WDS hardware ideas, improvements

[sckuehn]

Possibly a crazy idea....

You may know about the JEOL light element detector which uses a custom diffraction element and a CCD detector to do elements from Li on up. This received one of the 2015 MAS Society Awards: http://www.microbeamanalysis.org/docs/MicroNews%202015%20Summer.pdf

What about putting a similar detector behind a normal WDS crystal in place of the usual detector? Presumably, the JEOL system converts the CCD readout into a spectrum display based on which pixels/lines on the CCD recorded each X-ray that came in. The same kind of processing with a regular WDS spectrometer might be a way to do simultaneous peak and background measurement. Perhaps this could also give integrated peak counts, compensate for peak shape/shift changes (chemical environment or thermal drift), and maybe even do a form of multi-point background or EDS like background subtraction.  Assuming that the CCD outputs different pulses for different incoming X-ray energies so some form of PHA would be possible, there would be a lot of PHA data to manage (one per detector line).

[Mike Matthews]

I love the idea of doing peak and background simultaneously, and using peak integrals. The ccd doesn't have any pha discrimination though.

[sckuehn]

So, if no PHA, then interference corrections would have to be used instead.

[Probeman]

Possibly a crazy idea....

You may know about the JEOL light element detector which uses a custom diffraction element and a CCD detector to do elements from Li on up. This received one of the 2015 MAS Society Awards: http://www.microbeamanalysis.org/docs/MicroNews%202015%20Summer.pdf

What about putting a similar detector behind a normal WDS crystal in place of the usual detector? Presumably, the JEOL system converts the CCD readout into a spectrum display based on which pixels/lines on the CCD recorded each X-ray that came in. The same kind of processing with a regular WDS spectrometer might be a way to do simultaneous peak and background measurement. Perhaps this could also give integrated peak counts, compensate for peak shape/shift changes (chemical environment or thermal drift), and maybe even do a form of multi-point background or EDS like background subtraction.  Assuming that the CCD outputs different pulses for different incoming X-ray energies so some form of PHA would be possible, there would be a lot of PHA data to manage (one per detector line).

Hi Steve,
If you replaced the analyzing crystal with a variable pitch grating it would diffract a region of the spectra, but the Rowland circle and detector geometry engineering is non-trivial. A similar geometry was been proposed a number of year ago by Dave Wittry. See papers attached below.   One discusses your ARL spectrometer design...

Here another interesting WDS idea patented by Wittry in 1986:

https://www.google.com/patents/US4599741

john

[jon_wade]

swap the CCD for a drift detector and maybe...

one of the things thats always puzzled me is the general lack of development in the EPMA market. My suspicion is it reflects the lack of competition in the hardware market and the relative low number of sales in comparison to, say,  the MS market.  For instance, you can look at both  current  FEG offerings and wonder why significant parts of the designs are stuck in the early/mid 80's in different ways.

[Probeman]

swap the CCD for a drift detector and maybe...

Hi Jon,
I think Steve is thinking of WDS hardware for acquiring position sensitive intensities at the peak position in order to perform a peak integration and background correction *without* moving the spectrometer.  It's a good idea- obtain more photons and deal with chemical state shift effects and correct for background all at once, but the devil is in the engineering details. 

One related thing I've been hoping for is a tandem flow and pin-diode detector design that can handle the full energy range as described here:

http://probesoftware.com/smf/index.php?topic=193.msg1831#msg1831

one of the things thats always puzzled me is the general lack of development in the EPMA market. My suspicion is it reflects the lack of competition in the hardware market and the relative low number of sales in comparison to, say,  the MS market.  For instance, you can look at both  current  FEG offerings and wonder why significant parts of the designs are stuck in the early/mid 80's in different ways.

I could not agree with you more.  I'm hoping for some progress in EPMA hardware myself.  For example, it's criminal that we do not have sub micro-second deadtime counting electronics for the EPMA...  Cameca could simply use their existing IMS counting electronics for the SXFive for that. 

After all, they have already appropriated the IMS vacuum control electronics from the IMS for the probe!

[sckuehn]

I think Steve is thinking of WDS hardware for acquiring position sensitive intensities at the peak position in order to perform a peak integration and background correction *without* moving the spectrometer.  It's a good idea- obtain more photons and deal with chemical state shift effects and correct for background all at once, but the devil is in the engineering details. 

Exactly.

[Jeremy Wykes]

Interestingly, I was talking with a beamline scientist from the soft x-ray beamline here at AS, and he was surprised to hear that modern electron probes did not disperse x-rays on to an area detector of some sort. Apparently this is common among soft x-ray beamlines, though I am not sure if this is being done by medium energy (roughly Si to <10keV) beamlines. I suspect the synchrotron world is the place to look for different approaches to detectors.

[Probeman]

Interestingly, I was talking with a beamline scientist from the soft x-ray beamline here at AS, and he was surprised to hear that modern electron probes did not disperse x-rays on to an area detector of some sort. Apparently this is common among soft x-ray beamlines, though I am not sure if this is being done by medium energy (roughly Si to <10keV) beamlines. I suspect the synchrotron world is the place to look for different approaches to detectors.

