Accelerating Radioactive Ion Beams With REX-ISOLDE
F. Ames1, G. Bollen2, J. Cederkäll3, S. Emhofer1, O. Forstner3, D. Habs1, G.
Huber4, O. Kester1, K. Reisinger1, D. Schwalm5, T. Sieber1, P. Van Den Bergh6, P.
Van Duppen6, R. von Hahn5, F. Wenander3, B. Wolf4,
and the REX-ISOLDE collaboration
1) Sektion Physik, LMU, München, D-85748 Garching, Germany
2) NSCL, Michigan State University, East Lansing, MI 48824, USA
3) CERN, CH-1211 Geneva 23, Switzerland
4) Institut für Physik, J. Gutenberg-Universität, D-55099 Mainz, Germany
5) Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany
6) Instituut voor Kern- en Stralingsfysica, K.U. Leuven, B-3001 Leuven, Belgium
Abstract. The post accelerator REX-ISOLDE is installed at the ISOLDE facility at CERN, where a broad variety of radioactive ions can be addressed. Since the end of 2001 beams at the final energy of 2.2 MeV/u are available. REXISOLDE uses a unique system of beam bunching and charge breeding. First a Penning trap accumulates and bunches the
ions, which are delivered as a quasi-continuous beam from the ISOLDE target-ion-source, and then an electron beam ion
source (EBIS) charge-breeds them to a mass-to-charge ratio below 4.5. This enables a very compact design for the following LINAC, consisting of a 4 rod RFQ, an IH structure and three 7-gap-resonators. The later ones allow a variation
of the final energy between 0.8 and 2.2 MeV/u. Although the machine is still in the commissioning phase, first physics
experiments have been done with neutron rich Na and Mg isotopes and 9Li. A total efficiency of several percent has already been obtained.
counters for γ measurements, which are placed in 4 π
geometry around a target chamber. Several particle
detectors for the identification of the reaction kinematics are also included.
INTRODUCTION
Experiments with radioactive ions at an energy of
several MeV open up new fields not only in nuclear
physics but also astro-physics and solid-state physics.
In the case of nuclear physics reactions on nuclei far
off stability can be studied, which can enlarge our
knowledge on nuclear structure. Examples for such
experiments planed or already started are the study of
Coulomb excitation and one-nucleon-transfer reactions
on light nuclei around the magic neutron numbers
N=20 and N=28 or the study of halo nuclei like 9Li. In
astro-physics key processes in nucleo-synthesis like
for example the proton capture rate of 35Ar can be
studied accurately only with accelerated radioactive
beams. In solid-state physics the implantation of radioactive probe ions into the material in general helps
investigating the bulk properties more clearly by
avoiding surface related effects. Most of the nuclear
physics experiments at REX-ISOLDE [1] are planed
with the MINIBALL [2], an array of clustered Ge
With an end energy of 0.3 and 0.8-2.3 MeV/u the
post accelerator REX-ISOLDE [1] can fulfill most of
the requirements for experiments with radioactive nuclei with m<50 amu. It makes use of a broad variety of
radioactive ions, which are produced at the on-line
mass separator ISOLDE at CERN. Different target
ion-source combinations enable the production of
more than 600 radioactive nuclei by the impact of
high-energy protons on solid or liquid targets. The
ions, mostly single charged, are accelerated to up to 60
keV and mass separated. Despite the fact that the proton beam is pulsed the ion beam is quasi continuous.
Its time structure is governed by the diffusion time out
of the target and the half-life of the isotope under investigation. In contrary efficient acceleration requires
pulsed beams and higher charge states. REX-ISOLDE
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
245
and 238U. Typically an efficiency of 40% has been
achieved. Fig. 2 shows an example of a time-of-flight
spectrum of ions extracted from the trap after the injection of 29Na (T1/2=44.9 ms). Beside 29Na also ions
from the buffer gas or impurities can be seen. Moreover, a 29Al contaminant of the ISOLDE beam cannot
be distinguished from 29Na at this point.
is designed to work at a mass to charge ratio up to 4.5
and a pulse repetition frequency of up to 50 Hz. Thus
beam preparation is necessary. For charge state breeding we have chosen an electron beam ion source
(EBIS) and a Penning trap (REXTRAP) in front of it
for accumulation, bunching and cooling of the ions.
