News Archive 2017

24.11.17: Most precise measurement of the proton magnetic moment by an optimized double Penning-trap technique

The proton is one of the central building blocks of stable matter. For that reason the precise knowledge of its fundamental properties such as its mass (see our news of 18.07.17), lifetime, charge radius and magnetic moment is of great importance and thus the subject of many contemporary physics research programs. The mass, for example, is an input parameter for precise quantum electrodynamics calculations and the lower bound of the proton lifetime sets constraints on possible baryon number violation.
Apart from its fundamental nature, the proton is one of the few particles, which is comparable to its antiparticle – the antiproton. This allows for a test of the Charge, Parity, and Time Reversal (CPT) symmetry of the Standard Model of particle physics and Big Bang cosmology, which postulates that the fundamental properties of a particle and its antiparticle are exactly equal. Therefore, particles and antiparticles should have been created with same amounts. The lack of antimatter in our Universe, however, is in contradiction with this assumed matter-antimatter symmetry of the Standard Model. One possible explanation of the baryon asymmetry of the Universe is a violation of CPT invariance. Therefore, the measurement of minutest differences between the properties of a proton and its corresponding antiparticle, the antiproton, provides a precise test of the particle-antiparticle equality in the baryonic sector and of the CPT symmetry of the Standard Model.
The proton magnetic moment μ_p was directly measured in 2014 at the University of Mainz with a unprecedented fractional precision of 3.3 parts per billion (see our news of 28.05.14) using a novel double Penning-trap technique. Since then, the researchers at Mainz have developed techniques that enabled them to improve the measurement of the proton magnetic moment by more than one order of magnitude.

In a recent article published in "Science", G. Schneider et al. report on a direct high-precision measurement of the magnetic moment of the proton μ_p in units of the nuclear magneton μ_N. The measurement has been carried out in an improved double Penning-trap system at the University of Mainz, Germany.
A Penning trap is formed by a superposition of an electrostatic quadrupole potential and a homogeneous magnetic field B₀, which was at 1.9 T in the described proton experiment.
The measurement of the frequency ratio ν_L/ν_c of the Larmor frequency and the cyclotron frequency allows the determination of the magnetic moment of a single proton in the Penning trap independent from the magnetic field B₀. The cyclotron frequency ν_c can be determined from the three harmonic components ν_z, ν₊, and ν_- of the proton motion in the Penning trap using the invariance theorem: ν_c² = ν₊² + ν_z² + ν_-². The axial frequency ν_z of the stored proton is directly observed by non-destructive image current detection. The modified cyclotron frequency ν₊ and the magnetron frequency ν_- are measured by sideband coupling.
Since the Larmor frequency ν_L is not accompanied by an oscillating charge it cannot be determined by image current detection but by means of spin-transition spectroscopy. The superimposition of an inhomogeneous magnetic field, called a "magnetic bottle", couples the spin-magnetic moment of the proton to its axial oscillation frequency and thus allows to apply non-destructive detection of spin transitions (spin flips) using the continuous Stern-Gerlach effect.
However, the strong magnetic bottle broadens the Larmor resonance line significantly and limits the measurement to a relative precision of order 10^-6 (p.p.m.). The application of a double Penning-trap technique overcomes this problem by using two separate Penning traps which are interconnected by a transport section. A precision trap (PT) with a nearly homogenous magnetic field is used for high-precision measurements of ν_c and ν_L and an analysis trap (AT) with the magnetic bottle is deployed for spin state detection of the proton. By applying this method, the proton g-factor measurement in 2014 at Mainz reached a relative precision of 3.3 p.p.b..

With respect to the proton measurements in 2014 the recent proton experiment at Mainz used an optimized double Penning-trap technique. The trap geometry was optimized to reach a higher magnetic homogeneity in the precision trap. Together with a superconducting self-shielding coil this allowed to improve the cyclotron stability by one order of magnitude. The width of the g-factor resonance, a major limitation in the previous measurement, was reduced by carefully optimizing the Larmor rf drive to probe the resonance. The particle preparation time for each individual measurement cycle could be reduced by applying a significantly improved superconducting detector for the modified cyclotron frequency ν₊. This enabled a twice as high data acquisition rate which greatly increased statistics. Furthermore, the simultaneous measurement of ν_L and ν₊ instead of a sequential determination ensured that ν_L and ν₊ were measured at the same energies and times which eliminated many systematic shift contributions.
The final result of the determined proton magnetic moment in units of the nuclear magneton has a fractional precision of 0.3 p.p.b. (3·10^-10): μ_p = 2.792 847 344 62(75)(34) μ_N. The first value in parentheses is the statistical uncertainty and the second one the systematic uncertainty. The new value of μ_p is in agreement with the currently accepted CODATA (Committee on Data for Science and Technology) value, but a factor of ten more precise. It improves the 3.3 p.p.b. precision of the μ_p measurement in 2014 at Mainz by a factor of eleven.

