EURONS - JRA6
INTAG - Instrumentation for Tagging
Task 2 - Application of RDT method to ISOL beams
FINAL REPORT
1. Description of the Task
Apply the RDT method to ISOL beams. Improve in-beam and radioactive decay
studies of short-lived exotic nuclei, selected using (i) laser ionization, (ii)
magnetic separation before and/or after secondary reaction, (iii) RDT, for ISOL
beams. Requires better magnetic selectivity of the ISOLDE-HRS separator. Close
links will also be made with the LASER JRA. A design study will be made for a
magnetic spectrometer that will transport particular reaction products following
the secondary reaction.
J06-2.1 – “Magnetic pre-selection": Design of beam emittance meters and
upgrade of ISOLDE HRS also using beam cooling, provide accelerated radioactive
ion beams. Application of ion selection using laser resonance techniques.
J06-2.2 – "Tests with target detectors": Development of prototype large solidangle Bragg spectrometer for application to weak radioactive beams. Test of 12fold segmented Ge detector in weak radioactive beams and development of
tracking algorithms. Analysis of Coulex.
J06-2.3 – "Magnetic separator design": Design of magnetic separator for
selection of reaction products after secondary target.
2.
J06-2.1 – Magnetic pre-selection
2.1. Beam cooling
Beam cooling and bunching is one of the most important stages in the
enhancement of the magnetic separation at ISOLDE-CERN. With this aim a new
Radio Frequency Quadrupole cooler and buncher (ISCOOL) has been designed,
constructed and installed at ISOLDE-CERN. First injection tests on the RFQ cooler
took place in the summer of 2006, complemented by emittance measurements
and upgrade of the diagnostic instrumentation. The device was thoroughly
tested off-line before installation. After the improvement of the existing vacuum
and water-cooling systems, and the construction of a high voltage platform, the
ISCOOL was installed at the exit of the ISOLDE HRS at the end of 2007 (see
Figure 1). The device has been fully operational during the 2008 experiment
campaign.
Figure 1: The ISOLDE RFQ Cooler and Buncher installed after the High Resolution Separator
at ISOLDE
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
100
133
39
23
6
0
Cs
80
K
Transmission
Transmission
The tests performed in the off-line laboratory included the investigation of the
influence of the gas pressure and RF parameters (amplitude and frequency) on
the emittance and transmission of the ion beam in order to find the optimum
settings. The emittance of the ion beam after the RFQ-CB was measured to be
2-3 π mm·mrad. This low value makes it possible to achieve a smooth beam
transport after the ISCOOL to experiments in the ISOLDE hall. Transmission
efficiencies were measured off-line for different alkali ions: 6Li, 23Na, 39K, and
133Cs. The results are plotted in Figure 2. The transmissions achieved were 17%,
28%, 68% and 79% respectively.
Na
Li
20
60
on-line surface ionsource
on-line plasma ionsource
off-line surface ionsource
40
20
40
60
80
100
120
140
0
0
20
40
60
80
Mass
100
120
140
Mass
Figure 2: [Left] Off line transmission obtained for different alkali elements. [Right]
Transmission measured with the ISCOOL installed at ISOLDE, compared with the
measurements performed off-line.
After installation of the ISCOOL the transmission was measured with stable alkali
ions, as shown in Figure 2. In comparison to the off-line results the transmissions
was somewhat higher for masses smaller than 40. The space charge limit was
carefully investigated, since it is the main limitation to the amount of ions that
can be cooled and bunched in the ISCOOL. As shown in Figure 3 there is an upper
limit of 108 ions/bunch that can be stored
8
7
1.4x10
7
1.2x10
7
1.0x10
1.4x10
8
1.2x10
8
Extracted ions
Extracted ions
1.0x10
6
8.0x10
6
6.0x10
6
4.0x10
7
8.0x10
7
6.0x10
7
4.0x10
7
2.0x10
6
2.0x10
0.0
0.0
0.0
7
5.0x10
8
1.0x10
8
1.5x10
8
2.0x10
8
2.5x10
0
9
1x10
9
2x10
9
3x10
9
4x10
9
5x10
Injected ions
Injected ions
1.60E+008
1.40E+008
Extracted ions
1.20E+008
1.00E+008
8.00E+007
6.00E+007
4.00E+007
2.00E+007
0.00E+000
-2.00E+007
0.00E+000
4.00E+008
8.00E+008
1.20E+009
Injected ions
Figure 3 : Space charge limit measured for
ions per bunch was measured.
23Na, 39K
and
85Rb.
