectation appeared to be that while these systems were somewhat disappointing at the present time, continuing development of these devices may improve them to the point where they will be useful as parallel detectors. Other parallel detection systems based on conversion to photons were also discussed. At present, the standard method of obtaining spectra is to sweep the dispersed electrons across a slit and the scintillator/photomultiplier detector combination appears to be the preferred detection method. It was pointed out the maximum count rate for this system is in the region of 10 MHz. Electronics capability up to -200 MHz does exist but the limiting feature is probably the photomultiplier tube. The virtues of pulse counting as opposed to analogue counting were debated, and it was suggested that the need for a large dynamic range could be avoided by the use of analogue recording for the highintensity low-loss peak and digital recording for the low-intensity high-energy loss peaks. Difficulties in adjusting the photomultiplier tube were identified in this respect. Two techniques for handling the statistics problems were mentioned. Spence mentioned the use
session 2
of variable energy windows and Batson mentioned the technique of recording counts until a fixed count is reached and recording the time needed to reach that level. Reasons for recording the zero-loss data were requested. One answer was that relative concentrations do not require zero-loss data but that absolute concentrations do require them. A second reason was that any attempt to invoke multiple scattering compensation or Fourier analysis of the data would need such data. Variable channel recording (above) would necessitate variable channel Fourier analysis programs. Discussion of the count statistics elicited two observations that suggested that Poisson statistics were not being obeyed. A suggestion that data processing techniques commonly used in Auger electron spectroscopy (AES) might be valuable in ELS studies brought a report from Newbury that a special session on spectral techniques in AES, ELS and EDS was held at the Microbeam Analysis Society in Ann Arbor (20-24 June, 1978). The use of differentiated electron spectra (N’Q vs. 5”) is declining in AES in cases where meaningful background subtraction is required. Application of NQ vs. E is preferred for the application of deconvolution based on physical models. Application of differentiation degrades S/N and therefore sensitivity. General discussion led to the conclusion that recording the spectrum directly led to the least degradation and is to be preferred for numerical work. The derivative technique is valuable in recognizing peak positions but can often be handled quickly by numerical differentiation of the recorded data if suitable computational facilities are available. Moreover, differentiation is only useful if the spectrum is not S/N limited. Finally, control aspects of computers were discussed. The advent of cheap microprocessors is rapidly broadening the capability of sophisticated control, local storage and higher level on-line processing. Current control uses also provide for compensation for drift in angle, changes in emission current and possible spatial drift. One drawback to microprocessors at present is that assembly language is necessary to program most currently available systems.