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 solid-angle Bragg spectrometer for application to weak radioactive beams. Test of 12-fold 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
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.
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
Figure 3: Space charge limit measured for 23Na, 39K and 85Rb. A space charge limit of 108 ions per bunch was measured.
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.
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 post-accelerated 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 gamma-decaying 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 Z-identification 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 cross-section 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.