FSU Home  
FSU Experimental Nuclear Physics  

The John D. Fox Superconducting Accelerator Laboratory

Professor John D. FoxIn March 2007, FSU’s superconducting linear accelerator laboratory was named for John D. Fox, a longtime FSU faculty member who was instrumental in its development.  Shortly after then-Florida Gov. LeRoy Collins established FSU’s program in nuclear physics in 1958, Fox joined the faculty in 1960 and went on to play a key role in building up the accelerator laboratory into a position of worldwide renown. In particular, his collaboration with FSU Professor Donald Robson on isobaric analog states put the accelerator lab on the world map of nuclear physics.

The John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University is a busy, teeming place with the radioactive beam facility RESOLUT, the construction of advanced detector systems, nuclear spectroscopy using a sophisticated array of Compton-suppressed clover gamma-ray detectors and experiments with the finest source of polarized lithium beam in the world. 

Laboratory History

 

The FSU Accelerator Laboratory began operation in 1960 following the installation of an EN Tandem Van de Graaf accelerator.  It was the second of its type in the United States.  The first useful acceleration of negatively-charged helium ions was achieved at FSU in 1961, and isobaric analogue resonances were identified at the facility in proton-induced reactions in 1963.   

The laboratory entered its second development stage in 1970 with the installation of an Super-FN Tandem Van de Graaf accelerator.  At that time, the research program turned to an emphasis on heavy-ion beams.  A superconducting linear post-accelerator was funded by the National Science Foundation in the mid-1980’s, with the first experiment on the completed facility run in 1987. 

The Laboratory Facility 

Brian Schmidt at the OPPLIS polarized Li source

 

The Super-FN tandem is injected by either a National Electrostatics SNICS-II cesium sputter ion source or a laser-pumped polarized lithium ion source.  Among the beams available from the sputter source is the radioactive isotope 14C.  The FSU lab is the only one in the world presently using a 14C beam.  In addition, helium beams can be produced using an rf-discharge and the same cesium charge exchange canal as the polarized lithium source. 

 FSU’s FN Tandem Van de Graaff accelerator

 

 

The FN tandem is equipped with a Pelletron charging system.  Both the usual carbon foil strippers and a turbo-pumped recirculating gas stripper are located at the terminal of the FN tandem. 

FSU's Superconducting Linear Accelerator

 

The superconducting linear accelerator consists of twelve accelerating resonators installed in three cryostats, plus buncher and re-buncher each located in their own cryostats.  The resonators are niobium-on-copper "split-ring" resonators produced by Argonne National Laboratory.  The cryostats were designed and built at FSU.  All the resonators are designed for b=0.1, except for the buncher, which is designed for b=0.06.   

The Radioactive Beam Facility RESOLUT

The radioactive beam facility RESOLUT

The laboratory has constructed an in-flight radioactive beam facility named RESOLUT.  This technique utilized for RESOLUT allows the production of large quantities of exotic ions without the limitations of the efficiency of an ion source or the chemistry of the beam material.  One of the concerns associated with an in-flight facility – one that can be particularly important in experiments near the Coulomb barrier – is that the energy definition of the secondary beam is relatively poor because of the kinematic broadening induced in the production reaction.  However, K.E. Rehm addressed this issue at Argonne National Laboratory by placing a superconducting resonator downstream of the production target.  This resonator is used to sharpen the energy resolution of the secondary beam.  This technique was adapted and improved for RESOLUT, which provides an increased angular acceptance for the recoil products and uses an intermediate dispersive focal plane to select the beam of choice more cleanly. 

A neutron wall consisting of plastic position-sensitive scintillators has been constructed for use at the end of RESOLUT.  The neutron wall is particularly useful for detecting neutrons from inverse kinematics (d,n) reactions with the radioactive beams produced by RESOLUT.

Polarized Lithium Source

To prepare a beam of polarized 7Li in the FSU optically pumped polarized lithium ion source (OPPLIS), an atomic beam of lithium is produced by heating a lithium oven with a nozzle diameter of about 0.5 mm to temperatures of about 750 șC. While the resulting lithium atomic beam is passing through a region with a weak magnetic holding field of about 10 G, it is irradiated transversely with up to 150 mW of circularly-polarized laser light at a wavelength of 670.8 nm. This single frequency laser is obtained from a dye laser which is tuned midway between the 2S1/2, F= 1, 2 → 2P1/2, F’ = 2 hyperfine components of the D1 resonance line in 7Li. Before interacting with the lithium atomic beam, the laser light is electro-optically modulated at half the 2S1/2 hyperfine splitting (402 Mhz for 7Li), causing the first order laser frequency sidebands to have the proper frequencies for exciting the D1 transitions from both lithium hyperfine levels with the same laser beam. If the circularly polarized laser light has σ+ polarization with respect to the magnetic holding field, then only ΔMF = +1 atomic transitions occur to populate hyperfine states in the 2P1/2, F’=2 atomic energy levels. Spontaneous decays from these states then repopulate the 2S1/2 atomic states. Through multiple interactions of the atoms with the laser light, the atoms are pumped into the 2S1/2, F=2, MF=+2 state, which corresponds to the pure nuclear substate MI = +3/2. If the laser light instead has σ- polarization with respect to the magnetic holding field, only ΔMF = -1 atomic transitions occur leading in a similar way to population of the 2S1/2, F=2, MF=-2, which corresponds to the pure nuclear substate MI = - 3/2. Beams of the remaining two possible nuclear substates can be obtained by interacting the polarized lithium atoms with a longitudinal rf magnetic field with amplitude of 1 G at 20 Mhz, while the atoms are contained in a transverse magnetic field of about 30 G. In this environment, adiabatic transitions occur from the 2S1/2, F=2, MF=+2 hyperfine state to either the MF=1 (MI = 1/2), or MF=0 (MI = -1/2) states. Thus, 7Li atomic beams that are spin polarized in any of the four nuclear substates can be produced. The resulting polarized beam is then ionized to 7Li+ by a heated tungsten strip, and then passed through a cesium vapor charge exchange cell where a small fraction (~5%) of the atoms emerge as Li-. These negatively charged, polarized ions are then into the Tandem Van de Graaf for experiments. 

 FSU g-ray Detector Array

FSU’s array of Compton-suppressed gamma-ray detectors

 

The FSU Compton-suppressed g-ray detector array consists of three Compton-suppressed clover segmented germanium detectors and ten Compton-suppressed single-crystal germanium detectors.  A segmented silicon particle detector array, liquid scintillator neutron detector and plunger for Recoil Distance Measurements are available as auxiliary detectors.