Small neutron source

[Richard Hull]

At such a low beam current, and with the Russian 3He tube's low NV value. I would be surprised if you saw anything much, even if the tube was good.

Richard Hull

[Peter Schmelcher]

Thomas your equipment dilemma is probably very common. Effectively does my used stuff actually work? I have suffered from a non functioning Russian tube, failing electronics, as well as bad welds, and many rookie mistakes. Put them all together and progress is challenging.

I built several preamp designs that you can find on this site. These exposed the system dilemma, if it works great, but if it doesn’t how do you know which part is not working. Ultimately I went “All In” and designed my own high performance preamp which I could independently functionally verify, and then I knew the Russian tube was bad. So all dressed up and no place to go.

In hindsight if I were to do it again I would simply buy a new commercial preamp circuit board for about $50 and avoid suffering. Regrettably the very best low noise preamps are the easiest to destroy.

My preamp design was engineered, it is overkill with 15KeV rms of noise in the output signal (CERN would be closer to 12KeV). I used a Geiger mode He3 tube biased as a proportional tube with no issues.

The big advantage of proportionality IMHO is that you can verify that the neutron detection system is working using cosmic ray generated neutrons. Without a moderator your signal includes the initial neutron kinetic energy. With a moderator the neutrons are stripped of their kinetic creation energy and the output signal amplitudes show a distinct grouping or banding at the He3 neutron capture and dissociation energy. The catch is that fast neutron detection probability decreases dramatically with increasing kinetic energy so after you know the system is working you want a moderator.

FWIW
-Peter

[Rapp Instruments]

Hi,
I have builded preamps myself, which worked very well with a home-made prop counter for EDX analysis (1.5keV res at Mn K alpha)

http://www.rapp-instruments.de/Radioakt ... ounter.htm

The amplifiers should also work with the corona tube. I just ordered a new tube from Ebay.

Anybody knows about the spatial distributions of the neutron radiation from a flat target. If I suspect a homogen distribution I can calculate the minimun flux from the scintillators data.
Distance from source 25 cm > surface of the sphere 1963 square cm
surface of the scintillator 1 square cm
flux 14 n/s * 1963 = 27,000 n/s
assuming 100% detection effiency of the scintillator,
the real flux value should be somewhat higher.

Thomas

[Peter Schmelcher]

Nice project and preamp. I used the same preamp approach but picked the ADA4817 op amp.

My pre amp is battery powered and gets used with a scope that has a telecom test mask feature. The mask gets violated every time the displayed preamp signal crosses a mask boundary, and this increments a counter. So the scope becomes a multi channel analyzer with 8 adjustable channels.

-Peter

[Rapp Instruments]

hi,
at the end I got the corona counter to work. First I measured the voltage/current field of the tube (CHM 42)


The tube behave like it should regarding the data sheet (Starting voltage nom 600V, max 700V)
for the neutron counting I used a rather high working resistance with 1 Gigaohm.


The parafin moderate tube gave following counts

I supose the strong decay of counts after switching the target voltage on results from the large time constant of the RC-network or a change at target surface.
The neutron pulses have about twice the height then the corona noise amplitudes.

Happily after the measurement I broke the tube by handling it a little bit to hard. Anyone also using the CHM42, be carefull, there is a weak spot at the lower end of the tube, maybe the spot where it was evacuated and filled.
After the accident I opened the tube to have a look inside.

The tube is constructet like a normal counter tube with a thin central wire. What I'm wondering about is the black powder layer on the inside wall. The data sheet clearly says that the CHM42 should be a He3 counter, but this looks like a boron lined counter

Thomas

[Chris Bradley]

It might just be sputtered material from the wire, as these work by a continuous corona generation.

Yes, I bought a pile of these too and wondered what I had wasted my money on! After 10's of hours of effort, decided they didn't work. Some months later looked again once I had access to an oscilloscope of sufficient input impedance. Then I saw the signal, and then I could make some electronics to detect those signals!

Don't give up trying to make them work, simply that you have to get the right electronics in place. They do seem quite delicate.

[Richard Hull]

This discussion has moved into the area of neutron detection and might be better suited to the Radiation detection forum.

Richard Hull

[Rapp Instruments]

Okay, back to the accelerator,

I have increased the target bis from -50kV to arround -90kv, actually the voltage jumps between 80kV and 100 kV because of micro discharges along the tube walls. The increase of indium activation is remarkable, arround threefold



What would be the next, easy to activate element, beside silver and indium ?

Thomas

[Dennis P Brown]

Your current is low but you might want to check the x-ray output just to be on the safe side.

[Rapp Instruments]

Hi Dennis,

I don't worry about x-rays because I have a 5 mm lead shield to the operator side. Thick enough for low intensity 50 to 100 keV xrays I found with geiger counter.

Are there really no more elements beside silver and indium to activated with low neutron flux?

Thomas

[George Schmermund]

Have you given any thought to detecting protons from the DD fusion? That should be doable and you could then petition to be the first candidate to be enrolled into the 'Proton Club'!

[Richard Hull]

Rhodium is excellent if you can get it! But those are the big three.

