Distinguished EPJ Referees

EPJ D Highlight - Buckyballs release electron-positron pairs in forward directions

Impacting positrons release positronium from C60. Credit: Benjah-bmm27, public domain.

Theoretical calculations reveal that when impacted by positrons of particular energies, spherical nanoparticles release unstable electron-positron pairs, with signals dominating in the same direction as the incoming positrons.

When electrons collide with positrons, their antimatter counterparts, unstable pairs can form in which both types of particle orbit around each other. Named ‘positronium’, physicists have now produced this intriguing structure using a diverse range of positron targets – from atomic gases to metal films. However, they have yet to achieve the same result from vapours of nanoparticles, whose unique properties are influenced by the ‘gases’ of free electrons they contain in well-defined, nanoscopic regions. In new research published in EPJ D, Paul-Antoine Hervieux at the University of Strasbourg, France and Himadri Chakraborty at Northwest Missouri State University, USA, reveal the characteristics of positronium formation within football-shaped nanoparticles, C60, for the first time. At specific positron impact energies, they show that positronium emission dominates in the same direction as the incoming antiparticles.

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EPJD has appointed a new Editor-in-Chief

 Sylwia Ptasinska

EPJ is pleased to announce that Prof Sylwia Ptasinska of the University of Notre Dame, USA has been appointed as an Editor-in-Chief for EPJ D, effective January 2020. She will be responsible for the plasmas section of the journal, and succeeds Prof Holger Kersten, who steps down after five years in the role. A faculty member at Notre Dame since 2010, her research focuses on understanding the variety of processes occurring in heterogeneous systems, including plasmas and interfaces. Though experimental investigations in her laboratory address fundamental questions, the goal of her team is to apply this research in areas such as energy, medicine, and industry. Sylwia Ptasinska is a member of the Executive Committee for the Gaseous Electronics Conference (GEC) and is also the local chair of the next POSMOL meeting. She has been a member of the Editorial Board for EPJD since 2015.

EPJ D Highlight - Laser-based prototype probes cold atom dynamics

Apparatus for cold atom inertial sensing

A new prototype design doubles the frequencies of widely used telecommunications lasers to study the dynamics of cold atoms while in space.

By tracking the motions of cold atom clouds, astronomers can learn much about the physical processes which play out in the depths of space. To make these measurements, researchers currently use instruments named ‘cold atom inertial sensors’ which, so far, have largely been operated inside the lab. In new work published in EPJ D, a team of physicists at Muquans and LNE-SYRTE (the French national metrology laboratory for time, frequency and gravimetry) present an innovative prototype for a new industrial laser system. Their design paves the way for the development of cold atom inertial sensors in space.

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EPJ D Highlight - Colliding molecules and antiparticles

Graph showing the transfer of rotational momentum between positrons and molecules of methane (CH4)

A new theoretical study of the interaction between positrons and simple tetrahedral and octahedral molecules agrees with experimental work and could have useful implications for PET scanning techniques.

Antiparticles - subatomic particles that have exactly opposite properties to those that make up everyday matter - may seem like a concept out of science fiction, but they are real, and the study of matter-antimatter interactions has important medical and technological applications. Marcos Barp and Felipe Arretche from the Universidade Federal de Santa Catarina, Brazil have modelled the interaction between simple molecules and antiparticles known as positrons and found that this model agreed well with experimental observations. This study has been published in EPJ D.

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EPJ D Highlight - Proton-hydrogen collision model could impact fusion research

Models could aid nuclear fusion projects. IAEA Imagebank [CC BY-SA 2.0 (https://creativecommons.org/ licenses/by-sa/2.0)]

A new theoretical model predicts how protons will collide with hydrogen atoms which have been excited to higher energy levels, over a wide range of impact energies

The motions of plasmas may be notoriously difficult to model, but they can be better understood by analysing what happens when protons are scattered by atoms of hydrogen. In itself, this property is characterised by the size of a particular area surrounding the atom, known as its ‘cross section’. In new research published in EPJ D, Anthony Leung and Tom Kirchner at York University in Canada used new techniques to calculate the cross sections of atoms which have been excited to higher energy levels. They analysed the behaviour over a wide range of impact energies.

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EPJ D Highlight - Retrieving physical properties from two-colour laser experiments

Extracting ionisation yields following ultrafast interactions.

