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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, 2020

The elephant in the room: COVID19

As we all face this challenging time together, please stay safe and healthy. We look forward to a return to normal operations. As almost everyone is aware, the San Francisco Bay Area is under a 'shelter in place' order, and Lawrence Berkeley National Laboratory has curtailed on-site operations.

For the Nuclear Science Division (as for the lab as a whole), safety is our highest priority. So, most of us are working from home, on data analysis, simulations, writing, and other tasks. The 88- inch cyclotron has stopped running, and most other laboratory work is in suspension. CERN has also largely shut down, so the ALICE ITS2 work has shifted from commissioning to software. Most meetings have been either cancelled or shifted to on-line only. The March 11-15th STAR Collaboration meeting, which was hosted by the RNC group, became on-line only.

California coalesces for the Electron Ion Collider (EIC)

As the EIC moves forward on the national and international level, California is ramping up its efforts. LBNL, Livermore, Los Alamos and four University of California campuses – Davis, Riverside, UCLA and UC Berkeley – have joined together to form the EIC "California Coalition," banding together to pursue several specific physics and detector topics. The coalition has had several in-person meetings to find common ground, and recently, received funding from the University of California Office of the President (UCOP) to cement this multi-institution initiative.

The coalition is focusing on three physics topics – jets in ep/eA collisions, polarized proton parton distributions, and vector meson photo & electroproduction. The jet work builds on studies done at STAR and ALICE, where jet energy loss in hot and cold nuclear matter is of great interest; the nucleon spin work follows polarized parton studies with the STAR detector; and the vector meson studies follow naturally from current work on ultra-peripheral collisions at RHIC and the LHC.

Figure 1. An Υ->ee event from the eSTARlight Monte Carlo in the simulated silicon detector. Each red dot is a hit in one of the silicon barrel layers, which are not shown here; the forward and backward disks are shown.

One common feature of these reactions is that they rely on charged-particle tracking. Accordingly, the group has chosen as a hardware focus a large-acceptance silicon tracking detector, which will likely have a central barrel, plus forward and backward disks. Figure 1 shows one possible arrangement, showing an Υ(1S)->ee event from the eSTARlight Monte Carlo in the simulated detector. The technology flows from the Monolithic Active Pixel Detector (MAPS) sensors developed for the STAR (at RHIC) Heavy Flavor Tracker (HFT) and ALICE (at the LHC) Inner Tracking System (ITS). Improvements in both chip and overall detector design are being actively studied; some of this work will be in cooperation with the effort within the ALICE Collaboration to develop a replacement for the two inner ITS layers, with improved performance, and, critically, a smaller material budget.

This work has naturally led to a heavy involvement in the nascent EIC "Yellow Report," with group members becoming co-convenors for several of the working groups: Ernst Sichtermann (jets, heavy quarks), Spencer Klein (exclusive, diffractive and forward tagging) and Leo Greiner (tracking including vertexing). These efforts should position California to play key roles in an EIC detector.

CUORE sets new limits on 130Te neutrinoless double-beta decay

Figure 1.  The 988 TeO2 crystal bolometers, arranged in nineteen towers, of the fully assembled CUORE detector. The total mass of the bolometers is 742 kg, including 206 kg of the ββ decay isotope 130Te.
Figure 2. The energy spectrum in the region-of-interest with the best-fit curve (solid red) and the best fit-curve with the 0νββ decay component fixed to the 90% CI limit (dashed blue).

