The Florida State University
Superconducting Linear Accelerator Laboratory

The Florida State University superconducting linear accelerator provides energetic beams of heavy ions for use in fundamental experiments in nuclear physics. The two most important features of the accelerator are that it can accelerate isotopes of most elements from hydrogen up to bromine (i.e. the 35 lightest elements), and that the final beam velocity is easily and continuously variable. The layout of the accelerator facility is shown in Figure 1.

Fig. 2 Ion sources. The SNICS-II sputter source can produce negative ions of all elements with the exception of the noble gases. A radio-frequency ion source is used to provide beams of helium. A third ion source provides highly polarized beams o f lithium isotopes. In a polarized ion beam, most of the nuclei have their spin axes aligned in the same direction. Such beams are used to investigate the important effects of nuclear spin in nuclear reactions. The polarized lithium ion source uses the technique of "optical pumping" with a specially developed laser and optical system. The laser system is located in another room, and laser light is transported to the source using a fiber optic. Other capabilities include an RF ion source operated in the central terminal of the tandem which provides beams of N, He, Ne; and a sputter source dedicated to radioactive ¹⁴C

Fig. 3 The tandem Van de Graaff accelerator is shown schematically. The name "tandem" derives from the two thrusts it gives the ions as they travel down its 50-foot long evacuated acceleration tube. Halfway along this tube is a point (in side the "terminal") in which the negative ions accelerated towards the positively charged terminal are stripped of many of their electrons and become positive ions. This electron stripping is accomplished by passing the ions through a very thin carbon foil placed in their path. The second acceleration is accomplished as the positive ions are repelled by the positively charged terminal. The amount of acceleration can be varied by changing the terminal voltage. This voltage is maintained by continuously transferring charge using an endless insulating belt carrying positive charge between ground potential and the terminal. The terminal can be charged to a maximum potential of 9 to 10 million volts.

Fig. 4 The tandem accelerator appears on the outside to be a large tank. The tank contains the charging belt, the acceleration tube, the terminal, and an electrically insulating support structure. It can be pressurized to 6 atmospheres with sulfur hexafluoride insulating gas to prevent sparking.

The LINAC consists of 12 superconducting accelerating resonators built by Argonne National Laboratory. Each is about 35 cm long and is capable of accelerating ions by the equivalent of a potential difference of 1 million volts. By making the resonators out of the superconducting metal niobium - a superconductor is a material which loses its electrical resistance at low temperatures - and cooling them to liquid helium temperature (4.5 K or -451 degrees F), the power losses due to resistive heating inside t he resonators can be eliminated. If the resonators were made out of a "normal" conductor such as copper, maintaining such high radio-frequency electric fields would be prohibitively expensive due to the enormous electric power requirements. A picture of a resonator is shown in Fig. 5, and the manner in which it accelerates a bunch of positive ions is explained in Fig. 6.

Fig. 5 Technician polishes the niobium surface of an accelerating resonator just before the end plates are mounted. The outer housing is made of copper to which a thin sheet of niobium superconductor has been explosively bonded. The interior structure, two metal arms joined at the base, is made or pure niobium tubing. The ion beams pass through the hole in the center of the tubes at the end of the arms.

Fig. 6 Bunches of positive ions, "beam pulses", arrive at the entrances to each resonator when the first arm is negatively charged (a). Thus the ions are attracted toward the arm. As the pulse passes through the tube at the end of the first arm, the negative charge is draining away and being replaced by positive charge. At the same time, the second arm is becoming negatively charged. When the pulse emerges from the tube at the end of the first arm (b), the ions are repelled by the positively charged first arm and attracted by the negatively charged second arm. By the time the ions emerge from the tube at the end of the second arm (c), that arm is again positively charged and repels the ions. The amount of acceleration can be varied by changing the amount of radio-frequency power fed into the resonators. The beam pulse frequency is adjusted so that each pulse arrives at just the right time to be accelerated.

To maintain the resonators at such low temperatures, they are housed in carefully designed cryostats and cooled with liquid helium. The liquid is supplied from a 1000 liter dewar which is filled by a large helium liquefaction system. One of the three large LINAC cryostats, each of which contains four resonators and two superconducting solenoids for beam focusing, is shown in Fig. 7. In addition, there are two smaller cryostats, each housing a single resonator, located at either end of the LINAC. These are used to produce extremely short ion beam pulses (a tenth of a billionth of a second) going into the LINAC, and at the experimenter's target station. The complex task of monitoring and controlling the resonators in the LINAC is carried out by computers

After leaving either the tandem Van de Graaff accelerator or the LINAC, the beam is focused by a number of electromagnetic lenses and steerers. At the entrance to each of the two target rooms is an electromagnet which allows the beam to be switched into any one of several beam lines to a target station in that target room. Each station houses a target, usually a very thin self-supporting film made of the element or enriched isotope the beam is to interact with, inside an evacuated chamber. Very often exotic isotopes of a particular element are created for special study by the fusion of the beam and target nuclei to form rare nuclear systems. Detectors are positioned around the target so that radiation produced when the beam particles react with the target may be measured.

