1051_1.pdf

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