All,
Here is what I have to offer on this topic.
1. One should not use assumed ("factory") settings for deadtime on their microprobe. To do so can result in up to percent level systematic errors resulting from and incorrect deadtime constant, the magnitude of which depends on the difference in count rate between sample and standard.
2. The use of element specific detector bias values on the Jeol in order to have the PHA pulse at 4 volts (and an equivalent gain setting on the Cameca to put the pulse at 2 V) is an attempt to remove the dependence of deadtime on X-ray photon energy. It also provides the same clearance from baseline noise, room to set the baseline, and headroom for coincidence pulses above the 4 V main pulse.
3. Attention should be paid to the existence of escape peaks and appropriate baseline setting used.
4. So, "what are typical deadtime values". I have observed on production Jeol 8x00, 8530 instruments that the 1.1 usec value is sometimes correct but the deadtime can be as high as 1.7 usec or so. I have seen an 8900 with 1.1 usec deadtimes approximately. I have no information from Cameca users on typical values because it is complicated by the 3 usec enforced dt value, but still it would be worthwhile to see values of 3 + X where X is the apparent deadtime value.
5. Not only should you measure the deadtime, but monitoring the long term changes is important. There is a small change in the deadtime for P-10 counters as the Ar:CH4 ratio changes when the tank slowly empties; there is perhaps a greater change during tank evolution than when you change the tank.
6. Probably most important is to observe the linear range of your deadtime plot which demonstrates to you what the dynamic range of the counting system is. I use a count rate range up to at least 200k cps to see where nonlinearity starts and possible paralyzable behavior exists -- you want to know where that is now rather than discovering it as an artifact later. Stage map runs are likely data sets where a large dynamic range is sampled due to the high probe current that you should be using.
7. Pulse processing and alignment issues are problem areas for probe users and instruments. It is really important to have the counting system characterized for WDS, compare to using an EDS system where the calibration was incorrect and the baseline set too high (cutting out low energy photons) and the detector parameters (resolution, energy calibration) changing as a function of count rate. You would raise hell about those problems with the EDS system, so why not do the right thing and characterize your WDS system. I applaud Dave for doing so and sharing the numbers here.
8. Last thing. I think that a lower dynamic range is used by Jeol when evaluating the deadtime. The 1.1 usec value should be viewed as a placeholder until you update it with an actual measured value. The dt is not measured on any system to my knowledge prior to or during installation, regardless of manufacturer. This is because resolution and WDS crystal performance, leak rate, blah blah, all that stuff is a spec value and a deliverable benchmark for the installation.
Cheers,
Paul "deadtime" Carpenter
