NUCLEAR ALCHEMY, ACHIEVED NOT BY ANCIENT MAGIC but through a new state-of-the-art accelerator system, is the purpose of the IsoSpin Laboratory (ISL) that is being developed by researchers with LBL's Nuclear Science Division (NSD). Designed primarily to produce intense beams of radioactive ions that are rich in either neutrons or protons, the ISL could help expand the number of different atomic nuclei available for study and possible exploitation from the approximately 270 that Nature provides to more than 5,000. This should not only create an entirely new field of nuclear physics, but could also lead to advances in the materials and life sciences, such as exotic new semiconductor devices and new radioisotopes for diagnostics and medical treatments.
"Isospin" is the term for the ratio of neutrons to protons in the nucleus of an atom. Among the known elements and their long-lived isotopes that are indigenous to Earth, there is a balance between the two forces that shape and govern nuclear structure and interactions. One is the "strong force" which holds together protons and neutrons and the other is the Coulomb force, the electromagnetic repulsion between particles with the same charge. The strong force is the more powerful of the two but has a much shorter range. If too many neutrons are added to a stable nucleus, the nucleus is disrupted beyond the strong force's ability to hold it together. On the other hand, if too many protons are added, the Coulomb force blows it apart. The upper and lower limits of the neutron-proton ratio represent the extremes of stable nuclei, the driplines, so to speak.
To date, the most common technique for making beams of radioactive nuclei is one called "projectile fragmentation." As the name implies, this involves smashing a beam of heavy ions that have been accelerated to high energies into a tiny sliver of a target (less than one gram per square centimeter). The collision fragments the projectile ions into radioactive nuclei that continue to travel along at the same relativistic speeds as the original projectiles. After separation by mass spectrometry, these fast-moving radioactive nuclei can be directly focused into beams. These beams will be low in intensity, however, because the collision target has to be so thin in order to fragment without annihilating the projectile ions. The radioactive beams are also predominantly made up of proton-rich nuclei because so many of the more-loosely bound neutrons get thrown clear of the nucleus when a projectile strikes the target.
A newer technique just beginning to come into its own can produce beams of perhaps three orders of magnitude greater intensity than projectile fragmentation. Called the "isotope separator on line" (ISOL) approach, this technique involves the bombardment of large (200 grams per square centimeter) targets with high energy protons. The bombardment triggers a great number of nuclear reactions within the target, (a process known in the physics world as "spallation") resulting in the release of radioactive nuclei from the target's surface. These nuclei are released at low energies and must be accelerated to beam velocities before they can be used to do research. However, the sheer volume of reactions ensures that beams of neutron-rich as well as proton-rich nuclei will be produced for the full exploration of isospin properties.
ISL-type accelerator systems designed to optimize the ISOL approach are in various planning stages at several facilities around the the world, but the project proposed for LBL would be the first. As conceived by NSD physicist Michael Nitschke, the ISL at LBL would consist of a primary beam accelerator for producing high energy protons, an isotope separation system, and a secondary accelerator system for energizing beams of radioactive nuclei.
LBL scientists and engineers are at present recommending that the most cost-effective primary accelerator would be either a cyclotron or a rapid-cycling synchrotron that could produce an intense beam of protons at energies up to 1 GeV (billion electron volts). The proton beam would be fired into one of several target stations and the spallated radioactive nuclei spewing out would then be directed into what is called a Broad Range Atomic Mass Analyzer (BRAMA). Featuring a large magnetic mass spectrometer, BRAMA can analyze and sort through the entire spectrum of radioactive nuclei simultaneously, thereby allowing dozens of experiments to be carried out concurrently. Those nuclei selected for further acceleration are ushered through a series of bunchers, radiofrequency cavities, and small linacs, which focus and accelerate them into a beam, before finally being sent through a large linac that can boost them up to the 25 MeV per nucleon energies needed for nuclear science and astrophysics experiments. LBL's ISL could be built on the floor of the old Bevatron building. Nitschke estimates the cost of construction will be on the order of $150 million.
THE FINAL AND BY FAR THE smallest member of LBL's next generation of particle accelerators is the mini-cyclotron, or Cyclotrino.
AFRD's Tony Young (foreground) led the team that developed the mini-cyclotron. Behind Young from left, are Glen Ackerman, Mary Stuart, Dave Clark, Wulf Kunkel, Luke Perkins, Ka Leung, Klaus Halbach, Chaoyang Li, Tom McVey, Al Rowlings, and Steve Wilde.
