Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
Conner Ruhl
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Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Tue Aug 28, 2012 6:40 pm

I am wondering if medical isotopes could be produced using a neutron moderator and a fissionable capsule that surrounds the target material. Does this sound reasonable?

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Carl Willis
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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Carl Willis » Wed Aug 29, 2012 4:06 am

Hi Conner,

A fissionable reflector can increase neutron flux, although practically speaking, not dramatically (perhaps a factor of 2-3 has actually been demonstrated experimentally using depleted uranium pieces of man-portable size). The shape and size of the fissioning matter and its disposition among moderating materials affect the probability that neutrons will create more neutrons by fission. Since the NRC limits how much "source material" a hobbyist can fool around with, one is therefore also limited in how much multiplication can take place.

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Conner Ruhl
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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Wed Aug 29, 2012 4:08 am

Would isotope amounts as small as milliliters be possible?

P.S., Thank you for discussing this with me. I have been checking the post for replies continuously since I posted the question. If this can be used as an experiment I will be very excited.

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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Wed Aug 29, 2012 4:13 am

Also, I don't want to increase the count of the entire reactor, but a very confined area.

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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Wed Aug 29, 2012 2:07 pm

Here is a diagram for easier understanding:



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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Carl Willis » Wed Aug 29, 2012 7:14 pm

The use of low-capture moderators like heavy water and fissioning materials in the vicinity of the target can have a positive impact on the neutron flux in some geometries, but can produce inferior results in other geometries. There's much to consider. In the absence of material interactions, the neutron flux decreases according to the inverse square law as one gains distance from the source, so one of the desirable goals of an activation experiment is to get the target close to the source while still accomplishing a useful degree of moderation. For many--probably most--cases, light hydrogenous plastics or water are better than heavy water for this reason. And the same kind of balance would have to be struck if there is fissionable matter in the mix. An optimum geometry may contain none at all. Optimizations can be performed quickly and with a high degree of confidence using a neutron transport code like MCNP (or maybe FLUKA or GEANT).

Keep in mind the practical considerations of actually doing this experiment. How would you obtain and work with Zircalloy, and what does it accomplish? If it's just an exterior container for the rest of the stuff, then its low thermal neutron cross-section would be irrelevant. All the neutrons entering the geometry from outside will be at 2.5 MeV and for all practical purposes will go through stainless steel, for example, just as efficiently as Zircalloy.

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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Wed Aug 29, 2012 7:58 pm

Thank you for the excellent points! The sketch was just me putting down my thoughts on paper to be destroyed by constructive criticism from this forum . The choice of heavy water and zircaloy was from my experience with fission reactors, and was largely arbitrary. I will probably be seeking extensive advice from fusor.net for the actual design that is used. I am hoping to use this concept as my science fair experiment.

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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Carl Willis » Wed Aug 29, 2012 10:16 pm

Making isotopes presupposes a fairly reliable and intense neutron source. Seasoned fusor builders often point out the time commitment necessary to make it that far. It's a major project in itself (and of course fusors are now becoming familiar entrants in science fairs).

You might want to focus on the core science and engineering that is central to your idea--making isotopes with the aid of a "fission blanket"--and get right into the meat of that idea with an existing source of neutrons (although you can pitch your project as intended to be paired ultimately with a fusor neutron source if you want to emphasize certain advantages of the fusor). Existing "off-the-shelf" neutron sources include nuclear reactors, accelerators, and isotope sources having at least the intensity of the most intense hobby fusors. You are perfectly capable of getting access to any of the above--take your pick--with a carefully-composed project proposal. Your local cancer therapy centers have electron linacs that can be a source of neutrons as potent as the best hobby fusors with the right targets. University research reactors suffer from under-utilization around the country and you would be welcomed with open arms at many of them. So if you want help connecting with these kinds of resources, I can help. (And of course you can build a fusor in your spare time to add icing to the cake later if all works out.) Just a suggestion to help focus your thoughts about what to do.

Designing the neutronics for isotope production typically involves modeling of the kind I mentioned earlier. If you want to try your hand writing MCNP "decks" I will contribute my advice and run them for you.

-Carl
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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Conner Ruhl » Wed Aug 29, 2012 11:41 pm

I would really like to use a fusor as the neutron source. It is reletivaley small and very cheap compared to other sources of neutrons used to make medical isotopes. It provides the unique opportunity of being able to manufacture isotopes onsite at a hospital without the costs of transporting short lived materials or operating a full fission reactor. I wouldn't need to create much, just enough of the isotope to detect. Is such an amount not possible? I see posts on here about silver activation, if silver can be activated why not other materials that are used to make isotopes?

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Re: Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

Post by Frank Sanns » Thu Aug 30, 2012 1:46 pm

Run the numbers to see what you are up against. The best of the amateur fusor will run a few million neutrons per second. That sounds like a large number until you consider that one mole of an element is 6.02 x 10^23 atoms. Even a microgram of an isotope would take on the order of 1x10^11 seconds (>3,000 years) assuming 100% efficiency. Not exactly a practical use of a fusor.

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