What technique could one use to create a 3 dimensional image/composition analysis of a material that is made of two isotopes of the same element? For instance, if I make a disc of magnesium, and then coat it with a thin layer of a magnesium isoptope, what technique/instrument could I use to tell me the dimensions/volume/area/thickness of the isotope layer, and could also render an image that differentiates the two? The only thing I have found is neutron scattering (XRD for isotopes), but I want a volumetric analysis.
hmmm .... atom probe tomography maybe? http://en.wikipedia.org/wiki/Atom_probe#Atom-probe_tomography_.28APT.29
Also maybe SIMS? http://en.wikipedia.org/wiki/Secondary_ion_mass_spectrometry
I'm not sure if either of these techniques have enough mass resolution to differentiate isotopes but perhaps this is a starting point. I know some people at Northwestern University who do APT and they may be able to answer any more questions if you have any.
There is something like high resolution SIMS with a microprobe: SHRIMP:) (http://en.wikipedia.org/wiki/Sensitive_high-resolution_ion_microprobe). This device is usually used for 3D isotope analysis in geological samples.
Take care,
Lukasz
How thin is your film? How smooth is your substrate? What isotope are you putting on the surface? As you said, neutrons do this well. You can use Neutron Reflectometry to determine the film thickness, roughness, and density. In the analysis of NR data you end up with a scattering length density profile. which is a depth profile of the thin film. At a neutron source their instruments typically probe a specific area on the sample, so you can combine the thickness of the film from NR with the surface area and you get the volume.
In looking at Mg scattering lengths, this would work if you were using natural Mg as a substrate and either Mg-25 or Mg-26 as the film. Mg-25 would work better because it would have better contrast to the natural Mg than Mg-26. Mg-24 wouldn't work. If your film is below 1000 Angstroms, and your surface roughness is somewhere below 80-100 Angstroms then NR can give you what you want.
A couple assumptions... The surface must be uniform, you can't have regions of inhomogeneity, warp of the substrate should be minimal, your sample size should be on the order of 1.5" diameter at a minimum, and you would need to make sure the surface roughness isn't too large by doing something like AFM.
Unlike SIMS, NR gives you a big-picture film thickness instead of a very localized result.
The film/coating is not homogenous, at the moment it will be a hand mixed and applied coating between 0.5 and 2mm thick. At later stages it will be applied in patches and nanofeatures. Also, why won't Mg24 work, is it because it has no net spin?
For NR you need to have contrast between layers in order to identify them as being distinct. Since natural Mg is mostly Mg-24 you wont have enough difference. To put that into numbers. Natural Mg at a density of 1.738 g/cc (wikipedia) has a scattering length density (SLD) of 2.315e-6 A^-2 (where A is an angstrom), and Mg-24 at the same density (it would be slightly different) has an SLD of 2.396e-6 A^-2. Those two are so close that I don't think you would have enough contrast to see the individual layer, or at least not fit the difference with any real confidence. For Mg-25 and Mg-26 the SLD's are 1.516e-6 A^-2 and 1.970e-6 A^-2. Mg-25 certainly has enough contrast to see the layer. Mg-26 isn't so good, but I made a quick model and I think it would provide enough contrast to fit a model to experimental data.
However, given the sample as you described it NR wouldn't work.
Hi Michael,
Atom probe tomography will distinguish between two isotopes of the same element and also provide a 3D reconstruction. I routinely conduct atom probe tomography and one of outputs is atom counts as a function of mass/charge ratio. So, one can identify all the naturally occurring isotopes and also obtain data in the form you require. Please feel free to write back if you have more questions.
Monica
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