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News Archive 2015

04.12.15: ISOLTRAP precision mass measurements enhance the reliability of r-process nucleosynthesis description

The origin of the elements heavier than iron remains one of the major quests of today's observational, experimental, and theoretical physics. Various isotopes are created in radically different environments by the slow neutron capture process (s process) which takes millions of years or within seconds by the rapid neutron capture process (r process).
Peaks on the solar abundance curve are associated with nuclear structure i.e. the closed nuclear shells at neutron magic numbers N = 50, 82, and 126. Thus, astrophysical simulations have a strong sensitivity to the underlying nuclear structure. Nuclear data of very exotic isotopes, serving as crucial input for the astrophysical models, are still lacking due to the difficulties in production and measurement of the short-lived nuclei. Therefore, nuclear theory is indispensable and many theoretical approaches have been proposed. Since there are still significant deviations of predictions from experiment, considerable efforts have been devoted to improve the production yields and selectivity of exotic nuclear species, as well as the sensitivity of experimental mass spectrometry.

In a recently in Physical Review Letters published article D. Atanasov et al. report on the precision mass measurement of the closed shell waiting-point nuclide 130Cd as well as the first mass determinations of its neighboring isotopes 129Cd and 131Cd, allowing further examination of the strength of the N = 82 shell closure beyond the doubly magic 132Sn.
The new mass measurements were performed at the online radioactive ion beam facility ISOLDE/CERN external Link, Geneva using the ISOLTRAP external Link mass spectrometer. Depending on the half-life and production yield of the ion of interest, the mass determination at ISOLTRAP is performed either by the time-of-flight ion cyclotron resonance technique (TOF-ICR) using the precision Penning trap or by performing time-of-flight mass spectrometry with the multireflection time-of-flight mass separator (MR-TOF MS). For the cases of 129,130Cd+, a Ramsey-type TOF-ICR mass measurement in the precision Penning trap was performed. Considering the low production and the short half-life of 131Cd+, the mass measurements were performed by using the faster MR-TOF MS technique.
From the measured mass excesses of the cadmium isotopes their neutron-separation energies Sn were deduced, which are a measure of the binding of the resp. nucleus. The experimental value of the empirical shell gap, defined as Sn(N = 82) - Sn(N = 83) was determined and a significant reduction of the N = 82 shell gap for Z < 50 was found.

As an important result, the new data provide additional constraints for nuclear theory, considering the diverging predictions of mass models concerning the N = 82 empirical shell gap below the doubly magic 132Sn. The new measurements bring reliability to the description of r-process nucleosynthesis by reducing the uncertainty from the nuclear-physics input. Given the large volume of data required for r-process calculations, it is remarkable that only three masses (129-131Cd) make an observable impact on the predicted elemental abundances, highlighting the importance of precision measurements in this region of the nuclear chart.

Please read more in the article ... >

Further information also in the press releases of the MPIK external Link, the idw external Link, the University Greifswald external Link, and Pro-Physik external Link.

23.11.15: Review article about the Baryon Antibaryon Symmetry Experiment (BASE) published

The novel Baryon Antibaryon Symmetry Experiment (BASE) external Link at the Antiproton Decelerator (AD) external Link of CERN, Geneva aims at performing a stringent test of the combined charge, parity and time reversal (CPT) symmetry by comparing the magnetic moments of the proton and the antiproton with high precision. The discovery of minutest differences between the magnetic moments of the proton and the antiproton could lead to an understanding of physics beyond the Standard Model which is known to be incomplete and thus potentially contribute to the solution of the observed matter/antimatter imbalance in the universe.

In a review article recently published in "The European Physical Journal Special Topics" the BASE collaboration external Link describes and summarizes the physical and technical aspects of BASE.
Within BASE the proton/antiproton g-factors are determined by measuring the respective ratio of the spin-precession frequency νL (Larmor frequency) to the cyclotron frequency νc. The spin precession frequency is measured by non-destructive detection of spin quantum transitions using the continuous Stern-Gerlach effect, and the cyclotron frequency is determined from the particle's motional eigenfrequencies in the Penning trap using the invariance theorem: νc2 = ν+2 + νz2 + ν-2. The modified cyclotron frequency ν+, the axial frequency νz, and the magnetron frequency ν- are measured via image current detection.