A space charge limit of 108
Finally the dependence of the width of the ion bunches (when the ISCOOL is
operated in bunched mode) with the trapping time in the cooler before release
was studied. A change of 10 to 30 μs is observed with increasing trapping times,
as depicted in Figure 4. For one second of trapping time a bunch of about 30 μs
was measured.
-3
2x10
-3
MCP signal
1x10
0
-3
-1x10
-3
-2x10
10 ms
100 ms
1000 ms
-3
-3x10
-3
-4x10
-3
-5x10
-3
-6x10
-3
-7x10
-3
-8x10
-40 -20
0
20
40
60
80 100 120 140
Time [s]
Figure 4: [Left] Ion bunch width as a function of trapping time [Right] Transmission efficiency
measured during the on-line campaign of ISOLDE in 2008.
During the on-line campaign of 2008 at ISOLDE the ISCOOL had already been
fully integrated and has been operated as a part of the facility. The transport of
the ion beam to experiments from the ISCOOL has been made easier due to the
improved beam emittance. Transmission efficiencies were measured on-line.
Transmissions of 70-80 % for masses >40 were achieved for ions produced with
a plasma ion source. Somewhat higher efficiencies were obtained when a surface
ion source was used. The results are graphed in Figure 4.
2.2.Laser resonance techniques. Provision of accelerated RIBs
Isomeric beams of 68,70Cu have been successfully separated using resonant laser
ionization and post-accelerated at REX-ISOLDE for Coulomb Excitation
experiments. Using the 6-fold segmented MINIBALL germanium detector array
and the segmented silicon CD detector, Coulomb excitation on the different
isomers was successfully performed. This is the first time isomeric postaccelerated beams have been produced and used for an on-line experiment. The
outcome shed new light on the fragility of the N=40 sub-shell gap around 68Ni.
Figure 5 shows part of the results showing Coulomb excitation on the different
isomers.
Figure 5: The particle-gamma-ray coincidence spectrum acquired with the 6- beam of 68Cu
(Top). The partial level scheme and de-excitation gamma rays are shown in the upper right
corner. Energies are given in keV. Levels drawn with thick lines represent the gammadecaying states. Particle–gamma ray coincidence spectrum acquired with the 1 + beam are
shown below. No Doppler correction was applied.
Using the same set-up other Coulomb excitation campaigns have been initiated,
amongst others in the region of the odd mass Cu isotopes as well as with 80Zn.
For all these experiments a new data acquiring strategy making optimal use of
the laser ionization was developed. This work has lead to the following scientific
publications:




Stefanescu et al., Phys. Rev. Lett. 98, 122701 (2007)
J. Van de Walle et al., Phys. Rev. Lett. 99, 142501 (2007)
N. Bree et al., Phys. Rev. C78, 047301 (2008)
Stefanescu et al., Phys. Rev. Lett. 100, 112502 (2008)
2.3. Design upgrade of the ISOLDE High Resolution Separator
The design upgrade of the ISOLDE High Resolution Separator has been
accomplished. A staged approach is proposed for the upgrade. After the
successful installation of the ISOLDE RFQ cooler and bencher, and upgrade in the
instrumentation is foreseen next. The next step would comprise the upgrade of
the 90-degree dipole including multipole field corrections. Design simulations
for this stage are shown in Figure 6. The upgrade will be finished by the
reinstallation of the ISOLDE RFQ before the new HRS and a complete redesign
and construction of the matching sections.
Figure 6: Phase space and separation of Sn and Cs beams for A=120 assuming 3800:1
production for the new design of the 90º dipole of the HRS.
3. J06-2.2 – Tests with target detectors
3.1. Test of segmented Ge detectors
Beta decay studies on laser ionized and mass separated species using a new
detection set-up including segmented germanium detectors have been
performed at the LISOL facility. The selectivity achieved for laser ionized
66,67,68Fe and 54Ni decay and the use of multi detectors allowed for  correlation
measurements extending into the seconds range without having a implantation
trigger from e.g. an energetic ion beam. Crucial in this experiment was the
availability of extremely pure beams (laser ionization) and multi-detector
systems to reduce the random count rate. A new (about 500 ms) isomer in the
neutron-rich 67Co nucleus was discovered and was interpreted as a proton
intruder state. This work has lead to the following scientific publications:
 D. Pauwels et al., Phys. Rev. C 78, 041307 (2008)
 D. Pauwels, O. Ivanov et al., NIM B266 (2008) 4600
3.2. Large solid-angle Bragg spectrometer
A prototype large solid-angle Bragg spectrometer for application to weak
radioactive beams has been designed. The design is shown in Figure 7.