The BASE collaboration recently measured the antiproton magnetic moment with a fractional precision of 1.5 p.p.b. (see our news of 18.10.17).
The recent results for the magnetic moments of the proton and the antiproton enable a test of the fundamental symmetry between matter and antimatter in the baryonic sector at the 10^-9 - 10^-10 level. The researchers expect that proton/antiproton magnetic moment measurements on the parts per trillion level (10^-12) will become possible by reducing the particle preparation times by more than two orders of magnitude using sympathetically cooled protons/antiprotons by coupling them to laser cooled beryllium ions.

Please read more in the "Science" article ... >

A radio report on the most precise measurement of the proton magnetic moment (start at 2:15 min.).

Further press releases:

07.11.17: Mass measurements on copper isotopes strengthen the prediction of a new island of inversion

In atomic nuclei the protons and neutrons occupy quantum levels that are separated by energy gaps. This leads to the ordering of the energy levels of the nucleons in the simple nuclear shell model. It successfully describes the nuclear structure near the valley of stability and explains the exceptional stability of nuclei with completely filled proton or neutron shells (i.e. magic proton or neutron numbers). However, over the years studies of exotic neutron-rich nuclei have shown that the postulated stability of nuclei with magic numbers can disappear. Today it is known that the shell-model magic numbers N = 8, 20, 28, and 40 are broken at the so called islands of inversion. Improved shell-model calculations predict a fifth island of inversion for Z < 28. Thus, the Z = 28, N = 50 region of the nuclear chart is a focus of today’s experimental and theoretical nuclear structure research. The doubly magic exotic nuclide ⁷⁸Ni is of special interest and it is an open question, whether it retains the exceptional stability of the classic closed-shell nuclides. Furthermore, recent studies by laser spectroscopy offered strong evidence, that normal near-spherical and deformed structures coexist (so called shape coexistence) close to ⁷⁸Ni. Copper isotopes in the vicinity of ⁷⁸Ni provide an excellent proxy for probing the structure of ⁷⁸Ni.

In a recently in Physical Review Letters published article A. Welker et al. report on high-precision mass measurements of the Z = 29 copper isotopes ^75–79Cu (N = 46-50). The measurements were performed using the high-resolution multi-reflection time-of-flight mass spectrometer/separator (MR-TOF MS) ISOLTRAP at CERN, Geneva.
A purified copper-ion ensemble was injected in a precision Penning trap, where the high-precision mass measurements were carried out by time-of-flight ion-cyclotron resonance (TOF-ICR). The TOF-ICR yields the atomic masses of the copper ions of interest by determination of the cyclotron-frequency ratio between the reference ⁸⁵Rb⁺ ions and the copper ions. The use of the MR-TOF MS allowed measuring at a lower yield than would have been possible solely by Penning-trap mass spectrometry.
From the determined mass excesses the two-neutron separation energies S_2n of the nuclides in the ⁷⁸Ni region were deduced. The S_2n values allow to probe the evolution of nuclear structure with neutron number. The S_2n value is theoretically expected to drop between N = 50 and N = 52, which would be a well-known sign for magicity. Since the new masses do not extend beyond N = 50, it is not possible to observe the expected S_2n drop. However, the new data differ significantly from that of previous results and the effect of the magic number is already noticeable in the S_2n difference between N = 48 and N = 50. Thus, the high-precision mass measurements on ^75–79Cu allow to define the mass surface above ⁷⁸Ni and offer evidence for its doubly magic nature.

In addition to the mass experiments, large-scale shell-model calculations with the recently developed PFSDG-U interaction were performed. The new calculations predict shape coexistence in a doubly magic ⁷⁸Ni and a new island of inversion for Z < 28. The calculated S_2n values are in excellent agreement with the experimental results for Ni and Cu isotopes. This gives confidence in the underlying shell-model description of these exotic nuclides.

Please read more in the article ... >

The article has been selected for a Viewpoint in Physics. Please read also the viewpoint on the article by Daniel Bazin.

Further information also in the press releases of the idw and CERN .

