H+ Ion Gun Design and then some

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Post Reply
alltn
Posts: 2
Joined: Tue Jan 06, 2009 2:57 am
Real name:

H+ Ion Gun Design and then some

Post by alltn » Tue Jan 06, 2009 3:35 am

Hello fusor.net forum.

I am considering building a cockcroft-walton accelerator for protons to demonstrate the following reactions in sequence:

p + 7Li --> 2a (at the energies I would be using the neutron releasing reaction is too endothermic to occur)
a + 9Be --> n + 12C (it is my understanding the above alpha particles would have enough energy for this)
n + 235U ---> X + Y + 2-6n

I have a rather complex apperatus designed for the latter part which I have already run by a proffessor, but it is the first two reactions that are my chief problem.

I decided that a cockcroft-walton accelerator with successive 7Li and 9Be targets would allow for a greator neutron flux than a fusor or smoke detector-sized AmBe source while costing less than a Betatron or Linac with a Be or D2O target. My first question is if this assertion is correct and what the potential neutron flux of such a device would be.

I figure that if I run at around 1 mA i get about 6 x 10^15 protons/s, and then X fraction of the time a proton will cause the first reaction, 1.2X x 10^16 alpha particles will be released into the 9Be target, releasing 3.6X x 10^11 neutrons. Naturally my second question is, what on earth is X?

Now if I do go forward with my plan, I need a few more questions answered:

3. How do I build an ions source for the protons needed for operations? (this is my biggest question, the reason I posted this in this section)

4. What size, shape, and type of electrodes should be used, and how far appart do they belong at say, 200KeV?

5. How far would protons at 200KeV penetrate through lithium foil?

6. What pressure would I need to run at?

If anyone could answer any of these questions it would be much appreciated.

By the way, is this is an impractical neutron source (as the third reaction is what I'm really after), what would be a better alternative on a budget?

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: H+ Ion Gun Design and then some

Post by Carl Willis » Tue Jan 06, 2009 4:36 am

Welcome.

I prefer to see people use their real name (or some semblance thereof) on this forum. That's just a personal preference. Anyway...

>successive 7Li and 9Be targets would allow for a greator neutron flux than a fusor or smoke detector-sized AmBe source while costing less than a Betatron or Linac with a Be or D2O target.

Possibly, depending on circumstances, but not likely. The DD reaction done in fusors has a higher neutron yield at most energies than the Li-7(p,a)He-4 reaction has alpha yield, and that's not taking into account the lossy Be-9(a,n) step or the other losses in your proposed process. I can say with some certainty that you won't save on $$ / neutron. If I had to advise you on making the greatest number of neutrons possible regardless of spectral characteristics, at the lowest cost, the answer would be to use a DD generator of some sort.

>Naturally my second question is, what on earth is X?

"X" is properly called the "thick target yield." It is presumed that the lithium is thick enough to entirely stop the protons entirely. Thick target yields can be calculated from two series of easily-available data: (1) the energy-dependent cross-section for the reaction Li-7(p,a)He-4 and (2) the energy-dependent stopping power for protons in the target material. The calculation involves numerically integrating the quotient of the cross-section and the stopping power with respect to energy, where the bounds on energy are 0 (the protons are stopped entirely in the target) and Emax, your incident beam's energy. The full equation is usually called the "thick target yield equation" and it can probably be Googled.

Cross-section data are often available on government-sponsored nuclear data websites. For instance the plot below is from a cross-section data set [S.N.Abramovich,B.Ja.Guzhovskij,V.A.Zherebcov, A.G.Zvenigorodskij. Estimated values of total and differential cross sections of proton interactions with nuclei Li-6 and Li-7. Journ.: Vop. At.Nauki i Tekhn.,Ser.Yadernye Konstanty, Issue.4, p.17 (1984)]
retrieved from www.nndc.bnl.gov, the National Nuclear Data Center. You'd do well to acquaint yourself with this site and its tools.

The stopping power can be calculated with a freeware tool called "SRIM." This program is a must-have in my opinion.

>3. How do I build an ions source for the protons needed for operations? (this is my biggest question, the reason I posted this in this section)

This question is too broad to answer in a single paragraph. There are many kinds of ion sources, all with their virtues and drawbacks. You will have to be a self-directed learner for most of that. Some of us have built ion sources. I have one, recently built, that is described in this forum. It is an old but simple and reliable style of RF electrodeless discharge source noted for high atomic ion yield (H+ rather than H2+).

>4. What size, shape, and type of electrodes should be used, and how far appart do they belong at say, 200KeV?

There is no simple answer. The "Kilpatrick Limit" is sometimes invoked in the design of very high gradient accelerator components. It is an approximation of the field strength at which high-vacuum breakdown occurs. But in your apparatus it will probably not be the limiting factor.

>5. How far would protons at 200KeV penetrate through lithium foil?

Answer this with SRIM, as mentioned above

>6. What pressure would I need to run at?

The ion source pressure depends on the type of source. A general range might be 0.1 - 10 mtorr. The rest of the apparatus should be under high vacuum, ~1E-6 torr.