Useful information about ultrafast light-matter interactions is buried deep in the signals produced by two-colour pump-probe experiments, and requires sophisticated techniques to disentangle it.

When photons of light interact with particles of matter, a diverse variety of physical processes can unfold in ultrafast timescales. To explore them, physicists currently use ‘two-colour pump-probe’ experiments, in which an ultrashort, infrared laser pulse is first fired at a material, causing its constituent electrons to move. After a controllable delay, this pulse is followed by a train of similarly short, extreme-ultraviolet pulses, ionising the material. By measuring the total ionisation following the pulses along with the resulting electron energy spectra, physicists can theoretically learn more about ultrafast, light-matter interactions. In new research published in EPJ D, an international team of physicists, led by Eric Suraud at the University of Toulouse, discovered that these signals are in fact dominated by the less interesting interplay between electrons and the initial infrared laser. They show that more useful information is buried deeper within these signals, and requires sophisticated techniques to disentangle it.

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EPJ D Highlight - Modelling ion beam therapy

https://commons.wikimedia.org/wiki/ File:Hadrontherapy.jpg Anna.puliaieva [CC BY-SA 4.0 (https://creativecommons.org/ licenses/by-sa/4.0)]

Recent analysis shows precisely how beams of charged particles transfer their energy to water, which has important implications for how these beams are targeted in ion beam cancer therapy.

Hadron beam therapy, which is often used to treat solid tumours, involves irradiating a tumour with a beam of high-energy charged particles, most often protons; these transfer their energy to the tumour cells, destroying them. It is important to understand the precise physics of this energy transfer so the tumour can be targeted precisely. Pablo de Vera of MBN Research Center, Frankfurt, Germany and co-workers in the Universities of Murcia and Alicante, Spain, have produced a consistent theoretical interpretation of the most accurate experimental measurements of ion beams energy deposition in liquid water jets, which is the most relevant substance for simulating interactions with human tissue. Their work is published in EPJ D.

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EPJ D Highlight - Fragmenting ions and radiation sensitizers

Mass spectrum of 5-fluorouracil showing ions produced by impact with high-energy electrons.

A new study using mass spectrometry is helping piece together what happens when DNA that has been sensitized by the oncology drug 5-fluorouracil is subjected to the ionising radiation used in radiotherapy.

The anti-cancer drug 5-fluorouracil (5FU) acts as a radiosensitizer: it is rapidly taken up into the DNA of cancer cells, making the cells more sensitive to radiotherapy. However, little is known about the precise mechanism through which radiation damages cells. A team of scientists led by Peter van der Burgt at the National University of Ireland in Maynooth, Ireland have now used mass spectrometry to shed some light on this process; their work was recently published in EPJ D. A full understanding of this process could ultimately lead to new ways of protecting normal tissues from the radiation damage caused by essential cancer treatments.

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EPJ D Highlight - Enabling longer space missions

A Hall thruster in operation. Image by the user Dstaak at Wikimedia Commons .

Hall thrusters, which are already used to propel spacecraft and satellites on long missions, could be used for even longer ones if models for minimising surface erosion were taken into account.

The 50th anniversary of the Apollo 11 moon landing has reignited interest in space travel. However, almost any mission beyond the moon, whether manned or unmanned, will require the spacecraft to remain fully operational for at least several years. The Hall thruster is a propulsion system that is often used by craft involved in long missions. A recent study by Andrey Shashkov and co-workers at the Moscow Institute of Physics and Technology, Russia has shown how the operating lives of these systems can be further extended; their work was recently published in EPJ D.

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EPJ D Highlight - Quantum momentum

Schemes for measuring time-of-flight in classical mechanics (top) and quantum mechanics (bottom). In quantum mechanics, the classical particle is represented by a wave packet. Values of X indicate position and t time.

A new quantum-mechanical model has been developed that allows the momentum of quantum particles to be measured using a variant of the classical time-of-flight.

Quantum mechanics is an extraordinarily successful way of understanding the physical world at extremely small scales. Through it, a handful of rules can be used to explain the majority of experimentally observable phenomena. Occasionally, however, we come across a problem in classical mechanics that poses particular difficulties for translation into the quantum world. A new study published in EPJ D has provided some insights into one of them: momentum. The authors, theoretical physicists Fabio Di Pumpo and Matthias Freyberger from Ulm University, Germany, present an elegant mathematical model of quantum momentum that is accessible through another classical concept: time-of-flight.

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