Electrical Any interesting reactions at 500kV? This might be DIY-able

[halfelven]

I just realized a Van de Graaf generator around 400...500 kV might be easily doable, by an average amateur working in a garage.

I was also thinking these days about a DYI cyclotron project, which I scrapped because the magnet is just too hard to make and feed. But then I figured the Van de Graaf too could be used to accelerate particles, within the limit indicated above.

I'm just not sure if anything interesting happens at that energy level.

Assume the ion source is something simple (hydrogen, helium, etc). Assume a simple two-electrode linear design for the "accelerator". Can anything be done within these constraints?

(A 1 MV VdG generator, while possibly achievable by a dedicated and resourceful amateur, is a much more difficult project that requires very significant effort, so let's ignore anything at that level.)

 

[bcrowell]

Helium is not simple. To get a negative ion source to make helium, you need a special device called a duoplasmatron. IIRC the helium atom won't accept an extra electron in its ground state, so there is a trick involving an excited state.

I believe the energies you're talking about are quite a bit below the Coulomb barrier even for protons on protons, so you won't be able to do anything like nuclear transmutation. (Remember that what you get is really the energy in the c.m. frame, not the energy in the lab frame.)

You could probably do Coulomb excitation, a.k.a. "coulex." Coulex is a process in which the beam nucleus interacts electrically with a nucleus in the target, causing it to rotate. It generally has huge cross-sections compared to nuclear reactions, so it's relatively easy to study. The rotating target nucleus then deexcites by emitting gamma rays, so you would need some kind of gamma-ray detector. I'm guessing that a NaI would be the cheapest, although if you have some money, small planar HPGe detectors are really not that expensive these days.

Another possible application is mass spectrometry. Here is an example: http://arxiv.org/abs/0907.0126 I believe this can be done with basically no detectors or electronics, but you might have to build some big magnets.

Another possibility is studying the stopping power of materials and the biological effects of ionizing radiation. E.g., ion beams can be used to treat cancers; you could probably design experiments using plants, insects, etc. that might actually produce some interesting science.

 

[halfelven]

Why negative ions? I thought it's easier to make positive ions - maybe a hot filament would suffice? (I'm pretty ignorant in this area, so that's why I'm asking)

And if making positive ions is easier, how about heavier ions of elements easy to vaporize (Hg, iodine, noble gases... the list is big), completely stripped of electrons. Those should gain more kinetic energy from the given potential, provided I could make them somehow.

Coulex does sound interesting. I'll investigate it.

 

[bcrowell]

Oh, sorry -- I worked with a *tandem* van de graaf in the past, and I just subconsciously assumed that was what you had in mind. From the point of view of a DIY-er, I don't know whether a tandem or non-tandem design makes more sense. A tandem gets you a lot more energy for the same voltage, but it does require negative ions, not positive ones.