ISOLDE Facility Research Articles

Potentials of the ECR 1+ n+ charge breeding for radioactive ions

During the past three years, a Phoenix ECR charge breeder has been tested on-line with various beams delivered by the ISOLDE facility. The performances obtained so far are compatible with most of the requirements for future production of intense radioactive charge bred beams. However, there are aspects that are could be improved. These concern beam purity, injection efficiency and the capability of raising high charge state in continuous or pulsed mode. We summarize here the results obtained with the present Phoenix ECR charge breeder and a few simple upgrades that are foreseen to improve its performances.

New Binding Energy for the Two-Neutron Halo ofLi11

The extended radius of a halo nuclide is very sensitive to the minute binding energy of its valence nucleons. The binding energy of 11Li has been measured with high precision by using the radio-frequency spectrometer MISTRAL at CERN's ISOLDE facility. The new two-neutron separation energy of 378+/-5 keV is 25% higher than the previously accepted value with an uncertainty 5 times smaller.

Towards a magnetic field stabilization at ISOLTRAP for high-accuracy mass measurements on exotic nuclides

The field stability of a mass spectrometer plays a crucial role in the accuracy of mass measurements. In the case of mass determination of short-lived nuclides with a Penning trap, major causes of fluctuations are temperature variations in the vicinity of the trap and pressure changes in the liquid helium cryostat of the superconducting magnet. Thus systems for the temperature and pressure stabilization of the Penning trap mass spectrometer ISOLTRAP at the ISOLDE facility at CERN have been installed. A reduction of the temperature and pressure fluctuations by at least an order of magnitude down to Δ T ≈ ± 5 mK and Δ p ≈ ± 5 Pa has been achieved, which corresponds to a relative magnetic field change of Δ B / B = 2.7 × 10 - 9 and 1.1 × 10 - 10 , respectively.

A pulsed drift cavity to capture 30 keV ion bunches at ground potential

To capture radioactive ion beams of tens of keV in an ion trap, the use of a pulsed drift cavity allows one to avoid placing the complete setup on a high voltage (HV) platform. By pulsing down the voltage on a long electrode while the ion bunch is inside it, the electric potential zero level can be shifted down over a range equal to the initial beam energy, thus allowing the ions to be captured in a trap at ground potential. The pulsed drift cavity of the WITCH setup, which is completely HV-platform free, is described here. It is demonstrated that bunched beams with energies of typically 30–60keV, as will become available to all users at the ISOLDE facility after the installation of an RFQ buncher, can be transported over distances of tens of meters to the experiments and by means of a pulsed drift cavity directly be injected into a purification Penning trap. Simulation work showing the efficiency of such a system is discussed and tests showing the feasibility of such a system are presented. An estimate of the pulse down time is made from TOF spectra. An outlook on how the system can be improved is presented too.

Mass measurements of the shortest-lived nuclides à la MISTRAL

At Princeton in the 1960’s, L.G. Smith invented an instrument of astonishing accuracy and rapid measurement time, derived from his so-called mass synchrometer. Using the same principle, a radiofrequency spectrometer was constructed in Orsay to measure masses of the shortest-lived nuclides at Cern’s Isolde facility. Smith’s spectrometer is now a museum piece, making the Orsay version (since baptized, MISTRAL) the sole example of such an instrument and the only one ever to be used on-line. Here we report on a measurement of the 65 ms half-life, N = Z nuclide Rb 74 performed with MISTRAL. The measured mass excess of − 51944 ( 117 ) keV is compared with that obtained by ISOLTRAP, since independent measurements using different techniques assure a healthy gene pool for the recommended masses of the atomic mass evaluation. The nuclide Rb 74 is the heaviest for which a precise mass is of importance for the so-called Wigner energy. A discussion is presented concerning this Wigner energy, perhaps the last component of nuclear mass formulas resisting microscopic treatment.

Accurate mass measurements of 26Ne, 26–30Na, 29–33Mg performed with the Mistral spectrometer

The minuteness of the nuclear binding energy requires that mass measurements be highly precise and accurate. Here we report on new measurements of 29−33Mg and 26Na performed with the Mistral mass spectrometer at CERN's Isolde facility. Since mass measurements are prone to systematic errors, considerable effort has been devoted to their evaluation and elimination in order to achieve accuracy and not only precision. We have therefore conducted a campaign of measurements for calibration and error evaluation. As a result, we now have a satisfactory description of the Mistral calibration laws and error budget. We have applied our new understanding to previous measurements of 26Ne, 26–30Na and 29,32Mg for which re-evaluated values are reported.

