Charge State Breeding for the Acceleration of Radioactive Ions at TRIUMFa)
F. Ames,1,b) R. Baartman,1 P. Bricault,1 K. Jayamanna,1 T. Lamy,2 and M.
McDonald1
1
TRIUMF, 4004 Wesbrook Mall, Vancouver BC, V6T 2A3, Canada
2LPSC, UJF-IN2P3-CNRS, 53 Av. des Martyres, 38026 Grenoble, France
(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX)
(Dates appearing here are provided by the Editorial Office)
A 14.5 GHz ECRIS (PHOENIX from Pantechnik) has been set up at the ISAC facility at TRIUMF for the
charge state breeding of radioactive ions. After extensive testing and optimization on a test bench it has been
moved on-line and put into operation. During a first test in 2008 a beam of 80Rb14+ was successfully created
from 80Rb1+ and accelerated by the ISAC post accelerator. Further tests with different stable and radioactive
isotopes from the ISAC on-line sources and from a test source with stable Cs have been carried out. Until
now an efficiency of 1.4% for 124Cs20+ has been obtained.
I. ITRODUCTION
Charge state breeding is an important ingredient of most post accelerators at ISOL facilities. It implies the transformation of singly or
low charged ions extracted from the on-line target ion source system into highly charged ions, which are better suited for acceleration. An
overview on charge breeding techniques and results from different facilities can be found for example in a recent article by F. Wenander1. At
the ISAC facility at TRIUMF radioactive nuclei are produced in either one of two solid targets by bombarding it with up to 100 μA of
protons at 500 MeV. The target is kept at a temperature up to 2200 °C depending on the target material so that the products can diffuse out of
it into an ion source. Until now surface ion sources, resonance ionization laser ion sources or FEBIAD ion sources have been used and an
ECR ion source is being developed2. The singly charged ions are extracted, accelerated to several 10 keV and the desired species are filtered
out by a magnetic mass separator. Typical intensities for radioactive ions produced at ISAC span a range from single ions up to about 10 9
ions per second. The mass separated ions can be guided directly to experiments at low energy or to a linear accelerator for post acceleration
up to 5 MeV/u. The acceptance of the first part of the accelerator, a 4 rod RFQ, requires a maximum A/q value of 30. It accelerates the ions
to 150 keV/u. After this further stripping of the ions is done to reach a charge state leading to an A/q value <7, necessary for the final
acceleration in a room temperature drift tube accelerator and superconducting cavities. If ions with a mass greater than 30 amu are to be
accelerated their charge state has to be >1 and in order to avoid further losses due to the stripping the charge state breeding should go to
charge states with an A/q value <7.
II. IMPLEMENTATION OF THE CHARGE STATE BREEDER AT ISAC
A 14.5 GHz ECRIS (PHOENIX from Pantechnik) has been chosen as it is well adapted to the continuous mode operation of the ISAC
facility and is capable of handling relatively high intensity. The source has been tested at an off-line test bench3 before being integrated into
the ISAC on-line system. Both the injection and extraction ion optics have been exchanged from the original design to a two step system to
increase the efficiency and to simplify the change of the operating source bias voltage. The latter is necessary to adapt the ion velocity to the
acceptance of the accelerator, which is at 2.04 keV∙(A/q). Detailed results from the test set-up have been reported earlier3. The efficiency
obtained at the test bench was up to 3.5% for Cs20+ and 6.3% for Kr12+. The discrepancy can be mainly explained by the low surface sticking
probability of the noble gases on the plasma chamber walls.
