Needing more than a spark test?

[graham-xrf]

Alpha particles have to be accelerated to a very high velocity in order to generate x-ray fluorescence in a target. This would require a linear accelerator. In addition, the beam would have used in a high vacuum. The Americium 241 source emits a relatively low energy alpha particles, easily stopped by a few sheets of paper. It is the 59.7Kev gamma ray that is responsible for the fluorescence in target materials. The fluorescence photons re not light. They are much higher in frequency , above UV and in the x-ray region where their "frequency" is measured in terms of energy( e =hf).

To detect these x-rays, they are passed through the scintillation crystal where again the photoelectric process takes place and visible light photons are generated. The sum of the energies of the photons generated is equal to the energy of the x-ray photon.

Yes - and I should have seen it earlier. The spectrum for AM241 in post #15 shows it has the 59.7keV emission line on the right, which is enough to excite the K and L lines of all the elements.

I am guessing it it is the role of the scintillator crystal material to capture these beyond-visible wavelengths, and re-emit light photons into a photo-multiplier or other technology detector. It must be a special effect, and not just from excitation of it's own elements.

Thanks also for the explanation of experience with solutions. I did think that route would have some downsides.

 

[RJSakowski]

Yes - and I should have seen it earlier. The spectrum for AM241 in post #15 shows it has the 59.7keV emission line on the right, which is enough to excite the K and L lines of all the elements.

I am guessing it it is the role of the scintillator crystal material to capture these beyond-visible wavelengths, and re-emit light photons into a photo-multiplier or other technology detector. It must be a special effect, and not just from excitation of it's own elements.

Thanks also for the explanation of experience with solutions. I did think that route would have some downsides.

Here is a fairly thorough explanation of the scintillation process.https://en.wikipedia.org/wiki/Scintillator

 

[rwm]

RJ- I thought you were just a fisherman!? But wow! You guys are very impressive. However, I am not at all shocked that we have this kind of talent on HM! This is a very interesting project that I would like to hear more about. I have always wanted an XRF since I got into casting (metal, not trout.)
I am staring at a photomultiplier tube from a scrapped gamma camera. I think I have some Americium in the basement. Is this really possible? Talk about a quarantine project!
Robert
Edit: I believe all the current gamma cameras are solid state detectors. Is that an option?
Robert

 

[hman]

Graham -

Thanks for your explanation. I really goofed when I said "monochromator" ... actually meant to ask about way to distinguish the wavelength/energy of the fluorescence (Xray) photons. Your mention of a spectrum analyzer did set me straight. Bottom line, though, is that I'm an old geezer and sometimes kinda dinosaurian in my thinking. Ah, well ...

 

[graham-xrf]

Graham -

Thanks for your explanation. I really goofed when I said "monochromator" ... actually meant to ask about way to distinguish the wavelength/energy of the fluorescence (Xray) photons. Your mention of a spectrum analyzer did set me straight. Bottom line, though, is that I'm an old geezer and sometimes kinda dinosaurian in my thinking. Ah, well ...

Old geezer me too, but I nevertheless am still surprised and irritated when the stuff that reminds us of it gets in the way of a bit of "growing old disgracefully".

As I understand it so far..

1. You start with some of these smoke detector sensors. Here is one I extracted from an old smoke detector.
(Yeah - nearly live. On the desk in front of me. Making a mess!)

Not that I expect any Brownie points from HM for doing this in their cause, but I am very aware that I am handling a by-product of plutonium decay that is a fissile material with a large neutron absorption cross section 5700 barns (yes - as in "big as a barn door"), which gives it a critical mass of around 9kg to 14kg. This is not quite what I mean when I say "growing old disgracefully, but you get the idea.

That said, the amount in there is around 2.62E-7g or 262 nano-grams. It is in the little pressed metal setting "button", which comes out and bounces on the desk, and makes it to the floor, and ..yeah!



Of course, the great YT resource shows it much better than I can, and one in particular comes with fun sparks and zap stuff..
--> How Radiation Works using Americium 241, Alpha Particles and Gamma Rays

and another where at about 3:05, we actually see the radioactivity measured (no gloves)
--> Extraction of Americium 241 from Smoke Detector

It occurs to me that one might get up an effective source from grinding up a Coleman lamp gas mantle. You can get the thorium version, which work better than the yttrium type as lamps, from eBay.

2. We arrange the Am241 buttons in a circle, around a shield tube end. The radiation from the buttons strikes the metal under test, and out comes photons you can't see, back at the detector in the middle of the tube. Yes - the energy that exits the metal atom does have a wavelength, and it might not be at some convenient visible (wavelength).