Brief History of the Laboratory

The Florida State University accelerator laboratory began operation in 1960 following the instillation of a 6 million volt tandem Van de Graff, the second of its type in the U.S. The laboratory and the accelerator were funded by the Florida Legislature. T he first useful acceleration of negatively-charged helium ions in the world was achieved here in 1961. An exciting development of the period was the identification of isobaric analogue resonances in proton-induced reactions. The resonances demonstrated the equivalence of the nuclear forces between neutrons and protons.

The laboratory entered its second period in 1970 with the installation of the present 9 million volt tandem accelerator funded by the National Science Foundation. Research with this accelerator has emphasized the use of heavy ion beams, especially those from lithium to sulpher. An important contribution to the success of the program was development of the inverted sputter ion source in this laboratory.

Preparations for the superconducting linear accelerator began in 1980 with a study of the most cost-effective way to improve the capabilities of the laboratory. Florida State University and the State of Florida provided crucial funding in the early stages for the laboratory building addition and a demonstration accelerator. Scientists at Argonne National Laboratory shared their expertise and experience. The LINAC was funded by the National Science Foundation at a total cost of about $3,500,000. Experiments using the LINAC began in June 1987.

Fig. 8 The field shaping electrodes for the Bragg curve spectrometer, a gas filled detector which was designed and built at FSU. The electrodes are sections of spherical surfaces, and carry voltages ranging from -20,000 volts to zero. T hey shape the electric field so that it points at the target everywhere inside the detector volume. Charged particles ionize gas atoms in the detector and the liberated electrons are collected by the strong electric field. Computer analysis of the signal reveals the energy and the nuclear charge of the detected particle.

Fig. 10. Students and faculty celebrate the completion of the first experiment using the new gamma-ray detection array. This detector assemblage, one of the largest in the world based at an university laboratory, consist of 11 Compton-suppressed Ge spectrometers, 3 GLOVER Ge detectors, and 28 BGO scintillator detectors for use in studies of complex gamma-ray decays of highly excited nuclei. The array can be coupled with a mini-orange spectrometer allowing the detection of coincident electrons and gamma-rays emitted following a nuclear reaction. The array is a collaborative effort with the University of Pittsburgh and Notre Dame University.

Nuclear and Atomic Experimental Physics

The Nuclear Physics program at FSU involves research into a wide range of exciting frontiers in nuclear science. These forefront activities involve experiments performed locally using the Superconducting Linear Accelerator Facility as well as at many national facilities, such as MIT-Bates, Brookhaven (AGS and RHIC), GAMMASPHERE, Michigan State (NSCL) and the Thomas Jefferson Laboratory. 

Strong collaborations exist between the experimental and theoretical groups creating an ideal environment for nuclear science research. 

High-spin gamma-ray spectroscopy measurements seek to understand the varied and fascinating changes in nuclear structure that occur with increasing angular momentum values. Nuclear correlations (superfluidity), exotic shapes (superdeformation), the interplay of single-particle and collective degrees of freedom and also vibrational motion (octupole modes) are some of the topics investigated. The world's most powerful gamma-ray "microscope" GAMMASPHERE is employed in these studies as well as the latest Ge detector technology at FSU. 

High powered CO2 lasers and other spectroscopic equipment are used to measure transition energies in highly charged ions produced by the accelerator system. These precison measurements provide fundamental tests of QED and aim to yield a new value of the fine structure constant. Other unique facilities such as the laser-pumped polarized lithium ion source allow state of the art measurements of nuclear reaction mechanisms. Cluster structures of light nuclei are being investigated by the innovative method of resonant particle decay spectroscopy. 

FSU is playing a major role in the construction of a detector system for use at the Relativistic Heavy Ion Collider where very heavy nuclei at 100 GeV/nucleon will collide, producing extremely hot and dense nuclear matter. These collisions will re-create conditions that were believed to be present in the first moments after the Big Bang. It is hoped that a new phase of matter, the predicted quark-gluon plasma, will be discovered. 

Electron scattering experiments probing the quark structure of nuclei will be undertaken at the new Thomas Jefferson National Accelerator Facility and at MIT-Bates. New detector developments are being undertaken and in addition, the FSU group is leading the effort to record, archive and process the unprecedented amount of data that will be forthcoming from certain key experiments. 

Many future measurements of the nucleus under extreme conditions will involve radioactive nuclear beams. The FSU group has already begun to take part in this exciting area at Michigan State and Oak Ridge National Laboratory as well as beginning development of its own radioactive beam capability. 

The nuclear physics laboratory at Florida State University involves nine faculty members, several research scientists, ten engineers and skilled technicians, and many graduate and undergraduate students. Scientists from a number of other institutions regularly use this facility in collaborative research. Over 120 graduate students have completed their Ph.D. research in the nuclear physics laboratory, and countless undergraduates have gained valuable experience working in an advanced research laboratory on campus. Today, those graduates occupy prominent positions in education, research, industry, and health care.