The CUORE (Cryogenic Underground Observatory for Rare Events) experiment recently announced an update of its search for the neutrinoless double-beta (0νββ) decay of 130Te. The search used the 988 TeO2 crystal bolometers shown in Fig. 1. An observation of 0νββ decay would demonstrate lepton number violation and that the neutrino is a Majorana fermion (i.e., is its own antiparticle), as well as providing constraints to the absolute neutrino mass scale. This search, from the data collection period from April 2017 to July 2019 - about fourfold the exposure of CUORE's first result, found no signal, as can be seen in the energy spectrum in Fig. 2. With no evidence for 0νββ decay, the collaboration set a new 90% CL lower limit on the 130Te 0νββ decay half-life of 3.2 × 1025 yr, corresponding to an effective Majorana mass of 75-350 meV. This new limit approaches the so-called inverted neutrino mass hierarchy band that will be probed by the next generation CUPID (CUORE Upgrade with Particle ID) detector. CUORE is a near ton-scale bolometric detector located underground in the Gran Sasso lab in Italy, and the collaboration consists of members from US, Italian, French, and Chinese institutions. The US DOE Office of Nuclear Physics supports the US component of CUORE, and Berkeley Lab is the lead DOE laboratory on CUORE. UC Berkeley physics professor and NSD senior faculty scientist Yury Kolomensky is the US Spokesperson of CUORE, and NSD staff scientist Brian Fujikawa leads the CUORE group at Berkeley Lab. UC Berkeley postdocs and NSD affiliates Giovanni Benato and Laura Marini led the data analysis team as well as the writing committee. NSD postdocs Ben Schmidt and Brad Welliver made significant contributions to this analysis.


Reference
D. Q. Adams et al., (CUORE Collaboration), Physical Review Letters 124, 122501 (2020).

Fission fragment yields calculated for a wide range of heavy elements

Figure 1. The potential-energy surface for 236U as a function of the overall elongation of the fissioning nucleus (left to right) and its reflection asymmetry (across, with reflection symmetric shapes along the middle).  The spheroidal ground-state shape is the low minimum on the left.  Shell effects give the fission barrier landscape a second (isomeric) minimum corresponding to a more deformed (but still reflection symmetric) shape, separated from the ground-state by a barrier over which the shape must diffuse on its way towards fission.  Beyond the second minimum, reflection-asymmetric shapes become energetically favorable (the system prefers one of the fragments to be near the doubly-magic nucleus 132Sn), so the second saddle is asymmetric.  Once the random walk passes the second barrier, the system splits asymmetrically into two fragments.
Figure 2. This chart shows the overall width of the mass yield, defined as the number of fragment mass numbers A for which the (normalized) yield Y(A) exceeds 1% [1].

NSDer Jorgen Randrup and collaborators have calculated a complete set of primary fission fragment yields, Y(Z,A), for heavy nuclei across the chart of nuclides, including those of particular relevance to the rapid neutron capture process of nucleosynthesis. They assumed that the nuclear shape evolution is strongly damped which gives the fission process the character of Brownian motion that can be simulated as a random walk on the multi-dimensional deformation-energy surface of the fissioning system. These surfaces have been (previously) calculated by augmenting the macroscopic (liquid-drop like) deformation energy with the shell and pairing effects associated with each specific shape. (Fig. 1.)

Using their recently developed method, Randrup and collaborators calculated a comprehensive set of fission-fragment yields from over 3,800 nuclides bounded by 80 ≤ Z ≤ 130 and A ≤ 330; these yields have been made available to the community as ASCII files. The resulting yields compare well with the experimental data (where available). Generally, the fragment mass yields tend to be rather wide in the r-process region, a feature with important implications for the fission recycling process (Fig. 2).

Reference
M.R. Mumpower, P. Jaffke, M.Verriere and J. Randrup, arXiv:1911.06344.

Fragments

NSD Director Barbara Jacak has been appointed to a committee to update the EIC Users Group charter, the for the next phase of the EICUG after the DOE CD0.

Congratulations to Paul Barton (top left), Jordan Myslik (top right), Jeff Bramble (bottom left) and Tom Gallant (bottom right) for receiving SPOT awards for their key contributions to the division. Paul and Jordan received their awards "for their outstanding efforts to complete the fabrication of the Low-Mass Front End for the upgrade of the Majorana Demonstrator experiment under a compressed time schedule." Jeff Bramble was recognized "for taking the initiative to organize systematic chemical inventory and disposal within the Nuclear Science Division and for leading successfully the Division to carry out this months-long plan efficiently and safely." Tom Gallant was thanked for his efforts "on behalf of Berkeley Lab's ERGs [Employee Resource Groups -ed], the first ever Bay Area ERG Leadership Summit was held on Feb. 24, 2020, with LBNL, LLNL, SLAC and Sandia to huge success." The awardees received a certificate and a monetary award.

Congratulations to Volker Koch for reaching 25 years of service at the University of California.

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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