Temperature dependent PAC studies with the rare earth 172Lu in ZnO

Abstract Wide band-gap semiconductors have become an important base material for applications in optoelectronics and in high power, high temperature electronics. After doping with various rare earths, electroluminescence throughout the whole visible spectrum has been observed. We have studied the implantation behaviour of the rare earth Lu in ZnO. Our samples were implanted at the ISOLDE facility at CERN. In order to recover the sustained implantation damage the samples were treated in a rapid thermal annealing furnace. In a first attempt to measure the temperature dependence of the electric field gradient at the site of 172Lu/172Yb in ZnO a large jump in the quadrupole interaction frequency was observed between measurements at room temperature and 200 °C. Above 200 °C the frequency only changed very little. In order to understand this unusual behaviour we prepared another sample and studied the temperature range between room temperature and 200 °C in more detail. The results obtained previously could be confirmed. The new data show a smooth increase of the quadrupole interaction frequency from about 146 MHz at room temperature to almost 390 MHz at 400 °C. This behaviour is likely to be due to the influence of the rare earth’s partially filled 4f shell.

Purification of radioactive neutron-rich argon beams using an ion source in charge breeding mode

Measurements of nuclear properties of some neutron-rich asymmetric fission isotopes at an ISOL laboratory present a severe problem when a plasma ion source is used as the primary ion-source. The problem is associated with intense multi-charged symmetric fission contaminants that cannot be mass-separated from asymmetric fission products. In this work we demonstrate how this problem can be overcome on-line by using an electron cyclotron resonance ion source (ECRIS) as a charge-breeding secondary ion-source. Mixed Ar + / Kr 2 + / Xe 3 + radioactive beams from an plasma ion-source were injected into the PHOENIX ECRIS that currently is installed at the ISOLDE facility at CERN in order to improve the ratio of Ar to the background contaminants of Kr and Xe. The signal to noise ratio was improved by more than two orders of magnitude after charge-breeding and mass-separation of the Ar 8 + , Ar 9 + beams. The information on the decay of the 48Ar nucleus obtained in these tests is useful for a better understanding of the origin of the 48Ca/ 46Ca meteorite isotopic anomaly. Potential applications of the method are discussed.

ISOLTRAP pins down masses of exotic nuclides

The mass of radionuclides contribute to a variety of fundamental studies including tests of the weak interaction and the standard model. The limits of mass measurements on exotic nuclides have been extended considerably by the Penning-trap mass spectrometer ISOLTRAP at the ISOLDE facility at CERN. Recent ISOLTRAP measurements are summarized and current technical improvements are outlined.

New structure information on 30Mg, 31Mg and 32Mg

The fast timing βγγ(t) method was applied to investigate the level lifetimes in 30,31,32Mg. Levels in Mg have been populated in β and β-delayed neutron emission of Na at the ISOLDE facility. From the γγ coincidences a number of new states have been identified and new level schemes were constructed for 30,31,32Mg. The following preliminary half lives have been determined: T1/2 = 3.9(4) ns for the 1789 keV state in 30Mg, T1/2 = 133(8) ps and 10.5(8) ns for the 221 keV and 461 keV states in 31Mg, respectively, and T1/2 = 16(4) ps for the 885 keV level in 32Mg. The 1789 keV level was established as a candidate for the intruder 0+ configuration in 30Mg with a possible strong E0 branch to the ground state.

β-decay of 22O

A mass-separated 12C22O molecular ion beam from the ISOLDE facility was used to study the decay of neutron-rich 22O. The experimental results were compared with the results from an earlier experiment and predictions by shell-model calculations using various effective interactions. The mechanism leading to the vanishing decay strength to the first 1+ level of the 22F nucleus, predicted with the USD effective interaction but not supported by the experimental data, is analysed.

Application of Low Energy Spin Polarized Radioactive Ion Beams in Condensed Matter Research

The new ISAC facility at TRIUMF (Canada), produces some of the world's most intense radioactive ion beams (RIBs). Although the primary scientific motivation for these facilities lies in nuclear physics and nuclear astrophysics, RIBs also have applications in condensed matter (CM). Many pioneering CM experiments have already been performed at the ISOLDE facility (CERN). Continuing on this path at ISAC we have recently polarized a beam of 8Li+ to be used as a magnetic probe of ultra-thin films and interfaces. We report here on progress in developing this new application of low energy RIBs, which is based on the technique of β-detected nuclear magnetic resonance (β- NMR)

ISOLTRAP mass measurements of exotic nuclides at [formula omitted

The ISOLTRAP experiment at the ISOLDE facility at CERN is a Penning trap mass spectrometer for on-line mass measurements on short-lived radionuclides. It allows the determination of atomic masses of exotic nuclides with a relative uncertainty of only 10 −8 . The results provide important information for, e.g. , weak interaction studies and nuclear models. Recent ISOLTRAP investigations and applications of high-precision mass measurements are discussed.

βdecay ofNe26

A pure neutron-rich $^{26}\mathrm{Ne}$ beam was obtained at the ISOLDE facility using isobaric selectivity. This was achieved by a combination of a plasma ion source with a cooled transfer line and subsequent mass separation. The high quality of the beam and good statistics allowed us to obtain new experimental information on the $^{26}\mathrm{Ne}$ $\ensuremath{\beta}$-decay properties and resolve a contradiction between earlier experimental data and prediction of shell-model calculations.

