Bremsstrahlung Radiation Detection

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Tom McCarthy
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Bremsstrahlung Radiation Detection

Post by Tom McCarthy »

Hi all,

One idea I'm considering for entry of the Fusor (not built) is an examination of Bremsstrahlung radiation in the Fusor.

From what I've managed to find on the forum, the main thing to get is a mica-windowed GM detector and then probably begin filtering what x-rays I detect. Can anybody elaborate on this or link some good reading material - other than the Wikipedia page on Bremsstrahlung radiation...

Apologies if this is classed as "spoon-feeding," I've found very little information in older threads. It might be there, but so far I've seen very little.

Kept it short and sweet as it's bed-time here in Ireland.

Thanks,
Tom
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Richard Hull
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Re: Bremsstrahlung Radiation Detection

Post by Richard Hull »

Bremsstrahlung is usually associated with turning or a change in vector motion of fast moving mater particles due to electrostatic or magnetic fields. This is not found in the fusor. However, it can be used a bit freely to describe x-radiation from mater particles smashing into a metal wall or grid and this happens with great frequency and in wholesale numbers in a fusor. So let us call it x-radiation as it is produced in a single instantaneous dead stop of fast moving particles.

A full exposition is given on x-ray issues in many FAQs in a number of the forums here.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Chris Bradley
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Re: Bremsstrahlung Radiation Detection

Post by Chris Bradley »

Richard Hull wrote:This is not found in the fusor.
Are you sure that is true, RIchard? There is a continuum of soft X-rays from UV up which, I have always previously thought, was strongly associated with gas excitation alone, this being a mix of recombination and electron deceleration events. I do not know the relative contributions, but that would be a research point and I'm not sure it cannot be found at all.
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Dennis P Brown
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Re: Bremsstrahlung Radiation Detection

Post by Dennis P Brown »

While no expert on this subject, I do know and have seen devices to read the complex x-ray spectrum of a plasma and frankly, such a setup is a very expensive and delicate device requiring complex crystals specially cut and set at precise angles with special detectors (assuming a small system is bright enough); even then, there are complex issues of calibration and reading of the data. Getting the correct line for the desired Bremsstrahlung events isn't defined as some universal and fixed energy (like say the double line for sodium) but requires calculations that depend on the plasma energy, species and other factors that can strongly affects the actual energy spectrum of that process. However, if you desire to address this project in a real way, there are excellent papers published on this subject all the time and also some grad work relative to fusors - try google.
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Re: Bremsstrahlung Radiation Detection

Post by prestonbarrows »

Chris Bradley wrote:There is a continuum of soft X-rays from UV up

Just because one detects a continuum of photons does not mean your source is initially emitting that same spectrum. Even starting from decay of an isotope with a well-defined mono-energetic gamma emission, you will measure some spectrum of energy on your detector. This is known as the Compton continuum and is basically due to photons getting downscattered around the environment before entering the detector. You end up with a main peak at the original gamma energy and a relatively flat plateau across lower energies with smaller peaks at either end due to the scattering.

Image

To first order a fusor is basically an inverted x-ray source. Bremsstrahlung scales very heavily with mass, so is almost entirely from electron motion. Since there is basically zero electron confinement in a fusor, I would expect almost all the x-rays to come from surface impacts of electrons rather than the bulk plasma. This is assuming there are negligible electron-D collisions during the electrons single pass towards ground; you would have to look up the collision cross sections for that. It would be energy dependent too.

Because of this, Bremsstrahlung is probably not a very useful diagnostic for an IEC device as compared to magnetic confinement etc.

These electrons will have some spectrum of kinetic energy before collision. The actual impact will also be made of some chain of sub-collisions. Both of these should make the x-ray output have some fairly broad spectrum with a peak energy and intensity that correlates with the grid voltage.
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Chris Bradley
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Re: Bremsstrahlung Radiation Detection

Post by Chris Bradley »

I'd make a guess cross section wouldn't be too far off 10^-16 cm^2 at 100 eV, +/- an oom, with the background atoms.

So we'd be looking at 10^14/cm^3 density, and so a probability of interaction of around one per 100 cm. In a 10cm radius fusor at 10mA, I can't see why we wouldn't be looking at >10^15/s oom electron collisions. I'd imagine most of it would be small incident scattering so any Brems would be low energy.

I'd suggest you might discriminate this from any other continuum by modulating the pressure, keeping the current the same, and seeing how far off the voltage versus count rate changes in a different way that you'd expect from a plot of differential cross-section, but that rather depends on having good data on what the cross-section should be rather than my oom guessing... just thinking aloud....
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Re: Bremsstrahlung Radiation Detection

Post by Tom McCarthy »

Thanks for the comments everybody.

Right now, the plan is to get at least one PMT and associated crystal - either BaF2 or lithium iodide and hook it up to some form of rate meter. From what's been said, I assume I'd have to filter what I'm detecting somehow. I'm not familiar with x-ray detection and scintillation, so would a good initial plan be to use some filter materials (e.g: zirconium, molybdenum) and see what each material lets through - any ideas on that front?

Thanks for all the input,
Tom
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Re: Bremsstrahlung Radiation Detection

Post by prestonbarrows »

Typical gamma detectors will output a pulse from each detection. For most detectors, height of the pulse is proportional to the energy of the photon detected. If you are interested in a specific energy of photon, you will need some form of filtering.

The simplest way is a multichannel analyzer (MCA) which will spit out a nice binned histogram in terms of energy like pictured above. You can then post-process this in software however you like. These are usually expensive, but the most straightforward for a beginner.

