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News from the Nuclear Science Division at Berkeley Lab
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Nuclear Science Division Newsletter
In this issue:April, 2016

Magnesium nuclei gain inertia

Art McDonaldSeven GRETINA detector modules of arranged in the close-packed configuration. The photo, taken during the NSCL campaign, shows the front face of each module as viewed from the target position.

The study of nuclear structure and reactions is a key to understanding the origin of the elements and how complex many-body quantum systems are organized. Much of what we know about nuclei comes from measurements of excited states. With the state-of-the-art g-ray tracking array, GRETINA, it is now possible to measure excitation spectra in increasingly exotic, short-lived nuclei, which exist far from the known stable isotopes and possess unusual ratios of neutrons to protons.

Most exotic, neutron-rich nuclei are produced and studied using beams of very short-lived isotopes travelling at about 30-40% the speed of light. The power of GRETINA to study exotic nuclei comes from a combination of good energy and position resolution and high efficiency. The position sensitivity is key to combating the Doppler broadened energies of g rays emitted in flight and to obtain good energy resolution while also maintaining high efficiency.

GRETINA’s unique capabilities were used in a recent experiment carried out at the National Superconducting Cyclotron Laboratory to measure the spectrum of rotational states in the neutron-rich nucleus 32Mg. Three strong transitions are seen at 886, 1438, and 1773 keV, corresponding to the 2+, 4+, and 6+ rotational band states. The decrease in the energy spacing between the transitions corresponds to an increasing moment of inertia.  This is a clear signal that the internal structure of the nucleus is changing with increasing spin. By comparing to theory, and noting that the moment of inertia reaches the rigid-body value, we can quantify how the nucleon “wavefunction” changes with increasing spin. Surprisingly, the increased spin quenches the superfluid-like nuclear pairing correlations by a factor of about 2.

graph(a) Gamma-ray spectrum of the 32Mg rotational band detected with GRETINA, (b) Background subtracted spectrum of Gamma-ray in coincidence with the 1773 keV, 6+ transition (red shaded region). (c) Similar to (b), but now in coincidence with a “background" region. The 886 keV and 1438 keV transitions are visible in the 1773 keV gated spectrum, but not in the “background" gated spectrum. (d) Experimental excitation energies for the known levels in 32Mg, including the rotational band states observed in this work (bold lines), together with theoretical predictions (SDPF-U-MIX ).

SNO+ begins commissioning

Art McDonald SNO+ partially filled with water.

After successfully repairing a leak in the cavity liner, the SNO+ neutrino detector is now gearing up for data. The experiment is the successor to the Sudbury Neutrino Observatory, in which NSD played a leadership role. Along with upgrades to the readout and electronics, the vessel will be filled with ~780 tons of liquid scintillator. The low energy threshold and ultra-low levels of radioactive contamination open up a wide research program (solar neutrinos, reactor antineutrinos, geoneutrinos, supernova neutrinos and nucleon decay). The primary physics goal is to search for neutrinoless double beta decay: an extremely rare (half-lives >1025 years) and still undetected decay possible only if the neutrino is a Majorana fermion (its own antiparticle).

The detector will be filled with water by summer for an initial data taking phase. Scintillator fill begins in the fall.  Background levels in the scintillator will be characterized before introducing 0.5% of natural Tellurium (1300 kg of target isotope Te-130). The Q-value for the process is 2.53 MeV, a region in which radioactive backgrounds can be rejected at high efficiency using coincidence tagging.  The low background and high loading fraction results in a highly competitive sensitivity of 1.9*1026 years at 90% CL with 5 years of data.  The search for double beta decay commences in 2017. Plans are underway to significantly increase the isotopic mass with the goal of covering the inverted hierarchy neutrino mass parameter space.

Art McDonaldUC Berkeley graduate student Ben Land examines the Cherenkov-light source during construction.

The LBNL group has significant roles in SNO+ including leadership of the international analysis effort (PI Gabriel Orebi Gann), as well as heading up the PMT calibration group (postdocs Freija Descamps and Javier Caravaca) and spearheading DAQ development (Freija Descamps). The team has also designed and constructed a new calibration source, which will provide a stable source of Cherenkov light that can be deployed in scintillator in order to deconvolve the scintillator light yield from PMT response.  This is a critical step in understanding the detector response and energy resolution.

EIC Planning comes to Berkeley

Some 120 plus participants from the U. S., Europe, Asia and Australia gathered in Berkeley on Jan. 6th - 9th to discuss plans for a future electron-ion collider (EIC).   Buoyed by the strong recommendation in the recent U.S. NSAC Long Range Plan for Nuclear Physics, participants discussed strengthening and/or broadening the physics case, detector and accelerator design, and forming an Electron-Ion Collider Users Group (EICUG) to help coordinate planning as the EIC moves toward reality.

The meeting began with overviews of EIC physics, detector and accelerator design status.  This was followed by presenations from NSAC chair Don Geesaman (Argonne) and DOE Associate Director of Science for Nuclear Physics Tim Hallman, who discussed the status of EIC in the nuclear physics community and the DOE respectively.   The prospects for partial foreign funding were introduced by Rolf Ent (Jefferson Lab), who surveyed models for international funding, using as examples, the German HERA collider, the FAIR project (also in Germany) and the U.S. SURF facility for neutrino and dark matter physics.  This was followed by formal and informal comments by physicists from the U. K., France, Italy, Germany and Japan. 