Due to the high charge state of the ions the following
LINAC can be built up very compact. It consists of a
4-rod RFQ an IH structure, and three 7-gap resonators.
All resonators are working at 101 MHz at a maximum
duty cycle of 10%. A scheme of the set-up can be seen
in Fig. 1.
250
20
200
Ne
ions
150
29
100
29
Na, Al
3
H2O
50
1.3 10 ions/pulse
22
Ne
0
0
20
40
60
80
100
TOF [µs]
FIGURE 1. Schematic set-up of REX-ISOLDE
FIGURE 2. Time-of-flight (TOF) spectrum of ions ejected
from REXTRAP
If the number of simultaneously stored ions exceeds some 106 space charge effects have been observed, resulting in a shift in the centering frequency
and finally in a decrease of the efficiency [4]. In most
cases this is not a severe limitation, as the radioactive
beams from ISOLDE are much weaker. However, it
can be a limitation if for high masses the accumulation
time has to be higher, due to the longer breeding time
in the EBIS or the incoming beam contains a high
number of contaminating isobaric ions.
BEAM PREPARATION
The Penning Trap REXTRAP
The task of this first device is to accumulate the
ions during the breeding time of the EBIS, typically 20
ms, and finally release them as a narrow bunch. Furthermore, for an efficient injection into the EBIS the
transversal phase space volume has to be reduced from
typically 30 π mm mrad at 60 keV delivered by
ISOLDE to less than 10 π mm mrad. A gas filled cylindrical Penning trap has been chosen [3]. It is located
in a 3 T superconducting magnet on a high voltage
platform slightly below the ion energy. Final stopping
is done via collisions with the buffer gas atoms, typically up to 10-3 mbar Ne or Ar. After the ions have
accumulated in the potential minimum they can be
centered in radial direction by side-band cooling at
their cyclotron frequency. This method enables a mass
selective and very fast cooling of the ions. The time
constant only depends on the damping force of the gas
and in the case of REXTRAP is about a few ms. To
release the ion bunch the electrical potential is
switched and the ions are accelerated again to ground
potential for an efficient transport to the EBIS. The
trap has been operated with several stable and radioactive ions covering the entire periodic table between 7Li
The Electron Beam Ion Source (REXEBIS)
At REX-ISOLDE an electron beam ion source has
been chosen for the charge breeding [5,6]. Besides the
possibility of reaching very high charge states in a
small breeding time the total current from restgas ions
is very low, ≈ 1 nA. This allows the handling of very
weak radioactive beams. The REX-EBIS is again
placed on a high voltage platform. For injection of the
ions this is set to nearly the ion energy from the trap.
For ejection it is switched to a lower voltage to match
the velocity acceptance of 5 keV/u of the following
RFQ accelerator. To increase the ions charge state
inside the EBIS they are superimposed by a strong
electron beam, which is compressed by the field of a 2
T superconducting magnet to about 200 A/cm2. This
246
ments of the following IH structure. This 20-gap drift
tube device brings the ions up to 1.2 MeV/u. Its internal structure is divided into first an accelerating followed by a rebunching and finally again an accelerating section. Between the first accelerating and the rebuncher section a magnetic quadrupole triblet lens is
included inside the resonator tank.