Physics and present status of the WITCH experiment

The WITCH experiment (Weak Interaction Trap for CHarged particles) is starting measurements at the ISOLDE facility at CERN at present. It has been set up to measure the energy spectrum of the recoiling daughter ions after nuclear beta decay for precision tests of the Standard Model of weak interactions. However, many other topics of interest are accessible. In this article, the possibilities of recoil spectroscopy with the WITCH experiment are discussed, as well as the principle of the setup and its present situation.

Beta-decay measurements of neutron-rich thallium, lead, and bismuth by means of resonant laser ionisation

Neutron-rich thallium, lead, and bismuth isotopes were investigated at the ISOLDE facility. After mass separation and resonant laser ionisation of the produced activity, new spectroscopic data were obtained for 215,218 Bi and 215 Pb. An attempt to reach heavy thallium had to be abandoned because of a strong francium component in the beam that gave rise to a neutron background through (α,n) reactions on the aluminium walls of the experimental chamber.

Beta-decay measurements of neutron-rich thallium, lead, and bismuth by means of resonant laser ionisation

Neutron-rich thallium, lead, and bismuth isotopes were investigated at the ISOLDE facility. After mass separation and resonant laser ionisation of the produced activity, new spectroscopic data were obtained for 215,218Bi and 215Pb. An attempt to reach heavy thallium had to be abandoned because of a strong francium component in the beam that gave rise to a neutron background through (α,n) reactions on the aluminium walls of the experimental chamber.

New information on the β-decay of 11Li from Doppler broadened γ lines

The γ-ray spectrum following β-decay of 11Li has been remeasured at the ISOLDE facility at CERN. Two new transitions were observed through the use of large Ge-detectors. Most γ-decays will follow β-delayed neutron emission. Information on the energy of the neutron is derived from analysis of the γ line-shape and used to construct a partial decay scheme for 11Li. Lifetime values for the 1 − and 2 − levels in 10Be are also derived. A new partial decay scheme is presented.

Coupling a proton and a neutron to the semidoubly magic nucleusNi68: A study ofCu70via theβdecay ofNi70andCu70

The $^{70}\mathrm{Ni}$ nucleus has been produced at the LISOL facility and its $\ensuremath{\beta}$ decay to $^{70}\mathrm{Cu}$ has been observed. In parallel, the $^{70}\mathrm{Cu}$ nucleus has been produced at the ISOLDE facility. A new third $\ensuremath{\beta}$-decaying isomeric state in $^{70}\mathrm{Cu}$ has been identified, partly with the aid of the in-source laser spectroscopy method. Its measured half-life is ${T}_{1∕2}=33(2)\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. In addition, two isomeric transitions in $^{70}\mathrm{Cu}$ have been observed at energies of 101.1(3) and $141.3(3)\phantom{\rule{0.3em}{0ex}}\text{keV}$, and it allowed the relative placement of the three $^{70}\mathrm{Cu}$ isomeric states and their tentative spin and parity assignments. The new $^{70}\mathrm{Cu}$ isomer was found to be weakly populated in the $\ensuremath{\beta}$ decay of $^{70}\mathrm{Ni}$. It allowed the construction of the $^{70}\mathrm{Ni}$ decay scheme. Furthermore, the $\ensuremath{\beta}$ decay of the three $^{70}\mathrm{Cu}$ isomers to $^{70}\mathrm{Zn}$ has been measured and their decay schemes are presented. The $^{70}\mathrm{Ni}$ $\ensuremath{\beta}$ decay to the isomeric states in $^{70}\mathrm{Cu}$ and their subsequent isomeric decay and $\ensuremath{\beta}$ decay to $^{70}\mathrm{Zn}$ are discussed within the extreme shell-model picture of two valence nucleons outside the semidouble magic $_{28}^{68}\mathrm{Ni}_{40}$ core and it yields satisfactory results. Large-basis shell-model calculations using $_{28}^{56}\mathrm{Ni}_{28}$ as the core and a realistic effective interaction support rather well the suggested interpretation.

WITCH: a recoil spectrometer for weak interaction and nuclear physics studies

An experimental set-up is described for the precise measurement of the recoil energy spectrum of the daughter ions from nuclear beta decay. The experiment is called WITCH, short for Weak Interaction Trap for CHarged particles, and is set up at the ISOLDE facility at CERN. The principle of the experiment and its realization are explained as well as the main physics goal. A cloud of radioactive ions stored in a Penning trap serves as the source for the WITCH experiment, leading to the minimization of scattering and energy loss of the decay products. The energy spectrum of the recoiling daughter ions from the β-decays in this ion cloud will be measured with a retardation spectrometer. The principal aim of the WITCH experiment is to study the electroweak interaction by determining the beta-neutrino angular correlation in nuclear β-decay from the shape of this recoil energy spectrum. This will be the first time that the recoil energy spectrum of the daughter ions from β-decay can be measured for a wide variety of isotopes, independent of their specific properties.