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Electron Microprobe Lab, University of Minnesota

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*

JD,
 
    When Bill Chambers set up our system he decided to fix the baseline
and window settings for the detection system and change the bias voltage
to center each energy curve in the window.  An advantage of this method
is that almost all curves are about the same size within the window .
If I set the DT values with one element, the others on the same
spectrometer track well (especially for specs 2, 3, &4 with LiF-PET
pairs) with the possible exception of spec. 1 which has TAP, PET, and
LiF crystals.  I primarily use spec. 1 for quants on TAP or  low
magnification, semiquantitative x-ray photo work.  (Spec 5 is similar -
quants on TAP and quals to semiquants on syn. layered "crystals".)
 
    Now, perhaps my technique does not attain perfection but I have to
keep my requestors happy with a lot of throughput.  I cannot spend a lot
of time tweaking my numbers all the time.  How much of an error are you
and Paul C. talking about?
 
 
Paul

Paul F. Hlava
Electron Microprobe Laboratories
Department 01822 - Materials Characterization
505-844-1890
[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 John J. Donovan
Sent: Friday, October 21, 2005 5:07 PM
To: [log in to unmask]
Subject: Re: [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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* 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
	[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 
	
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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]
University of Oregon                             (541) 346-4632 (office)
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97403-1272              

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