The Sericho Pallasite Meteorite - XRF Analysis

I obtained 2 fragments of the Sericho Pallasite Meteorite, "discovered" in Habaswein, Eastern Kenya in 2016. 

The meteorite has been known to local populations for many years but it wasn't until 2016 when the meteorite was officially classified as such. This was a huge meteorite - so far 2.8t were recovered.

A highly sculpted complete fragment of the Sericho meteorite. This specimen exhibits the "classic" meteorite look and the fragment is complete, not cut from a larger piece.

The second specimen is an end-cut piece from a larger fragment. It exhibits the typical "fusion crust" from the entry in Earth's atmosphere. 

The back side of the second piece with straight polished cut reveals the pallasite nature of the meteorite and a structure of olivine crystals.

The XRF Analysis setup - the polished cut of the meteorite is exposed to the 59.54 keV X-Ray source and the X-Ray detector. I placed the source a bit further away decreasing the intensity in order to eliminate parasitic peaks coming from Np, Au and Ag in the source itself. The weaker beam resulted in a long acquisition time - nearly 6 hours but produced a fairly clean spectrum.

XRF analysis plot.

As it turns out from an XRF point of view, much like most other meteorites, Sericho is typical and quite boring - no exotic metals are present - just Iron, Nickel and traces of Cobalt and Chromium.

The plot prominently features the Kα1 and Kβ1 peaks of Iron and smaller peaks of Nickel. Cobalt is in very low concentration (0.8%) and masked but if one looks for it, it can be seen in the irregular shape of the base (on the right side) of the Ni Kα1 photopeak. The Ni Kα1 at 7.48 keV is too close to the 7.65 keV of the Kβ1 of Cobalt just at the edge of the detector resolution. The 6.93 keV Kα1 line of the Co is dominated by the Kβ1 Fe at 7.06 keV and can not be differentiated. Chromium can not be detected at all with my setup due to the trace amounts (0.03%)

My XRF Setup - Part 3 - Exciter

The exciter is the second main component of an XRF setup - this is the source of the primary X-Rays.

Two type of Exciters are generally used - X-Ray tube or Radioactive Isotope.

X-Ray tubes 

Pros:

- provide high-intensity beam

- low limit of element detection

- easy on/off capabilities 

- fast integration times

- fairly clean and uniform spectrum

- very small spot of irradiation / sampling 

Cons:

 - big, heavy, very delicate

- require additional cooling

- large, hazardous HV power supplies

- need for safety interlock system

- heavy beam collimators

- substantial shielding is required

- consideration must be made about beam scattering and reflection

- Not as portable

Radioactive Isotope source 

Pros: 

- smaller, lighter and simple to use

- 100% reliable

- very portable for field use

Cons:

- low intensity beam requires long acquisition times

- shielding is required as well a shutter-type on/off system

- highly regulated

- danger of contamination if source is damaged

- spectrum is not as clean and can contain various peaks

 The holder of the exciter was designed with TinkerCAD and 3D printed

While this method works and it is a convenient way to use a number of small individual sources, the main problem is that they need to be placed at some distance from the Object Under Test which decreases the flux and irradiates a larger area of the specimen. One can not easily select the area being sampled.

I made a small, single, directed source with a  Lead collimator / shield which works very well and I can place it much closer to the specimen without the detector picking up the primary X-ray.

Update: X-Ray tube is added as yet another option to do XRF excitation and I built a custom controller for it. See THIS post.