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Historical Perspectives on Contemporary Physics

EPJ A Highlight - Mechanisms of two-proton emission seen in three-body correlations

Sequential two-proton decay of the 16Ne Er=7.57 MeV state. The fractional energy distribution (left) gives resonance energy in 15F while the angular distribution (right) determines Iπ of the initial state.

Hitherto three-body correlations between decay products of nuclear resonances, unstable to the emission of two neutrons have been a very effective tool in the analysis of GSI-experiments on 5H, 10He, 13Li, and 14Be. Here the first report is given about the mechanisms for two-proton emission from states in 16Ne, representing the presently most complete study of this nucleus. One-neutron knockout from 17Ne populated the 16Ne(g.s.) (Er=1.39 MeV, Γ=0.08 MeV) above the 14O+p+p threshold, and resonances at Er=3.22 MeV and 7.57 MeV. The decay mechanisms were revealed analysing three-body energy correlations in the 14O+p+p system. It was found that the 16Ne(g.s.) undergoes a democratic three-body decay. In contrast to this, the 16Ne(21+) state emits protons through the 15F(g.s.) sequentially. The decay of 7.57 MeV state is well-described assuming emission of a proton from the d5/2 shell to 15F(5/2+), which decays by d5/2 proton emission to 14O(g.s.). By using R-matrix analysis and mirror symmetry this state was unambiguously identified as the third 2+ state in 16Ne.

EPJ A Highlight - Achieving high resolution in binary nuclear reactions with outgoing fast neutrons – at last!

To date, the two-nucleon pick-up and stripping counterparts of the (p,t) and (t,p) reactions, the (3He,n) and (n, 3He) reactions, have been poorly investigated due to the difficulty in performing high-resolution measurements of fast-neutron energies. The best time-of-flight (3He,n) measurements report resolutions not better than 250 keV. This lack of experimental resolution has hindered a full understanding of the role of proton pairing in nuclei.

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EPJ A Highlight - Modern three-body forces make neutron stars collapse

Density profile of the collapsed state of 10000 neutrons in the X-Y-plane along the symmetry axis Z=0 (schematic illustration). Polarized neutrons, which interact through incorrect three-body forces, concentrate in small spheres separated by 0.9 fermi. © Dmitry K. Gridnev et al.

Nuclear systems ranging from light nuclei to massive neutron stars can be well described by nucleons interacting through two-body and three-body forces. From electrostatics we know that two identical uniformly charged spheres repel at any distance but the repulsion disappears when the spheres completely overlap. Similarly, in some modern expressions of nuclear three-body force it is assumed that the nuclear repulsion between the three nucleons is zero when they occupy the same position in space.

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EPJ A Highlight - Photocouplings at the Pole from Pion Photoproduction

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Тelected fit results for the beam asymmetry Σ (left) and the double polarization E (right)

While the strong force is well understood at high energies in terms of perturbative QCD, the precise mechanism responsible for the confinement of quarks and gluons in color-neutral hadrons at low energies remains a mystery to date. The intermediate energy region is characterized by rich and complex spectra of excited baryons and mesons. Its phenomenology provides a key to our understanding of the fundamental properties of matter.

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EPJ A Highlight – COMPASS: Radiative widths of the a2(1320) and π2(1670) mesons

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Intensity of the 2-+ partial-wave in π^- γ→π^- π^- π^+, interpreted as radiative width of the π2(1670).

Radiative transitions are among the most important and insightful processes for the investigation of atomic, nuclear and hadronic systems. They reveal the electromagnetic substructure of the involved particles. The a2(1320) meson is known since the 1980s to decay radiatively with a branching of about 0.3% into a pion and a photon. Theoretically this can be linked, for example through the vector meson dominance model, to the main hadronic decay channels.

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EPJ A Highlight - MINOS: A vertex tracker coupled to a thick liquid-hydrogen target for in-beam spectroscopy of exotic nuclei

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View of the MINOS device inside the DALI2 gamma array at the RIKEN Radioactive Isotope Beam Factory.

MINOS is a new apparatus dedicated to in-beam nuclear structure experiments with low-intensity exotic beams at energies above 150 MeV/nucleon.

It is intended to provide increased luminosity compared to standard solid-target experiments in hydrogen-induced studies, while simultaneously improving experimental resolution. This article exposes the concept of the device developed at the CEA in France and reviews in detail the associated recent technical advances. MINOS is composed of a thick finger-shaped liquid hydrogen target, from 50 to 200 mm thick, combined with a compact time projection chamber serving as a vertex tracker, the first of its kind in low-energy nuclear physics. This innovative setup offers access to the first spectroscopy of a new range of very exotic nuclei beyond our current reach. An exciting program on the search for new 21+ states in neutron-rich even-even nuclei, spectroscopy of unbound oxygen nuclei and di-neutron correlations in borromean nuclei will be performed with MINOS at the RIKEN Radioactive Isotope Beam Factory in Japan over the next few years. MINOS is funded by the European Research Council.