We also heard about the status of the two EIC designs: eRHIC at Brookhaven, and JLEIC (formerly MEIC) at Jefferson Lab and on straw-man designs for detectors focused on these designs.   Other talks covered multiple aspects of the planned EIC physics program.    Because of the intense interest, organizers were forced to include on session of parallel sessions to accommodate the many requested talks.

The formation of an EIC users group was another focus of the meeting.   There was considerable discussion about both the role and the format of the group.  Would it be a coordinating group, or would it play a more central role in detector selection?  To what extent should accelerator physicists and theorists be involved?  How can we ensure that both small and large institutions, including the two possible host laboratories are appropriately represented?

Of course, the workshop included an outreach event: a well-attended public lecture by Nobel Prizewinner Frank Wilczek, speaking on “The Intersection of Art and Science.”  Wilczek made a particular connection to the EIC, showing movies of lattice gauge theory calculations of the ever-changing gluon distribution in free space.

The meeting was jointly organized by LBNL and UC Berkeley.    The talks are available on the web, at https://conferences.lbl.gov/event/56/

Fragments

New laboratory director Dr. Michael Witherell.

LBNL welcomes Dr. Michael Witherell as the new laboratory director.  Witherell comes to LBNL from UC Santa Barbara, where he was Vice Chancellor for Research.   Before that, he served as the director of Fermilab.  Witherell is a particle physics by training, with many interests, some of which overlap with NSD,  that abut those in NSD, most notably collider physics and low-background experiments.

Two new postdocs have joined the nuclear structure group in the past few weeks.  Dr. Marco Salathe (right) comes from the Max-Planck-Institut für Kernphysik Heidelberg, where he worked on detector development as a member of the GERDA collaboration.  He is now working on an LDRD project investigating a novel detector design for future gamma-ray tracking arrays using segmented inverted point contact detectors.  Dr. Michael Jones joins the group from Michigan State University and the National Superconducting Cyclotron Laboratory, where he did his PhD research in neutron spectroscopy.  He’s now switching gears to the wonderful world of gamma-rays and the analysis of GRETINA data, studying the nature of the low-energy enhancement in the gamma-ray strength functions of 56Fe.

Dr. Leonardo Milano has joined the RNC group, where he is working with Peter Jacobs on particle and jet correlations in ALICE, studying Pb-Pb collisions and in smaller systems such as high multiplicity p-Pb and pp collisions. After receiving his PhD from the University of Turin in 2013, where he worked on low pT particle identification, he joined CERN as research fellow. He is the coordinator of the “Correlation and event-by-event” analysis group of the ALICE Collaboration.

The NSD CUORE group welcomes two new postdocs: Drs. Benjamin Schmidt (left)  and Brad Welliver (right). Dr. Schmidt received his Ph.D. in 2015 from the Karlsruhe Institute of Technology where he searched for dark matter with EDELWEISS.  Dr. Schmidt was awarded the prestigious Helmholtz Doktorandenpreis (Helmholtz Doctoral Award) for his thesis. Dr. Welliver received his Ph.D. in 2016 from the University of Florida for a dark matter search with SuperCDMS.  Drs. Schmidt and Welliver will be involved with CUORE commissioning, operations, and data analysis. They will also apply their experience with cryogenic detectors in performing R&D for CUPID (Cuore Upgrade with Particle ID), the next generation CUORE-like bolometric experiment to search for neutrinoless double beta decay and other rare events.

Several new people have joined the Nuclear Theory Program.  Last September Dr. André Walker-Loud (left) returned to LBNL from William and Mary, where had been a tenure track assistant professor, and a holder of a DOE career award. André will lead our effort in Lattice QCD for nuclear systems.  His early career award supports Dr. Chia Cheng (Jason) Chang (right), who has joined the lattice effort. Jason received his PhD from the University of Illinois at Urbana-Champaign, where he calculated the matrix elements for neutral heavy-light meson mixing using lattice QCD methods.

The Nuclear Theory program also welcomes two visiting postdoctoral fellows, Drs. Yong Zhao (right) and Nils Strodthoff.  Yong graduated in 2015 from the University of Maryland, where he studied the QCD structure of hadrons with Prof. Ji.   Nils arrived in January for a two-year stay, supported by a fellowship from the Deutsche Forschungsgemeinschaft (DFG). He received is PhD from the University of Darmstadt Germany in 2012 and spent the last three years in Heidelberg as a postdoc. His research focuses on applications of functional renormalization group methods to explore the properties of strongly interacting matter, in particular the QCD phase diagram and aspects of chiral symmetry.

New ANP program officer Erika Suzuki.

NSD’s Applied Nuclear Physics Program and the Institute for Resilient Communities welcome Erika Suzuki, as a Program Officer working with Kai Vetter. She formerly served as the Program Manager of the Nuclear Science and Security Consortium and Deputy Director of the Nuclear Policy Working Group at the University of California, Berkeley.

Newsletter Notes
Please send any comments, including story suggestions to Spencer Klein at srklein@lbl.gov.
 
Previous issues of the newsletter are available at:
https://commons.lbl.gov/display/nsd/NSD+Newsletter.
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