LEBIT - a low-energy beam and ion trap facility at NSCL/MSU S. Schwarz∗ , G. Bollen∗ , D. Davies∗ , D. Lawton∗ , P. Lofy∗ , D. J. Morrissey∗ , J. Ottarson∗ , R. Ringle∗ , P. Schury∗ , T. Sun∗ , D. VanWasshenova∗ , T. Sun∗ , L. Weissman∗ and D. Wiggins∗ ∗ NSCL / MSU, East Lansing, MI, 48824 Abstract. The Low Energy Beam and Ion Trap (LEBIT) Project aims to convert the high-energy exotic beams produced at NSCL/MSU into low-energy low-emittance beams. A combination of a high-pressure gas stopping cell and a radiofrequency quadrupole (RFQ) ion accumulator and buncher will be used to manipulate the beam accordingly. High-accuracy mass measurements on very short-lived isotopes with a 9.4 T Penning trap system will be the first experimental program to profit from the low-energy beams. The status of the project is presented with a focus on recent stopping tests of 100-140 MeV/A Ar18+ ions in a gas cell. INTRODUCTION The coupling of high-precision nuclear and atomic physics experiments to ISOL-facilities has expedited the investigation of the structure of nuclei far from stability considerably (for an overview see [1]). Applying these experiments at fragmentation facilities will allow the exploration of very exotic and short-lived nuclei which are not available with ISOL-techniques. Matching the beam properties of fragment separators (i.e. highly energetic, large emittance) and the requirements of trap-type experiments (i.e. cold ion bunches nearly at rest) provide a major challenge. At NSCL/MSU a low-energybeam and ion-trap facility (LEBIT) is being set-up as outlined in fig. 1. The project aims to stop the fragmentation products in a gas stopping cell and to use radio-frequency quadrupole (RFQ) ion-guide techniques to form a (bunched) low-energy ion beam with excellent beam properties. This beam will be transported to the experimental area, where a Penning trap system for mass measurements will be the first experimental installation to use the LEBIT beams. THE GAS STOPPING CELL A schematic diagram of the gas stopping station is shown in fig. 2. Solid degraders, the entrance window and the gas cell containing high-purity helium gas at a pressure of up to 1 bar slow down, stop, and thermalize the highenergy beam coming from the A1900 fragment separator. The combination of DC electric fields, created by a set of focusing electrodes, and gas flow through a nozzle is used to extract ions from the gas cell. A series of RFQion guides has been built to transfer the ions into high vacuum and to form a continuous low-energy ion beam. Stopping range measurements Recently on-line tests of stopping energetic ions in the gas cell have been continued with beams of 100.4 MeV/A 40 Ar18+ and 125.4 MeV/A 36 Ar18+ . The ion energy was reduced by a set of two glass degraders with a total thickness of about 3.5 mm and 4.7 mm for the 100 MeV/A and 125 MeV/A beams respectively. The effective thickness of the degraders can be changed during the measurement continuously by varying the degrader angle with respect to the beam axis. After passing a 1.5 mm beryllium entrance window ions were detected in the gas cell by a set of two Si-detectors of 100 and 500 micron thickness. Using two detectors allowed to discriminate the primary ions from the lighter radioactive nuclei produced via reactions in the degrader and the window (typically a few per cent of the flux). For the tests with the 125.4 MeV/A 36 Ar beam the detectors were set at 43 cm distance from the Be window. The counting rate of the primary ions was measured as a function of the degrader thickness for the gas cell filled with helium at 800 Torr and for the evacuated gas cell. The results of the measurements are compared in fig. 3a) with the ATIMA stopping power calculations [2]. The CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan © 2003 American Institute of Physics 0-7354-0149-7/03/$20.00 1051 9 .4 T P e n n in g tr a p sy ste m d e c a y s tu d ie s N 4 v a u lt m a ss m e a s u re m e n ts te s t b e a m io n s o u rc e b e a m fro m A 1 9 0 0 Conceptual layout of the LEBIT-project G a s s to p p in g c e ll R F Q 1 R F Q 2 E x tra c tio n e le c tro d e s T o A 1 9 0 0 b u n c h e r E n tra n c e w in d o w N o z z le S u p e rs o n ic g a s je t S k im m e r P u m p FIGURE 2. O rific e P u m p 0 T o rr 0 .6 8 0 0 T o rr T ra n s m is s io n 0 .8 0 .4 0 .2 0 .0 1 5 0 0 T o rr 1 0 0 2 0 0 b ) 3 0 0 D is ta n c e fro m 4 0 0 e n tra n c e w in d o w [m m ] 1 .0 0 .8 0 .6 0 T o rr 8 0 0 T o rr 0 .4 0 .2 0 .0 4 .7 a ) 4 .8 4 .9 5 .0 5 .1 D e g ra d e r th ic k n e s s [m m ] FIGURE 3. a) Transmission of a 125.4 MeV/u 36 Ar beam to the detector as a function of the degrader thickness (normalized to the transmission at 0 degree). The calculated values (lines) were obtained by integrating over stopping ranges obtained with the ATIMA code [2]. b)Transmission