Explain This If You Can!

[Probeman]

This is a beam damage issue related I think to the beam damage issues in SiO2 as described here:

http://probesoftware.com/smf/index.php?topic=418.msg2269#msg2269

But this is zircon, a much more robust material I think everyone will agree. And the data was acquired at 100nA but with a 5 um defocussed beam. To me it looks like a 5 um carbon ring, with a smaller central area that appears more damaged. So what I'm wondering is whether the damage pattern seen here:



represents an uneven distribution of electrons in the defocussed beam or if the central area that appears damaged is simply an effect of the heat being concentrated in the center, since the outer regions are also being heated by the defocussed beam so the center area should get the hottest...

I guess my question is: has anyone attempted to profile the electron distribution in a defocussed beam spot?  In other words, is the electron density distribution profile across the defocussed beam like A or B:



I assume we all hope it is like A, but I suspect it is a lot more like B...

I recently ran across this issue again on a very beam sensitive sample. This was a thin film of ZnSn oxide on a polyester material that showed significant damage in the center of the beam spot, even when the beam was defocused to over 20 microns. Even when the beam current was reduced to 15 nA!

I ended up having to use a scanning mode at 10,000x to avoid damaging the sample. The difference was that the beam distribution seems much more uniform with the scanning beam than the defocused beam.

I wonder if anyone has tried to measure the beam flux profile on our EPMA instruments...  perhaps using an imaging CCD array as a sample?  Or would that just cook the CCD?

[Probeman]

I recently ran across this issue again on a very beam sensitive sample. This was a thin film of ZnSn oxide on a polyester material that showed significant damage in the center of the beam spot, even when the beam was defocused to over 20 microns. Even when the beam current was reduced to 15 nA!

I ended up having to use a scanning mode at 10,000x to avoid damaging the sample. The difference was that the beam distribution seems much more uniform with the scanning beam than the defocused beam.

I wonder if anyone has tried to measure the beam flux profile on our EPMA instruments...  perhaps using an imaging CCD array as a sample?  Or would that just cook the CCD?

I just chatted with Ed Vicenzi and he pointed out that by looking carefully at a defocused electron beam on a fluorescent sample, we should be able to see if the electron flux is uniform over the defocused area... and now that I think of it, this is what I think we in fact see.

So that might mean that the damage in the center of the defocused beam area must be due to differential heating of the inner area by the outer area.  That is consistent with seeing less sample damage when we scan the beam over the same size area since we are distributing the heating over the scan area...

He also mentioned that any finite element modeler should be able to calculate the temperature distribution in a material given the electron flux, area and the thermal conductivity of the material.  Anyone out there interested?

[Nick Bulloss]

Is the image an otolith?

[Probeman]

Is the image an otolith?

Hi Nick,
How's it going?

Which image are you asking about?  This one?

http://probesoftware.com/smf/index.php?topic=144.msg871#msg871

john

[Nick Bulloss]

Is the image an otolith?

Hi Nick,
How's it going?

Which image are you asking about?  This one?

http://probesoftware.com/smf/index.php?topic=144.msg871#msg871

john

Hi John,
I was looking at the image in the third post on page one, now I look more at the attachment I don't think it is an otolith.
Cheers,
Nick

[Probeman]

Hi John,
I was looking at the image in the third post on page one, now I look more at the attachment I don't think it is an otolith.
Cheers,
Nick

Ah, OK. 

That is a SIMS etch pit in a zircon grain.  The area around the hole is coated with gold, but the etch pit is uncoated and somehow creates a charging cavity of some type.

The thing that is really weird is that this charging artifact is quite stable and persistent.
john

[Les Moore]

Regarding the odd spots.

The C rich contamination spot deposited at high currents on a steel sample is 'as expected' nicely Gaussian.

The C rich contamination spot on a refractory mineral under the same conditions is a donut.
Well, a Gaussian donut made by spinning the distribution about one tail on the sample.
I have some maps showing this behaviour somewhere (a long time ago).

The only idea that makes sense is that the sample is becoming so hot that it stops contaminating right under the beam but deposits on the cooler regions immediately adjacent. The high conductivity of metals does not allow sufficient elevation in temp for this to occur.

