The RESOLUT Facility
Dr. Ingo Wiedenhöver, Dr. Lagy T. Baby, E. Diffenderfer, J. Fridmann, A. Bernstein,
P. Peplowski, A. Rojas

Florida State University operates a superconducting linear accelerator laboratory, which is the central facility of the nuclear physics group and which hosts a vigorous, student-driven research program. The group aligns its priorities to those identified by the national Nuclear Science Advisory Committee (NSAC), which in its most recent report recommended to build the Rare Isotope Accelerator laboratory as its highest priority for a new facility. This is a recognition of the importance that measuring certain nuclear reactions has on astrophysics and gave us the motivation to focus our research in this field and construct the RESOLUT facility at FSU's linear accelerator laboratory. The field of nuclear astrophysics exemplifies the connectivity of science with its investigations being motivated by questions in astrophysics and cosmology. Its topics range from investigations of the origin of the chemical elements to understanding the physics of stars and ultimately the structure of space and time in the universe.

Figure 1: RESOLUT in April 2006. The beam of stable isotopes enters from the right, bombarding the production target, from where the reaction products are focused into the RF resonator by means of a superconducting solenoid magnet. The superconducting RF resonator bunches the velocities of the reaction products, which in effect concentrates the wanted products at the focal plane of the magnetic spectrograph. Here, slits will select desired radioactive beam and suppress other products. This beam of exotic isotopes is then refocused by a second superconducting solenoid into the experiment chamber, where its secondary reactions are measured.

The construction of RESOLUT is a group effort with heavy involvement of the graduate students Joel Fridmann, Eric Diffenderfer and Patrick Peplowski and, very importantly, Dr. Lagy T. Baby, who joined the effort as a post-doctoral researcher. He has now become a staff employee of the laboratory, in charge of the operation of RESOLUT. Several undergraduate students participated in development projects around RESOLUT and its detector systems. During Summer 2003, 2004 and 2005, high school students from the “Young Scholars Program” performed research projects on cryogenic sensors for RESOLUT and contributed to the automated cryogenic supply system used in the facility.

RESOLUT is featured on the title page of the June 2006 issue of “Nuclear Physics News” and was described in a radio program of “FSU headlines” and in local TV as a CBS local news program in spring 2004. Weighing more than 16 tons and occupying approximately 42 square meters in the laboratory RESOLUT will be the lab's major instrumental workhorse.

Commissioning and Initial Operation of RESOLUT.

Figure 1 shows RESOLUT facility in its completed state. We performed a number of commissioning experiments with the facility, in various stages of construction. The optics of the spectrograph were calibrated during the Summer of 2004 and a first radioactive beam of ⁶He at an energy of 32 MeV was produced from the ¹¹Li(d,³He) ⁶He reaction in the Fall of 2004. A cooled, degraded beam was produced in April 2005, proving the principle of operation for the superconducting RF Resonator. The setup and control of RESOLUT is based on beam optics simulations, which are integrated into the interactive control program.

The biggest challenge in achieving the reliable and continuous operation required for secondary beam experiments was found in the cryogenics supply systems. Three components of RESOLUT are operated at liquid Helium temperature, two 6T solenoid magnets and one superconducting RF-resonator. One of the magnets showed excessive Helium consumption and was returned to the manufacturer three times before it was delivered in working condition in January 2006. Furthermore, the existing cryogenic refrigerator proved beyond the limits of its capacity for a continuous operation of the full linear accelerator and RESOLUT. The cryogenic refrigerator was upgraded to a new system with increased capacity in December 2005. The system started operation in March 2006. The first experiment during which all components of RESOLUT were operated continuously was performed in April 2006.

During this experiment, we studied the reactions of a 94.5 MeV ¹⁴C beam bombarding a thin ⁹Be target. In a first run, the ⁹Be target was placed in the scattering chamber of RESOLUT, where the annular ion-chamber and Silicon detector was used to characterize the reaction products and their energies. A particle identification spectrum from this experiment is displayed in Figure 3. The cross sections for lighter reaction products, such as B, Be and Li are measured relative to the elastic scattering cross section. In a second experimental run with the same primary beam, the ⁹Be target was moved to the production target
location and the reaction products were analyzed with RESOLUT. We scanned the reaction products by varying the spectrograph rigidity. Analysis of this measurement will yield isotopic separation and the relative isotopic yields throughout a wide range of the reaction products displayed in Fig. 3. For instance, at 56 MeV, the composition of Beryllium isotopes observed in this reaction is 1/7 ¹²Be and 6/7 ¹¹Be. The fact, that ¹²Be is produced at all in this type of reaction comes as a solid surprise, given that the two-body reaction Q-value is -28 MeV. The data will allow us to investigate the mechanism for this type of deep-inelastic reaction.