The article has also been selected as "Editor's Suggestion". This is an award "based on the potential interest in the results presented and, importantly, on the success of the paper in communicating its message, in particular to readers from other fields" (see also here

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18.10.17: Most precise measurement of the antiproton magnetic moment by a novel spectroscopy method

The Charge, Parity, and Time Reversal (CPT) symmetry of the Standard Model of particle physics implies the exact equality between the properties of a particle and its antiparticle. Thus, precise comparisons of the fundamental properties of matter/antimatter conjugates provide sensitive tests of CPT invariance. They provide data for Standard Model Extensions and contribute to a better understanding of the matter-antimatter imbalance of our universe.

Since the magnetic moment of the antiproton is 660 times smaller than the one of the positron, it is very difficult to measure its magnetic moment with high precision. Therefore, the antiproton magnetic moment is so far only known to a fractional uncertainty at the parts per million (p.p.m.) level (see our news of 18.01.17), whereas other matter/antimatter properties have been compared on the parts per billion (p.p.b.) level or better.
In 2014 the magnetic moment of the proton was measured directly at Mainz with 3.3 parts per billion (p.p.b.) precision (see our news of 28.05.14). These high-precision proton measurements were based on the application of the challenging double Penning-trap technique. The implementation of this method promised an improvement of the precision in antiproton magnetic moment measurements to the level of a few parts per billion.

In a recent article published in "Nature" C. Smorra and the BASE collaboration reports on a measurement of the magnetic moment of the antiproton with a fractional precision of 1.5 p.p.b.. The measurement was performed at the Antiproton Decelerator (AD) of CERN, Geneva, utilizing a novel two-particle spectroscopy method in an advanced cryogenic multi-Penning trap system.
The determination of the magnetic moment of a single particle in a Penning trap is based on the measurement of the frequency ratio of the Larmor frequency (ν_L) and the cyclotron frequency (ν_c). The measurement was mainly performed in two separate Penning traps, an analysis trap (AT) with an inhomogeneous magnetic field and a precision trap (PT) with a factor ~10⁵ more homogeneous magnetic field B₀ = 1.945 T.
In contrast to the double-Penning trap technique used in the measurement of the proton magnetic moment in 2014 at Mainz, the novel two-particle technique utilized a hot cyclotron antiproton for measurements of the cyclotron frequency ν_c, and a cold Larmor antiproton to determine the Larmor frequency ν_L. Thus, the new method eliminated the need of cyclotron cooling in each measurement cycle, and increased the sampling rate.

The Larmor frequency measurement is based on spin-transition spectroscopy. To this end, the spin-state of the antiproton is non-destructively determined in the magnetic bottle of the analysis trap, which couples the spin magnetic moment of the antiproton to its axial oscillation frequency ν_z. After the initial spin state is defined, the Larmor antiproton is moved into the precision trap where a spin transition for the spin-flip resonance is induced. The final state of the spin transition is analyzed by moving the Larmor antiproton back to the analysis trap.

The cyclotron antiproton is used to determine the magnetic field before and after each spin-flip attempt of the Larmor particle in the precision trap. It is moved to a park electrode before the Larmor antiproton is transported to the precision trap. The cyclotron frequency measurement is also based on image current detection. The three oscillation frequencies of the trapped antiproton are measured and the cyclotron frequency is obtained using the invariance theorem: ν_c² = ν₊² + ν_z² + ν_-².
The axial frequency ν_z of the stored antiprotons is directly observed by the image current detector. The modified cyclotron frequency ν₊ and the magnetron frequency ν_- were measured by coupling the axial and the radial modes using radio frequency drives on the motional sidebands at ν₊ - ν_z resp. ν_z + ν_-.

The novel two-particle spectroscopy method yielded a highly improved value for the magnetic moment of the antiproton in units of the nuclear magneton μ_N: μ_p = -2.792 847 344 1 (42) μ_N. This new value is about 350 times more precise than the result of the previous best μ_p measurement. It is consistent with the proton magnetic moment measured at Mainz in 2014, μ_p = 2.792 847 350 (9) μ_N, and is at the reached level of precision in agreement with CPT invariance.

The BASE collaboration expects that with a technically revised apparatus, including an improved magnetic shielding, an improved resistive cooling system for the cyclotron mode with lower temperature, and a precision trap with a 10-fold more homogeneous magnetic field, a ten-fold improved limit on CPT-odd interactions from proton/antiproton magnetic moment comparisons can be obtained in the future.

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

Further information also in the press release of the MPIK .

