Extremely accurate experiments can be conducted when particles and nuclei are delicately guided using electric and magnetic fields. A recent conference surveyed the range of such research under way around the world.
The various techniques of mass spectrometry now provide atomic and nuclear mass measurements of such precision that, as well as enabling theoretical nuclear models to be stringently tested, they also allow the testing of higher-order effects in quantum electrodynamics (QED) – the most precise theory in existence.
Progress in the field was highlighted by the Atomic Physics at Accelerators (APAC) 2000 conference on mass spectrometry held at the Institut d’Etudes Scientifiques de Cargèse, Corsica. APAC2000 was the second of a series of three Euroconferences on atomic physics at accelerators. Completing the triad begun by APAC99 (held near Mainz), which covered laser spectroscopy, will be APAC2001 (to be held in Aarhus), which will cover spectroscopy with highly charged ions. The APAC conferences were initiated by Heinz-Jürgen Kluge, director of the Atomic Physics Division of the GSI Laboratory, Darmstadt, and they are interdisciplinary, linking the study of the nucleus with its influence on the atomic electron cloud.
Commitment to atomic masses
Atomic mass data are systematically evaluated because their impact on neighboring masses via reaction and decay energies can be considerable. This important job has long been carried out by Aaldert H Wapstra, now retired from NIKHEF, and Georges Audi (CSNSM-Orsay), who together periodically produce the benchmark Atomic Mass Evaluation publication.
A large part of the funding for APAC2000 was secured for the training and mobility of young European students and researchers, so a tutorial session of six lectures was included. It marked the 78th birthday of Aaldert Wapstra, honouring his unwavering commitment to the field of atomic masses.
The first lecture covered the evaluation of atomic masses (Georges Audi, CSNSM-Orsay), and was followed by an overview of the experimental techniques for such measurements (Alinka Lépine-Szily, Sao Paulo). Penning traps – magnetic storage devices that confine charged particles and determine their mass by making them “dance” at their cyclotron frequency – now dominate the field in precision measurement.
Atomic mass provides important information about nuclear structure via the binding energy. The correct treatment of the nucleon-nucleon interaction is required to make a correct calculation. Unfortunately, masses can still be measured with more than 100 times the accuracy that calculations can provide. Some of the reasons were explained by Michael Pearson (Montreal), who described the quest for a microscopic nuclear mass formula. Measurements are even accurate enough to warrant the inclusion of the atomic binding energy, thus requiring theorists to look beyond QED (Gerhard Soff, Dresden). The role of nuclear masses in explosive nucleosynthesis (Stéphane Goriely, UL Brussels) is critical, and the fact that most of the nuclides involved cannot be produced in the laboratory forces a dependence on nuclear models.
Accurate mass measurements also provide stringent probes of the underlying fundamental interactions – a domain that was once solely the concern of those conducting experiments with the world’s largest particle accelerators. The energy of a certain type of beta-decay (super-allowed) is sensitive to up-down quark transitions, and accurate measurements provide important constraints (John Hardy, Texas A&M).
The tutorial session was concluded by Aaldert Wapstra, who shared some of his memories of a long career dedicated to studying atomic masses.
Worldwide effort
Review talks from the numerous groups involved in mass measurement worldwide – all of whom were represented – revealed not only a variety of techniques but also a rich harvest of new results.
In Europe, several groups are pursuing mass measurements for nuclear physics: at GANIL in France the energy loss spectrometer SPEG (Hervé Savajols, GANIL-Caen) is being used to study the weakening of shell structure far from stability (Fred Sarazin, Edinburgh), and the CSS2 and CIME cyclotrons (Marielle Chartier, Bordeaux) are being used for the study of isospin symmetry in nuclei (Anne-Sophie Lalleman, GANIL-Caen); at GSI in Germany the Experimental Storage Ring (ESR) is being used in both Schottky pick-up mode (Yuri Litvinov, GSI Darmstadt, and Guenther Loebner, LMU-Munich) and isochronous mode for shorter half-lives (Marc Haussman, GSI-Darmstadt). At ISOLDE-CERN such studies are being made using the radiofrequency spectrometer MISTRAL (Dave Lunney, CSNSM/Paris Sud) and the Penning trap spectrometer ISOLTRAP (Georg Bollen, ex ISOLDE, now Michigan State), where a variety of physics themes are explored, notably isospin symmetry and the weak interaction (Frank Herfurth, GSI).
