So you agree that WDS dead time is non-extending? I agree this would seem to be true by definition, since all WDS systems count only for exactly as long as the specified count time. But then why does Brian make this claim:
It is not that I agree or don't agree (That is not a matter of an agreement).
1) on Cameca instruments I am 100% sure it is non-extendable as I am fully aware how the hardware is built, and on Jeol I argue that by seeing secondary observations (strong shifts of PHA) that it is designed with very similar hardware (and missing hardware part which is needed for extension of dead time - else there would be no PHA shifts). However, probably on Jeol it does have some unintended pralysable behavior misidentified as "extension" by some mechanisms/processes, which are not on Cameca instrument.
2) Before we dwell further we need to distinguish that most of dead time we observe on these instruments is intentionally designed to be there and it cover-over (with huge overlap) the unintentional dead-time (the missing counts from other processes which creeps into signal process depending from count rate)... I think confusion comes from that all (EDS and WDS) systems enforce some dead time in different ways, but I think it does that for a bit different reasons and thus it is more (EDS) or less (WDS) complicated/advanced.
3) lets look to
EDS "enforced" dead time. The main reason for EDS enforced dead time is energy accuracy. The counting system looks for all pulses (with very fast but low resolution pulse shapping amplifier) in parallel to the main (high resolution) shapping amplifier and rejects the currently processed pulse if any (accepted or rejected) pulse was close enough
before to overlap anyhow with currently processed pulse. As such counter is
keeping track of all incoming pulses it will keep rejecting pulses perpetually, unless there is enough of space before the current pulse and its amplitude then can be guaranteed to be accurate. That
ability to keep rejecting pulses, unless the height of incoming pulse can be guaranteed to be accurate, - that is what makes the dead time extendable by hardware design.
4)
WDS could look similar on the first glimpse, as it "enforces" some dead time. But, it does it differently: a) it enforces dead time
after the sensing pulse and
blinds itself from sensing any incoming pulses during the dead time (see the difference: EDS does not blind itself at the fast track - so it could keep a note of all incoming pulse, where WDS blinds itself completely) b) it could look that the reason is similrar to EDS: a simplified attempt to prevent to count the pulse coming after the sensed pulse (As there is normally negative tail of pulse, thus preventing overlapped pulse with tail) - thus only the pulse with accurate height would be counted. I initially thought that would be the reason - but, it fails completely, as system have no idea what happened before the sensed pulse (and thus we see PHA shifts on both Cameca and Jeol). Basically if it sees a pulse it holds the pulse and blinds itself (it is accepted or rejected by PHA) for "enforced" amount of time. c) I think the main reason for WDS "enforced" dead time is not accuracy (which we know fails miserable) but to have predictable dead time and overcome the bottleneck of sharing the part of pipeline by few spectrometers. In example on Cameca SX old WDS boards that is up to 3 spectrometers, where analog pulse signal is multiplexed to single shared ADC - The multiplexer requires 1µs for switch! setting dead time anything below 3µs with all three spectrometers on high count rate would not decrease the dead time! On new WDS boards multiplexing is shifted to digital domain (switching can be done at 50 MHz) on single digital bus (all five spectrometers); There setting the "enforced" dead time below 3µs shows the huge difference in count rates, even when all spectrometers are near fully saturated. Still because of multiplexing it should not be set below 1µs (and thus it is blocked from doing that) as the dead time would start to "float" depending from count rate of other spectrometers.
So it is not that "WDS systems count only for exactly as long as the specified count time" - You actually can force most of EDS systems to count for realtime and not live time, which would make it the same from that perspective. No, it is so because of the different counting design and hardware. But, why Brian brings in extending dead time? probably there is misunderstanding what is extending vs non-extending and paralyzing vs non-paralyzing. I think Jeol is at disadvantage and I think You had uncovered the reason in your other thread showing that Jeol is much more affected with PHA shifts than Cameca instruments, which introduce paralysable behavior where more and more pulses are rejected by baseline of PHA. I actually could simulate paralysable behavior at my Monte-Carlo simulation for diff mode (which demonstrates it (diff mode) is very unsuitable for high count rates), --the rejection by baseline would be a similar.
How can pulse pileup in a (JEOL) WDS system equate to an extending dead time model when the count time is fixed? Is the (JEOL) pulse processing electronics "saving" pulses to be counted later? But then you go on to say:
Again "fixing counting time" has nothing to do with extending - non-extending. As JEOL sees more rejected pulses by PHA baseline with increasing count rate, due to severe broadening and shifting of the PHA it starts to observe paralysable behaviour. It have nothing to do with extension of dead time as hardware is blind for any pulse-pileup and don't care (same as I had wrote before).
I agree with this, but then why does Brian say the JEOL WDS system is extending? How could it be different from the Cameca? Because of the "enforced" dead time of the Cameca electronics? I am somewhat confused by these seemingly conflicting statements.
First, I believe the Jeol has "enforced" dead time - the difference from Cameca is that on Jeol it is "cut-in-the-stone", where on Cameca it is "user-settable" with low boundary of 1µs (to prevent from "floating" dead time by multiplexing) and high 255µs (max of 8-bits). Anyway, as by default it is set to 3µs and most user don't change it -- that will produce less of PHA shift than on Jeol. Other reason is Jeol gain circuit looks rubish (sorry), and setting the PHA peak position centrally by changing the bias is not the best idea (the countermeasures of PHA shift by increasing bias just increase the very cause of the shift). Lastly I am not sure about that, I am near ready to test it out, but I think Jeol has "pseudo"-integral mode, where Cameca has real integral mode for counting, and that would do the huge difference introducing the paralysable behaviour for Jeol and no paralysable behaviour observed on Cameca.
The biggest confusion comes from mixing the "extending" and "non-extending" with "paralysing" and "non-paralysing" terminology.
It is not synonimous, but misunderstanding originates due to extending deadtime producing paralyzable behaviour. But it is not the same other way arround!
If it goes about mathematics: extendable dead time will revert the input count rate vs observed count rate curve at some point, and it will drop and drop untill will reach the 0 output counts at extremely high input count rate - which would be 100% dead time on the EDS.
Clearly this is extendable and paralysing.
To compare with EDS, on WDS with non-extendable deadtime, we can also see paralyzable behaviour at some point - in particular if using diff PHA mode. However, that paralyzable behaviour won't lead to 0 cps at very extremely high input count rates - it will never drop there, as that is only additional mechanism blocking some but not all pulses. It will start dropping but after some time then will go into plato. If paralysing behaviour is noted on any detector, going above that point is absolutely bad as it is not possible to calculate the real count rate (as it can be from both sides of parabolic curve). EDS gets away with that as due to tracking dead time (as it measures all incoming pulses) it knows on which side of such parabolic curve it is. WDS by not tracking the total number of pulses is blind and resolution is impossible.
As for experiment, I have access only for Cameca instruments. As soon I will have something to share I will do, hopefully someone owning Jeol probe will feel adventurous and knowledgeable enough (connecting the earth/ground clip of oscilloscope to wrong place can instantly fry the boards! be warned!) to do such experiments for Jeol.
P.S. above described "double track" pipelines on EDS was on previous detectors. Newest generation of EDS detectors most probably has no more of double tracking but resolve the pileups with terrific beefy Digital Signaling Processors on FPGA's (I am aware that some EDS vendors had moved there - the outcome is terrific: You wont see any pulse-pile ups even with 90% dead time!!!). That is what I would like to go with for WDS too.