Developed by AFRD researchers under the project leadership of physicist Anthony Young, this new accelerator is a cylcotron comparable in size to the early cyclotrons of LBL founder and namesake E.O. Lawrence. However, unlike Lawrence's machines, this new miniature cyclotron is not meant to do nuclear physics. Instead, it is designed to bring into the laboratory a highly sensitive technique for culling and identifying single atoms from a sample containing trillions. Called accelerator-based mass spectrometry (AMS), this technique can be used for a variety of purposes, including determining the age of fossils and archeological artifacts, measuring the concentration of pollutants in the atmosphere, sequencing the genetic code of life, or studying the metabolism of therapeutic drugs.
Young credits Richard Muller, an astrophysicist in LBL's Physics Division, for having launched AMS in 1976 as a way to make sample dating through analysis for carbon-14 (the radioactive isotope of stable carbon-12) much more sensitive. With a half-life of 5,370 years, carbon-14, or radiocarbon, serves as a sort of "second hand" on the geological clock, enabling scientists to determine the age of objects that date back no more than 50,000 years. Radiocarbon is especially useful for determining the age of fossils because, thanks to the steady influx of cosmic rays into earth's ecosystem, its ratio to stable carbon is constant in all living organisms. Once an organism dies, it no longer absorbs new radiocarbon and the ratio to stable carbon begins to change.
In conventional carbon dating, the age of a sample is estimated by taking a count of the one or more carbon-14 atoms that are decaying to nitrogen. In AMS, the total number of carbon-14 atoms in the sample are counted whether they are decaying or not. This increases sensitivity a thousand times or more more, which means a much smaller amount of sample is needed, i.e., a tenth of a milligram instead of a gram.
AMS was never widely adapted because of the size and expense of tandem Van de Graff accelerators, the machines of choice for radiodating samples. At this time, there are less than two dozen AMS facilities in the world. There have been several attempts, including a couple by Muller, to build a low-energy, relatively inexpensive miniature cyclotron -- also known as a "cyclotrino" -- that, like tandem accelerators, would be well-suited to measure the presence of carbon-14. In all cases, however, a variety of technical difficulties hampered further development.
Young and his colleagues have made several crucial improvements that should rekindle interest in cyclotron-based AMS technology (sometimes referred to as cyclotron mass spectrometry or CMS). Perhaps the most important has been the replacement of an electrical magnet with a series of permanent magnets in order to generate the cyclotron's magnetic field. In cyclotron-based AMS, a sample is vaporized into ions which are accelerated and sorted according to mass and charge as they pass through a magnetic field. A particle detector is then used to identify the ions. By generating this magnetic field with permanent magnets instead of the electrical magnets used in previous compact cyclotrons, Young and his colleagues reduced the overall size and weight of their machine and minimized its power and cooling demands. Permanent magnets also give this new mini-cyclotron a portability that older versions lacked.
There was a question as to whether permanent magnets could provide a field uniform enough to accelerate as well as sort the ions. Young and his colleagues were able to answer this challenge by arranging approximately 200 individual samarium cobalt magnets around a steel yoke. The magnets, which were designed by Klaus Halbach and Ross Schlueter of LBL's Engineering Division, can provide a highly uniform 1.0 Tesla strength field that accelerates ions to 50,000 electron volts of energy.
Another major improvement was in the injection line that delivers a beam of accelerated ions from its source to the cyclotron's magnetic field. Previous compact cyclotrons used a radial injection system that is straightforward but reduces spectrometric sensitivity and is slower than it needs to be. Young and his colleagues designed a spiral-shaped inflector with an electrostatic channel that twists and tilts as it quickly guides the ions down the axis of the machine and into the heart of the magnetic field
Sensitivity and detection efficiency were further improved by the use of a negative ion source designed by AFRD's Ka-Ngo Leung. This source not only is much easier to operate than the sputter sources used in previous mini-cyclotrons, it also yields a superior quality beam at a much higher current, which means there are more ions available for measurement.
Like the compact cyclotrons that preceded it, the new minicyclotron has been optimized to sort and identify carbon-14 ions. Unlike the previous machines, this new device is sensitive enough to distinguish carbon-14 from the numerous other isotopes and molecules that have the same mass. It should be noted that the new mini-cyclotron can not match the sensitivity of tandem accelerators and therefore is not viewed by its creators as a competitive technology.
"Our mini-cyclotron is an alternative for those who can't afford to use a tandem accelerator," says Young. "Building a machine like this is something that could not be done at many other places because its design and fabrication called for so many different skills."
Young's collaborators, in addition to those mentioned, included Kirk Bertsche, a former member of Muller's group, Dave Clark of the Nuclear Science Division, and AFRD's Wulf Kunkel, Chaoyang Li, Mary Stuart, and Russ Wells.
In the world of science, there may not be any rule requiring that great successes be followed by sequels, but there is no rule that says they can't. At LBL, the next generation of accelerators is ready to take center stage.