results in a very narrow charge state distribution
around the center, which depends only on the breeding
time. For example the maximum output current for Na
ions after a breeding time of 20 ms is at charge state 7
or 8 with more than 30 % of the total ion number in
one of these states. The EBIS has been operated with
injected stable or radioactive ions between 7Li and
153
Sm. An injection efficiency of about 40 % has been
achieved. Together with the charge state distribution
this gives a total efficiency for one charge state of up
to 15 % for Na7+. A typical spectrum of ejected ions
taken after the mass separator behind the EBIS is
shown in Fig. 3. It is clearly seen that due to the small
total current and the small emittance of the beam such
small currents below 1 pA can easily be detected even
with a separator with a relatively poor resolution of
(M/q)/(∆(M/q))≈100. The peaks beside the Na originate from restgas ions in the EBIS and Ne buffer gas
diffusing out of REXTRAP.
Ne5+, O4+, C3+
18
14
7+
Ne
C
Ne
4+
All resonators are working at 101.28 MHz at a duty
cycle of up to 10 % and a repetition frequency of 50
Hz. They have been designed and optimized using
simulations with MAFIA and TRANSPORT. After the
accelerator a switching magnet offers the possibility of
using two beam lines for experiments at a bending
angle of 20° or 65° respectively. The MINIBALL array is located at the latter one.
26
Na
A/q=4
16
The final acceleration to the maximum energy of
2.2MeV/u or deceleration to 0.8 MeV/u is done by
three 7-gap split ring resonators. The energy can be
adjusted by proper choosing the phases and amplitudes
of these three resonators.
6+
When accelerating radioactive ions, beam diagnostic to handle very weak beam intensities is an important issue. For beam intensities down to about 100 fA
sensitive Faraday cups are used. Below this and to
view the beam profile position sensitive channelplate
detectors in combination with phosphor screens and a
camera system are used. In the first part these detectors
are equipped with the possibility of an electronic readout to measure quantitatively small ion pulse intensities or to count the ions. In total 10 combinations of
Faraday cup and channelplate detectors are used at
REX-ISOLDE.
10
4+
Ne
8
Ne5+
Ar 9+
Na5+
26
26
N3+
Na6+
22
N4+
Ar11+
22
Ne6+
Na7+
2
26
Ar13+
22
Ne7+
4
Na8+
O5+
6
26
Ion current pA
12
0
15
16
17
18
19
20
21
22
Magnet current A
FIGURE 3. Part of a mass spectrum of ions extracted from
the EBIS after the injection of 26Na1+.
STATUS AND MEASUREMENTS
First tests with accelerated radioactive ions have
been done in the fall of 2001. At this time only 2 of the
7-gap resonators were operational because of a broken
rf-amplifier. Still the final energy reached was 2.0
MeV/u. One MINIBALL cluster to identify the accelerated particles and to look for first reactions has been
installed. 24,25,26Na ions have been successfully accelerated. After the winter shutdown of ISOLDE, the
accelerator and detector set-up have been completed
and the first physics experiments looking for Coulomb
excitation in heavy Na isotopes up to 29Na and in
heavy Mg isotopes have been carried out. With 9Li
particle transfer reactions could be studied. Additionally, an implantation of 153Sm into silicon carbide for
solid-state physics has been carried out at the 2nd
beamline. For this experiment a beam energy of 0.3
THE ACCELERATOR
The construction of the REX-ISOLDE Linac is
based on the set-up of the CERN Linac 3 and the High
current injector at the MPI für Kernphysik at Heidelberg. The first part is a 3 m long 4-rod-RFQ, which
accelerates the ions to 0.3 MeV/u. Its acceptance has
been chosen not only to meet the small emittance obtained with the EBIS but also to enable a high transmission when using an ECR source with much higher
emittance.
After the RFQ the ions enter a matching section,
which consists of a 3-gap rebuncher and two quadrupole triblet lenses. It is necessary to fulfill the require-
247
be increased. Therefore, in a next step an additional
resonator structure will be introduced after the 7-gap
resonators. It will be a 9-gap IH structure working at a
frequency of 202.56 MHz. The structure is nearly finished and test measurements on it have already started.