The source was moved from the test bench to the on line system during 2008. It is located in a shielded area below ground downstream
of the mass separator for the radioactive ions. A set of two switchable electrostatic benders is used to steer the ion beam from the existing
beam line into the charge breeder. Several quadrupole lenses allow a matching of the beam for injection. The extracted highly charged ions
are separated by a combination of a magnetic and two electrostatic sector fields. After this the beam is directed again by two switchable
electrostatic benders into the vertical direction, where it joins the existing beam line to the experimental hall and the accelerator. At the end
of the vertical section at the so called “yield station” the ions can be implanted into an aluminized Mylar tape which is surrounded by
detectors for α, β and γ radiation to allow identification and intensity determination of the radioactive isotopes. The tape can be moved in
order to transport accumulated activity from a previous measurement behind a lead shielding and thus provide a clean spot for each
measurement.
At the straight port of the second electrostatic bender in front of the charge state breeder a surface ion source (HeatWave Labs, Model
HWIG-250) has been installed. It provides a beam of singly charged stable Cs ions for set-up and optimization of the system independently
a)
Contributed paper published as part of the Proceedings of the 13th International
Conference on Ion Sources, Gatlinburg, Tennessee, September, 2009.
b)
Author to whom correspondence should be addressed. Electonic mail:
ames@triumf.ca.
from the on-line ion sources. This source is biased with the same high voltage power supply as the charge breeder. An additional offset
voltage allows to compensate for the ECR plasma potential and to optimize the injection energy. The experiments from the test bench have
shown that for optimum capture efficiency the energy of the incoming ions with respect to the high voltage of the charge breeder has to be
controlled to a level below 1 eV. For the injection of the radioactive ions this is achieved by the stability of the two power supplies used
(<0.5V).
III. RESULTS
A. stable ions
Figure 1 shows a mass spectrum obtained from ions extracted from the charge state breeder without injecting. They can be assigned to
different charge states and isotopes of residual gas constituents like H, C, N, O and the noble gases He, Ne and Ar and also some simple
molecules of them. The source has been operated with pure He as a support gas for the plasma. The total current extracted is about 100 μA.
-5
10
-6
10
-7
current [A]
10
-8
10
-9
10
-10
10
-11
10
0
10
20
30
40
A/Q
FIG 1 Mass spectra of ions from residual gas ions extracted from the charge breeder.
-6
10
-7
current [A]
10
-8
10
23
21
18
24
17
16
-9
10
15
14
-10
10
-11
10
4
5
6
7
8
9
10
A/Q
FIG 2 Mass spectrum obtained with 5 nA 133Cs1+ injected.
Figure 2 shows a part of a similar spectrum obtained with about 5 nA (3∙10 10 ions per second) of stable 133Cs1+ ions injected. Peaks
corresponding to different charge states of Cs between 14 and 24 are identified with arrows. Cs 19+, Cs20+, and Cs23+ can not be identified
because they coincide with strong signals from 7N2+, 40Ar6+ and 12C2+, respectively. This is a general problem associated with charge state
breeding especially when dealing with beams of low intensity or rare isotopes. The intensity of the highly charged ion beam of interest is
usually several orders of magnitudes lower than those of neighboring ions from the residual gas. Although, in the case of radioactive ions
they can be clearly identified by their decay, a high background of stable ions accelerated and impinging on the experimental station is not
acceptable for most experiments. This makes a good mass separation after the charge state breeding a necessity.
The importance of the energy difference in between the incoming ion beam and the potential of the charge state breeder source is
demonstrated in figure 3. Here the offset voltage for the bias of the test ion source has been varied and the efficiency for the production of
Cs17+ has been recorded. A sharp increase can be observed when the energy of the ions is high enough to overcome the plasma potential.
After this the signal decreases again as the energy of the ions becomes too high for being stopped in the plasma. The width of this curve
depends on the mass and atomic number of the ions, as it determines the stopping in the plasma. It also depends on the emittance of the
incoming beam as it may change the longitudinal energy spread of the ions when entering the high magnetic field of the ECR source or
causes them to be reflected.
1.0
efficiency [%]
0.8
0.6
0.4
0.2
0.0
10
15
20
25
30
deltaV [V]
FIG 3 relative efficiency for the charge state breeding to Cs17+ as function of the difference voltage between the test ion source and the charge
breeder.