Hi Jeremy,
Is it a commercial device or "home made"?  Can you obtain more details on this or maybe a contact there regarding this technology?

I'd be interested in learning more...
john

[Probeman]

Here's a schematic drawing of a tandem detector system (from one of our NSF instrument development proposals from a couple of years ago), that I think would be very useful to improve the energy range and sensitivity for WDS spectrometers in EPMA:



Please note that there is nothing new with the idea. We've seen this idea implemented in the distant past with Peak spectrometers for example.  Using new solid state technology this tandem method should be even more efficient than previous efforts.

Here's a mention of this tandem detector idea from a Bruker guide to XRF:

The  flow  counter  is  situated  inside  the  spectrometer  chamber  and  has  an  angle  range  of  2°  to  148°. Located  behind  the  flow  counter  and  outside  the  chamber,  separated  by  a  0.1  mm  Al  foil,  is  the scintillation  counter  with  an  angle  range  of  2°  to 110°.  Both  detectors  can  be  used  individually  or  in tandem.  In  tandem  operation,  the  intensity  in  the  flow  counter  is  measured  as  well  as  the  radiation  that passes through the flow counter and the radiation that is absorbed by the scintillation counter.

http://www.fem.unicamp.br/~liqcqits/facilities/xrf/%5BBruker_2006%5D%20Introduction%20to%20X-ray%20Fluorescence%20(XRF).pdf

Of course we probably shouldn't use a scintillation counter as described in the XRF guide above, because today's pin diode detectors are highly efficient and low cost. In addition, unlike SDDs, pin diode detectors don't need to be cooled. 

Also the 127 um thick Be exit window provides a nice low energy filter for the pin diode. That is, only x-rays greater than 6 keV or so will get detected, while the gas flow portion of this tandem design will very efficiently detect low energy x-rays down to Be or possibly lower.

If this tandem design was implemented, every WDS spectrometer on your EPMA instrument could capture the complete x-ray energy range!

[Probeman]

Here's a schematic drawing of a tandem detector system (from one of our NSF instrument development proposals from a couple of years ago), that I think would be very useful to improve the energy range and sensitivity for WDS spectrometers in EPMA:



Please note that there is nothing new with the idea. We've seen this idea implemented in the distant past with Peak spectrometers for example.  Using new solid state technology this tandem method should be even more efficient than previous efforts.

Here's a mention of this tandem detector idea from a Bruker guide to XRF:

The  flow  counter  is  situated  inside  the  spectrometer  chamber  and  has  an  angle  range  of  2°  to  148°. Located  behind  the  flow  counter  and  outside  the  chamber,  separated  by  a  0.1  mm  Al  foil,  is  the scintillation  counter  with  an  angle  range  of  2°  to 110°.  Both  detectors  can  be  used  individually  or  in tandem.  In  tandem  operation,  the  intensity  in  the  flow  counter  is  measured  as  well  as  the  radiation  that passes through the flow counter and the radiation that is absorbed by the scintillation counter.

http://www.fem.unicamp.br/~liqcqits/facilities/xrf/%5BBruker_2006%5D%20Introduction%20to%20X-ray%20Fluorescence%20(XRF).pdf

Of course we probably shouldn't use a scintillation counter as described in the XRF guide above, because today's pin diode detectors are highly efficient and low cost. In addition, unlike SDDs, pin diode detectors don't need to be cooled. 

Also the 127 um thick Be exit window provides a nice low energy filter for the pin diode. That is, only x-rays greater than 6 keV or so will get detected, while the gas flow portion of this tandem design will very efficiently detect low energy x-rays down to Be or possibly lower.

If this tandem design was implemented, every WDS spectrometer on your EPMA instrument could capture the complete x-ray energy range!

Just had a very nice chat with Nick Barbi (who founded Peak Instruments and is now at PulseTor):

http://www.pulsetor.com/

He said that at Peak Instruments they mainly focused their work on the light elements and so their WDS spectrometer only utilized a gas flow detector, but he said that what might work even better in this tandem design than a pin diode detector, for extending the high energy sensitivity, might be an intrinsic (high purity) Ge detector. Such an HPGe detector, when used as a high energy photon detector would not require cooling.

Interesting...  apparently these HPGe detectors (sensors?) used to be manufactured by PGT, but are still manufactured by someone else.  A quick search reveals:

http://www.canberra.com/products/detectors/germanium-detectors.asp

http://www.ortec-online.com/Products-Solutions/RadiationDetectors/Overview.aspx

john

[Probeman]

Here is a abstract on a parallel dispersion controlled WDS spectrometer from a few years ago.

Also a logarithmic spiral WDS diffractor and a review paper by Wittry and Barbi.

[Brian Joy]

I agree with Mike that Cameca has done a considerable amount of innovation (large area crystals also being an important one) over the last 10 to 15 years, but I think Nicholas' point is that with Cameca now (mostly) not selling microprobes, those innovations will essentially fall by the wayside.