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EPJ A Highlight - Prompt x-rays emitted in neutron-induced fission help unveil the evolution of fission fragment charge yields as a function of incident neutron energy

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Charge distribution determined from the x-ray yield measurements (symbols) for different incident neutron intervals: threshold to 6MeV ~3MeV, from 6 to 11MeV ~8MeV, from 11 to 20MeV ~14MeV, from 20 to 50MeV ~32MeV and from 50to 400MeV ~180MeV. For more detail see text.

Nuclear fission is accompanied by the prompt emission of neutrons, gamma rays and x-rays. It has been known since the sixties that fission prompt x-rays originate essentially as a consequence of the internal conversions occurring in the prompt gamma deexcitation cascades of fission fragments.

This work presents for the first time a measurement of the prompt fission x-ray yields in 238U(n,f) for average incident neutron energies ranging from 3 to 200 MeV. These results provide new information on fission fragment deexcitation and allow testing the current knowledge of fission fragment nuclear structure. These results provide also a means to investigate the evolution, as a function of incident neutron energy, of fission fragment charge yields and elemental prompt x-ray emission probabilities.

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EPJ A Highlight - Dissecting Deuteron Compton Scattering I: The Observables with Polarised Initial States

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Sensitivity of a double-polarisation-asymmetry observable to the E1-M2 spin polarisability.

The electromagnetic polarisabilities of the nucleons characterise their responses to external fields. The simplest are the electric and magnetic polarisabilities that describe the induced dipole moments. For spin-1/2 particles there are also four spin polarisabilities, analogous to rotations of the polarisation of light by optically active media. The best experimental window on them is Compton scattering of photons, which has provided good determinations of the electric and magnetic polarisabilities of the proton. Future experiments with polarised protons will give access to its spin polarisabilities. In contrast, much less is known of about the neutron since it does not exist as a stable target. Nonetheless, its properties can be obtained from Compton scattering on light nuclei, most notably the deuteron -- a weakly bound proton and neutron. A new generation of experiments is planned to provide beams of polarised photons on targets of polarised deuterons. If the spins of the final particles are not observed, there are 18 independent observables. This work provides, for the first time, the complete set of these, which will be needed for the experimental analyses. More importantly, it also examines their sensitivities to the various polarisabilities, which will be crucial for the design of the experiments.

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EPJ A has a new Editor in Chief for experimental physics

Nicolas Alamanos new Editor-in-Chief of EPJ A as of 1 January 2013

From January 2013 Nicolas Alamanos succeeds Enzo De Sanctis as Editor in Chief of EPJ A for the experimental physics section.

Professor Alamanos is Deputy Director of the Institute of Research into the Fundamental Laws of the Universe (IRFU) and Research Director at CEA Saclay working in the domain of fundamental research in Nuclear Physics. During his long scientific career, he has served on many scientific and program advisory committees and has occupied different managerial positions. Most notably he has been president of GANIL’s scientific council and director of Saclay Nuclear Physics Division. He is a member or evaluator of many national committees – ARISTEIA (GRECE), FRS-FNRS (Belgium), ANR (France). He is currently a member of the GANIL/SPIRAL2 scientific council, of GANIL’s program advisory committee, and scientific counselor of the European program “CEA-Euro talents” in the domain of high energy physics and physics of the universe.

In addition to his various scientific and administrative duties, Professor Alamanos has always been very active editorially: beyond having been a member of the editorial board of EPJ A for many years, he is the Editor of the Scholarpedia Encyclopedia of Nuclear Physics.

EPJ A Review - Coherent Investigation of Nuclear Data at CEA DAM: Theoretical Models, Experiments, and Evaluated Data

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Figure 1: (Bauge et al’s Fig. 46). Jezebel (a plutonium sphere) reactivity induced by components of the evaluated BRC09 and ENDF/B-VII.0 239Pu file, demonstrates that two different evaluated data sets predict the same k-eff criticality for different underlying reasons; one or more of them (probably both) are in need of improvements.

Dr. Eric Bauge et al describe a body of work accomplished by the CEA/DAM. Their goal is to determine accurate nuclear reaction cross sections for use in neutron transport codes. This work integrates theory and modeling, experiment, computer simulation, and statistical analysis. It involves researchers who thrive on multidisciplinary work, and who are motivated to achieve realistic simulation predictions in nuclear technology applications. Not only has the group succeeded in creating databases of accurate cross sections, but in every aspect of the work significant progress has been made in advancing our understanding of the underlying nuclear physics.

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Editors-in-Chief
M. Eckert and J.D. Wells
ISSN (Print Edition): 2102-6459
ISSN (Electronic Edition): 2102-6467

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