The BASE apparatus is an extension of the Mainz proton double-trap experiment. The advanced cryogenic Penning-trap system of BASE consists of four cylindrical traps: a precision trap (PT) for precision frequency measurements, an analysis trap (AT) for analysis of the spin state of the particle, a reservoir trap (RT) for capturing low energy antiprotons from the Antiproton Decelerator and as particle reservoir, and a cooling trap (CT) for fast and efficient cooling of the cyclotron mode of the trapped antiproton.
The novel four-Penning trap system is installed in the homogeneous center of a superconducting magnet (1.945 T). Its magnetic field defines the measured frequencies νL and νc. Transport electrodes connect the individual traps and allow for fast adiabatic particle shuttling along the trap axis and thus to perform fast high-precision measurements of the cyclotron frequency ratio of two particles.

In the baryon sector, the magnetic moment comparison of protons and antiprotons currently reaches a precision of a few parts per million (10-6). By application of the double Penning trap method using the precision trap (PT) and the analysis trap (AT) of BASE, a fractional precision of δg/g 10−9 is expected to be achieved. Thus, the successful application of the novel method to the antiproton will lead to a 1000-fold improved g-factor comparison and test of the CPT symmetry.

Please read more in the review article ... >

04.09.15: Progress in p-process studies by application of novel storage ring method

In order to investigate the origin of the elements in the universe, nuclear reaction processes that are involved in stellar evolution and explosions are studied. The 35 naturally occurring proton-rich isotopes of the heavy elements from selenium (74Se) to mercury (196Hg), the so called p nuclei, are produced in the p-process. The main production processes are (γ,n), (γ,p), and (γ,α) reactions, and subsequent β decays in the so-called γ process (photodisintegration). Thus, for p-process network calculations it is essential to know the cross sections of these photodisintegration reactions or their inverse reactions, e.g. (p,γ) reactions. Most of the used cross sections rely solely on predictions of Hauser-Feshbach calculations. Only a few of the thousands of nuclear reactions involved in network calculations for p-process nucleosynthesis have already been experimentally studied. Most of the existing experimental data for the p-process were measured in direct kinematics using stable isotope targets. A direct measurement on unstable radioactive nuclei is still a major challenge.

In a recently in Physical Review C published article B. Mei et al. present the first direct measurement of the 96Ru(p,γ)97Rh cross section between 9 and 11 MeV for the p-process with a heavy-ion storage ring. Since radioactive nuclei decay rapidly there is little time for data acquisition. To overcome this experimental challenge, a novel technique has been developed at the experimental storage ring ESR external Link of GSI, Darmstadt, Germany. It is based on the collision of stored heavy ions with a hydrogen target to measure cross sections of proton capture reactions in inverse kinematics. The great advantage of the novel method is that unreacted 96Ru44+ ions are recycled and repeatedly impinged on the hydrogen target for reactions. This increases the chance of observing proton capture reactions within the short lifetime of the unstable nuclei. The novel method is well suited for measurements on unstable nuclei with half-lives longer than several minutes. It provides unrivaled opportunities for the direct measurement of (p,γ) reactions around the energy range of astrophysical interest, particularly for previously unreachable radioactive ions.
The directly measured (p,γ) cross section allowed to pin down the γ-ray strength function as well as the nuclear level density model. Moreover, the proton optical potential could be constrained and an excellent prediction of the stellar rates for 96Ru(p,γ)97Rh over a large temperature range for p-process network calculations can be made.

Please read more in the article ... >

Please also read the Synopsis by Matteo Rini ... >

12.08.15: Novel direct high-precision test of CPT invariance

The fundamental Charge, Parity, and Time Reversal (CPT) symmetry of the Standard Model of particle physics implies exact equality between the properties of a particle and its antimatter equivalent. CPT invariance tests aim to find minutest differences between the fundamental properties of a particle and its antiparticle. Such differences could lead to an understanding of physics beyond the Standard Model which is known to be incomplete and thus potentially e. g. contribute to the solution of the observed matter/antimatter imbalance in the universe.
To date only a few direct high-precision tests of CPT invariance are available. In the lepton sector, the magnetic anomalies of electron and positron were compared with a fractional uncertainty of at the parts per billion level. In the baryon sector, the magnetic moment comparison of protons and antiprotons reached a precision of 4.4 parts per million (10-6). It is planned to outperform this measurement by at least a factor of thousand within the Baryon Antibaryon Symmetry Experiment (BASE) external Link at the Antiproton Decelerator of CERN, Geneva. The most precise test of CPT invariance with baryons is the comparison of the proton/antiproton charge-to-mass ratios with a fractional precision of 90 p.p.t. by measuring the cyclotron frequencies of single trapped antiprotons and H- ions in a Penning trap.