Looking forward can we expect JEOL to start innovating more?  I don't know, but let's try and list a least 10 ways JEOL could improve their current microprobe. I'll start:

1. Implement solid state detectors for WDS spectrometers
2. ...

Solid state WDS X-ray counters do exist.  JEOL partnered with Bruker ca. 2010 to develop them, and three such counters were built and were supposed to be installed on the instrument here.  Charlie Nielsen (now retired) was in charge of the project.  Although throughput was very high, the active area of the Si wafer was too small, and this would have resulted in a ~50% decrease in count rates at given potential and beam current.  Because of this, I declined to accept them.  Bruker was unwilling to commit additional resources to the project, and so it was scrapped.

[Probeman]

I agree with Mike that Cameca has done a considerable amount of innovation (large area crystals also being an important one) over the last 10 to 15 years, but I think Nicholas' point is that with Cameca now (mostly) not selling microprobes, those innovations will essentially fall by the wayside.

Looking forward can we expect JEOL to start innovating more?  I don't know, but let's try and list a least 10 ways JEOL could improve their current microprobe. I'll start:

1. Implement solid state detectors for WDS spectrometers
2. ...

Solid state WDS X-ray counters do exist.  JEOL partnered with Bruker ca. 2010 to develop them, and three such counters were built and were supposed to be installed on the instrument here.  Charlie Nielsen (now retired) was in charge of the project.  Although throughput was very high, the active area of the Si wafer was too small, and this would have resulted in a ~50% decrease in count rates at given potential and beam current.  Because of this, I declined to accept them.  Bruker was unwilling to commit additional resources to the project, and so it was scrapped.

Yeah, I know about that dead-end development effort.  How about something *better* than what we have today?  I will continue...

1. Implement solid state detectors for WDS spectrometers with *improved* counting statistics.
2. Implement faster counting electronics with sub microsec dead times.
3. ...

[Probeman]

I agree with Mike that Cameca has done a considerable amount of innovation (large area crystals also being an important one) over the last 10 to 15 years, but I think Nicholas' point is that with Cameca now (mostly) not selling microprobes, those innovations will essentially fall by the wayside.

Looking forward can we expect JEOL to start innovating more?  I don't know, but let's try and list a least 10 ways JEOL could improve their current microprobe. I'll start:

1. Implement solid state detectors for WDS spectrometers
2. ...

solid state detectors... again. Can't understand that obsession. It would make sense only if diffraction crystals would be focusing to point and not line. As Solid state detectors are round. The G(F)PC has few key advantages for being used in WDS:
    1) The active volume where X-rays can be registered is very narrow cylinder around the biased wire. That allows more simple diffraction crystal manufacturing process and peak is not blurred by large detection area (which large SDD would do). The active volume is used in its fullest. For SDD It would need some "line"-like aperture in front, so only very little fraction of active detector surface would be used.
    2) Cooling. G(F)PC does not need that. SDD working at room temperature and higher would have comparably terrible resolution.
    3) solid state detectors produce unnecessary spectral artifact like tails i.e. with incomplete charge collection. Si Ka escape line kicks in with lower energy lines than Ar Ka escape (or Xe escape). It is in particular important for Pb, Th, U in geochronology where Silica detector would introduce artifacts which are not on GFCP.
What I propose is combining advantage of G(F)PC with advantage of SDD (or more precise learn from advantages of current solid state detector counting electronics). Currently implemented GFPC counting is very strongly affected with pileups and does not allow to filter out 2nd order lines fully. current PHA is passive dumb PHA. That is like selecting regions for mapping on EDS for overlapping peaks (i.e. Zr La and P Ka; SKa and Pb Ma). But that problem (EDS mapping) already was solved a decade ago with introducing of deconvolution method! That is the main missing point for total WDS PHA empowerment. Currently PHA is set like those old time EDS maps at baseline and window size - the region of histogram. But PHA curve could be collected during counting and saved like EDS spectra for later treatment. Currently integral and diff counts are preserved at WDS board during and after counting. With such curve preserved the exact window, or with deconvolution by fitting Gaussian curve the precise part of counts corresponding to selected measured line could be much more precisely quantisized. Also this would allow to have much better peak-pile up correction than what EDS normally have by utilizing deterministic fixed dead time (as currently is). Actually we don't need much shorter deadtime (shorter deadtime means shorter shaping time and it comes at cost of worse energy resolution). SDD does not have much shorter dead time and still can cope much better with higher count rates than WDS. (i.e. 1Mcps)

I am not at all suggesting an SDD detector for WDS. I agree that would be silly. 

Instead I am imagining the use of a simple photon detector at room temperature, perhaps a pin-diode detector. Even just to eliminate the hassle of dealing with P-10 gas bottles would be great!   

I am not an electronics expert at all, so I will defer on the technical details, but I wonder if the Bragg crystal gives us all the energy resolution we require. Obviously, we need to filter the thermal noise from the detector, but that shouldn't be difficult I am hoping. 

I am sure you could come up with something pretty interesting if you thought about it.