Editorial on the Research Topic: Modern Advances in Direct Reactions for Nuclear Structure

Alan H. Wuosmaa^1*Sean J. Freeman²Benjamin P. Kay³

• ¹University of Connecticut, Storrs, United States
• ²EP Department, CERN, Geneva CH-1211, Switzerland
• ³Argonne National Laboratory, Lemont, United States

The study of nuclei far away from stability, with large imbalances between the number of protons 13 and neutrons, is a central topic in modern nuclear physics today. New experimental facilities now 14 provide access to nuclei that were previously inaccessible, revealing new and unexpected behaviors. 15Older models of nuclear structure fail to explain the properties of such nuclei, spurring major new 16 theoretical developments. Key to testing these new theories are new data. Direct reactions have, for 17 decades supplied the underpinnings of nuclear structure models. The necessity to use radioactive 18 beams from the new facilities introduces a host of technical difficulties, including experiment count 19 rates, and complicated kinematics that adversely affect experimental resolutions. 20In this collection, we draw together works from experimentalists and theorists that show just some of 21 the many new developments in detector and spectrometer technology, experimental approaches, and 22 structure and reaction theory to confront the challenges that will drive studies in the field into the 23 next decade and beyond. These collected papers illustrate how modern experimental techniques can 24 yield data on nuclei far from stability, and how modern theory can help understand those data. 25 New instrumentation is key to addressing the physics of exotic isotopes, and modern methods can 26 access data that were previously inaccessible. Ayyad and collaborators describe advances using the 27Active-Target Time-Projection Chamber (ATTPC), a highly sensitive device that is both a thick 28 gaseous target and a detector capable of untangling complex multi-particle final states with good 29 resolution. The ATTPC permits sensitive measurements with beams of very low intensity, extending 30 the reach of direct-reaction studies. Silicon detectors have long been a workhorse for direct-reaction 31 measurements, and new, complex multi-segmented arrays play an important role. In his paper, Pain 32 describes the ORRUBA, GODDESS and associated instruments used for a wide variety of 33 measurements, for both nuclear structure and nuclear astrophysics. The coupling of the silicon-34 detector array with large, modern arrays of germanium gamma-ray detectors adds to the capabilities. Many direct-reaction studies involve nucleon transfer between a light ion, typically 1,2 H or 3,4 He, and 36 a heavier nucleus; with unstable nuclei, the heavy nucleus is the beam, and the light species is often 37 in the form of a solid plastic or metal foil. Such targets have impurities that can complicate the 38 measurements, but a high-purity gas-jet target that can alleviate such problems. Chipps describes a 39 powerful alternative to solid targets, the JENSA device, which has facilitated a number of high-40 resolution studies of hydrogen-and helium-induced reactions. 41With new instruments come exciting new measurement techniques. Sobotka and Charity have 42 pioneered a groundbreaking approach to studying a wide range of nuclear phenomena using 43Invariant-Mass Spectroscopy. They apply this powerful method to light proton-rich nuclei to study 44 the nature of the proton dripline, and exotic unbound systems beyond that dripline. The data 45 characterize unbound states that can help understand the shell structure of exotic nuclei. Another 46 aspect key to the evolution of shell structure in neutron-rich systems is the nature of the spin-orbit 47 interaction. In her contribution, Chen examines the trends in spin-orbit splitting for neutron-rich 48 silicon and tin nuclei. Her analysis can distinguish between effects of the spin-orbit potential itself, 49 and from the wave functions of weakly bound nucleons. The data she describes for the 32 Si(d,p) 33 Si 50 reaction were obtained using the SOLARIS spectrometer, one of three spectrometers that exist 51 worldwide with a novel design based on a solenoidal magnetic field. The helical motion of the light 52 ions emitted in two-body reactions within the magnetic field provide an innovative way to avoid 53 kinematical factors that degrade the Q-value resolution obtained when making such measurements 54 with radioactive beams in inverse kinematics with more conventional techniques. 55The data from direct-reaction measurements do not immediately yield information about nuclear 56 structure; they must be understood in the context of theoretical analyses of the reactions.