daveb said:
Yes, that's true. But most medical isotopes are fission products rather than activation products.
daveb,
WRONG!
Medical isotopes are NOT fission products. Medical isotopes are the capture
products of irradiation by reactors of a precursor. If you used fission products
as the seed material; you would have a whole host of radioisotopes that you
didn't want. Many of these would be isotopes of the same element as the one
you sought. The only way to separate the isotopes would be an isotopic
separation process.
However, you would be doing this on radiologically "hot" material. That's complex,
expensive, and unnecessary. You do the isotopic separation BEFORE you irradiate.
You isotopically separate out the stable isotope, that when irradiated, will give you
the desired target radioisotope. [If the desired precursor is a large fraction of the
natural abundance; then this isotopic separation step is not neccessary.]
If one were to scavenge medical isotopes from fission products; i.e. spent fuel -
then one would need to do so shortly after the fuel was removed from the reactor.
Spent fuel fresh from a reactor is too radioactive to handle without the massive
facilities for reprocessing as were to be found at Hanford and Savannah River.
Those facilities weren't called "Canyons" for nothing - they are truly
massive.
There's simply no need to scavenge medical isotopes from the "whitch's brew"
that are fission products. Not when one can make medical isotopes directly
by irradiation in research reactors or the High-Flux Isotope Reactor [HFIR] at
Oak Ridge:
http://web.ornl.gov/sci/rrd/pages/wedo.html
I assume you mean the reaction is 98Mo(n,gamma)99Mo which then immediately starts decaying to Tc-99m (about 87% branch ratio) and Tc-99 (13%). Then you get a mixture of Mo-98, Mo-99, Tc-99, and Tc-99m once the Mo-99 starts decaying. My point is that if you want an isotpically pure radionuclide, it's impossible.
Why would you "want" something isotopically pure to put into the patient?
You have this misunderstanding that we want something isotopically pure
for the patient. No - you want it isotopically pure for the reactor.
What goes into the patient doesn't need to be isotopically pure. If you had
a mix of of Tc-99 and Tc-99m in the patient, that's no problem! As long
as the dose from the fraction of Tc-99m is enough. The fact that some
stable Tc-99 is "tagging along" is not a big issue.
However, Tc-99m in the medical industry is exclusively made by 4 manufacturers. These manufacturers make Mo-99 generators by separating out the Mo-99 that is a fission product from other fission products.
WRONG!
Those manufacturers prepare Mo-99 "cows" from Mo-98 targets that have
been irradiated in reactors. For example, when I was a graduate student at
MIT, one of the jobs that the MIT research reactor does is to do these
irradiations.
If one were to separate radionuclides from fission products, one would have
to chemically reprocess spent fuel. However, reprocessing spent fuel in the
USA is FORBIDDEN by LAW! The reason is that another of the byproducts
of fission reactions in the fuel is Plutonium. Plutonium that is not co-mingled
with other radiologically "hot" isotopes is a nuclear weapons proliferation risk.
That's the reason for banning reprocessing on spent fuel.
When you irradiate a sample of Molybdenum in a research reactor like the
MITR-II, you obtain a sample which consists of the desired radionuclides
and some of the original material. You don't get fission byproducts like
Plutonium; so there's no proliferation risk.
Perhaps you thought the feed material for the 4 companies was spent fuel.
However, that's incorrect. Their feed material are targets of a precursor
element that has been specifically irradiated in a reactor with neutron
irradiation facilities - like a research reactor.
It's not 100% efficient, so there are some impurities. The neutron absorption cross section for Mo-98 is only 127 millibarns, so it's not very efficient to use neutron activation as a means of production.
First NOTHING is 100% efficient, and that's fine because it doesn't have to
be. As far as the cross-section for Mo-98 being a little more than a tenth of
a barn - SO WHAT! The neutron fluxes available in a research reactor are
quite high, and one can leave the target in the reactor for a long time and it
doesn't affect the reactor to any appreciable degree.
Regardless of how "efficient" you think it is; that's how it is done!
Regardless, it's still impossible to make it isotopically pure Mo-99 because once that first decay event happens, there is some Tc-99 (or Tc-99m) in the mix.
Again, since it is so short lived, it's impractical to separate out the Tc from the Moly unless the hospital happens to be right next to the reactor.
You are a veritable FOUNT of MISINFORMATION today!
What the hospital has is a "Tc-99m generator". It has at its heart a bunch
of Mo-99 that is constantly decaying into Tc-99m. That Tc-99m is also
decaying away - however at any given instant, there is a certain amount of
Tc-99m in the generator. It is THAT Tc-99m that the hospital taps off when
they need Tc-99m to give to a patient:
http://www.orau.org/ptp/collection/nuclearmedicine/tc99mgenerator.htm
"Tc-99m is a versatile scanning agent that is often considered the workhorse of nuclear
medicine. It is obtained by elution from a generator ("cow") that contains the radioactive
parent of Tc-99m, molybdenum 99.
The generator is simply a column containing a resin to which Mo-99 is attached. The Mo-99
decays to produce the short-lived Tc-99m (6 hr half-life). To obtain the Tc-99m, a solution
(the eluent) is injected into the top of the column - the shield plug for the top of the column
can be seen in the two photos to the right. The Tc-99m comes out the bottom of the column
into the sterile collecting vial seen in the above photo. The collecting vial has a short
breather needle to allow air out of the vial as the eluent and Tc-99m enter."
You see it's
TRIVIALLY EASY to separate the Tc-99m from the Mo-99.
It's done merely by passing an eluent over the Mo-99. That's hardly "impractical"
as you stated above, and doesn't require the hospital to be anywhere near the
reactor.
What would be impractical is to separate out Tc-99m directly and ship that.
The Tc-99m isotope is short-lived - you don't want to be giving long lived
radionuclides to patients. So a hospital would need an essentially constant
stream of Tc-99m replenishing their supply.
No - instead they are given the Mo-99 precursor to Tc-99m. That way they
have a constant supply of Tc-99m being made for them by radioactive decay
of the longer lived Mo-99. That Mo-99 comes from targets that are irradiated
in research reactors. As the above linked article states, the Mo-99 "cow" is
replaced weekly.
Sure that's "inefficient" in that there are Tc-99m atoms that decay without
ever getting used - but SO WHAT!
Dr. Gregory Greenman
Physicist