3. It hits the material of a scintillator material. A crystal of cesium iodide.
They come at all sorts of crazy prices asked, from USA to Ukraine, from 20 bucks to a couple of hundred $$.
Like this --> eBay CsI

Now we discover there are various scintillation materials, like Na(TI), and every kind of combination fitted up with detector photomultipliers, etc. I have not explored this enough, but know that the electron tube type is the only sort that can get down to the lowest energy detection without generating a lot of competing electrical noise.
The flash given off by the scintillator material is proportional to the photon energy arriving.
I like the Hamamatsu tube with the cesium iodide scintillator already bonded to the tube.

4. The light pulse triggers electrons from a photo-sensitive cathode in a photomultiplier tube. The nature of the thing is to produce in proportion, perhaps up to a hundred million more. An electrical pulse, which can be measured. Clever gadget, but the circuits are simple.





This is, so far, the most expensive part. It is also the place where we get the most variety in DIY approaches. You don't have to get this particular one. The majority of enthusiasts out there are looking to make radiation detectors of various kinds. They want the clicks, and the meter showing the counts per second rate.

What we are after is more subtle. We want to measure the pulse height, duration, and shape.
That is - I think so. Everything I read looks to be about that.



5. If you are still with me so far, this amounts to some fairly simple, and not too expensive kit, given that we are trying to make a nondestructive point-and-shoot general materials detection and analysis gadget.

According to the link from @RJSakowski , the Wikipedia on Scintillators, you get a pulse. The stuff is very much hard sums about scintillation science, but it is worth scanning even the indigestible , just for perspective.

6. Measuring the pulse(s).
This is where we need not get all entangled with old project stuff like the "Therimino" thing. I have not yet untangled it all, but it seems they were using the audio input microphone channel on a computer to sample the signal as a 192kHz A-to-D converter, in the same manner as ham radio enthusiasts would contrive to make spectrum and waterfall displays of signals. They also had a PIC micro-controller USB adapter.

I think it is a series of pulses from each scintillation, triggered, sampled, and averaged.

I have not yet figured it all out, but I am thinking that yes - one could use the audio channel as a signal waveform sampler, though my preference would be something more made for the purpose, such as a dedicated A/D converter instrumentation card. Or use the Audio channel. Either way, all you need for final hardware is £35 or so of Raspberry Pi, or Arduino. It could even be a smartphone app.

I freely admit I have not got it all figured out yet, but a steels alloys analysis without breaking the bank seems kind of possible

 

[graham-xrf]

Edit: I believe all the current gamma cameras are solid state detectors. Is that an option?
Robert

Robert: No, I don't think so. What the detector can do depends on the scintillation material. You can get gamma detectors in several technologies.

The ranges of energies, and wavelengths that solid state (Si) detectors can do seems to have a lower end that is electrical noise limited - that the thermal noise from molecules wiggling about as in any semiconductor material. The electron tube types can apparently work at low noise to much lower levels.

Again - I am in the "soak-up" phase of learning much of this science. When I know enough to give a better answer, I will say what has been shown to be so, and who says, and so on.

I am less into fully getting to all the theory expounded , and more into wringing out the practical stuff one needs to make something like this work. I am not at all sure I am up do doing something like this all by myself. I may need some help. You can be sure that if I get to the point of getting up a working, proven verified, demonstrable build, then it will be posted here, with all the instructions.

 

[RJSakowski]

It looks like you are getting the hang of it. In bullet 1., I wouldn't mess with the Th 232 It has a much longer half life, 1.39e10 yrs. compared to 275 yrs for Am 241 which it will be a much weaker emitter/gram of material. Also, the largest danger with alpha emitters is from ingestion. Although they are extremely short range, alpha particles are extremely active. Any process that would have the potential of creating small particulates should be avoided. In contrast, the AM 241 is in the form of a pellet and and can be used that way which minimizes any potential ingestion risk. As I see it, the form found in smoke detectors is ideal for this application. Think of the array light a camera light, illuminating the target. If mounted on a lead backing plate, all of the beam will be directed forward toward the target with almost no radiation from the source interfering with the desired signal.

The combination of scintillation crystal and photomultiplier make up a radiation detector. As I recall, radiation detectors operate in three distinct modes, the Geiger region, a proportional region,and the avalanche region. Which region of operation you're in is determined by the mukltiplication rate of the individual dynodes in the multiplier which is controlled by the potential difference between the dynodes.