A sample spectrum of the isotopic separation achieved with RESOLUT is displayed in Figure 4. Here, ¹¹Be was detected centrally in the focal plane, while the RF-resonator remained switched off. In Figure 5 we show the same rigidity setting, with the resonator bunching the ¹¹Be recoils towards the 61 MeV rigidity setting of the magnetic spectrograph. The product yield is increased four-fold relative to the setting without the resonator. This experiment shows, that RESOLUT can be successfully used as a zero-degree spectrometer for recoiling nuclei with rigidities very close to that of the primary beam. The production rate for ¹¹Be is at this point too low to consider secondary beam experiments, which are mainly limited by the available primary beam intensity of ¹⁴C. However, this experiment demonstrates the use of RESOLUT as a recoil mass spectrometer and shows the potential for a program in the spectroscopy of light neutron-rich isotopes.

First Astrophysical Experiment

At present, the RESOLUT group is in the process of developing an ²⁵Al beam. The beam production is based on thereaction at an energy of 130 MeV. The ³He target is realized as a a thin-window gas cell, which is cooled by liquid Nitrogen in order to increase the effective target thickness.
The ²⁵Al beam itself is used to measure the proton pickup reaction leading to excited states of ²⁶Si. The spectroscopy of the lowest resonances in ²⁶Si is addressing an important problem associated with research in γ-ray astronomy, performed for instance with the Integral satellite, launched in 2002 by the European Space Agency ESA. Integral measures γ-rays emitted from radioactive ²⁶Al, which is used to estimate the rate of supernova events in the galaxy. The amount of ²⁶Al produced in a supernova explosion is strongly dependent on the rate, at which the lowest nuclear resonances in ²⁶Si capture protons, which at this point is not known and the assumed value is based on a model calculation. We expect to provide experimental information on this capture rate, which in turn will lead to a refinement of the analysis of γ-ray astronomy data.

RESOLUT: Principle of Operation

Although the scientific program at RESOLUT is beginning just now, we have reported on its concept and proposed an application as a recoil detector for the RIA laboratory at invited talks at the RIA facility workshop of 2005 East Lansing and as an invited talk at the Fall 2005 meeting of the American Physical Society in Chicago. RESOLUT uses one superconducting RF-resonator synchronized in phase with the linear accelerator, which is providing the primary beam. The operating principle of RESOLUT is illustrated in Figures 6 and 7. Without the RF-resonator acting (Fig. 6), RESOLUT constitutes a large-acceptance magnetic spectrograph, creating a (mv)/q -dispersive focal plane. For the calculation of Fig. 7, the resonator is set up to bunch the velocities of the reaction products. This effectively creates a mass-dispersive focal plane.

An undergraduate student, Clifford Burchfield developed a computer simulation of RESOLUT's ion optics with the COSY-infinity program package. This program was used to calculate the m/q dispersion for different ions under realistic beam conditions. It was further refined by the graduate student P. Peplowski and is now used to calculate the magnet and resonator settings during the operation of RESOLUT. The simulations show that RESOLUT constitutes a mass-spectrograph with high angular and moderate energy acceptance. In order to investigate its usefulness as a recoil separator for heavier beams, we calculated the separation of isotopes around ¹³²Sn at 5 MeV/u at the focal plane of RESOLUT. The simulation predicts a mass resolution of 180, based on the assumptions of ±2% energy spread and a ±2.4°opening angle for the recoils with a 200 ps time resolution of the initial beam.
Unlike electrostatic mass separators, RESOLUT is optimized to be used in inverse kinematics reactions, which makes it useful as an in-flight radioactive beam facility as well as a recoil detector for experiments in inverse kinematics, such as the majority of experiments proposed for RIA.

Instrument Development

The experiments planned for and performed at RESOLUT require state of the art particle detectors, multi-parameter data acquisition systems and modern analysis methods. We developed a multi-strip Silicon detector array with four annular detectors and an annular ion chamber (Left picture in Figure 8).
An array of NaI γ-detectors and the corresponding electronics for RESOLUT was commissioned by the groups of Dr. R. Kaye at the Calumet campus of Purdue university on the basis of a “Major Research Instrument” (MRI) grant for “Research at Undergraduate Institutions”. Another grant of this type was awarded to Dr. D. Gay at the University of North Florida in Jacksonville, who constructed a microchannel plate detector for tracking radioactive beams at the focal plane of RESOLUT (Right picture in Figure 8). Both electronics and detectors were in use during the commissioning experiments of RESOLUT and the undergraduate students Ognjen Gabor and Sarah Dekat took part in the first experiments using the respective systems.