News Archive 2021

23.09.21 | Precision measurements
Precision mass measurements benchmark modern ab initio calculations of ¹⁰⁰Sn

According to the simple nuclear shell model, each shell can be filled with a certain maximal number of protons Z or neutrons N, the so-called "magic" numbers (Z, N=8,20,28,50,82 and N=126). Such nuclear shell closures are associated with increased stability comparable with the stable electron configuration of the noble gases.
The "doubly magic" tin-100 (¹⁰⁰Sn) nucleus consists of 50 protons and 50 neutrons and thus shows so-called "mutually enhanced magicity" (i.e. increased stability associated with neutron magic number when the proton number is magic, and vice versa). Due to its extreme neutron deficiency ¹⁰⁰Sn forms the limit of proton stability and is the heaviest self-conjugate nucleus (Z=N) on the nuclear chart. Tin-100 transforms into indium-100 via beta-plus decay. Because of its comparatively simple inner even-even structure, ¹⁰⁰Sn is well suited to experimentally test modern nuclear models.
In recent years, two measurements (Hinke et al. and Lubos et al.) of the beta-plus decay Q-value (i.e. released energy) were used to derive the ¹⁰⁰Sn mass. These experiments yielded the largest strength of a Gamow-Teller transition (emission of a positron-neutrino pair with parallel spins) so far, showing the "superallowed" nature of the tin-100 beta-plus decay.
However, the two discrepant obtained Q-values and deduced atomic mass values of ¹⁰⁰Sn demonstrate the need to also address the odd-proton ¹⁰⁰Sn neighbors. They are inherently more difficult to describe but crucial for a complete test of modern ab initio calculations.

In a recent article published in "Nature Physics", M. Mougeot et al. present the high-precision mass measurements of neutron-deficient indium isotopes with ISOLTRAP , produced at the ISOLDE on-line isotope separator at CERN in Geneva. By direct determination of the nuclear binding energy, high-precision atomic mass measurements provide a crucial model-independent probe of the structural evolution of exotic nuclei. The new precision mass data allowed for the first time extending the experimental knowledge of binding energy to only one proton below ¹⁰⁰Sn. The mass measurements of ^{99,100,101g,101m}In were made possible only by employing the most advanced mass spectrometry techniques.
The mass of ⁹⁹In was measured for the first time. Since the production yield of ⁹⁹In was too low, ISOLTRAP's Multi-Reflection Time-of-Flight Mass Separator/Spectrometer (MR-ToF MS) was used to perform the mass measurement.
The rate of ¹⁰⁰In and ¹⁰¹In after the MR-ToF MS was sufficient to perform Penning-trap mass measurements. For ¹⁰⁰In the conventional Time-of-Flight Ion-Cyclotron-Resonance (ToF-ICR) technique was used. The additionally determined MR-ToF MS mass value for ¹⁰⁰In was in good agreement with the Penning-trap value.
The A=101 indium beam delivered to ISOLTRAP was a mixture of the ground and isomeric state (^101gIn and ^101mIn) so that the novel Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique (see e.g. our news of 11.02.14 and 05.03.20) had to be used to resolve them and ensure the accuracy of the ground-state mass value. The additional ToF-ICR measurement of ^101gIn was in excellent agreement with the value measured using PI-ICR.

The tin-100 mass excess value can be derived from that of indium-100 and the Q-value of the beta-plus decay of ¹⁰⁰Sn into ¹⁰⁰In. This Q-value was determined by Hinke et al. (2012) and Lubos et al. (2019) with significantly different results. In the new Penning-trap mass measurements, ¹⁰⁰In was found to be 130 keV more bound compared to the literature value while the mass uncertainty was improved by almost a factor of 90.

An analysis of the two neutron empirical shell-gap showed that the Q-value reported by Hinke et al. follows the expectation of a doubly-magic ¹⁰⁰Sn, whereas the more recent value reported by Lubos et al. yields a ¹⁰⁰Sn mass value that doesn't follow the simple extrapolation of experimental trends.