Cool.

Quite what this would do to the C yield is a worry as the C is deposited right where you don't want it re secondary fluorescence.
 

[Probeman]

I just finished re-reading Victor's Weisskopf's most excellent book "The Privilege of Being a Physicist" (1990):

https://www.amazon.com/Privilege-Being-Physicist-Victor-Weisskopf/dp/0716721066

And this isn't really an "Explain This if You Can" but it does make one wonder what explanation Victor Weisskopf could have for leaving out energies of 10^4 eV in his "Quantum Ladder":



This is after all, the realm of microanalysis!  He even says in the text that atomic physics deals with energies up to several thousand eV, but then jumps to 10^5 eV, calling that the beginning of the nuclear realm.  He must have heard of inner shell ionizations!

[Probeman]

Here's a fun science trivia quiz question: there are three places in the periodic table where the atomic number increases, yet the atomic weight decreases.  Can you name them without looking at the table?

But the more interesting question is: why does this occur in those three places?   Explain this if you can!   

[Probeman]

Here's a fun science trivia quiz question: there are three places in the periodic table where the atomic number increases, yet the atomic weight decreases.  Can you name them without looking at the table?

But the more interesting question is: why does this occur in those three places?   Explain this if you can!   

OK, I guess you all better check the periodic table if you don't have it memorized!   

The element pairs with an increasing atomic number and a decreasing atomic weight are:

Ar-K
Co-Ni
Te-I

Did anyone know this bit of science trivia?  In fact the Te-I pair is historically interesting because Mendeleyev, who arranged his periodic table by atomic *weight* (because the concept of atomic number had not yet been invented!), made a not often remembered (and wrong) prediction as seen here in a slide I prepared some years ago:



But now the question is why does this decrease in atomic weight occur in these three places only in the periodic table?  Seriously.  I'm asking for help here!

[Mike Matthews]

The atomic weight is the sum of the nucleons minus the amount of ‘mass’ that’s been taken up as binding energy in the nucleus (it’s that e=mc2 equation) so I presume there must be a larger relative increase in binding energy at these points.

[Probeman]

The atomic weight is the sum of the nucleons minus the amount of ‘mass’ that’s been taken up as binding energy in the nucleus (it’s that e=mc2 equation) so I presume there must be a larger relative increase in binding energy at these points.

Hi Mike,
That's what I thought also. But I wrote to my two "card carrying" physicist friends, Andrew Westphal and Zack Gainsforth at Berkeley, and asked about these binding energies, and Zack responded that the binding energies are not that different at all, but that is has more to do with the "magic" shells number, or as Andrew subsequently confirmed:

Zack is right, it's because of the nuclear closed shells for both protons and neutrons, at the numbers that he gave:  2, 8, 20, 28, 56, 82, 126, ...

This why there are s-process abundance peaks at 56Ni (which decays to 56Fe), Ba, Pb, and r-process peaks at Sn and Pt (for these there is pile-up at Z=56 and 82, then the beta-decay after the r-process is over shifts the peak down by a few elements.

He also adds:

In the case of K and Ar, it's not because of binding energy, but just because 40Ar is the most abundant isotope of Ar in the Earth's atmosphere and 39K is the most abundant isotope of K.  The reversal is not universal, it's just because most of the Ar in the atmosphere comes from the decay of 40K.   If you used the average atomic weight of the elements based on the *cosmic* abundances, e.g., in interstellar gas, you would not see a reversal -- that is, the *cosmic* atomic weight of Ar is close to 36, not 40.  So the sequence starting from S would be 32, ~35.4, 36, 39, ...

So the atomic weight is not a universal number, but varies according to the isotopic composition of the material that you're considering.

I have to say, I'm still not sure I understand completely, but I'm thinking about it.

[Probeman]

Explain this if you can!

