XRF update

 It's been some time since I posted -- for some reason, although Covid meant we spend much more time at home, I have remained busy.  But it's time for a quick update on my home-made XRF setup.

I started by finding some relatively inexpensive scintillator crystals that looked suitable for XRF, but got hung up on the detector side.  Photomultiplier tubes are fairly inexpensive but require well regulated high voltage, on the order of 1,000V.  They also are fairly bulky, a disadvantage if you want to perform XRF in the field.  A number of semiconductor manufacturers make something called silicon photomultipliers, commonly referred to as SiPM's.  They typically require something on the order of 30V to operate, much friendlier -- and they are much smaller than PMT's.  A 6x6mm SiPM on an evaluation board costs about $100.

However, I came across a thing called a pocket geiger radiation detector, sold by Sparkfun.  It uses a 10x10mm detector that by itself costs about $100 -- but they're selling it on a circuit board for only $69.95.  In its as-delivered condition it can't be used as the detector for XRF because the design uses comparators, which remove the pulse-height information needed.  However, the circuit board has some pads (possibly used for test purposes) that DO make the analog signal available.  So I bought a pocket geiger and started experimenting.  I used the 60Kev gamma rays from Americium (found in ionization type smoke detectors) as the radiation source, to excite XRF in a thin brass sheet.  I was hoping to see pulses of various heights coming out of the amplifier (from the mixture of copper and zinc that make up brass), and sure enough, I did.

While this result is encouraging, it's not quite enough.  To analyze ferrous metals the copper shield has to be removed, so the ~10Kev x-rays aren't absorbed.  But removing the shield results in a large 60Hz signal coming in from all the power lines.  The detector circuit has very high gain so this is an unavoidable problem with an unshielded detector.  So currently I'm making an aluminum box that will house the detector, and also serve as a shield to block 60Hz and those pesky 60Kev x-rays.  The box will have a partition with a hole in it to admit the x-rays emitted by the sample, and a removable end that, when installed, will fully shield the detector from all that power line noise.  The Americium disks will be placed around the hole so the detector will be shielded from them, but can "see" the fluorescence x-rays.  I have some 1/8" thick lead sheet that will line the interior of the box, just to make absolutely sure that I have no exposure to x-rays.

Then there is the software needed to process the pulses and assign their peak height to individual channels.  That information, in turn, will be used to determine what element(s) are present in the sample.  First things first though -- I need a robust test platform I can depend on before spending the effort on S/W.