The designed detector has an active area that spans 10 to 50 degrees in the
laboratory frame measured from the beam axis and can work in conjunction
with the MINIBALL array. A vacuum tube at the centre of the annular detector,
from 0 to 10 degrees measured from the beam axis, is incorporated in order to
allow unscattered radioactive beam nuclei and the associated radioactive decay
to be transported to a beam dump.
Such a large angle of acceptance created challenges for Z-resolution due to the
acceptance of different scattering angles leading to projections of ionisation
along the electric field lines. These ionisation projections could lead to
misidentified Z of the ions. Solutions were devised which allows for optimal Zidentification for scattered beam or recoiling target. The Bragg spectrometer is
devised as six identical modules in an azimuthal arrangement around the beam
line. Each module has an independent gas volume and has two sets of electric
field shaping rings. This essentially means that each module has two
independent Bragg spectrometers, one from 10 to 30 degrees and the second
from 30 to 50 degrees measured from the beam axis, allowing a maximum of a
10-degree projection of ionisation upon the electric field. The complete design
contains 12 independent Bragg spectrometers. Furthermore, in order to allow
higher counting rates to be achieved, the anode pads of each Bragg spectrometer
are segmented and shielded by a Frisch grid. Each segment would likewise act
like an independent Bragg spectrometer, allowing for up to 48 working
Z-resolving signals for the entire spectrometer.
Figure 7: Technical drawing of one module of the prototype PGAC and Bragg spectrometer. A
side view cross section is presented in the left showing the PGAC mounted at the entrance
while the two field shaping rings of each independent spectrometer follows in the tapered
volume. On the right is a drawing of this one module on a support stand coupling to a target
chamber compatible with the MINIBALL array. The Bragg spectrometer component is shown
in assembly position outside of the gas volume.
To assist with Z-identification and to provide a fast logic signal and quantify
angular resolution of scattered ions for coincident requirements with arrays,
such as MINIBALL, a position-sensitive Parallel Grid Avalanche Counter (PGAC)
at the entrance to the Bragg Detector volume was incorporated in the design.
The angular resolution will be better than 2 degrees for Doppler correction
purposes, and the position resolution will allow for corrections to be made to
any projection of ionisation upon the electric field in the Bragg spectrometer.
Like the Bragg spectrometer, there are 12 independent PGACs within the entire
detector.
To assist with the design of the spectrometer as well as to understand the signals
from the anode pads and achieve optimal Z-resolution of the spectrometer, a
simulation package has been developed. This incorporates the GARFIELD
[Veehof, GARFIELD, a drift chamber simulation program, Version 5.35, CERN]
simulation package used principally by particle physicists, and a Monte-Carlo
energy loss and ionisation calculations using TRIM [Ziegler et al. The Stopping
and Range of Ions in Solids. Pergamon (1985 – new edition 2009)] that is
applicable for these nuclear physics experiments. The energy loss and straggling
can be simulated from an ion scattering from the target, passing through the gas
detector windows and PGAC, and passing into the Bragg spectrometer gas
volume. The anode signals in the Bragg spectrometer can then be simulated
using GARFIELD from the drift of electrons resulting from ionisation of the gas
within the spectrometer. Tests of this package will run concurrently with tests of
the prototype Bragg spectrometer.
The design is completed and construction will begin soon. Time constraints have
not allowed the construction of a prototype module to be tested within the
timeframe of the project. Testing and use will commence in 2009.
3.3. Analysis of Coulex
A new version of the Coulomb excitation code GOSIA, GOSIA2, has been
developed. GOSIA2 is a special version of GOSIA that is intended to handle both
target and projectile excitation simultaneously. This avoids introducing free
parameters (normalization constants), which is important when only a very
limited number of experimental data results from the experiment, like in case
using radioactive beams and when the Rutherford scattering cross section is not
measured simultaneously to provide a normalization.
GOSIA2 requires two parallel inputs describing both collision partners. It was
chosen to keep separate inputs rather than merging them to form a single one to
preserve maximum compatibility with the regular GOSIA input. All geometric
factors are included in the calculations without arbitrary renormalization. In the
best case the experiments should be coupled together, like in GOSIA (although
this is not required). In this situation there is only one constant remaining
related to the “flux” or the total number of particles impinging on the target.
The newly developed GOSIA2 has been used to extract the E2 matrix elements
from the measured gamma yields in a low-energy Coulomb excitation
experiment was performed in GANIL with a neutron-rich 44Ar beam from SPIRAL
to study shape evolution in the vicinity of the N=28 shell closure. The present
version of the code does not allow calculating uncertainties of strongly
correlated matrix element. Therefore a new approach has been developed to
extract the quadrupole moment of first excited state in 44Ar, profiting from the
fact that its influence on the Coulomb excitation probability strongly depends on
the scattering angle.