Richard Hull

[Dennis P Brown]

Protons from any fusion events wouldn't travel through any significant amount of material (a few microns to tens of microns at best) if I recall from my calculations; be rather difficult to measure any considering that limitation even if they were accessible. Detector windows except for maybe beryllium wouldn't generally work for such low energy protons (a few MeV.)

Glad the x-ray radiation is not an issue!

Again, very nice experiment both for the design and confirmation method - your neutron flux is much higher than I would ever have expected considering such a simple apparatus.

I am curious about your target of zirconium - how did you fill it with deuterium?

[Rapp Instruments]

Hi George, hi Dennis

in the early days of nuclear research desintegration was mostly proved by protons. Researchers used a scintillator screen (ZnS) and counted the flashes with a microscope. The screen was mounted on the vacuum side of the tubes, you so only need a optical window. They were able to discriminate the protons flashes from the continous glow from x-rays, human multi chanel analyzers.
But I think with the most probably D-D reaction D + D > n + He3 there no proton emitted, the second reaction chanel with much lower probability D + D > p + H3 a proton would be emitted.

Zirkonium like some other metalls ( Titan, Palladium) readily absorb hydrogen/ deuterium from surrounding atmosphere, also the ion beam helps to load the surface layer with deuterium.

Hi Richard

altough the price for rhodium has fallen the last years it is still very expensive, but I can try my luck with rather cheap rhodium plated coins, maybe there enough material to see it.

Thomas

[Chris Bradley]

But I think with the most probably D-D reaction D + D > n + He3 there no proton emitted, the second reaction chanel with much lower probability D + D > p + H3 a proton would be emitted.

Those two routes are approximately (not quite) equal. It is the D+D->4He + hv reaction that is rare, about 1/20000 as likely as the others.

It could be a challenge to look out for the very high energy 24MeV gamma out of that rare reaction.

If you have that level of gamma spectrometry sufficient to discriminate this, another possible case for an interesting experiment is to mix your D with H, and try to detect the D+H->3He + hv gammas. Your linear target setup might lend itself better to that reaction. That would be the first detected amateur non-DD fusion if you could.

[Rapp Instruments]

Hi Chris,

I agree that I was wrong with my assumption about D +D < p +H3. It should be possible to see the proton with a scintillator screen.

I only have a small volume NaJ (Tl) scintillator and a homemade mca
http://www.rapp-instruments.de/Radioakt ... lation.htm
tested to 1.4 MeV (K40), I don't know if I can see very high energy quants, but I'm going to try.

I will buy hydrogen because I already planned to do desintegration of Li, B, C and F and so I can do the H/D mixing experiment.

For the desintegration I need some higher voltage. I found that arround 100..120 kV ist the highest voltage for the actual design of the single stage tube. I plan to build a multi stage tube like Dennis Brown has done. The individual stages will be feed by the stages of a Cockcroft-Walton multiplier

Thomas

[Chris Bradley]

Thomas, as I am sure you already know, a conventional CW multiplier does not provide equal current at each node (thus unequal acceleration voltage).

If you would like to improve that then you might like to take a look at an invention of mine:
viewtopic.php?f=11&t=4887

[Rapp Instruments]

Hi Chris

a smart design.
But if I understand the circuit right then the feeding capacitors C1,C2,C3... have to have increasing voltage ratings. For examble, in a 200 kV suppy the last feeding capacitor should have at least 200kV plus the value of the feeding voltage, such high voltage types are not easy to find. If one use serial connection of capaitors to increase the voltage rating one have to use a lot of them. To get the same capacitance at every stage you need one for the frist stage, four for the second, eight for the third and so on.

Thomas

[prestonbarrows]

Have you given any thought to detecting protons from the DD fusion? That should be doable and you could then petition to be the first candidate to be enrolled into the 'Proton Club'!

This is extremely difficult to do since protons have practically negligible range through any material. This means the detector must be inside the vacuum vessel. Since there is all sorts of other, non-fusion based, charged particles flying around in the plasma environment, you need a pretty sophisticated filter/analyzer to reject all the noise. This usually involved some type of bending magnet and faraday cup; given the MeV energy levels, this requires some pretty serious bending magnets.

It has been done before in the literature for fusors, but only at the University level that I am aware of. The main point is detecting fusion from advanced fuels like He-3 which don't readily put out neutrons. For DD, there is not really a need to mess with proton detection since neutrons are so much simpler to find.

[George Schmermund]

pb - It looks like your confused again. This thread is about a beam-on-target accelerator, NOT a fusor. OF COURSE a detector would have to go into a vacuum chamber at the target end of the system. That's why they make scattering chambers for BoT experiments. The chamber need only be large enough to accommodate the experiment, which in this case could be rather small.

The 2" bending magnet I demonstrated (some time back) for use with the micro-cyclotron could be adapted to analyze ~3 Mev protons with a very simple arrangement of slits. The basic construction of the slits could follow the plan of the uC and be adjusted accordingly.

In fact, if a scattering chamber seems too ambitious, the whole magnet assembly and slits w/detector could sit outside the target chamber and still be pumped to high vacuum with a suitable flange. It doesn't get easier than this!