Friday, April 30, 2021

My XRF Setup - Part 2 / The Hardware

 Amptek (Amptek) is one of the leading companies for Space instrumentation, experimental and research XRF equipment.

They have a fantastic line of products called X-123 - (1) detector element and preamplifier, (2) Digital Pulse Processor and MCA and (3) Power supply.  It is all-in-one device which requires external power and connection to a computer. The computer is used to configure and control the X-123 and receive the accumulated spectrum. Once the acquisition process is started, X-123 doesn't even need the computer connection until it is time to receive the data integrated by the internal MCA. 

X-123 is offered as Si-PIN, SDD, Fast SDD or CdTe variants and different length extenders between the case and the detector element. Fast SDD is their top-of-the-line model, while Si-PIN is more of a general use detector.

I got my detector from George Dowel (GEO Electronics). Internally, the unit  is identical to the commercial Amptek Si-PIN X-123 unit - George uses the OEM modules and installs them in a custom machined enclosure. The enclosure is a bit larger than the commercial version but this is an advantage - the alloy enclosure is actually a giant heatsink for the heat pumped by the TEC inside the detector element and larger area means better heat dissipation.

The "business end" of the unit - the 25 mm2 / 500 μm Si-PIN X-Ray detector element (model FSJ32MD-G3SP) with a thin, 1 mil Beryllium window. 
(!) This window must never be touched or come in contact with any object or it will be a costly mistake. 
There is actually very little reason for the red protective cap to be removed and the detector works with the cap on. I would expect to see some attenuation in the very lower end of the range when the cap is on but even with this cap, Calcium K-lines are detectable.
(George supplied a spare modified cap with built-in thin Kapton window. )

Energy resolution is 190 - 225 eV FWHM @ 5.9 keV, peaking time 25.6 μs and Peak-to-Background ratio: 2000/1 (typical).

Plot showing the efficiency as a function of energy for Si-PIN detector. 
The optimal energy range is 1 to 25 keV with efficiency >25%. Below 1 keV the loses from X-Rays traveling thru the air are significant - only 1cm of air will stop 90% of the X-Rays.
Above 25 keV the detector is still useable up to around 100 keV with a decreasing efficiency.

The X-123 unit supports USB 2.0 (mini-USB), RS-232 (2.5mm jack) and Ethernet (RJ45) computer connections. 
USB works just fine and it is fast so I never had the motivation to try any other of the interfaces. The Ethernet connectivity might require a future software release for full implementation, according to one Amptek document, but the orange data light is a useful indicator - it is lit solid if the data acquisition is stopped and it is blinking when the MCA is running. There is also a well documented auxiliary connector for gated counts and other functions.
External power is supplied with a very small, proprietary connector (George provides a spare connector in the kit). The power adapter is regulated and rated for 5V / 2.5A. 
The current rating is very important - while the unit only needs 500-700 mA during normal operation, there is a short, high-current transient of 2A during the boot up sequence and current limiting could damage the internal power supply module.
All of the power conditioning is done internally by the PC5 power supply module.

The Amptek software - DppMCA is quite good and I really like it! It is available on their web site for free. It is fairly easy to use and provides extensive set of tools for data acquisition and analysis.
There are a few features I wish it had but overall it does its job very well and it is well integrated with the hardware DP5 Pulse Processor.
Speaking of the DP5 module, the built-in hardware MCA in X-123 is quite impressive - 256 to 8192 channels (I normally use it in 4096 channels configuration) and 24 bits per channel (16.7 million counts). Acquisition time is selectable from 10 ms to 466 days. 

The Si-PIN detector is quite linear and 2 point calibration is all that is needed for most applications.
I use pure 99.9% Copper (Cu) foil - the Kα1 line at 8.05 keV and the Am-241 X-rays at 59.54 keV at the other end of the spectrum are sufficient for channel/energy calibration but more points can easily be added if necessary.

Gadolinium (Gd) is a Rare-Earth Element which also can be used as a calibration aid - all 4 peaks - Kα1, Kβ1, Lα1 and Lβ1 are visible in the plot.
My sample seems to have an impurity of Neodymium as seen here.

to be continued....

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