Friday, April 30, 2021

My XRF Setup - Part 2 / X-Ray Spectrometer

 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 Spectrometer - (1) detector element and preamplifier, (2) Digital Pulse Processor and MCA and (3) Power supply.  It is all-in-one device which only requires an external power and connection to a computer. The PC software by Amptek called DppMCA is used to configure, control the X-123 operation and receive & visualize the accumulated spectrum.

Once the acquisition process is started, X-123 doesn't even need the computer connection until it is time to receive, save and display the data, integrated by the internal to X-123 multi-channel analyzer (MCA) located on the DP5 module. 

X-123 Spectrometers are offered with a variety of detectors -  Si-PIN, SDD, Fast SDD or CdTe and can employ 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. CdTe detectors are great for the higher energy region - up to 150 keV at the expense of resolution and internal noise.

I got my detector from George Dowel (GEO Electronics) as Geo-123. 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 Amptek version but this is an advantage - the aluminum alloy enclosure actually acts as a giant heatsink for the heat pumped by the TEC module, located inside the detector element and larger surface area results in better heat dissipation.
If there is one thing I wish for, is to have at least 1" or more extension between the detector and the main enclosure - this could help a lot with detector placement in relation to the sample and the exciter.

The "business end" of the unit - the 25 mm2 / 500 μm Si-PIN X-Ray detector element (model FSJ32MD-G3SP) with a thin, very fragile 1 mil Beryllium window.
(!) This window must never be touched by hand or come in contact with any object - such thing could turn into a very costly mistake!

Out-of-the-box there is a red polyethylene protective cap installed. 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 low end of the range (0 to 2 keV) when the cap is on but even with this cap, Calcium K-lines are actually detectable.
(George supplies a spare modified cap with a 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 for a SiPIN detector is 1 to 10 keV. 
The range of 10 keV to 25 keV exhibits a drop in efficiency to ~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 60 keV with a rapidly decreasing efficiency.

The X-123 Spectrometer supports USB 2.0 (mini-USB Connector), RS-232 (2.5mm jack) and Ethernet (RJ45) computer connections. 

USB works just fine and it is very fast so I never had the motivation to try any of the other interfaces. The Ethernet connectivity might require a future software release for full implementation, according to one Amptek document, but the orange data light on the port is a useful indicator - it is lit solid if the data acquisition is stopped and it is blinking when the MCA is running and storing data.
Other connectors on the back are the proprietary jack for the External Power supply and there is also a well documented auxiliary connector for gated counts and other functions.

The "sandwich" of DP5 Digital Pulse Processor (top board) and PC5 power supply module on the bottom. A ribbon cable connects DP5 to the PA230 Pre-amplifier board.

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 around 2A during the boot up sequence and any current limiting bellow 2A could damage the internal power supply PC5 module.
All of the power conditioning and the generation of various voltages 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 the AmpTek web site for free. The software is fairly easy to use and provides extensive toolset for data acquisition and analysis. The peak identification feature using energy reference libraries is very useful. The UI is logical and easy to use and ability to customize the color schemes.

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 MCA can be set to work in two modes - NORMAL and DELTA. In Delta mode it shows the spectrum, refreshed every second with pulses integrated over the past 1 second.

Combination of coarse and fine amplifier gain yields an overall Gain, continuously adjustable from x0.84 to x127.5 - the amount of preamp Gain determines the spread of the spectrum over a specified number of channels in the MCA.
For example, when using 4096 Channels, a Gain of x18.5 allows coverage of 0 to 62 keV range.

The Si-PIN detector response 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 high end of the spectrum are sufficient for channel/energy calibration but more intermediate points can easily be added if necessary using different pure metals.

Gadolinium (Gd) is a Rare-Earth Element which is very interesting to XRF with its many peaks and also can be used as a calibration aid since both L and K-lines show up nicely at the low and high energy range of the detector.

XRF of a 99.9% pure 1" disk of Gadolinium (Gd).

Most of the Gd peaks can be easily identified - Kα1, Kα2, Kβ1, Kβ2, Lα1/Lα2, Lβ1, Lβ2 and even Lγ1, Lγ2 and Ll are visible in this plot. Obviously, Lα1 and Lα2 can not be separated - they are only 30 eV apart - way too close for the 190-225 eV resolution of the Si-PIN detector.

The very low count "hash" above the group of Gd L-lines is caused by Np-237 L-lines coming from the native spectrum of the exciter - 25 μCi of Am-241. Am-241 decays to Np-237 and the L-lines of the Neptunium are Rayleigh scattering and somewhat visible in the spectrum. 
The Exciter's X-Ray beam is collimated and reduced down to about 3mm spot for a precise sampling so the overall count rate is low as expected but the spectrum is nice and fairly clean. Longer integration times are to be expected with this type of exciter and I use a different exciter with a broader beam for general purpose.

1 comment:

Boris said...

How much did it cost you to build the XRF setup?