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Electron Microprobe Lab, University of Minnesota

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I think I've mentioned this before but there is a philosophical difference 
in the way the Cameca and Jeol deadtime electronics works. In the Jeol the 
measured deadtime (as you just did) is just that. What you measured. It 
includes a contribution from the detector gas amplification and the pulse 
shaping electronics. However, it is NOT a constant. As Paul Carpenter has 
demonstrated and I have confirmed, the deadtime is affected by a number of 
factors including the bias voltage and the incident x-ray energy.

Because these effects can contribute to the need for a "variable" deadtime 
constant, Cameca microprobes incorporate a "pulse stretcher" circuit for 
extending the deadtime to a moderate but known (and constant) value that 
essentially "masks" the intrinsic (but variable) deadtime of the detector 
and electronics.

The native (unstretched) deadtime of the SX50/51 Cameca electronics and 
detectors is slightly larger than the Jeol and is about 1 to 2 microsecs, 
but on my new SX100 the software would not let me set the "pulse stretcher" 
electronics to a zero deadtime and so I could not directly measure the 
intrinsic deadtime. However, I did measure it when the "pulse stretcher" 
was set to 1 microsec and it gave 1.03, 1.38, 1.44, 0.91, and 1.13 
microsecs using Ca Ka.

So on my new instrument the intrinsic deadtime is probably around 1 
microsec but I then reset the "pulse stretcher" to 3 microseconds and then 
measured the ACTUAL pulse widths which are then used for the software 
correction. The beauty of this method is that although the underlying 
intrinsic deadtime may change due to changes in bias voltage and x-ray 
energy, the measured pulse width (and hence actual deadtime) never varies.

At 10:26 AM 10/21/2005, you wrote:
>     I have measured the deadtimes on my 8600 a few times (when I think I 
> detect a slight change).  Here's what I now have on my system but I have 
> not changed these in at least a year or so.
>Spec 1 - 1.17 u-seconds
>Spec 2 - 1.47 u-seconds
>Spec 1 - 1.35 u-seconds
>Spec 1 - 1.55 u-seconds
>Spec 1 - 1.27 u-seconds
>     How accurate are they?  Hey, I have them out to the 2nd decimal point 
> for a reason!  BTW - Cameca locks in their deadtimes to 3 u-seconds and 
> that really restricts the count rate they can use without problems.  If 
> ya need a description of my simplistic technique for determining these 
> numbers, let me know.
>Paul F. Hlava
>Electron Microprobe Laboratories
>Department 01822 - Materials Characterization
>[log in to unmask]
>mail to
>Sandia National Laboratories
>MS - 0886
>Albuquerque, NM 87185-0886
>From: JEOL Probe Users List [mailto:[log in to unmask]] On Behalf Of 
>Alfred Kracher
>Sent: Thursday, October 20, 2005 11:33 AM
>To: [log in to unmask]
>Subject: [PROBE-USERS] detector deadtime
>JEOL Probe Users Listserver
>Moderator: Ellery Frahm, [log in to unmask], Electron Microprobe Lab, 
>University of Minnesota
>Post a message: send your message to [log in to unmask]
>Unsubscribe: send "SIGNOFF PROBEUSERS" to [log in to unmask]
>On-line help and FAQ:
>* Hello,
>I am curious how many JEOL 8x00 users have actually measured the deadtime 
>of their detectors? And if so, what were the results? Has anyone figured 
>out how precise such a measurement could be?
>Alfred Kracher
>Ames Laboratory (USDOE)
>Iowa State University
>227 Wilhelm Hall
>Ames, IA 50011-3020
>Tel.: 515 294 7097

John J. Donovan                                   [log in to unmask]
University of Oregon                             (541) 346-4632 (office)
1260 Franklin Blvd                              (541) 346-4655 (probe)
Eugene, OR                                        (541) 346-4692 (FAX)

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