For a pulse height analyzer, we want to operate in the proportional region. In this region, the output current generated by each event will be proportional to the energy of the x-ray photon.

From there, we need to sort out the energy, as represented by the output current. I expect that this could be done by converting the current signal to a voltage and using an A/D converter to create a digital value. A gate would activate the A/D converter and lock out any additional input while the pulse information was being processed. The processing could be done by a series of conditional cases in the program sorting the event into an appropriate accumulator array and the array would be scanned to provide the count vs. energy display. The processing part would need to fairly fast to minimize dead time and loss of events. From my experience, the events aren't to closely timed, at least with fairly weak sources. The energy spectrum would be built up, pulse by pulse until it was reset and displayed on some suitable output device.

A simpler scheme would be look at a single energy, knowing that a particular element would be emitting x-rays at that energy. A fairly simple pair of comparators could define the voltage box, V1< Vsample< V2, and if the sample voltage is in the box, a counter is incremented. Ideally, You would compare the signal to a background signal and calculate the ratio. The ratio would be compared to the ratio from a known sample to give a thumbs up or down to the identity.

It would seem that we have existing devices that could be hacked. I have an app on my Android phone called Physics Toolbox Suite which has, among other interesting tools, an audio spectrum analyzer. It is sorting a real time audio signal into a frequency spectrum which can be frozen.

 

[graham-xrf]

@RJSakowski :
Entirely agreed. I did say we should not eat the stuff! The bits of fragile (used) thorium mantle sticks to fingers, and floats about in the air, and generally much harder to control until one gets it held down somehow.

That said, the smoke detector little "button" did it's best to become inaccessible. I had to crawl around on the floor, looking all over the place to retrieve it. After all - it might eventually become 1/8 of my steel alloy sensor!

Now that we are beginning to figure this out, even to the extent of a design of our own, it is maybe time to look at the wish list of features. Can we aspire to know more than which elements are present? Might it deliver approximate percentage proportions? Is it, or need it be battery powered? Is it a gadget with a USB lead to a smartphone app? Are there any low cost Silicon photodiodes that might do the job?

This needs a pause for HM contributions. Feel free - no matter how critical, or crazy. If it is just clouded by wishful thinking, I am happy to blame the lockdown effect!

 

[homebrewed]

Scintillators and PMTs aren't used in commercial EDX tools these days. Instead, they use large-area silicon detectors, basically high voltage diodes. As in the case of a scintillator/PMT, the pulse height is proportional to the x-ray photon energy. That, by the way, is what "EDX" stands for -- energy dispersive x-ray spectroscopy. There is another approach that uses a crystal as a kind of diffraction grating, where the grating is the crystal lattice itself. As the crystal is rotated the incident x-rays are diffracted at different angles toward a fixed detector. Very much like a visible-light monochromator. This type of system offers much higher resolution but, because it acquires one wavelength at a time, it is much slower. Also, you STILL need some kind of detector. This type of system is called a WDX, or wavelength-dispersive x-ray spectrometer. Our old WDX used an ionization detector, a lot like a Geiger tube -- but IIRC the active gas was either a mixture of methane or carbon monoxide (!).

Also check these links out: http://www.noah.org/science/x-ray/detector/ https://benkrasnow.blogspot.com/2012/11/large-area-detector-for-x-rays.html ; and you might be able to acquire some of the hardware here. I haven't gone so far as to check prices from the latter so it still could be out of the range of what hobbyists would be willing to pay.

 

[RJSakowski]

@RJSakowski :
Entirely agreed. I did say we should not eat the stuff! The bits of fragile (used) thorium mantle sticks to fingers, and floats about in the air, and generally much harder to control until one gets it held down somehow.

That said, the smoke detector little "button" did it's best to become inaccessible. I had to crawl around on the floor, looking all over the place to retrieve it. After all - it might eventually become 1/8 of my steel alloy sensor!

Now that we are beginning to figure this out, even to the extent of a design of our own, it is maybe time to look at the wish list of features. Can we aspire to know more than which elements are present? Might it deliver approximate percentage proportions? Is it, or need it be battery powered? Is it a gadget with a USB lead to a smartphone app? Are there any low cost Silicon photodiodes that might do the job?

This needs a pause for HM contributions. Feel free - no matter how critical, or crazy. If it is just clouded by wishful thinking, I am happy to blame the lockdown effect!

You need a scintillation detector to find those wayward buttons.

The PHA we have been discussing are capable of measuring quantity provided there is a means of calibration. The easiest way would be to have a known sample.