Radioisotope production

[Mark R]

The major methods of production of industrial and medical isotopes are either via:
1) Cyclotron
2) Fission

In some cases the product has significantly different characteristics to the target or carrier. E.g. 18O-water is irradiated by a proton beam to produce 18-F which dissolves as F-, so can easily be separated.

However, what sort of techniques are used for purifying fission produced isotopes? E.g. probably the most important isotope supplied for use in radio-pharamaceuticals is Mo-99 (this is allowed to decay on-site to produce the pharamceutical isotope Tc-99m).

The problem is that Mo-99 is only produced from fission of Uranium (there are cyclotron methods, but they're impractical for commercial use). The problem I see is that fission will produce a complex mixture of isotopes and elements - yet an extremely pure material is required. What sort of separative procedures are needed to select only one isotope of one element?

 

[Born2bwire]

Despite the fact that they are usually only a few particles different, isotopes do have differing masses. I know for the enrichment of uranium, they exploit the fact that the U-235 isotope is slightly ligher than the natural U-238. So they have used a variety of techniques to separate the two using the mass differences. They can use gas centrifuges, and they can also use diffusion. Either diffusing the isotope in a gaseous form through membranes or they use heat to draw up the U-235. Mass-spectrometers can be used to collect a certain isotope. They ionize a gaseous form of the uranium and the lighter U-235 has a smaller radius in the mass-spectrometer and thus is separated.

 

[Gibsons]

Originally posted by: Born2bwire
Despite the fact that they are usually only a few particles different, isotopes do have differing masses. I know for the enrichment of uranium, they exploit the fact that the U-235 isotope is slightly ligher than the natural U-238. So they have used a variety of techniques to separate the two using the mass differences. They can use gas centrifuges, and they can also use diffusion. Either diffusing the isotope in a gaseous form through membranes or they use heat to draw up the U-235. Mass-spectrometers can be used to collect a certain isotope. They ionize a gaseous form of the uranium and the lighter U-235 has a smaller radius in the mass-spectrometer and thus is separated.

I think MarkRs point is that mass separation of isotopes as described above is far too expensive/inefficient to account for most radioisotope production, particularly if high purity is needed. Sure, you can do it if you have a DoD-scale budget. But it seems unlikely on a hospital/HMO/small grant research kind of budget.

 

[Born2bwire]

The problem I see is that fission will produce a complex mixture of isotopes and elements - yet an extremely pure material is required. What sort of separative procedures are needed to select only one isotope of one element?

I just kinda caught his last line. I don't know, I mean, if you can afford to use fission to produce your materials, I think you can probably afford a gas centrifuge.

 

[Mark R]

Originally posted by: Gibsons
I think MarkRs point is that mass separation of isotopes as described above is far too expensive/inefficient to account for most radioisotope production, particularly if high purity is needed. Sure, you can do it if you have a DoD-scale budget. But it seems unlikely on a hospital/HMO/small grant research kind of budget.

Well, that's exactly my point.

While there are proposed methods of producing Mo-99, fission is by far the simplest and cheapest.

Alternatives, include producing enriched Mo-100 and then irradiating it with high energy X-rays to disintegrate the nucleus. Unfortunately, the isotopic separation of Mo is rather more difficult than that of Uranium. And the accelerators needed to produce the X-rays are also expensive and much sought after pieces of equipment, so no one has ever attempted this as a practical method of production. By contrast, Mo-99 is a major fission product.

Of course, the whole point of producing Mo-99 is that the hospital doesn't have to do anything difficult. Essentially the hospital buys a 'Tc generator' which is a vial containing Mo-99 oxide sealed in a lead or uranium box. All a technician needs to do is inject a bit of sterile NaCl solution into the vial. Wait a bit, then drain it out - and they've got sodium pertechnetate solution ready for injection. Once the Mo-99 decays too much (about 2 weeks) they either send the generator back for a credit, or let it decay for another 6 months, before removing the shielding and dumping the remainder in the trash.

Anyway, having thought about it a bit more, maybe all you need to do is seperate out the Mo, from everything else. Of all the Mo isotopes, none are going to do anything bad if they get mixed in with the Mo-99 in a generator. They've either got really short half-lives so they could they could decay away before the generator is shipped out, or such long half-lives that they will neither contaminate the generator with decay products nor pose a significant disposal hazard.

I suppose that the reason you're supposed ot hang onto a dead generator for 6 months could be because a crude separation method is used, and all sorts of other crap gets in - not really enough to pose a signifcant purity problem, but enough to cause a disposal problem.

 

[CycloWizard]

Originally posted by: Gibsons
I think MarkRs point is that mass separation of isotopes as described above is far too expensive/inefficient to account for most radioisotope production, particularly if high purity is needed. Sure, you can do it if you have a DoD-scale budget. But it seems unlikely on a hospital/HMO/small grant research kind of budget.

The radiation sources that we use are simply purchased from a large-scale distributor. I don't use any of them personally, so I'm not sure what kind we have, but it seems that an individual hospital will not produce its own sources. There are probably only a few such distributors in the country if I had to guess.

As for how they might be separated, I can only speculate and I'm too tired to wave my hands any more tonight.

 

[imported_bearfx]

Born2bwire has it right.

The most effective (and by effective I mean cost efficient) method is usually centrifuge.

Their are very few methods available to seperate material based solely on mass. Since the isotopes will (almost) always behave the same chemically, you have to seperate based solely on physical characteristics... Not an easy task.

---------------
Athlon 64 X2 4400+ (2200Mhz -> 2600Mhz)
4xKingston HyperX DDR400 1GB