||Nuclear Science Division researchers glimpsed a
new experimental future one Sunday last summer when a supply of the
radioactive isotope carbon-11, created in the Life Sciences Division's
medical cyclotron, was piped downhill to the 88-Inch Cyclotron. Within
minutes the radioactive carbon had been ionized to the 4-plus charge state
-- stripped of four electrons for a high positive charge -- and
accelerated to 110 MeV (million electron volts) in a beam that delivered a
hundred million radioactive carbon-11 ions per second onto a gold target.
It was the first full scale test of the BEARS project: Berkeley Experiments with Accelerated Radioactive Species.
Over the years, nuclear scientists have observed over three thousand of the six thousand atomic isotopes thought to exist, but only 263 of these are stable. Experiments at the 88-Inch Cyclotron using beams of stable isotopes continue to yield important basic knowledge -- and occasional major surprises, such as last spring's identification of elements 118 and 116 and new isotopes in their decay products -- yet virtually all possible combinations of beam and target using stable beams have already been explored.
"Back in 1989 the 88-Inch Cyclotron researchers posed themselves a challenge, namely how to make the 88-Inch into a uniquely valuable instrument for future nuclear research," says Peter Haustein, a visiting nuclear chemist from Brookhaven National Laboratory who since 1997 has worked periodically on the BEARS project with its leader Joseph Cerny, a member of the Nuclear Sciences Division (NSD) and a professor of chemistry at the University of California at Berkeley.
Haustein says the 88-Inch group came up with several ideas, among them the coupled-cyclotron proposal, "a relatively cheap and simple method of producing radioactive beams for experiments that cannot be done with beams of stable ions." For example, mechanisms of energy production in some stars -- such as the carbon-nitrogen-oxygen cycle, which involves several short-lived isotopes -- require studying nuclear reactions in which one of the reaction partners is radioactive.
"Our first proposal was to install a second high-current cyclotron right in Building 88. We spent six months in 1989-90 designing that system before other projects took precedence. In 1995 we learned that the Life Sciences Division was installing its own cyclotron for the purpose of producing short-lived isotopes for medical imaging."
Haustein says that when Joe Cerny approached Thomas Budinger, Henry Vanbrocklin, and Jim O’Neil of the new Biological Isotope Facility, "they indicated they would be happy to join BEARS and share their isotopes with us."
The medical minicyclotron -- its magnet is just 90 centimeters, one yard, in diameter -- produces isotopes by bombarding gas or foil targets with a beam of protons accelerated to 11 MeV. Carbon-11 and oxygen-14, two isotopes of interest to the BEARS researchers, are produced by bombarding nitrogen gas.
One problem was how to get the isotopes downhill fast enough to feed them into the ion source at the 88-Inch. The half-life of oxygen-14 is only 70.6 seconds -- after one minute and 11 seconds, half the oxygen-14 atoms mixed with the nitrogen have decayed. Carbon-11's half-life of 20.3 minutes makes it a bit more convenient.
Before a delivery system could be designed, however, the coupled-cyclotron concept had to be tested. Dennis Moltz of NSD led initial work at the 88-Inch, and recently James Powell of NSD and UC Berkeley has spearheaded a team including Rainer Joosten, Mike Rowe, Daniela Wutte, Z.Q. "Dan" Xie, and other researchers.
They developed a method of trapping isotopes cryogenically in a coil of stainless steel tubing submerged in liquid nitrogen. When the coil is connected to the ion source, its temperature is raised, and the carbon-11 is released in a controlled fashion.
In preliminary tests in "batch mode," carbon-11 in a heavily shielded container was brought down the hill from Building 56 by truck. Transfer was quick enough that useful amounts of the isotope could be injected into the 88-Inch's ion source to test proof-of-principle. In this way Joosten studied the yield of astatine isotopes produced by bombarding gold targets with beams of carbon-11.
Meanwhile work proceeded on a direct method of transporting the isotopes: the capillary transfer line, a 300-meter-long pipe down Blackberry Canyon. NSD’s Eric Norman worked closely with Max Ostas and others in the Facilities Division on materials and design, while Health Physicist Christine Donahue and Radiation Safety Technician Bob Fairchild of the Environment, Health and Safety Division helped with a thorough program of integrated safety management.
The amount of radioactive material in a capillary at any time is modest, and the capillaries are doubly contained inside a continuously monitored, two-inch inner line -- which is under vacuum and equipped with redundant interlocks -- inside a six-inch outer line. The risk of radiation exposure, on or off site, is extremely low "under every possible scenario," says James Powell. Under the National Environmental Protection Act, the California Environmental Quality Act, and rigorous safety reviews at Berkeley Lab, the system has been approved at every step of the way.
Finally, computer-automated systems built by Powell and Joosten were installed to track and route the pressurized gas mixed with the isotopes from the medical cyclotron to the cold trap feeding the ion source. Transfer time is approximately 20 seconds. In the efficient new AECR ion source, more than 10 percent of arriving carbon-11 atoms can be ionized to the 4-plus state.
"When we were feeding the ion source with carbon-11 in batch mode, there was no time for the operators to tweak the beam to improve its intensity," says Haustein. "In continous mode, there's plenty of time. During our first full test on August 1, it was impressive to see the beam start at low intensity and then grow progressively stronger."
At a hundred million atoms per second on target, the carbon-11 beam is less intense than stable beams, but it's two or three orders of magnitude stronger than the radioactive beams other U.S. facilities have achieved.
Swift transfer time will allow BEARS to develop and deliver other isotopes with short half-lives such as nitrogen-13, oxygen-14 and 15, and fluorine-17 and 18.
Radioactive beams open many experimental possibilities in addition to astrophysics. In some interactions with targets, radioactive beams are expected to have a significant advantage over stable beams in producing desired isotopes, with fewer unwanted species to clutter the mix of reaction products. A survey of reaction yields is thus a priority.
A class of uniquely interesting experiments includes the scattering of "mirror nuclei," isotopes of different elements that have the same total number of nucleons -- for example, carbon-11, with 6 protons and 5 neutrons, and boron-11, with 5 protons and 6 neutrons. Because a mirror pair's members have similar nuclear structure, theorists have suggested that unusual resonance scattering effects may occur in their interaction.
"Meanwhile we are beginning to work on what we will call BEARS II," says Joe Cerny, "similar to our original thoughts about installing a second cyclotron inside Building 88. With proton energies up to 30 MeV, we could make several dozens of short-lived radioactive species, up to an atomic mass number of about 80. And with a transfer line only a tenth as long, we could deliver these to the ion source very quickly. Beams of these isotopes could take full advantage of the 88-Inch Cyclotron's capabilities."
BEARS II could be in operation as soon as 2002. Meanwhile BEARS I -- funded mostly by a Laboratory Directed Research and Development Program grant, and an example of a true team effort involving several Berkeley Lab divisions and the collaborative efforts of researchers from other institutions -- will soon be working up to 100 hours a month with beams of highly charged radioactive ions, helping to keep the 88-Inch Cyclotron in the forefront of nuclear research for years to come.