Sophisticated calculations
SMILETRAP in Stockholm (see following article) uses a Penning trap for measuring masses of highly charged stable nuclides (Tomas Fritioff, Stockholm) to test sophisticated atomic binding energy calculations for various atomic charge states. Another key SMILETRAP goal is a fundamental test concerning neutrinoless double beta decay, where an accurate mass difference can help in determining whether neutrinos are their own antiparticles or not. At the GSI on-line facility, masses come as a by-product of extensive nuclear spectroscopy studies (Ernst Roeckl), and similarly at the Swedish Studsvik reactor facility (Konstantine Mezilev, NPI-Gatchina).
Mass-yielding spectroscopy is very important for short-lived isotopes that are inaccessible by other techniques, as shown by a report (Mark Huyse, KU Leuven) on neutron-deficient polonium isotopes measured at SHIP-GSI and at RITU-JYFL in Finland. Although some nuclides are known to be “schizophrenic”, these measurements indicate that some species may have not just two but three shape “personalities”.
In North America, masses are also included in large data harvests of alpha-particle and proton emission from Argonne (Cary Davids) where these nuclides are formed offshore of the island of nuclear stability, then shedding protons to beach themselves and decay along the valley of stability.
Similarly, beta-particle endpoint measurements at Yale’s Wright Laboratory (Daeg Brenner, Clark) provide masses of proton-rich nuclides of importance to the astrophysical rapid proton capture process (Ani Aprahamian, Notre Dame). Argonne will soon initiate an ambitious programme for studying nuclides of interest for testing the fundamental properties of weak interactions, using the Canadian Penning Trap (Guy Savard, Argonne).
An important aspect of using mass values for the better definition of physical constants is pursued at the University of Washington in Seattle (Robert Van Dyck) and at MIT (Simon Rainville). These groups give mass measurements of record accuracy – parts per trillion. A Harvard group working at CERN has used the excellent environment of Penning traps for an ultra-precise comparison of the proton and antiproton masses – an appetizer for the imminent synthesis and study of antihydrogen (Gerald Gabrielse).
A further high-precision Penning trap result, on bound-electron magnetism (Guenther Werth, Mainz), puts the ball back in the court of Penning trappers measuring masses, because the electron mass uncertainty now dominates the error of this measurement. Complementing the masses of these fundamental particles was that of the pion (Guenther Borchert, IKP-Julich), which is of importance for cosmology.
New standards
The very mass unit itself, the kilogram – the last fundamental standard defined by an artefact – was the subject of a prizewinning presentation (Annette Paul, PTB-Braunschweig) on the AVOGADRO project to redefine the kilogram using silicon atoms counted in a lattice.
Atomic masses can also be determined via nuclear reactions with heavy ions (Yuri Penionzhkevich, JINR-Dubna) and with neutrons (Cyriel Wagemans, Gent). New schemes were presented for measurement techniques using tabletop storage rings (Hermann Wollnik, Giessen) and small cyclotrons (Oleg Kozlov, JINR-Dubna).
The sessions on measurements and techniques were complemented by reports on advances in theory. On the atomic physics side, higher-order QED corrections to atomic binding energies dominate the overall errors. These calculations are among the most precise possible (Vladimir Shabaev, St Petersburg; Eva Lindroth, Stockholm; and Paul Indelicato, LKB-Paris). On the nuclear physics side, more affordable computing power opens up the scope and validity of mass predictions. Recent advances on the nuclear theory front include mean-field calculations (Paul-Henri Heenen, UL Brussels), including a separable monopole Hamiltonian (Jirina Rikovska, Oxford), the Monte Carlo shell model for light nuclides (Takaharu Otsuka, Tokyo) and semi-empirical shell model calculations for superheavy nuclides (Nissan Zeldes, Jerusalem).
Future projects
The conference ended with a look to future projects – all involving ion traps – for the DRIBS facility (Nicolai Tarantin, JINR-Dubna) and GSI’s new HITRAP for atomic physics (Wolfgang Quint) and SHIPTRAP for nuclear physics (Gerrit Marx). Some of these are covered by European Research and Training networks, notably EUROTRAPS and EXOTRAPS (Ari Jokinen, Jyvaskyla), and have begun to yield promising results.
The concluding speaker, Catherine Thibault (CSNSM-Orsay), was one of the pioneers of direct on-line mass measurements via mass spectrometry. She lauded the tremendous progress made in recent years, notably thanks to Penning traps.
Abstracts of the conference presentations, as well as all of the posters, are available at the conference Web site. The conference proceedings will be published this year in the journal Hyperfine Interactions.
APAC2000 was funded by the EU Program for Training and Mobility of Young Researchers. Additional support came from the French Institut National de Physique Nucléaire et de Physique des Particules (IN2P3), from the German GSI heavy ion laboratory and from the French host institute, Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, at the Université de Paris Sud, Orsay.