Further press releases:

18.07.17: Most precise measurement of the proton mass

One of the fundamental constants, which is crucial to develop precise quantitative understanding of nature and its symmetries is the mass of the proton m_p. Its value is among others required for the precise comparison of the proton and antiproton mass, in order to perform a precise test of the fundamental Charge, Parity, and Time Reversal (CPT) invariance.
Mass measurements are generally based on the comparison of the mass of interest with a precisely known reference mass. The cyclotron frequency ν_c of an stored ion in a Penning trap is proportional to its charge-to-mass ratio q/m: ν_c = 1/(2π) (q/m) B. Thus, precision Pennig-trap mass measurements of the proton are performed by comparing the cyclotron frequency of the proton with the cyclotron frequency of a reference ion in the same homogeneous magnetic field B.

In a recently in Physical Review Letters published article F. Heiße et al. report on a high-precision measurement of the proton mass m_p in atomic mass units, which was based on cyclotron frequency comparisons of protons and highly charged carbon (¹²C⁶⁺) ions. The mass of ¹²C⁶⁺ can be related to the mass of ¹²C resp. the atomic mass unit by correcting for the mass of the missing six electrons and their respective binding energies.
The measurements have been carried out in a highly optimized, purpose-built cryogenic (4 K) Penning-trap setup, dedicated to mass measurements on light ions. The superconducting magnet and the liquid helium cryostat of the preceding Mainz g-factor experiment for highly charged ions could be re-used. The trap section as well as the cryogenic electronics and detection circuitry have been newly developed. The new trap tower includes two separate storage traps and the measurement trap (MT).
The ions are produced in-situ using a miniature electron beam ion source (EBIS). By shuttling the ions between the two storage traps and the measurement trap, the time between successive measurements in the MT is minimized. For the first time two independent superconducting detection circuits for the proton and for the carbon ion were implemented, which needed to fit very precisely. They allowed for measurements at the identical electrostatic and magnetic field configurations.

The cyclotron frequency ν_c is determined by the three particle's motional eigenfrequencies in the Penning trap using the invariance theorem: ν_c = (ν₊² + ν_z² + ν_-²)^1/2. Of the three independent eigenmotions, the tank circuit can only detect the axial motion directly. In order to determine the axial frequencies of the stored ions, the image current the ions induce on the trap electrodes when oscillating with the axial eigenfrequency ν_z of about 525 kHz for ¹²C⁶⁺ and 740 kHz for the proton, respectively, was measured. The axial frequency ν_z was determined from a dip spectrum, the modified cyclotron frequency ν₊ (and similarly the magnetron frequency ν_-) was measured with a "double-dip" method. Here the modified cyclotron motion was coupled to the axial motion via radio frequency drives on the motional sideband at ν₊ - ν_z. This leads to the so called “double-dip”.
Since the uncertainty of ν₊ is dominant in the invariance relation, the about an order of magnitude more precise phase-sensitive PnA (Pulse and Amplify) technique was also used for determination of ν₊ with highest precision and very low kinetic energy of the ion, and thus low systematic frequency shifts.

Taking into account the statistical uncertainties (1st bracket) as well as the systematic shifts and their uncertainties (2nd bracket) of the PnA data, the ratio ν_c(¹²C⁶⁺)/ν_c(p) = 0.503 776 367 662 4(77)(146) was obtained. Here the statistical and systematic uncertainties are listed separately. The by a factor of around four less precise result from the double-dip data was in excellent agreement with the PnA result.
From this cyclotron frequency ratio the proton mass in atomic mass units m_p = ν_c(¹²C⁶⁺)/ν_c(p)·m(¹²C⁶⁺)/6 = 1.007 276 466 583(15)(29) u was calculated. The value of m_p has a relative precision of 32 parts-per-trillion (ppt), which is a factor of three times more precise than the current CODATA (Committee on Data for Science and Technology) value. Moreover, it shows a deviation from the literature value by more than three standard deviations.

In addition, using the independently measured electron mass m_e, the new value of m_p yielded a factor of two more precise proton-electron-mass ratio m_p/m_e = 1 836.152 673 346(81).

More information can be found in the Physical Review Letters article ... >

Further information also in the press release of the MPIK and the Synopsis of Michael Schirber.

Further press releases:

17.07.17: First photodetachment spectroscopy on a fast OH^- beam in the CSR

Molecular anions play an important role in chemical reactions in interstellar space and planetary atmospheres and were, for example, observed in the ionosphere of Saturn's moon Titan. Ion traps provide radiation shielding and often extremely low pressures for laboratory experiments with ions at rest. For molecular ions, several studies recently focused on their cooling in laser fields or in collisions with other particles. Investigations where ions are kept in isolation and studied over long time have been rare. Recently constructed cryogenic storage rings for fast ion beams have opened up such studies since ions can be stored for very long durations and individually detected through their fast motion. For anions, in which a neutral atom or molecule binds an excess electron, the photodetachment of this electron offers sensitive single-ion detection.