Recently I made some simultaneous k-ratio measurements on our PET crystals at high sin theta using F Ka on CaF2 (natural) and BaF2 (synthetic) in order to look at the effective takeoff angles as described here:

https://probesoftware.com/smf/index.php?topic=1569.msg12191#msg12191

But I found that were some significant peak shape issues that fortunately could be corrected for quite well using the area peak factor (APF) method, as described here:

https://probesoftware.com/smf/index.php?topic=536.msg12187#msg12187

What I did not mention was that there were also some significant time dependent intensity (TDI) issues during the measurement that also needed to be corrected for, even with a 20 um diameter beam and at 30 nA!

Here is what spectrometer 1 (TAP) intensities looked like:



And spec 2 (LTAP):



And spec 4 (TAP):



You have probably noticed that that the TDI effects were largest in spec 1, less in spec 2 and essentially nonexistent in spec 4... now why would that be?  Since all three spectrometers were measuring the same points at the the same time!?

And these effects were repeated in both BaF2 and CaF2. Here are the (unaggregated) statistics for BaF2:

St  835 Set   2 BaF2 (barium fluoride), Results in Elemental Weight Percents
 
ELEM:        F       F       F       F      Ba
TYPE:     ANAL    ANAL    ANAL    ANAL    SPEC
BGDS:      LIN     LIN     LIN     EDS
TIME:   100.00  100.00  100.00   80.00     ---
BEAM:    29.95   29.95   29.95   29.95     ---

ELEM:        F       F       F       F      Ba   SUM 
XRAY:     (ka)    (ka)    (ka)    (ka)      ()
   215  22.674  22.297  21.931  23.055  78.330 168.286
   216  23.205  22.544  22.081  24.294  78.330 170.454
   217  22.329  22.308  22.248  24.241  78.330 169.456
   218  22.605  22.285  22.165  21.838  78.330 167.223
   219  22.816  22.459  22.380  21.856  78.330 167.841
   220  22.167  22.280  22.000  21.100  78.330 165.877
   221  23.259  22.438  22.447  21.617  78.330 168.092
   222  23.287  22.043  22.171  25.752  78.330 171.583

AVER:   22.793  22.332  22.178  22.969  78.330 168.601
SDEV:     .429    .152    .177   1.646    .000   1.825
SERR:     .152    .054    .063    .582    .000
%RSD:     1.88     .68     .80    7.17     .00

PUBL:   21.670  21.670  21.670  21.670  78.330 100.000
%VAR:     5.18    3.05    2.34    5.99     .00
DIFF:    1.123    .662    .508   1.299    .000
STDS:      831     831     831     831     ---

STKF:    .1545   .1545   .1545   .1545     ---
STCT:    15.90   47.06   20.35   18.22     ---

UNKF:    .1959   .1919   .1906   .1974     ---
UNCT:    20.16   58.48   25.11   23.29     ---
UNBG:      .25     .67     .31     .00     ---

ZCOR:   1.1636  1.1636  1.1636  1.1636     ---
KRAW:   1.2682  1.2426  1.2340  1.2781     ---
PKBG:    82.44   88.08   83.15     .00     ---

TDI%:    9.584   1.675    .140    .000     ---
DEV%:       .4      .2      .2      .0     ---
TDIF:  HYP-EXP HYP-EXP LOG-LIN    ----     ---
TDIT:   130.13  131.75  132.38     .00     ---
TDII:     24.3    72.1    32.3    ----     ---
TDIL:     3.19    4.28    3.47    ----     ---

Note that the TDI effects are *not* correlated with intensity!

And here (again unaggregated) for CaF2:

St  831 Set   4 Fluorite U.C. #20011, Results in Elemental Weight Percents
 
ELEM:        F       F       F       F      Ca
TYPE:     ANAL    ANAL    ANAL    ANAL    SPEC
BGDS:      LIN     LIN     LIN     EDS
TIME:   100.00  100.00  100.00   80.00     ---
BEAM:    29.95   29.95   29.95   29.95     ---

