UNRESTRICTED | ILLIMITÉ
There are several news articles this week describing the possibility of using cyclotrons for mass producing Mo-99 rather than the current reactor-based method (which generates Mo-99 as a fission product). I asked a colleague (a radiochemist) about the different processes; hopefully the following comments correctly reflect his comments.
A cyclotron can be used to irradiate Mo-100 with high-energy electrons, which knock off (spallate) a neutron from the Mo-100 and leave radioactive Mo-99 behind. Mo-99 has a 2.75 day half-life, decaying to Tc-99m. It is the Tc-99m (“m” for meta-stable) which decays (6.0 hours half-life) and gives off a gamma useful for medical imaging. The resulting decay product Tc-99 is also radioactive, with a half-life of 213,000 years (Tc-99 decays to stable ruthenium-99).
While Mo-100 is “a naturally occurring” isotope, it must be enriched from natural molybdenum (which is a mixture of seven stable or very-long-lived isotopes). Mo-100 represents 9.63% of natural Mo. and is the heaviest of the isotopes. Since there is a mass gap of 2 neutrons between Mo-100 and Mo-98 (the next isotope in naturally-occurring Mo), it makes it a bit easier to separate Mo-100 in a cyclotron (note that U-238 and U-235 are separated by a mass of 3 neutrons, and can be separated in a cyclotron, but the higher masses make the difference smaller on a fractional basis). Does anyone know if the Mo-100 will be (or can be) recycled into new targets after the Tc-99m is extracted?
One only needs small amounts of Mo-99 to produce enough Tc-99m (according to Wikipedia, “a few micrograms of Mo-99 can potentially [be used to] diagnose ten thousand patients”). Thus there is no need to produce massive quantities of Mo-100, although there would be an optimum amount of irradiation of the Mo-100 targets. As the irradiation occurs, Mo-99 is produced and some will start to decay to Tc-99m. As irradiation continues, radioactive decay and spallation of the already-produced Mo-99+Tc-99m will then cause the loss of some of these nuclides (plus there are now fewer remaining target Mo-100 atoms). Anyone know the optimum fraction of Mo-100 to be consumed in a target before it is sent to processing?
In a fission reactor (the current primary source), Mo-99 is primarily produced as a fission product of thermally-fissioned U-235. A Mo-99 atom is produced in 6.1% of all U-235 fissions (this is almost at the peak of one of the U-235 FP production peaks, and one of the Pu-239 FP peaks. See http://en.wikipedia.org/wiki/File:ThermalFissionYield.svg ). According to http://atom.kaeri.re.kr/ton/nuc5.html (type in Mo-99 in the “Nuclide” input box), Mo-99 is produced in 5.7 to 6.1% of fissions (thermal and fast) of U-235 and Pu-239, and in 6.2% of fast fissions of U-238.
Strictly speaking, the above fission product yields are for the accumulated production of Mo-99 from all the 99 AMU (atomic mass unit) direct fission products that decay to Mo-99. Several 99 AMU nuclides are produced directly by fission, but they tend to have very short half-lives (ms to s) as they beta decay. Mo-99, with a much longer half-life (2.75 days) thus builds up in the radionuclide inventory from the very-short-half-life 99 AMU FPs. Again according to http://atom.kaeri.re.kr/ton/nuc5.html, the direct production of Mo-99 as an FP is 2 to 4 orders of magnitude less than the accumulated production (i.e., the summation of all the short-lived 99 AMU precursors that decay to Mo-99).
High enrichment uranium targets are typically used for Mo-99 production in a reactor. This is so that reasonable amounts of Mo-99 can be produced prior to being lost to decay, and the concentration in the target matrix is high (i.e., relatively little waste). With lower enrichment or natural U targets, the Mo-99 concentration would be much lower than in highly enriched targets and more waste (3-4 times?) is produced.
The proof of the proposed cyclotron process will be in the success of long-term Mo-99 production capability and cost of using cyclotrons.
New finding may solve isotopes shortage
21 February 2012
Canadian scientists have shown they can make radioactive medicine without nuclear reactors, a new process that could go a long way toward solving the world's shortage of medical isotopes.
The process uses hospital cyclotrons to make the compounds, bypassing the need for reactors. "It's essentially a win-win scenario for health care," Dr. Francois Benard of the B.C. Cancer Agency told a news conference Monday at the annual meeting of the American Association for the Advancement of Science.
"We have found a practical, simple solution that can use existing infrastructure."
The team, led by the TRIUMF nuclear lab based at the University of B.C., has produced technetium-99m in cyclotrons in Ontario and B.C. The scientists describe it as a "major milestone" in the international race to come up with new ways to make the critically important isotope.
Technetium-99m is used to help detect cancers, blocked arteries and heart disease in millions of people around the world each year. Yet the supply is often disrupted because 75% of the technetium-99m is now made at the trouble-prone Chalk River reactors near Ottawa and another aging reactor in the Netherlands.
Canada, which pioneered nuclear medicine, is seen as largely responsible for the precarious state of the global supply. New MAPLE reactors built at Chalk River were to supply the world with medical isotopes, but were mothballed, at a cost of over $500-million to Canadian taxpayers, because of technical flaws.
Several countries are now looking for new ways to make the isotope, and the Harper government last year handed the country's nuclear medicine experts $35-million. It challenged them to produce the isotope without using a reactor or weapons-grade uranium, which is now imported from the U.S. to make isotopes in the Chalk River reactor.
"It's a friendly competition," Dr. Benard said of the competing Canadian teams.
One of the big advantages of his team's approach is that they can use existing cyclotrons - there are 12 across Canada - regardless of brand or type of machine.
"The goal was to develop a technical solution that would work for many people, not just one machine or one brand of machine," said Dr. Benard.
Cyclotrons are essentially large electromagnets that accelerate streams of charged particles to incredibly high speed.
The technetium-99m was made in the cyclotrons from molybdenum-100, a naturally occurring compound mined in many parts of the world. Small discs of molybdenum-100 were strategically placed in the cyclotrons and the beams of energy stripped off subatomic particles, transforming the molybdenum-100 into technetium-99m.
It has been known since 1971 that it was possible in principle, but the idea was shelved. "A lot of people were saying this cannot be done, there were too many obstacles," said Dr. Benard.
Paul Schaffer, head of TRIUMF's nuclear medicine division, said it was quite a technical challenge. The team had to figure out how to package molybdenum-100 to withstand the intense irradiation and devise a way to automatically extract the radioactive disc and move it so it could be clinically processed.
The researchers don't see scaling up production as a problem. Discussions are underway with several industrial partners and regional health authorities about ramping up isotope production, said Ruth. "The science and the technology are essentially ready."
The technetium-99m from the cyclotrons appears to be identical to isotopes made from enriched uranium in nuclear reactors, Dr. Benard said, but he expects Health Canada will require clinical trials.
Morgan Brown, P.Eng., FCNS