Date: Wed, 17 Apr 1996 20:57:53 -0500
Subject: Re: Deadtime and "DTIM"
From: Carl Henderson

X-Listprocessor-Version: 6.0b -- ListProcessor by Anastasios Kotsikonas

Status: OR



On Wed, 17 Apr 1996, John J. Donovan wrote:



> Don't know about MBX, but Sam Pindrys stated that the "DTIM" parameter is

> the length of time in microseconds that the hardware pulse counter will

> refuse subsequent counts. That's why I prefer to call it a "recovery time

> parameter".

>

> Note however, that this DTIM hardware correction is NOT a deadtime

> correction per say.



Yes, you are correct, it is not purely a correction.



My understanding is that instead of trying to *measure* the inherent

deadtime in the detection/electronic system of each spectrometer and then

using that deadtime in software to perform the correction, CAMECA decided

to *impose* a deadtime (ideally greater than the fixed hardware

deadtime). With this fixed and known (!) deadtime set by software, the

same deadtime number is used in the software to perform the correction.

Increasing the DTIM number will lower the countrate, but it should be

corrected by software.



> Assuming deadtime values without measuring them is not a good idea unless

> you ALWAYS keep your count rates very low. With a typical Cameca deadtimes

> of 2 - 3 or even 4 microseconds (3.4 for one of my detectors), this is

> difficult.

>

> Look at these % errors for various count rates and deadtimes:

>

> CPS 1K 2K 5K 10K 20K 50K

>

> deadtime 1 us .1 .2 .5 1 2 5

> deadtime 2 us .2 .4 1 2 4 10

> deadtime 3 us .3 .6 1.5 3 6 15

> deadtime 4 us .4 .8 2 4 8 20

>



Point taken. But the analytical errors aren't so bad if your standard and

unknown count rates are similar. My count rates are usually << 10K,

except for perhaps Si and Al. And then, the standards are pretty close to

unknowns. I agree that using a pure metal standard to measure minor or

trace amounts in an unknown *would not be a good idea* (actually for more

reasons than just the deadtime correction).



Cheers,



Carl







Carl Henderson

Electron Microbeam Analysis Laboratory

University of Michigan

2005 C.C. Little Bldg.

Ann Arbor, MI 48109-1063 USA

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