In recent years, there has been great progress advancing ab initio calculations in medium-mass nuclei (see e.g. our news of 19.06.13 and 08.02.16) up to the tin isotopes based on modern nuclear forces derived from chiral effective field theory (EFT) of the strong interaction. Thus, the new experimental results could be confronted with new ab initio predictions. For this purpose, the experimental three-point estimators of the neutron and proton odd-even staggering were compared with theoretical results, employing the valence space formulation of the in-medium similarity renormalization group (VS-IMSRG) and the shell-model coupled-cluster (SMCC) methods.
The new ab initio calculations agree with the experimental trend, yielding a neutron and proton staggering of similar magnitude and differing only in absolute values. At N=51 it is highlighted that the Q-value of Hinke et al. is more in line with the new theoretical results than the result of Lubos et al.. At Z=49 the evolution of all theoretical trends also clearly favor the Hinke et al. Q-value over that of Lubos et al..

Thus, the new precise mass values of neutron-deficient indium isotopes provide a successful experimental benchmark and increased confidence for the modern theoretical approaches concerning the iconic ¹⁰⁰Sn.

Please read more in the "Nature Physics" article and the associated article in News & Views .

Further information also in the press release of the MPIK (idw ).

Further press releases:

25.08.21 | Precision measurements
First sympathetic laser cooling of a single proton in a Penning trap

Comparisons of fundamental particle-antiparticle properties provide stringent constraints on possible violations of the charge-parity-time reversal (CPT) symmetry and aim to explain the observed overabundance of matter to antimatter in the universe. The Baryon Antibaryon Symmetry Experiment (BASE) at CERN in Geneva, for example, aims at performing a stringent test of CPT symmetry by comparing the magnetic moments (g-factors) of the proton and the antiproton with high precision (see our news of 23.11.15).
Since such high-precision g-factor measurements are limited by cryogenic particle temperatures (see our news of 12.08.15, 18.10.17, and 24.11.17), efficient cooling of trapped charged particles is essential in the mentioned but also in many other fundamental physics experiments.
The lowest temperatures for proton and antiproton are presently reached by resistive cooling in a cryogenic Penning trap system. This allows cooling the proton or antiproton to the environment temperature of about 4 K.
Protons and antiprotons have no electronic structure and thus are not suitable for direct laser cooling in order to further reduce their temperature. Thus, coupling of laser coolable ions to protons or antiprotons (or other systems with no optical structure) has to be achieved. This has long been desired for precision spectroscopy (see e.g. our news of 20.09.19), mass measurements, quantum information, and the realization of novel quantum systems.
In 2018, it has already been proposed to sympathetically cool single protons or antiprotons by using two Penning traps for a proton/antiproton and laser cooled beryllium ions connected by a common endcap (see our news of 19.03.18).

In a recent article published in "Nature", M. Bohman and the BASE collaboration demonstrate the first sympathetic cooling of a single proton using a cryogenic two-Penning-trap system. To this end, the single proton was stored in the proton trap (PT) and a cloud of Be⁺ ions in a separate beryllium trap (BT). In contrast to the mentioned common endcap technique, the coupling was realized by connecting the two Penning traps to a superconducting cryogenic LC circuit with resonance frequency near their axial frequencies.
LC resonators with a high quality factor (in this case Q ~ 15 000) are commonly used for image-current detection of single trapped particles. In the novel experiment, the resonator was connected to both traps so that the two ion-trap systems were coupled via image currents and the proton, Be⁺ ions, and LC resonator formed a system of three coupled oscillators.
Since the coupling does not rely on a shared electrode as in the former non-resonant proposal, the energy exchange rate is not limited by the trap capacitance. This allows much stronger coupling and the cooling scheme can be realized over long distances and with several distributed ion traps. In the described experiment, the proton trap and the beryllium trap were separated axially by around 9 cm.
The researchers demonstrated the successful coupling in two ways. Firstly, they showed that the temperature of the proton can be modified by coupling to a cloud of excited (heated) Be⁺ ions, typically consisting of around 15 ions. To quantify the energy transferred to the proton, the axial frequency of the proton was measured before and after coupling to the excited Be⁺ ions. Secondly, to demonstrate the sympathetic cooling, similar axial frequency shift measurements of the proton were employed in the presence of a continuously laser cooled Be⁺ ion cloud. For this purpose, the Be⁺ ions were cooled with the closed ²S_1/2 -> ²P_3/2 transition and tuned to resonance with the superconducting LC circuit and the proton. This new cooling technique allows to reach proton temperatures far below the environment temperature. The lowest proton temperatures are not found by minimizing the Be⁺ temperature, but by maximizing the coupling of the Be+ ions to the LC resonator. In the demonstration measurement, the proton temperature was reduced by 85%, from 17 K environment temperature to 2,6 K.