ELEM:        F       F       F       F      Ca   SUM 
XRAY:     (ka)    (ka)    (ka)    (ka)      ()
   223  33.512  32.461  32.533  32.362  51.200 182.067
   224  31.706  32.161  32.409  32.734  51.200 180.209
   225  31.520  32.164  32.536  32.728  51.200 180.149
   226  31.602  32.846  32.228  32.132  51.200 180.008
   227  32.700  32.635  32.333  32.447  51.200 181.314
   228  32.328  33.019  32.625  32.045  51.200 181.217
   229  33.551  32.394  32.476  32.702  51.200 182.323
   230  32.759  32.057  32.537  32.546  51.200 181.100

AVER:   32.460  32.467  32.460  32.462  51.200 181.048
SDEV:     .815    .345    .129    .268    .000    .875
SERR:     .288    .122    .046    .095    .000
%RSD:     2.51    1.06     .40     .83     .00

PUBL:   48.800  48.800  48.800  48.800  51.200 100.000
%VAR: (-33.48)(-33.47)(-33.48)(-33.48)     .00
DIFF: (-16.34)(-16.33)(-16.34)(-16.34)    .000
STDS:      831     831     831     831     ---

STKF:    .1545   .1545   .1545   .1545     ---
STCT:    15.87   47.06   20.31   18.20     ---

UNKF:    .1544   .1544   .1544   .1544     ---
UNCT:    15.87   47.06   20.31   18.20     ---
UNBG:      .13     .34     .16     .00     ---

ZCOR:   2.1022  2.1022  2.1022  2.1022     ---
KRAW:    .9998  1.0000   .9998   .9998     ---
PKBG:   121.53  141.66  125.91     .00     ---

TDI%:   11.326   6.243   4.077    .000     ---
DEV%:       .6      .2      .4      .0     ---
TDIF:  HYP-EXP HYP-EXP LOG-LIN    ----     ---
TDIT:   128.63  130.00  132.50     .00     ---
TDII:     15.9    47.4    20.5    ----     ---
TDIL:     2.77    3.86    3.02    ----     ---

Again, very similar trends in the CaF2 and also uncorrelated with intensity...

So I have a hypothesis, but I'm asking seriously: can anyone explain these observations?

[Probeman]

Explain this if you can!

Recently I made some simultaneous k-ratio measurements on our PET crystals at high sin theta using F Ka on CaF2 (natural) and BaF2 (synthetic) in order to look at the effective takeoff angles as described here:

https://probesoftware.com/smf/index.php?topic=1569.msg12191#msg12191

But I found that were some significant peak shape issues that fortunately could be corrected for quite well using the area peak factor (APF) method, as described here:

https://probesoftware.com/smf/index.php?topic=536.msg12187#msg12187

What I did not mention was that there were also some significant time dependent intensity (TDI) issues during the measurement that also needed to be corrected for, even with a 20 um diameter beam and at 30 nA!

Here is what spectrometer 1 (TAP) intensities looked like:



And spec 2 (LTAP):



And spec 4 (TAP):



You have probably noticed that that the TDI effects were largest in spec 1, less in spec 2 and essentially nonexistent in spec 4... now why would that be?  Since all three spectrometers were measuring the same points at the the same time!?
...

So I have a hypothesis, but I'm asking seriously: can anyone explain these observations?

No speculations on what could be causing these different TDI trends on 3 simultaneous spectrometers...?

[sem-geologist]

No speculations on what could be causing these different TDI trends on 3 simultaneous spectrometers...?

speculations... there is a bit too little information on other stuff to exclude the hardware effects. In case there is no hardware effect, I would speculate that maybe, maybe, this is crystallographic orientation thing. I cant remember now where I had read this, but as far I could remember it was somewhere shown that analyzing apatite-F at different orientations shows sever F loss where at other orientations shows nearly no F loss. I could get from that experiment the mind-shortcut that it is e-beam--crystallographic orientation controlled F loss. But Your experiment makes me start to think that maybe there is no loss at all (it was always for me illogical that negative (-1 valence) F ion would move toward negatively charged sample surface! There is probably only crystallographic repositioning of F, where for some spectrometer it gets hidden behind other atoms in lattice and thus absorption significantly goes up (decreasing intensity), while for different angle spectrometer they stay completely unhindered.

I think it would be easy to throw this hypothesis out the window If same behavior would be observed after redoing experiment with rotated sample.