At MPIK Heidelberg, the novel ultracold storage ring CSR for experiments under conditions as they occur in space was inaugurated in May 2016. It allows for experiments with negative ions with lifetimes of minutes or hours, e. g. small molecular anions like OH^-, in order to investigate their interactions in the lowest rotational states. Those studies are crucial for the formation of molecules in interstellar space and for low-temperature plasma chemistry in general.

In a recent article published in Physical Review Letters, C. Meyer et al. report on a state-selective photodetachment experiment on OH^- molecular anions in the cryogenic storage ring CSR at MPIK Heidelberg. In this experiment a beam of OH^- anions from a Cs sputter ion source was accelerated to 60 keV and injected into the ultracold storage ring. About 10⁷ ions were stored at an ambient temperature near 6 K (-267 °C) and extremely high vacuum (below 10^-14 mbar). The ions could be stored for up to 1200 s (20 min). This allowed to observe the spontaneous decay of low-lying excited rotational levels of OH^- and to follow the in-vacuo rotational relaxation over times long compared to the natural lifetime of the first excited rotational level.
Frequency and time dependent photodetachment spectroscopy was performed by overlapping nearly collinear laser beams with the ion beam. The fast neutral particles, produced by photodetachment at a steady small rate, were counted with a large microchannel plate detector about 3 m downstream from the interaction region.

An effective radiative temperature T_r = 15.1(1) K with about 90% of all ions in the rotational ground state was obtained.
The Einstein coefficients A_J, the natural lifetimes τ_J = (A_J)^-1 and the corresponding transition dipole moments were determined for the lowest OH^- rotational states (J = 1,2,3). The natural lifetime of the first excited rotational level J = 1 was found to be about 193 s. Such direct in-vacuo lifetime measurements on low-lying, purely rotationally excited states in small molecules have not been reported previously. The electric dipole moment was measured with 1.5% uncertainty. It differs significantly from the theoretical values available.
At photon energies close to the electron binding energy, the photodetachment cross section is a powerful probe for the internal states of the anion and the neutral daughter molecule as well as for the interaction of the outgoing low-energy electron with the neutral molecule. Thus, the relative photodetachment cross sections were additionally determined over a sample of near-threshold energies for individual rotational levels of OH^-.

The photodetachment spectroscopy in the CSR allows precise laboratory measurements of natural lifetimes and line intensities for extremely slow, purely rotational transitions in molecular ions. To date, line strengths for ionic rotational transitions are generally obtained from calculated molecular dipole moments. Thus, rotational lifetimes from such measurements add a further, so far unavailable experimental benchmark for quantum chemical calculations.
Moreover, in the future, the single-level sensitivity of time-dependent near-threshold photodetachment spectroscopy in the CSR can be used to probe rotational population changes by in-ring molecular collisions.

Please read more in the Physical Review Letters article ... >

Further information also in the press release of the MPIK and the focus story on the article.

23.06.17: Review article on the direct neutrino mass experiment ECHo

Already in 1930, a new neutral particle was postulated by Wolfgang Pauli to explain the β-decay, which was named "neutrino" by Enrico Fermi in 1933. In 1956 the existence of neutrinos was experimentally proved. Neutrinos were long believed to be massless, but experiments between 1997 and 2002 showed that the neutrinos change their flavour (electron, muon, or tau neutrino) periodically. According to theory, this so called neutrino oscillation can only occur if neutrinos have mass.
In neutrino oscillation experiments, only the squared mass difference can be extracted but no information about the absolute neutrino mass.
The determination of the neutrino masses is one of the most difficult challenges in the investigation of the fundamental properties of elementary particles. Thus, although hundred trillion neutrinos are traversing every human per second, their tiny 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 current Standard Model of particle physics. Thus, it is incomplete and an extension is needed to understand the structure of the universe, the hierarchy of masses and to achieve a grand unification.
Precision measurements of the kinematics of weak interactions represent the only model independent approach to determine the absolute scale of neutrino masses. The KArlsruhe TRItium Neutrino (KATRIN) experiment has been designed to measure the mass of the electron antineutrino directly with a sensitivity of 0.2 eV. In the Electron Capture 163-Ho experiment (ECHo) , the ECHo collaboration aims at extracting the neutrino mass from measurements of the energy emitted in the electron capture (EC) decay of the artificial holmium-163 isotope to the stable stable dysprosium-163.