In a letter recently published in "Nature" the BASE collaboration external Link reports on high-precision cyclotron frequency comparisons of a single antiproton and a negatively charged hydrogen ion (H-) carried out in an advanced cryogenic Penning-trap system. The frequency measurements were conducted at the Antiproton Decelerator (AD) external Link of CERN, Geneva. Since comparisons of the Antiproton-to-H- charge-to-mass ratio are equivalent to a direct antiproton-to-proton comparison H- was used as a proxy for the proton in the experiment. Its negative charge facilitated the experiment by eliminating the need to invert trap voltages. Thus, systematic shifts caused by polarity switching of the trapping voltages could be avoided.
The used advanced cryogenic Penning-trap system consists of a measurement trap (MT) and a reservoir trap (RT). Transport electrodes connect the individual traps and allow for fast adiabatic particle shuttling along the trap axis. From the AD particle cloud a single antiproton is extracted and kept in the center of the trap, as well as an H- ion which is parked in the downstream park electrode (DPE). The rest of the particles is shuttled to the reservoir trap. At 4K and ultra-low pressures antiproton storage times of more than a year can be achieved.
The charge-to-mass ratios of the trapped particles result from their cyclotron frequencies νc which are determined via the invariance theorem νc2 = ν+2 + νz2 + ν-2. The modified cyclotron frequency ν+, the axial frequency νz, and the magnetron frequency ν- are measured via image current detection. A single charge-to-mass ratio-comparison takes exactly two AD-cycles, corresponding to a typical measuring time of 220s to 240s. From 13000 frequency measurements the antiproton/proton charge-to-mass ratio 1(69)·10-12 was extracted. This result is in agreement with CPT conservation and its precision of 69 p.p.t. exceeds the energy resolution of previous antiproton-to-proton mass comparisons as well as the respective figure of merit of the Standard Model Extension (SME) by a factor of 4. In terms of energy sensitivity the result is the most stringent test of CPT invariance with baryonic antimatter performed so far.

Additionally, the experiment yielded two new important limits:
The high data accumulation rate of the experiment enabled the search for sidereal variations. A limit of < 720 p.p.t. on sidereal variations in the measured antiproton/proton charge-to-mass ratio was extracted.
If matter respects weak equivalence while antimatter experiences an anomalous coupling to the gravitational field, this gravitational anomaly contributes to a possible difference in the measured cyclotron frequencies. Thus, by following these assumptions the experimental result can be interpreted as a test of the weak equivalence principle using baryonic antimatter, and sets a new limit on the gravitational anomaly parameter αg: |αg - 1| < 8.7·10-7.

Please read more in the "Nature" letter ... >

Read also the comment on the article external Link by Klaus P. Jungmann.

Further information in the press releases of the MPIK external Link, the Max Planck Society external Link, the idw external Link, the CERN external Link (resp. here external Link), and of RIKEN external Link.

Further press releases:

06.08.15: Solved Q value puzzle paves way to the neutrino mass

Hundred trillion neutrinos are traversing every human per second, but their mass is still unknown. So far, only upper limits of the neutrino mass could be determined, confirming it to be tiny but not zero as predicted by the standard model of particle physics. In order to determine the neutrino mass, radioactive beta decay or electron capture in suitable nuclei are being studied. The ECHo collaboration external Link aims at extracting the neutrino mass from measurements of the energy emitted in the electron capture decay of the artificial holmium-163 isotope to the stable stable dysprosium-163. Thus, it is essential to known the 163Ho decay energy with highest precision. The reported Q values from indirect measurements span the quite large range from about 2400 to 2900 eV. This Q value puzzle has to be solved.

In a recently in Physical Review Letters published article S. Eliseev et al. report on the first direct high-precision Penning-trap determination of the atomic mass difference of 163Ho and 163Dy. The measurement was performed with the Penning-trap mass spectrometer SHIPTRAP external Link at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. To determine the masses of holmium and dysprosium, the phase-imaging ion-cyclotron-resonance technique (PI-ICR) was used. This novel high-precision technique is based on the projection of the radial ion motion in a Penning trap onto a position-sensitive detector.
The measurement has yielded a final value of the decay energy of 2833 eV with an uncertainty of only a few tens of eV. This is in perfect agreement with the Q values obtained with cryogenic microcalorimetry. The result provides confidence that a sensitivity below 10 eV for the neutrino mass will be reached in the first phase of the ECHo experiment external Link, which is more than a factor of ten below the current upper limit. Future mass measurements using the new PENTATRAP device will improve the accuracy of the decay energy value by an order of magnitude. This will pave the way to reach sub-eV sensitivity for the neutrino mass.