Otherwise, you will need a single channel analyzer which only counts pulses between a certain minimum and maximum voltage (photon energy). It can be tricky to get set up and reading the correct energy band if you are unfamiliar with such systems. An SCA can be used as a poor-mans MCA by using a small window and scanning its position across the energy band. Recording the counts during a given time at each channel position will essentially reproduce the MCA spectrum but take much longer and requires the source be stable over the entire time.

You could also try to physically block the photons outside of a given energy band before entering the detector using foils etc. This is tricky to calibrate the energy levels right and you will need a good amount of lead to block stray signals getting through the detector sides.
Tom McCarthy
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Re: Bremsstrahlung Radiation Detection

Post by Tom McCarthy »

Thanks for the advice Preston. I'm planning to get touch with the Irish branch of National Instruments, I hope they can sort out some sort of old MCA that's still functional or loan me one - as of now I don't trust myself/don't have the proficiency to set up my own one.

I'll do some more reading on the subject and maybe I'll eventually put together my own setup, but as of now I'm most likely going for a plug-and-play detector. Thanks for the pointers.

Cheers,
Tom
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Re: Bremsstrahlung Radiation Detection

Post by RealBorg »

It should be interesting to mount a piece of uranium glass in a fusor.
1.) You get x-ray fluorescence
2.) You get neutron activation

Tom
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Re: Bremsstrahlung Radiation Detection

Post by Rex Allers »

For a cheap but effective MCA, here's a link to to a popular Italian project.
http://www.theremino.com/downloads/radioactivity

I haven't used it myself but have seen others' good results posted. It is based on getting the signal input through a decent sound card. The software is free for download. If you browse around the links on the page above, they have a design for a simple interface board to power the PMT and do initial processing of the signal. I think you can buy the board too.

You still need to find yourself a probe (crystal/PMT combination). NaI(Tl) is probably the most common and available crystal for gamma spectrometry. If opened to air the NaI is very hygroscopic and absorbed water will destroy it. Just a warning that some of the stuff you find on eBay may not be good.

The Gammaspectrometry group on Yahoo Groups is probably one of the most accessible places to start acquiring basic knowledge.
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Richard Hull
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Re: Bremsstrahlung Radiation Detection

Post by Richard Hull »

Pure uranium in a fusor would fission, not activate. It has been tried and no one detected activation and fission detection would be difficult to detect without special internal sensors. There is just not enough neutron flux to do enough fission for easy detection or make good measurements with amateur equipment.

Uranium glass would most likely glow from UV Fluoresence. XRF data would be lost due to the shell, unless an internal sensor capable of detecting it were used. Even then the internal X-ray continuum might overpower the signal. There is only a tiny U mass in Uglass. Pure U would be the thing or a crystal of solid uraninite.

Still, no one would do this here as maybe only 3 people have the gear needed and they have been here long enough to read Carl Willis' report on the matter.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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RealBorg
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Re: Bremsstrahlung Radiation Detection

Post by RealBorg »

With activation I meant increased radiation due to fission products, not as in neutron capture.

Of course you would have to shield the uranium glass against the UV radiation, but given it's reaction to UV radiation and the amount of low energy x-ray produced in a fusor the result should be impressive.

Tom
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Richard Hull
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Re: Bremsstrahlung Radiation Detection

Post by Richard Hull »

Again, there will be no radiation due to fission products as the flux is not significant enough to produce enough fission products to be readily detectable once removed. It has been tried by one of our best fusioneers with good gear.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Dan Tibbets
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Re: Bremsstrahlung Radiation Detection

Post by Dan Tibbets »

If this threat is still active a perspective may be gained from this PDF. EMC2 measured Bremsstruhlung radiation in order to demonstrate what is called the Wiffle Ball effect. Initially the filtered x-ray output was low- buried in the noise floor, only after the density of electrons above a KE of 2000 eV increased was a signal achieved. There are reasons why the X-ray origins were from Bremsstruhlung, rather than electron - wall impacts, mostly, I think, from sensor geometry relative to the plasma, and the dynamics expected as the plasma confinement and thus density increased while the high energy (above 2000eV) electron input was constant. I don't know if these situations could be obtainable in a glow discharge fusor or even an ion gunned fusor. Without this dynamic I'm uncertain how Bremsstruhlung x-rays could be distinguished from electron - wall origins. I suspect this second source of X-rays dominates unless, as is paramount in this study, the energetic electron confinement can be pushed to where the Bremsstruhlung sourced x-rays can become dominate, or at least prevalent enough that the signal can be massaged to get convincing results.

http://arxiv.org/pdf/1406.0133v1.pdf

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Re: Bremsstrahlung Radiation Detection

Post by Reid Byron »

Speaking of economic and effective sound card MCA's the gamma spectacular may work wonderfully for the experiments you are trying to do.

http://www.gammaspectacular.com/gamma-s ... ry-drivers


and to my knowledge was developed by someone on this forum, which is very cool.
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Tom McCarthy
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Re: Bremsstrahlung Radiation Detection

Post by Tom McCarthy »

Thanks for the bump on the thread, Dan and Reid.

Dan,
I've taken a quick scan through the EMC2 paper..It seems to be mainly about the Polywell in general with very little on the actual Bremsstrahlung detection (that's just from a quick look, correct me if I'm wrong), I'll search around for related papers when I get a chance. Nonetheless, thanks for supplying the link.

Reid,
I have a GS-2000-PRO Gamma Spectacular system for neutron detection (with He-3 tube). Steven's a great guy to work with and if you're looking for something of that nature, I'd absolutely recommend Steven's wares. At the moment, I'm putting together a PMT set-up, but I haven't been able to get a NaI(Tl) crystal yet...I have a Hamamatsu R6233-05 PMT with a 76mm diameter face, so I'm looking for a 3" crystal. I'm not well versed in the details of PMT set-up, but I'm working towards it.

Cheers,
Tom
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