In a recent review article published in the "European Physical Journal Special Topics" the ECHo collaboration discusses the motivations, expectations as well as the first two phases of the ECHo experiment. It has been conceived to reach sub-eV sensitivity on the electron neutrino mass by the analysis of the endpoint region of the calorimetrically measured ¹⁶³Ho electron capture spectrum. In order to achieve this sub-eV sensitivity, a large number of cutting edge technologies have to be developed, e.g. the production of large sample of high purity ¹⁶³Ho as well as the development of large metallic magnetic calorimeters (MMCs) arrays and a precise parameterization of the ¹⁶³Ho spectrum.

The ECHo experiment will be divided into several phases. The first phase, the present medium scale experiment ECHo-1k, is a three-year project, started in 2015, which will allow to reach a sensitivity below 10 eV for the electron neutrino mass, employing about 1000 Bq (1 Kilobecquerel) of highly radiochemically pure ¹⁶³Ho, which will be divided into an array of about 100 low temperature MMCs detectors.
Already in the first phase of ECHo-1k very interesting results have been achieved, e.g. the discovery of spectral structures due to higher order excited states in ¹⁶³Dy. Furthermore, our working group at MPIK participated in the first direct high-precision determination of the atomic mass difference of ¹⁶³Ho and ¹⁶³Dy with the Penning-trap mass spectrometer SHIPTRAP using the phase-imaging ion-cyclotron-resonance technique (PI-ICR). The measurement yielded a Q_EC-value of 2833 eV with an uncertainty of only a few tens of eV, providing confidence that a sensitivity below 10 eV for the neutrino mass can be reached in ECHo-1k.

The second phase, ECHo-1M, is under study and will start at the end of ECHo-1k. ECHo-1M will be characterized by a ¹⁶³Ho activity of 1 MBq, embedded in large MMCs arrays and will allow to reach a sensitivity on the electron neutrino mass below 1 eV. In particular, this aimed sub-eV sensitivity requires a direct measurement of the Q_EC-value of the EC in ¹⁶³Ho with an unprecedentedly low uncertainty of approximately 1 eV. This will become possible by improving the accuracy of the decay energy value (Q_EC) by an order of magnitude using the Penning-trap mass spectrometer PENTATRAP, which is currently being built by our working group at MPIK Heidelberg.

Please read more in the review article ... >

24.04.17: Review article on new methods and highlights of collinear laser spectroscopy at ISOLDE

In a recent review article published in "Journal of Physics G" R. Neugart et al. report on new methods and highlights of collinear laser spectroscopy at the on-line isotope mass separator ISOLDE at CERN, Geneva.

Collinear laser spectroscopy at ISOLDE has a successful history of more than three and a half decades. The COLLAPS (COLlinear LAser SPectroscopy) setup has been introduced at ISOLDE for the investigation of fundamental properties of exotic nuclei such as nuclear ground state spins, electromagnetic moments and charge radii. In 1980 the first COLLAPS setup was completed and first successful experiments were performed on the neutron-rich barium isotopes followed by numerous studies of further isotopic chains until 1990, mainly in the heavier-mass region.
In 1992 a second period of COLLAPS operation started at the new ISOLDE location at the Proton Synchrotron Booster (PSB) of CERN. During this period highly sensitive experimental techniques were developed which exploited the properties of special classes of atomic systems.

The classical collinear spectroscopy at the COLLAPS beam line is based on a single resonant excitation by a continuous wave (cw) laser and the fluorescence decay from the excited level is used as a straightforward detection method. In order to improve the sensitivity of COLLAPS an rf quadrupole cooler and ion buncher (RFQCB), the ion cooler–buncher ISCOOL , was installed at ISOLDE to deliver radioactive beams of improved quality which allows a suppression of background by several orders of magnitude. Since the fall of 2008, nearly all optical detection experiments at COLLAPS have been performed using this new method on bunched beams from ISCOOL.

The combination of collinear laser spectroscopy with the principle of laser resonance ionisation is used in the new CRIS (Collinear Resonance Ionisation Spectroscopy) setup at ISOLDE. The collinear resonance ionisation method uses a second (or third) laser to further excite the level populated in the first high-resolution step to the continuum and thus ionise the atoms. This is a very selective process which allows the detection of optical resonance by counting ions very efficiently and virtually without any background. Since 2012, the novel CRIS technique is used with promising results for measurements on the francium isotope chain.

The introduction of ISCOOL and the new CRIS method at ISOLDE yielded a breakthrough in sensitivity for nuclides in wide mass ranges. The determination of nuclear ground state properties along isotopic chains from collinear laser spectroscopy is now indispensable in nuclear physics research with radioactive beams.