It is planed to install it at ISOLDE during the winter
shutdown period 2002/3 [8]. With this addition an energy of 3.1 MeV/u can be reached. Finally with the
introduction of an additional IH-structure instead of
two of the 7-gap resonators, an energy of 4.3 MeV/u
will be reached. To allow charge breeding of ions with
higher masses the breeding time in the EBIS has to be
increased in order to reach the necessary mass-tocharge ratio. Consequently, also the accumulation time
and thus the maximum number of stored ions in REXTRAP has to be increased. To overcome the abovementioned space charge limitations new cooling
schemes, like rotating wall compression of the ions are
already being tested. With this a maximum charge
density near to the Brilloin limit should be possible to
reach, i.e. up to 2 orders of magnitude higher than with
the presently employed sideband cooling.
MeV as delivered by the RFQ only has been chosen.
The data analyses of these experiments are still going
on and results will be published soon.
As the radioactive beams normally are too weak for
setting up and optimizing the machine from the beginning on this has been done with stable beams of ions
with a mass near by. For the trap and the injection into
the EBIS, a test ion source producing stable beams of
alkali ions or stable ions from the ISOLDE ion source
have been used. Additionally, for setting up the accelerator restgas beams from the EBIS with an m/q value
near to the desired one can be used. Therefore gas can
be let into the EBIS if there is not sufficient beam from
the ionization of residual gas. Finally, the parameters
for the accelerator cavities and lenses can be scaled for
the radioactive ions. Using stable beams from ISOLDE
the total efficiency from the exit of the ISOLDE mass
separator to the target within the MINIBALL set-up
has been determined. With stable 27Al a value of about
3 % has been already reached for 27Al7+. The total current for the 27Al+ ions from ISOLDE for this measurement was around 50 pA. After correction for the decay
half-life for the radioactive ions a similar transmission
within the accuracy of the measurements has been
determined from the counting rates at the experiments.
REFERENCES
SUMMARY AND OUTLOOK
1. Habs, D., et al., Hyperfine Interactions 129, 43-66
(2000).
Although still in the commissioning phase REXISOLDE has already proven to efficiently accelerate
radioactive ions for nuclear and solid state physics
experiments. Radioactive ions successfully accelerated
until now are 9Li, 24-29Na, 30Mg up to 2.2 MeV/u and
153
Sm to 0.3 MeV/u. For the light isotopes a total efficiency of about 3% has been achieved. In the case of
153
Sm, where a charge state of 28+ has been used, the
efficiency was about 1% due to the broader charge
state distribution after breeding in the EBIS for 30 ms.
2. Eberth, J., et al., Progress in Particle and Nuclear Physics 46, 389-398 (2001)
3. Schmidt, P., et al., Nuclear Physics A701, 550c-556c
(2002).
4. Ames, F., et al., Hyperfine Interactions 132, 469-472
(2001).
5. Wenander, F., et al., Proc. of the 6th European Particle
Accelerator Conference, Stockholm, Sweden, IOP Bristol, 1999, p.1412-1414.
Beside the use of accelerated radioactive ion beams
REX-ISOLDE offers the possibility of using the
bunched beams out of REXTRAP or highly charged
ions from the EBIS. The bunched structure for example makes it easy to further capture the ions in ion
traps. This will be used by an experiment to measure
the electron neutrino correlation in beta decays by a
precise determination of the recoil energy of the product nuclei [7]. The highly charged ions are interesting
for implantation at intermediate energies up to several
MeV.
6. Wolf, B., et al., “14th Conf. on ElectroMagnetic Isotope
Separators and techniques related to their applications”,
to be published in Nucl. Instr. Meth. B.
7. Beck, D. et al., Nuclear Physics A701, 369c-372c (2002)
8. Sieber, T., et al., “LINAC 2002” conference proceedings,
to be published.
For nuclear physics experiments with ions of
higher masses the final energy of the accelerator has to
248