Viborg Fusor Radiation Measurements

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Schistad
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Real name: Bernhard Schistad

Viborg Fusor Radiation Measurements

Post by Schistad »

In my second post, we will look at the radiation measurements in a little more detail.
Gamma Spectrum
After repeated measurements of very high x-ray levels with the Insepctor, I decided to see if it was possible to measure the spectral distribution of the radiation to see if it was bremsstrahlung or blackbody radiation. The spectral distributions of these two kinds of radiation are quite different.
We have a reasonably large 1.5" x 2.0" NaI(Tl) scintillation detector with attached photo multiplier (made by Steven Sesselmann who is a member of this forum). I have very good experience of doing gamma spectroscopy with a quality soundcard with software as a spectrum analyzer.
Therefore, I connected the detector with the PC and started the excellent Theremino_MCA software. Then I placed the detector in front of the viewport window, opened the deuterium supply, switched on the diffusion pump and turned the high voltage up to 20 kV.
This is a screen dump of what happened:
Spectrum screen dump.png
I then did a measurement of the background with the HV switched off and calibrated the energy scale using a known Cs137 source.
The data with calibration and with background subtracted looked like this:
Fusor scpectrum.png
Converting the photon energy to temperature (using Boltzmann’s constant as a conversion factor like the Iter people do) yields the following curve:
Plasma Temperature.png
Now, to see if this is blackbody radiation, I calculated the Planck spectrum of a black body at 75 MK.
This theoretical graph together with the observed graph of count vs. wavelength agree very well:
Blackbody Spectrum.png
Therefore, my conclusion is that the x-ray radiation from the fusor is blackbody radiation from a plasma, which is not red hot, but rather gamma hot!

Neutron measurement
For measuring the neutron output, we have acquired a Russian SNM18 proportional detector. We have made a paraffin moderator and connected the tube to a charge sensitive amplifier, based on a chip from Cremat. Supplied with a high voltage of 1350V this tube is an excellent neutron detector with a very high probability of count for thermal neutrons.
Again, we use the soundcard as a spectrum analyzer end the Thermino_MCA software for data acquisition and analysis.
The tube generates quite a bit of noise at the lower channels, as shown on the raw count (with high voltage switched off) as shown in the screenshot below:
Neutron Background.png
Based on this plot, we set a pulse height threshold at channel 100. We then did a 4-minute background count without HV the 4 minutes with HV on and finally 4 minutes with HV off.
We then plotted the spectrum with the highest background count subtracted.
A measurement series was performed at 15 kV and 20 kV. The reason for going higher is that we have contaminated the ceramic isolators of the HV supply line inside the vacuum chamber with tungsten deposits and we need to clean the system before we can go much above 20 kV again.
The raw data of the background runs and the HV run are shown here:
Raw Neutron Spectrum.png
The neutron counts per channel with background subtracted (always two background runs with the highest value subtracted) at 15kV and 20kV were the following:
Neutron spectrum.png
As we can see, the count at 20 kV is approximately 3 times the count at 15 kV as should be expected from the fusion cross section.
We also tested running without deuterium, but there was no difference between HV on and off if, no deuterium is supplied before the measurement.
Finally, we have calculated the average neutron count in the detector. In order to be sure of not counting background, we have ignored the lowest channel numbers. This gives a total count of 51236 neutrons in 240s or 213 neutrons/s.
With a 70% POD in the detector (according to the manufacturer) and the detector covering a slid angle of 0.2 Sr and 50% of the fusions yielding a neutron, we get a total fusion count of 38,000 fusions/s.
This is in remarkably good agreement with my “back of the envelope” theoretical calculation. The cross section for deuterium at 20kV is 0.3 mBarn. With an ion current of 9 mA and a small plasma sphere inside my internal grid, I calculate a fusion rate of approx. 48,000 fusions/s.
Finally, the output power is 200W while the contribution from fusion is just 21 nW.
So it is still very far from beak-even!
Cheers,
Bernhard Schistad
Bruce Meagher
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Re: Viborg Fusor Radiation Measurements

Post by Bruce Meagher »

Congratulations on your operational fusor. Very nicely done!

I do have a question about your x-ray spectrum. Have you considered the attenuation through your glass viewport and through the aluminum shell of your NaI(Tl) detector? Attached is a plot of attenuation vs photon energy through borosilicate and aluminum. I believe the true spectrum inside the fusor, or even outside the window, would be considerably different than what you measured. I also don’t follow the 30-40keV photons shown in your plot during your 20kV run. Is that something you expect?

What low energy gamma peaks did you use to calibrate your detector?

Bruce
(edit 5/20/20: fixed graph)
Attachments
Attenuation vs Energy.jpg
Last edited by Bruce Meagher on Wed May 20, 2020 7:06 pm, edited 1 time in total.
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Richard Hull
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Re: Viborg Fusor Radiation Measurements

Post by Richard Hull »

Your report satisfies me and I will place you in the Neutron club. What was the running pressure at 20kv during your timed run?

Your neutron count is virtually identical to my earliest fusion results back in 1999 when I was supply limited to about 22kv in fusor III.

Well done.

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
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Schistad
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Re: Viborg Fusor Radiation Measurements

Post by Schistad »

The pressure was 4 Microns. The sweet spot for our fusor is between 3 and 5 Microns.

Bernhard
Bruce Meagher
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Real name: Bruce Meagher
Location: San Diego

Re: Viborg Fusor Radiation Measurements

Post by Bruce Meagher »

I messed up the previous chart for aluminum (charted was 0.05mm not 0.5mm). "I must have put a decimal point in the wrong place or something. $#@$! I always do that…"

Below is the updated chart. I wish we could edit our posts instead adding clutter...
Attachments
Attenuation vs Energy.jpg
Last edited by Bruce Meagher on Wed May 20, 2020 7:02 pm, edited 1 time in total.
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Schistad
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Re: Viborg Fusor Radiation Measurements

Post by Schistad »

You can see very clearly the absorption of the Borosilicate viewport in the spectrum. If you compare the theoretical calculation of blackbody radiation with the measured spectrum, you will se that the long wavelengths are missing.
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Richard Hull
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Re: Viborg Fusor Radiation Measurements

Post by Richard Hull »

Over time, you will find, as many here have experienced, that lots of fusion demands lots of D2 pressure. My fusor needs conditioning after long periods of inactivity. I rarely can exceed 6 microns on the first runs and never hit voltages over 35kv or reach 100,000 n/s isotropic emission. over a couple of days operation my running pressures can be increased and the voltage levels can climb to 40+kv.

After about 5 days I can easily hit 16 microns of D2 with 42kv applied and 12ma to get 1,500,000 n/s or more. The secret is in learning to operate the fusor over time. It is like learning to ride a bicycle. Once learned, you never have to worry about falling off again and are warranted a smooth, fun ride everytime, provided the equipment within the bicycle performs well.

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
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