Medical Isotope Production Using a Neutron Moderator and a Fissionable Capsule

[Conner Ruhl]

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?

[Carl Willis]

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.

-Carl

[Conner Ruhl]

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.

[Conner Ruhl]

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

[Conner Ruhl]

Here is a diagram for easier understanding:

[Carl Willis]

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.

-Carl

[Conner Ruhl]

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.

[Carl Willis]

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

[Conner Ruhl]

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?

[Frank Sanns]

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.

Frank Sanns

[Conner Ruhl]

I don't want to produce pure isotopes in volumes that can be used, I just want to show that the isotopes are present (even if just barely detectable). I keep reading about silver activation on here. If silver can be activated why not other elements?

[Frank Sanns]

It is all about detection. The average lab could never detect by weight or chemical method, the levels of activation going on with a fusor. The number of atoms are just too low. Yes, there are methods to do so but short half lives and low concentrations are against you. With radiation detection though, single decays can be detected. This make it a perfect measuring tool for radioisotopes. The activation with a fusor has more to do with proving unequivocally that neutrons were produced and electrical noise from the fusor is eliminated. Quite and elegant detection scheme it is but for any real quantity of isotope it is not.

Frank Sanns

[Conner Ruhl]

If I can show there is radioactivity from an originally stable element, I will have proven that there is now some quantity of isotope in the material. Is this not possible?

[Frank Sanns]

It is easily possible with even a feeble neutron source, a moderator like water or polyethylene and a pancake detector.

Frank Sanns

[Conner Ruhl]

Very cool, thanks for the responses!

[Jim Kovalchick]

It's a common exageration among the high school science fair folks to claim that medical isotope production is a practical use of a fusor.

I believe that IEC fusors by current design concepts don't come close to that. Just because you activated a few atoms to the point of convincing them to whisper to a sensitive radiation detector doesn't mean you changed lead to gold enough that you can see it shine.

Medical isotope production takes some significant fluence. A typical research reactor has a thermal flux on the order of 10+14 n/cm2-sec. That's many orders of magnitude greater than even the big IEC devices that the University of Wisconsin has tinkered with. The flux just isn't there, and no one has a design that comes close to that.

Trying to boost the flux using a subcritical uranium capsule is further along, but still, to make enough stuff to demonstrate practical use as a concept still means fluxes that are a significant challenge. There are certainly regulatory implications of doing that with uranium and ultimately the uranium if exposed enough to do something like this will still end up like fuel out of a fission reactor...very messy.

[Richard Hull]

You are looking for two things is selecting a neutron activation target when working with a fusor.

1. An element of large cross section for thermal neutrons
2. An element that matches the above criterion and, in its activated state, has a very short half life.

At any given neutron flux you need about 5 half-lives exposure time of the activated element to saturate it to its maximum radioactivity. Elements with activated isotopes with half lives of seconds to a few minutes are all that the average fusor run can deal with. As the fusor has a pitifully low net flux, we seek to saturate what we are bombarding.

Elements that satisfy the above requirements are tough to find.

I have prepared a listing in order of ease of activation with the fusor that can just make about 1 million neutrons isotropic per second. All activation materials are best presented to the neutron bombardment as a metal foil. Some, like number 3 below, are gamma emitters and can do well dissoved in water as a saturated soluable compound.

1. Silver
2. Indium
3. Manganese

Rhodium foil would be the ideal, but is far too costly.

Silver is the easiest and cheapest of all to activate and detect, even with a weak fusor. This is why it is so talked about here. Those who can't afford a neutron detector can use silver activation as proof of performance, but they need to be making over 250,000n/s for easy detection, though 100,000n/s is certainly possible with good technique.

Richard Hull

[Carl Willis]

The following elements have been activated using amateur fusors and are discussed in this site's archives (search for "activation" in the Neutron Detection forum). Jon Rosenstiel's experiments are the lion's share, and mine pick up most of the remaining handful. The discussions on this subject are quite detailed about instrumentation and technique.

(in no particular order)
Aluminum
Gold
Rhodium
Europium
Silver
Indium
Iodine
Vanadium
Magnesium
Arsenic
Hafnium
Scandium
Selenium
Holmium
Neodymium
Ytterbium
Lanthanum
Germanium
Gallium
Molybdenum
Bromine
Sodium
Samarium
Tungsten
Copper
Manganese
Uranium

(Am I missing any?)

There's more than a decade of good, solid information in the archives.

-Carl

[Conner Ruhl]

Thank you for this! Molybdenum is what is important. I am always astounded by the wealth of information available at this site!

[Richard Hull]

Carl's list includes elements that are tough to activate unless you have a fusor producing well over 1 million n/s or can run your fusor for many minutes to an hour. Not the norm at all!...especially for a newbie.

Some activations in Carl's list are impossible to detect following activation even with a plus ultra fusor without special detection gear and techniques.

If you have a superior, plus ultra fusor that ranks among the best ever run here, working at its peak and a Ge-Li, HpGe detector, all the elements in Carl's list are open to you. Superior fusor = lots of $$$.... Special detectors = lots of $$$. If you are working on a truly low budget or out of a scrap heap for parts and have only a GM counter, you are pretty much stuck with silver and indium.

As Carl noted in an earlier post, you might be able to borrow or use more sophisticated gear if you hook up with some college or professional organization avoiding a number of costs.

Richard Hull

[Conner Ruhl]

I am working with people from the physics department at SIUE and the nuclear engineering department at NCSU. I am hoping that a large portion of the budget woes will be alliviated by these two sources and their materials.

[Jon Rosenstiel]

Carl-

A few more to add to the list.

Antimony
Cobalt
Dysprosium
Erbium
Gadolinium
Palladium
Rubidium
Strontium
Tantalum

Makes me think of Tom Lehrer's "Elements" song. (First heard this ditty in 1958... I was 16-years old at the time)
http://www.youtube.com/watch?v=DYW50F42ss8

Jon Rosenstiel

[Chris Bradley]

Have you managed to activate cobalt, Jon!?

I missed seeing that one, if it's here in the forum.

Is there a link, what are the run details?