of a 100.4 MeV/u 40 Ar beam to the detector as a function of the detector position (normalized to the average transmission at 0 Torr). The degraders were set to an angle corresponding to a thickness of 3.5 mm. A c c e le ra tio n o p tic s F ro m 1 .0 T ra n s m is s io n vertical error bars correspond to the fluctuation of the beam intensity and horizontal errors are associated with uncertainty in determination of the degrader angle. The major uncertainties in the calculations are the homogeneity of the gas cell window material and the degrader angle. The dependence of the counting rate on the position of the Si-detectors in the cell was measured with a 100.4 MeV/A 40 Ar beam. The degrader angle was set to 10.7 degrees. When the gas cell was evacuated, the counting rate was found to be independent of the detector position within the gas cell (fig. 3b). This indicates that, even after passage of the degrader and the window, the beam has a small divergence over the measured range. As expected, when the gas was introduced into the cell the transmitted fraction drops drastically with the distance to the detector. The dependence of the transmitted fraction on the distance to the detector is shown in Fig. 3b) for 800 Torr and 1500 Torr along with the ATIMA predictions. A more detailed report on these and other stopping tests will be published in [4]. D e g ra d e r la s e r s p e c tro s c o p y g a s s to p p in g c e ll a n d io n g u id e s y s te m FIGURE 1. G a s in le t R F Q io n tr a p fo r b e a m a c c u m u la tio n c o o lin g a n d b u n c h in g P u m p Conceptual layout of the gas stopping cell THE RFQ ION BEAM BUNCHER The ion accumulator and buncher in the LEBIT project is a linear Paul trap system designed to accept the 5 keV- 1052 & A c c u m u la tio n b u n c h in g s e c tio n C o o lin g s e c tio n the beam-line coming from the buncher. This will give free access to the traps from the rear end and will e.g. allow to combine TOF measurements with nuclear spectroscopy. E x tra c tio n e le c tro d e s M ic ro -R F Q R e ta rd a tio n e le c tro d e s SUMMARY R F D C FIGURE 4. Schematic of the buncher electrode layout DC beam from the gas cell and convert it into low-energy low-emittance pulsed beams. The cooler and buncher is designed as a two-stage system in order to separately optimize the cooling process and the extraction process. Figure 4 shows the conceptual electrode layout of the LEBIT-buncher. The cooler and the trap section are designed to be operated at LN2 -temperature. Besides an increase of the acceptance of the system and a decrease of the cooling time this will significantly reduce the emittance of the resulting pulse compared to an operation at room temperature. Details of the buncher system can be found in [3]. With the gas cell having demonstrated the feasibility of stopping 100 MeV/A beams preparations are in progress to test ion extraction out of the cell. The design of the ion cooler and buncher system is finished, it promises efficient pulse forming and excellent beam properties. The 9.4 T Penning trap system is in an advanced planning stage with the magnet about to be installed this fall. ACKNOWLEDGMENTS This work is supported by the National Science Foundation, grant PHY-01-10523, the Department of Energy, grant 00ER41144 and Michigan State University. REFERENCES 1. THE 9.4 T PENNING TRAP SYSTEM 2. A 9.4 T Penning trap system will be the first set-up in the experimental area of the LEBIT facility. Figure 5 shows a sketch of the beam-line including the Penning trap system. The superconducting magnet features highly compensated, self-shielding coils and yields a magnetic field with a maximum strength of B = 9.4 T. A hyperbolic trap for high-precision mass measurements will be placed in the highly homogeneous field center of the magnet. A general-purpose cylindrical trap for ion "parking" and decay studies will follow the hyperbolic trap. Both traps will be operated at 80 K temperature which is essential to avoid charge-exchange at long storage times. A particle detector for time-of-flight (TOF) analysis is foreseen in 3. E in z e l le n s D a le y d e te c to r H y p e rb o lic a l P e n n in g tra p F ro m b u n c h e r P u m p P u m p 9 .4 T m a g n e t FIGURE 5. C y lin d ric a l P e n n in g tra p P u m p Sketch of the Penning trap section 1053 4. Conference series "Atomic Physics at Accelerators": APAC 1999, Mainz, Hyperfine Interactions 127 (2000), APAC 2000, Cargese, Hyperfine Interactions 132 (2001) ATIMA 1.2, http://www-aix.gsi.de/~scheid/ ATIMA1.html S. Schwarz et al., conference proceedings "Electromagnetic Isotope Separators and Techniques Related to their Applications", Victoria, Canada, 2002, to be published L. Weissman et al., Nucl. Instr. and Meth. A. submitted for publication
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