Please read more details in the review article on the experimental principle of collinear laser spectroscopy, the new methods and highlights since 2000 as well as the main future directions at ISOLDE, e. g. an proposed electrostatic ion beam trap (EIBT) for ultra-sensitive collinear laser spectroscopy of radionuclides and the proposed installation of the heavy ion storage ring TSR from MPIK Heidelberg, that provides excellent opportunities to study challenging cases which could not be addressed at ISOLDE so far ... >

21.03.17: First observation of individual spin quantum transitions of a single antiproton

In the past years, spectroscopy experiments that were based on the observation of quantum phenomena in electron/positron systems enabled sensitive measurements with highest resolution and e.g. led to the development of first optical frequency standards, to precise measurements of the Planck constant, to the most precise measurement of the fine-structure constant, and to a stringent test of the fundamental charge-parity-time (CPT) invariance. In 2012 quantum transitions of a pure antimatter system (antihydrogen) have been observed for the first time.
In order to make comparable observations in the proton/antiproton system, a considerably higher experimental sensitivity is needed. Thus, the application of quantum-transition based spectroscopy schemes is more challenging compared to the electron/positron system. The recent observation of individual spin transitions of a single trapped proton led to a high-precision measurement of the proton magnetic moment with 3.3·10^-9 relative precision in 2015.

In a recently in Physics Letters B published article C. Smorra et al. report on the first non-destructive detection of individual spin transitions of a single antiproton. The experiment was carried out with the BASE Penning-trap system located at the antiproton decelerator facility (AD) of CERN, Geneva.
The BASE Penning-trap system consists of four Penning traps in the horizontal bore of a superconducting magnet. A reservoir trap (RT) serves as interface between the AD and the measurement traps and supplies single particles from the reservoir into the other traps when needed. The precision trap (PT) and the cooling trap (CT) are required for the precision frequency measurements, and efficient cooling of the modified cyclotron mode, respectively. The observation of individual spin quantum transitions was achieved by using the continuous Stern–Gerlach effect in the 5-pole analysis trap (AT) with a superimposed magnetic bottle of B_z = 272(15) mT/mm² generated by the superconducting magnet with field strength B₀ = 1.945 T. This couples the spin-magnetic moment of the stored antiproton to its axial oscillation frequency ν_z.
In the described antiproton experiment, the axial frequency fluctuation of the particle in the magnetic bottle was at 48.1mHz for 96s averaging time. Under these conditions, it could be demonstrated that 92.1% of the spin states detected in the measurement sequence are identified correctly.

To measure the antiproton g-factor with very high precision, the BASE collaboration aims at the application of the challenging double Penning-trap technique, which was successfully demonstrated with a single proton for the first time in 2013 at the University of Mainz. Spin-state initialization with >99.9% fidelity and an average initialization time of 24 minutes were demonstrated in the BASE antiproton experiment. This enables an antiproton double-trap g-factor measurement with high contrast and will allow to reach the intended relative precision on the part-per-billion level (10^-9). This high precision will allow for one of the most stringent tests of charge-parity-time invariance in the baryon sector.

Please read more in the article ... >

18.01.17: Most precise measurement of the g-factor of the antiproton by the BASE collaboration

The observable universe shows no matter-antimatter symmetry but is matter-dominated. However, in particle physics every particle is produced with its corresponding antiparticle and such particle-antiparticle pairs annihilate each other. To explain the evident matter-antimatter imbalance on cosmological scales scientists assume that at the origin of the universe matter particles outnumbered antimatter particles. Therefore, all antimatter particles were destroyed, leaving behind only matter. According to the fundamental Charge, Parity, and Time Reversal (CPT) symmetry of the Standard Model of particle physics the fundamental properties of a particle and its antiparticle are exactly equal. Therefore, the highly precise comparision of the properties of a particle and its antiparticle provides a test of the CPT symmetry and contributes to a better understanding of the matter-antimatter imbalance of our universe. This inspires physicists to invent novel techniques and experimental setups to find minutest differences between the properties of matter and antimatter.

In a recently in "Nature Communications" published article H. Nagahama et al. report on high-precision measurements of the magnetic moment of the antiproton at the Baryon Antibaryon Symmetry Experiment (BASE) at the Antiproton Decelerator (AD) of CERN, Geneva. The experiment aimed at performing a stringent test of the CPT symmetry by comparing the magnetic moments of the proton and the antiproton with high precision. The used spin-flip technique has already been successfully applied to electrons and positrons, however, its application to measure the magnetic moments of the proton/antiproton is much more challenging, since their magnetic moments are 660 times smaller. Thus, the detection of single antiproton spin transitions required a specially designed ultra-strong magnetic bottle with an inhomogeneity of 2.88·10⁵ Tm^-2. This is more than 1000 times stronger than the inhomogeneous magnetic field used in the electron/positron experiments.