B. radioactive ions
Test measurements with radioactive ions have been performed with Cs and Rb ions produced in a tantalum target equipped with a
surface ion source. The intensity of the ions has been first determined by sending them directly to the yield station. Then they have been
directed into the charge state breeder and the highly charged ions have been sent again to the yield station to determine their intensity. The
ratio of the two measurements gives the efficiency for the charge breeding. Figures 4 and 5 show the result of such a measurement for 80Rb
and 124Cs. Both isotopes have a half-life of T1/2 = 30 s. The intensity for the singly charged ions was 5.7∙107 ions per second for the Rb and
2.75∙107 for the Cs. The maximum efficiency obtained was 1.1% for 80Rb13+ and 1.4% for 124Cs20+. The numbers include also the efficiency
of the beam transport to the charge breeder to and from the charge breeder.
1.2
1.0
efficiency [%]
0.8
0.6
0.4
0.2
0.0
10
12
14
16
18
20
charge state
FIG 4 charge state distribution of highly charged 80Rb ions.
1.4
1.2
efficiency [%]
1.0
0.8
0.6
0.4
0.2
0.0
16
18
20
22
24
26
charge state
FIG 5 charge state distribution of highly charged 124Cs ions.
C. Acceleration
During the first of those tests in November 2008 1.1∙105 ions per second of 80Rb14+ were transported further on and injected into the
RFQ accelerator at an energy of 11.66 qkeV. The ions were accompanied by a strong beam of about 100 nA of 40Ar7+ at the same value of
A/q. Although, this will most probably not be an ideal case for an experiment it allowed an easy tuning of the accelerator for this test
experiment. Normally the accelerator is set-up with an ion beam at the desired A/q value from an off line ion source. The ions were
accelerated to a final energy of 150 keV/u, drifted through the following DTL accelerator structure and their energy was analyzed with a 90°
bending magnet after which they were stopped in a Faraday cup. A Ge detector was placed beside this cup to measure the γ radiation of the
implanted 80Rb ions. The efficiency for the γ detection was determined with calibrated sources of 60Co and 152Eu. With this the 80Rb intensity
could be determined to be 3.5∙104 ions per second. This results in an acceleration efficiency of 32 %. Comparing the current measured for the
40Ar before the acceleration to the one in the final Farady cup an efficiency of 33 % was determined. That means no additional losses for the
radioactive ions could be seen.
III. SUMMARY AND OUTLOOK
For the first time radioactive ions charge bred with an ECR ion source have been accelerated at the ISAC facility at TRIUMF. This will
open up a new area for experiments with accelerated heavy radioactive ions. With the present 14.5 GHz source the maximum in the charge
state distribution for ions up to Cs allows A/q values below 7, which are suitable to accelerate without additional stripping. Although, the
breeding efficiency during this first set of experiments was lower than the values achieved on a test bench it should be possible to improve
this by further optimization mainly of the injection ion optics. Optimizing on a charge state with a minimum of stable background ions will
be essential for most experiments.
Commissioning of the charge state breeder will continue with isotopes of different elements and half-lives. The next scheduled on-line
experiments will be carried out with a FEBIAD ion source for the production of the singly charged ions. This will allow injecting also
gaseous elements and molecular ions. The latter can be of interest for elements with a higher release probability from the target in the
molecular state or for purification from isobaric contaminants. The molecules will break up in the plasma and a high charge state of the
isotope of interest can be extracted.
IV. REFERENCES
1
F. Wenander, Nucl Instrum Meth Phys Res B266, 4346 (2008)
P. Bricault, F. Ames, T. Achtzehn, M. Dombsky, F. Labreque, J. Lassen, J-P. Lavoie, Rev. Sci. Instrum 79, 02A908 (2008)
3
F. Ames, R. Baartman, P. Bricault, K. Jayamanna, M. McDonald, P. Schmor, T. Spanjers, D.H.L. Yuan, Rev. Sci. Instrum 79, 02A902 (2008)
2