Please read more in the article ... >

Further information:

14.07.15: Precision test of bound-state QED in three-electron systems

Calculations of fine structure splittings in two-electron atoms provide a precise test of bound-state quantum electrodynamics. The helium fine structure currently serves as one of the most precise QED tests in two-electron systems. Recently the extension of such calculations to three-electron systems was performed. Lithium fine structure calculations represent the most accurate QED test with lithium atoms. The most accurate tests of QED in heavier three-electron systems are those of the 2s1/2 -> 2p1/2 transition energy in lithiumlike uranium and the g factor in Si11+. For further tests on low-Z ions, the Be+ and B2+ ions are suitable candidates.

In a recently in Physical Review Letters published article W. Nörtershäuser et al. report on a precise test of bound-state QED in three-electron systems. The total transition frequencies and 2p1/2,3/2 fine structure splittings in 7,9-12Be+ have been experimentally and theoretically investigated. The measurements have been performed at COLLAPS external Link at the radioactive beam facility ISOLDE/CERN external Link. An improvement of the accuracy of the Be-9 fine structure splitting by two orders of magnitude could be achieved. The experimental results were compared with new ab initio atomic structure calculations with a similar level of accuracy. Good agreement between theory and experiment was observed. This new results provide one of the most precise tests of bound-state QED in three-electron systems.

Please read more in the article ... >

20.05.15: First investigation of the N=32 shell closure below the magic proton number Z=20

The investigation of the evolution of the nuclear shell structure far away from stability is important to improve the understanding of the nuclear force. In 2013 ISOLTRAP mass measurements of the exotic calcium isotopes 53Ca and 54Ca unambiguously established a prominent shell closure at N=32 and thereby showed that shell effects do not smear out far from stability (see news of June 19, 2013). Since 52Ca (Z=20, N=32) is a doubly magic nucleus, the question remained unanswered whether the found shell closure is an isolated case or occurs systematically. Thus, exotic isotopes below the magic proton number Z=20 have to be investigated.

In a recently in Physical Review Letters published article M. Rosenbusch et al. report on the first measurement of the masses of the exotic potassium isotopes 52,53K. The measurements were performed using the high-resolution multi-reflection time-of-flight mass spectrometer (MR-TOF MS) of ISOLTRAP external Link (ISOLDE/CERN external Link, Geneva). It is the first investigation of the N=32 shell closure towards the neutron drip line for Z<20 and 53K is to date the shortest-lived nuclide investigated at ISOLTRAP.
The new ISOLTRAP masses allow the determination of the mass excesses of the measured isotopes, from which the two-neutron separation energies S2n can be deduced. These S2n values allow to study the evolution of nuclear structure with neutron number. The new S2n values show a significant drop of about 3 MeV from 51K to 53K compared to only about 1 MeV from 49K to 51K. The shell gap is understandably slightly lower (about 1 MeV) than for doubly magic 52Ca. It clearly illustrates the N=32 shell effect for potassium.

The new ISOLTRAP measurements were accompanied by theoretical considerations. Skyrme-Hartree-Fock-Bogoliubov calculations and the recently developed ab initio Gorkov-Green function (GGF) theory could reproduce qualitatively the observed shell effect.

Please read more in the article ... >

Further information:

13.04.15: First storage-ring measurement of the decay of a hydrogen-like isomer

Heavy mid-shell nuclei are understood to be favorable for the existence of high-energy, long-lived isomers that often decay via either internal conversion or γ decay, which are well studied processes in neutral atoms. However, it is difficult to create an excited nuclear state as a highly charged ion and storing it for long enough to observe its decay. Thus, for nuclei in highly charged ionic states the reliability of calculations to extract internal conversion coefficients is currently undetermined.

In a recently in Physical Review C published highlight article external Link A. Akber et al. report on the first storage-ring measurement of the decay of a hydrogen-like isomer. An 8.5 s, 10 isomer with an excitation energy of 2015 keV has previously been identified in 192Os. The decay of an excited metastable nuclear state of 192Os in a hydrogen-like charge state has been studied using the experimental storage ring (ESR) external Link at GSI, Darmstadt, Germany. This was performed by examining over 400 injections where hydrogen-like 192mOs was observed. Electron and stochastic cooling of the injected beam particles enabled Schottky mass spectrometry to be performed.

The first ESR measurement of the hydrogen-like 192mOs ion yielded a considerably longer lifetime (15.1+1.5−1.3 s) than the previously observed lifetime of neutral 192Os (8.5(14) s). The difference is explained due to hindering of internal conversion in hydrogen-like 192Os. The experimental measurement is in good agreement with the lifetime of 13.0(24) s predicted from the evaluation of conversion coefficients. Thus, the described ESR measurement provides a test for the reliability of internal conversion coefficient calculations in highly ionized systems.

Please read more in the article ... >