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.
Tom McCarthy
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Bremsstrahlung Radiation Detection

Post by Tom McCarthy » Fri Aug 01, 2014 11:17 pm

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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Re: Bremsstrahlung Radiation Detection

Post by Richard Hull » Sat Aug 02, 2014 6:18 am

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.

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Re: Bremsstrahlung Radiation Detection

Post by Chris Bradley » Sun Aug 03, 2014 9:54 am

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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Re: Bremsstrahlung Radiation Detection

Post by Dennis P Brown » Sun Aug 03, 2014 10:46 am

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 » Sun Aug 03, 2014 2:44 pm

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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Re: Bremsstrahlung Radiation Detection

Post by Chris Bradley » Sun Aug 03, 2014 3:17 pm

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 » Sun Aug 03, 2014 7:38 pm

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 » Sun Aug 03, 2014 9:14 pm

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.

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Re: Bremsstrahlung Radiation Detection

Post by Tom McCarthy » Wed Aug 06, 2014 6:42 pm

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 » Thu Aug 07, 2014 9:47 am

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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