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* Paul,
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.
john

At 10:26 AM 10/21/2005, you wrote:

*
Alfred,
 
    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

Paul F. Hlava
Electron Microprobe Laboratories
Department 01822 - Materials Characterization
505-844-1890
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mail to
Sandia National Laboratories
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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

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* 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

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]
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