Within BASE the proton/antiproton g-factors are determined by measuring the respective ratio of the spin-precession frequency ν_L (Larmor frequency) to the cyclotron frequency ν_c. 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: ν_c² = ν₊² + ν_z² + ν_-². The modified cyclotron frequency ν₊, the axial frequency ν_z, and the magnetron frequency ν_- are measured via image current detection.
The BASE experiment uses a cryogenic Three-Penning trap system, which is mounted in the horizontal bore of a superconducting magnet with field strength B=1.945 T. A cloud of antiprotons is stored in the reservoir trap (RT), which supplies single particles to the co-magnetometer trap (CT) and the analysis trap (AT) when required. The CT is used for continuous magnetic field measurements. The AT is the trap with the strong superimposed magnetic bottle, which is used to measure the cyclotron frequency and the Larmor frequency.

Six direct g-factor measurements of single antiprotons have been performed with a fractional precision of 0.8 p.p.m (10^-6). The new value of 2,7928465(23) is 6 times more precise than the value of the previous best measurement by the ATRAP collaboration at CERN in 2013. It is in agreement with the most precise proton g-factor value of 2,792847350(9) measured at University of Mainz in 2014 and therefore agrees with the fundamental charge, parity, time (CPT) invariance of the Standard Model of particle physics.
The 1000 times more precise proton measurements at Mainz are based on the application of the challenging double Penning-trap technique. The BASE collaboration plans the implementation of this method to further improve the precision in antiproton magnetic moment measurements to the level of a few parts per billion (10^-9).

Please read more in the "Nature Communications" article ... >

Further information also in the press releases of the MPIK , CERN , RIKEN and the Johannes Gutenberg University Mainz .

Further press releases:

10.01.17: High-precision laser spectroscopy of Be⁺ for nuclear structure investigations and many-body QED tests

The investigation of the atomic structure by high-precision laser spectroscopy is of great relevance for the understanding of fundamental physics problems. It e.g. gives detailed insight into ground state properties of short-lived nuclei as the nuclear charge radii which allows to test and improve nuclear structure models. Moreover, the combination of laser spectroscopy with theoretical calculations is an indispensable tool to explore many-body quantum electrodynamics in weak and strong fields.

In a recently in Applied Physics B published article A. Krieger et al.. report on high-precision measurements on stable and radioactive beryllium isotopes at the COLLAPS collinear spectroscopy beam line at CERN-ISOLDE in Geneva. These investigations aimed to obtain more detailed information on the nuclear structure and to provide an important test for bound-state QED calculations in three-electron systems.
For this purpose, the technique of conventional collinear laser spectroscopy was further developed and combined with a frequency comb to provide high-precision measurements of the transition frequencies of the D1 and D2 transitions 2s²S_1/2 → 2p²P_1/2,3/2 in Be⁺. To overcome the accuracy limitations of conventional collinear laser spectroscopy due to the uncertainty in ion acceleration voltage determination the quasi-simultaneous excitation by a collinear and an anticollinear laser beam was introduced. Thus, two frequency-stabilized dye laser systems were required which delivered UV beams that superposed the beryllium ion beam in opposite directions.
The isotope shifts were obtained in two beam times as difference of the transition frequency of the Be isotope of interest and the stable reference isotope ⁹Be. The use of a frequency comb enabled the determination of the laser frequencies with the required relative accuracy better than 10⁻⁹. This yielded the isotope shifts with an accuracy better than 10⁻⁵.
From its isotope shift the corresponding change in the mean-square nuclear charge radius of the isotope can be derived. Hence, the accurate isotope shifts led to a precise determination of the nuclear charge radii along the beryllium isotopic chain ^7,10−12Be relative to the stable isotope ⁹Be.

Additionally, the 2p fine-structure splitting as a function of the atomic number along the beryllium isotopic chain was extracted from the precise transition frequencies. Only recently the fine-structure splitting in three-electron atoms became calculable with high precision. Thus, the measurements also allowed a test of such high-precision bound-state QED calculations. The experimental results confirmed first calculations for the Z=4 three-electron system of Be⁺. The measured mass dependence of the fine-structure splitting of ^7,10−12Be also provided a check of the so-called splitting isotope shift (SIS) which can be calculated theoretically to very high accuracy.

Please read more in the article ... >