The described novel sympathetic laser-cooling technique will enable enhanced precision experiments of any charged species at lower temperatures. In particular, it can be readily applied to cool protons and antiprotons in the same large macroscopic traps that enable precision measurements of the charge-to-mass ratio and g-factor. This will allow for improved precision in matter-antimatter comparisons and dark matter searches (see our news of 13.11.19 and 25.01.21), performed by the BASE collaboration.
Furthermore, the successful extension of laser cooling to particles in spatially separated traps may contribute to develop quantum control techniques for previously inaccessible particles such as highly charged ions, molecular ions, and antimatter particles.

Please read more in the "Nature" article and the associated article in News & Views .

The innovative particle cooling technique of the BASE collaboration has been selected to be among the Physics World Top 10 Breakthroughs in 2021 .

Further information also in the press release of the MPIK .

Further press releases:

25.01.21 | Precision measurements
BASE's ultra-sensitive detector used in the search for axion-like dark matter

Already from the 1960s onwards, interpretations of analyses of the dynamics of galaxies provided strong indication that there could be about five times more so-called "dark matter" in our Universe in addition to the well-known visible "baryonic" matter. But the microscopic properties of this dark matter are still unknown today. Quantum chromodynamic (QCD) axions and new axion-like particles (ALPs), predicted by extensions to the Standard Model, are excellent dark matter candidates, since they would be produced in the early universe and form a cold dark matter halo consistent with astrophysical observations.

Axions and ALPs couple to two photons, which allows them to convert into photons in a strong external magnetic (or electric) field. This conversion is the basis for experimental searches for axions and ALPs. The searches are performed in haloscope experiments (for axions and ALPs in the galactic halo), in helioscope experiments (for solar axions and ALPs) and in purely laboratory-based experiments. A number of laboratory experiments and astrophysical observations have already placed limits on ALP masses and couplings in the neV/c² range.
Besides dedicated axion detectors, ultra-sensitive superconducting single-particle detectors of cryogenic Penning-trap experiments can be repurposed for the detection of axions and ALPs.

In a recent article published in "Physical Review Letters", J. A. Devlin et al. present the results of the investigation of the ALP-to-photon conversion using the axial detection system of the analysis trap of the BASE antiproton experiment at CERN. This analysis complements the previous BASE study of the possible interactions between ALPs and antiprotons (see our news of 13.11.19).
BASE is a cryogenic Penning-trap experiment located at CERN's Antimatter Factory, dedicated to testing charge-parity-time-reversal (CPT) invariance by comparing the fundamental properties of protons and antiprotons. BASE allows to detect femtoamp sized image currents induced by oscillating antiprotons inside their analysis trap. The used resonant LC circuit is also sensitive to changes in the magnetic flux, caused by an oscillating ALP field. This allows to extract ALP-photon interaction limits by searching the noise spectrum of the fixed-frequency resonant circuit for peaks caused by hypothetical dark matter ALPs converting into photons in the strong magnetic field (1.945 T) of the Penning-trap magnet.

For a narrow mass range around 2.791 neV, the researchers could place the so far strongest laboratory constraints for ALP-photon interactions, at a level which is comparable to that obtained from astrophysical observations with the Fermi-LAT space telescope and stronger than other current haloscope and helioscope experiments.
The new approach opens an avenue for many other Penning-trap experiments to search for ALP signatures. The scientists are considering to adapt the highly sensitive single-particle detectors used in Penning-trap experiments like BASE into more powerful ALP search experiments with higher detection bandwidth.

Please read more in the article ... >

Further information also in the press release of the MPIK (idw ).

Further press releases:

13.01.21 | Strong interaction and exotic nuclei
Limits of atomic nuclei predicted

In a new study, "Ab Initio Limits of Nuclei", published in the journal Physical Review Letters as an Editors' Suggestion with an accompanying synopsis in APS Physics, our Max Planck Fellow Professor Achim Schwenk of TU Darmstadt, together with scientists from the University of Washington, TRIUMF and the University of Mainz, succeeded in calculating the limits of atomic nuclei using innovative theoretical methods up to medium-mass nuclei.
The novel calculations have enabled the study of nearly 700 isotopes between helium and iron. The results are a treasure trove of information about possible new isotopes and provide a roadmap for nuclear physicists to verify them.

Please read more in the article and the press release of the MPIK .

Further information also in the Synopsis of the article .