-Carl
Attachments
X4sPlotZvd2.gif
X4sPlotZvd2.gif (4.44 KiB) Viewed 2091 times
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

alltn
Posts: 2
Joined: Tue Jan 06, 2009 2:57 am
Real name:

Re: H+ Ion Gun Design and then some

Post by alltn » Tue Jan 06, 2009 5:24 am

Thank you very much for that information.

To begin, I'll explain why I did not use my name for my user name. I prefer not to give out my name on the internet, just as a safety precaution. However, after looking around this site, and since you bring it up, I will from now on sign my posts with my name.

After reading your post and doing the research you suggested, I calculated the thick target yeild to be 2 x 10^-9 at 250KeV, placing the neutron flux at around 720n/s for the device I proposed. Even though this would be a more confined beam than produced by a fusor, I definitely have to agree that it would be an impractical neutron source after all. Thank you for all your help, and I think I'll give fusors a second look.

-Alex Becker

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: H+ Ion Gun Design and then some

Post by Carl Willis » Tue Jan 06, 2009 11:43 pm

Hi Alex,

Understood about your name, and it's up to you. I admit to being prejudiced by a person's username; I see lots of online science forums in which the users go by names like "Lorax," "1337_HaX0r," and "Trash," and the content tends to be what you might loosely classify as juvenilia. I perceive a correlation there.

Definitely consider a fusor as a neutron source. It occupies a uniquely accessible niche in terms of simplicity, cost, and ease of operation. Alternatively, if you have plans for a 200-kV electrostatic accelerator, you could just use it to generate deuteron rather than proton beams and run them into titanium targets like the standard neutron generator tubes do.

-Carl
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

lutzhoffman
Posts: 188
Joined: Tue Dec 08, 2009 1:59 am
Real name:

Re: H+ Ion Gun Design and then some

Post by lutzhoffman » Tue Dec 15, 2009 1:55 am

Hello:

Have you thought about raising the energy possible to the 400-500KV range? The reason I am mentioning this is that most low budget university physics labs shoot for a 400KV accelerator. The reason is that many reactions become possible in the 300-400KV range, like F + p- Gamma (6-8MEV) and others. At the same time the reaction yield from DD, and Li + P, and other common reactions go way up also.

At 500KV for example Be + D gives 10 to the 7th N/Sec. per uAmp of beam current, overtaking even the DD reaction in neutron production efficiency. Of course you have to balance the much higher radiation hazard with using 400-500KV, the point being that you do not have to run it at 400-500KV, but if that’s what its built for, then you have plenty of room for future upgrades.

I was leaning towards a fusor, when I thought about what I wanted to do? Since the labor is not that much more for building a DC linac, I went in this direction. For both a fusor and a DC linac, the same vacuum system needs to be made etc. The only big difference being the source of HV. But even here even a simple Van DeGraff generator with a 2 foot terminal will do the job. This is not much harder than a fusor sphere. The accelerator tube can be made from glass insulating rings with sheet metal disks with a central hole, PVA glued between them. Newport Glass will grind the Pyrex insulating rings for you, you just tell them you want "Mirror Blanks" with a big hole in the center to form a glass ring.

Another source of the glass insulator sections is a glass fabricator and the amateur astronomy community, 4 inch Pyrex blanks are everywhere, and you just need to have the core cut out. If you can find thick wall glass pipe then this can be cut into the sections.

For 400KV you will need a tube about 3 feet long with an accelerating electrode every 1-2 inches. Scientific American had this as a project in their amateur scientist column a while back, but the accelerator tube with the external twisted wires did not work well, (I tried this design), and the ion source was poor. Now with Carl's, and others ion source designs, and a segmented tube, this VanDeGraff powered unit could be a serious research tool. If you do attempt this radiation monitoring is a must, as is active secondary electron suppression: (You bias the target 150V so secondary electrons do not get accelerated up the tube).

I like both designs, for a science project I think the fusor is much better for two reasons: First it has a much lower radiation hazard, and the operating conditions / gas etc. can be changed very easy. Second a fusor just looks so much cooler, you can safely view the discharge, and it is just beautiful and amazing to watch. The fusor simply wins hands down in the WOW factor department.

The DC linac on the other hand is more versitile, you can even for example: Accelerate electrons, and deliver massive radiation doses to a sample, via direct electron irradiation through a foil window. You can cross link and change the properties of many plastics, and materials this way as is done commercialy. Plus a large variety of nuclear reactions now become possible, even He+ reactions can be done.

If you really want to impress everyone then use a modified Tesla coil as the power source for a DC linac. Some new designs do this, the Tesla coil secondary is divided into pancake style coil sections, each with its own built in HV doubling rectifier. These are then connected in series, and attached to the accelerating tube electrodes. The primary is conical around it, and replaces the capacitive tank electrodes. Recently a university 5 MEV dynamitron was converted this way (For more beam current) to make a neutron source for BCNT (Boron Capture Neutron Therapy), using the Li + P reaction, which gives close to thermal neutrons at 2.2 MEV.

Anyway, both fusors and DC linacs have their place, it just boils down to what you want to do, no matter what you decide just remember: That its better to build either, than neither : )

Good Luck.....

Post Reply