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| In front of the nuclear reactor at TU Wien Photo Credit: © TU Wien |
The copper isotope Cu-64 plays an important role in medicine: it is used in imaging processes and also shows potential for cancer therapy. However, it does not occur naturally and must be produced artificially — a complex and costly process. Until now, Cu-64 has been generated by bombarding nickel atoms with protons. When a nickel nucleus absorbs a proton, it is transformed into copper. At TU Wien, however, a different pathway has now been demonstrated: Cu-63 can be converted into Cu-64 by neutron irradiation in a research reactor. This works thanks to a special trick — so-called “recoil chemistry.”
From Nickel to Copper
Copper atoms contain 29 protons, while the number of neutrons can vary. The most common naturally occurring variant is Cu-63, which has 34 neutrons and is stable. Cu-64, by contrast, contains one additional neutron and is radioactive, decaying with a half-life of about 13 hours. This makes Cu-64 attractive for medical use: it remains stable long enough to be transported to its target location inside the body, but decays quickly enough to keep the patient’s radiation exposure low.
“Today, Cu-64 is typically produced in a cyclotron,” explains Veronika Rosecker of TU Wien. “You can produce Cu-64 by taking Ni-64 and bombarding it with protons. The nickel nucleus absorbs the proton, ejects a neutron, and is thereby transformed into copper-64.” This method works very well, but it is expensive — and it requires access to both a cyclotron and enriched Ni-64, itself a rare isotope.
Copper with One Extra Neutron
It is therefore natural to consider a simpler alternative: producing Cu-64 from Cu-63 directly. All that is needed is to add a single neutron — something a research reactor can provide. But this approach comes with a challenge: “When Cu-63 is irradiated with neutrons, Cu-64 nuclei are indeed produced, but it is almost impossible to separate them chemically from the ordinary copper atoms,” says Martin Pressler. “You end up with a mixture that consists mostly of ordinary copper, with only tiny traces of the desired Cu-64.”
Now, however, this problem has been solved using recoil chemistry. This effect has been known for nearly a century but has not previously been used to produce medically relevant radioisotopes. Before irradiation, the copper atoms are built into specially designed molecules. “When a Cu-63 atom within such a molecule absorbs a neutron and becomes Cu-64, it briefly holds a large amount of excess energy, which it releases as gamma radiation,” says Veronika Rosecker. The emission of this high-energy photon gives the atom a recoil — much like a rocket recoil when expelling exhaust. This recoil is strong enough to eject the copper atom from the molecule.
“This means that Cu-63 and Cu-64 can now be cleanly separated,” says Veronika Rosecker. “The Cu-63 atoms remain bound within the molecules, while the newly formed Cu-64 atoms are released. This makes it easy to separate the two isotopes chemically.”
Finding the Right Molecule
A key challenge was identifying a suitable molecule. It needed to be stable enough to withstand conditions inside a nuclear reactor, yet soluble enough to allow efficient chemical processing afterward.
“We achieved this using a metal–organic complex that resembles heme — the molecule found in human blood,” explains Martin Pressler. Similar substances had been studied before but were not soluble. The new complex was chemically modified to make it soluble, enabling straightforward recovery of the Cu-64 after neutron irradiation.
The method can be automated; the molecules can be reused without loss, and — instead of requiring a cyclotron — it only needs a research reactor such as the one at TU Wien.
Published in journal: Dalton Transactions
Authors: Martin Pressler, Christoph Denk, Hannes Mikula, and Veronika Rosecker
Source/Credit: Technische Universität Wien
Reference Number: mols120525_01
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