It has been possible to extract B(E2;2+  0+) value from the excitation crosssection of the 2+ state for the smallest angular range using the normalization to
known excitation probabilities in 109Ag. The quadrupole moment has been
obtained by subdividing the remaining data in angular bins using the obtained
B(E2) value, as shown in Figure 8. In this way the quadrupole moment of a
radioactive nucleus has been measured for the first time from the Coulomb
excitation data without need for constraints from complementary lifetime
measurements. The results were presented at several workshops and lead to the
publication M. Zielinska et al., Acta Physica Polonica B 39 519 (2008).
Figure 8. Influence of the quadrupole moment on the excitation cross-section of the first 2+ in
44Ar. The three curves correspond to negative, zero and positive value of the quadrupole
moment. Four ranges of scattering angles used in the analysis are marked.
4. J06-2.3 – Magnetic separator design
A meeting was held at Leuven to establish collaboration for the construction and
installation recoil separator after REX-ISOLDE. A European consortium exploring
possible synergies between the proposed vacuum mode separator projects at
CERN and JYFL was proposed. Various design options, pertaining to projected
scientific needs, have been presented and discussed. The work in 2007 was
concentrated on examining recent plans and experiments in major facilities such
as SPIRAL-II at GANIL and ISAC-II at TRIUMF. The developments and future
upgrade plans of the corresponding separators, PRISMA at LNL and EMMA at
TRIUMF, have been followed closely, with the goal of easier adoption of similar
ideas to a future device.
The planned High Intensity and Energy (HIE) upgrade of ISOLDE will enable
post-acceleration of radioactive beams up to energy of about 10 MeV/u thus
opening the door to nuclear reaction studies. Here one is often interested in
reactions where one or a few nucleons are transferred to the beam, resulting in
the reaction products (recoils) being forward focused, or in deep inelastic
transfer reactions where the preferred angle is often close the grazing angle. The
separation of recoils from beam has been achieved by dispersing the particles
according to their mass-to-charge ratio (A/q) using a combination of
electrostatic and magnetic elements. In this case the particles are detected at the
focal plane using position sensitive detectors with digital readout. The position
information gives the A/q of the detected particle. Mass identification must be
done using auxiliary detectors if this information is needed. Some examples of
separators of this kind have been studied in simulations keeping the HIE-ISOLDE
upgrade in mind. Few different reactions for three different beam energies (3, 5
and 10 MeV/u) with both the EMMA- and PRISMA-like designs using beam
parameters for HIE-ISOLDE.
Figure 9 shows simulation results the 22Mg(d,n)23Al reaction at 5 MeV/u with the
PRISMA design. The recoils are well separated at the focal plane and there is a
sufficient separation in time-of-flight. The estimated total transmission to the
focal plane is preliminary estimated at 15%. For the same 22Mg(d,n)23Al reaction
with the FMA/EMMA-like design 23Al13+ recoils are transmitted to the focal plane
only. The preliminary transmission estimate for this reaction is 3%. The
difference in these preliminary transmissions may be vital for the final design
decision. Simulations have been performed for the 132Sn(d,p)133Sn and other
reactions. In the 132Sn(d,p)133Sn reaction at 660 MeV with the FMA/EMMA-like
design the recoil has a too high energy after a 0.1 mg/cm2 PD target and must be
degraded. As expected, the recoils are well separated from the beam at the focal
plane. The estimated transmission of the recoils is 37.8 %. The energy spread of
the transmitted particles is ±3.1 MeV as compared to the total energy spread,
which is ± 3.3 MeV. The ray-tracing type of spectrometer has an advantage of
large acceptance. On the other hand the traditional type of mass recoil
spectrometer offers a simple data analysis.
Figure 9: Simulation of the 22Mg(d,n)23Al reaction. The recoils and the beam ions are
separated at the focal plane. The time-of-flight for 23Al and 22Mg versus the position at the
focal plane is shown to the right.
The beam line layout of the extension in the experimental hall at ISOLDE is being
designed. The approximate space available in the hall for a spectrometer is about
9 meters of length from the target position to the short end of the hall, and in
total about 8 meters to the left wall. The total length from target to focal plane
for the EMMA-like design is 9.04 meters. Figure 10 shows one of a few new
suggested beam line layouts for HIE-ISOLDE. EMMA-like designs of this type will
just fit at 0◦ but rotation may be difficult to achieve. The PRISMA type of design is
more compact as it has one bending element only. However, for deep inelastic
reactions it should be possible to allow rotation up to at least 90◦.
Figure 10: A suggested beam line layout for the new ISOLDE experimental area.