October 2, 2015 | VOLUME 35 NUMBER 39

20F BETA DECAY TO SEARCH FOR THEORY OF ROTATING TRIAXIAL NEW PHYSICS NUCLEI Contributed by: M. Hughes and O. Naviliat-Cuncic Contributed by: C. L. Zhang and W. Nazarewicz In the beginning of September, experiment E14020 was Atomic nuclei come in many different shapes and sizes. carried out using a Fluorine-20 beam produced in the Some are shaped like softballs, others like footballs, and A1900 fragment separator from fragmenting a primary some are so oddly configured that their three axes all have beam of Neon-22 from the Coupled Cyclotron Facility. different lengths (not unlike a football that’s been hit with The goal of the experiment was to perform a precision a hammer). The existence of nuclei with triaxial shape measurement of the shape of the beta-particle energy deformations has been a topic of active research since the spectrum in the decay of Fluorine-20, as a means to early fifties. So far, there has been no clear evidence for search or constrain signatures of new physics. nuclei that are triaxial in their ground states, and theoretical calculations predict very few such candidates having fairly The Flourine-20 particles were deposited deep inside small energy gain due to triaxiality. an implantation detector so that the beta-particles from the decay could not escape and deposit all of their According to theory, good prospects for low-spin triaxiality energy into the detector. An array of four large CsI(Na) are in the neutron-rich region of the chart of the nuclei modules surrounded the implantation detector to tag near molybdenum and ruthenium. Experimentally, the the beta particle in coincidence with a 1.6 MeV gamma clearest signature of triaxial shapes comes from the γ-ray ray following the decay of Fluorine-20. The comparison spectroscopy of rotating nuclei. Recently, transition between the measured beta spectrum with an accurately quadrupole moments in rotational bands of even-mass calculated theoretical spectrum enables the researchers isotopes of molybdenum (Mo) and ruthenium (Ru) nuclei to constrain exotic contributions to the weak interaction. have been measured with Gammasphere at ANL. The new The choice of Flourine-20 for this precision study was data have provided a challenge for theoretical descriptions motivated by the fact that nuclear structure effects of invoking stable triaxial deformations. Indeed, as concluded: this nucleus are very well-known. “Attempts to describe the observations in mean-field based models (…) illustrate the challenge theory faces and the difficulty to infer information on γ -softness and triaxiality from the data.”

The photo shows the array of the four large CsI(Na) modules surrounding the implanted beam.

Fig. 1: Calculated potential energy surface in the quadrupole moment plane (Q20,Q22) for 106,108Mo and 110,112Ru. The difference between contour lines is 0.5 MeV. To shed some light on this puzzle, and to further explore SEMINARS the importance of triaxial deformation in this mass region, the team carried out theoretical analysis of moments of • TUESDAY, OCT 06 AT 11:00 AM inertia, shapes, and transition quadrupole moments in Theory Trailer Conference Room nuclei around Ruthenium-110. They also explored the Erik Olsen, FRIB/NSCL/MSU variety of nuclear shapes at low and high angular momenta. 'Survey of Reflection-Asymmetric Nuclear The results were compared with predictions of the triaxial Deformations' projected shell model, and the calculations predict triaxial • WEDNESDAY, OCT 07 AT 10:00 AM ground-state deformations in Molybdenum-106,108 and NSCL Lecture Hall Ruthenium-108,110,112 and reproduce the observed low- Joshua Bradt, Graduate Assistant, NSCL frequency behavior of moments of inertia. The computed 'Resonant Proton Scattering on 46Ar with the AT-TPC' transition quadrupole moments vary with angular momentum, which reflects deformation changes with • WEDNESDAY, OCT 07 AT 12:00 PM rotation; those variations are consistent with experiment. NSCL Lecture Hall The results obtained with triaxial projected shell model Thomas Baumann, NSCL approach paint the same picture as density functional 'Are you sure you got the right isotope?' Rare theory (DFT) calculations, and strongly favor the triaxial Isotope Beam Delivery at NSCL interpretation. They concluded that the structure of • WEDNESDAY, OCT 07 AT 4:10 PM neutron-rich even-even nuclei around Ruthenium-110 NSCL Lecture Hall is consistent with triaxial shapes. FRIB will allow for Hans Otto Udall Fynbo, Aarhus University, Denmark the study of even more spectaular examples of triaxial 'Experimental Nuclear Astrophysics with neutron-rich nuclei, in particular in experiments at ReA6 Clustered Nuclei' and ReA12. This work constitutes a part of Ph.D. project of Chunli Zhang. PEOPLE AT THE LAB • Clementine Santamaria is a Research Associate who CCF UPDATE joined the Lab this week. Her line manager is Betty The cyclotrons emerged form their brief shutdown Tsang. Monday morning, and a Kr-86 beam was tuned up. On • Lakshmi Lalitha joined the Magnet Systems Group Tuesday, an S800 test run took place to shake down this week. Her line manager is Earle Burkhardt. the newly upgraded data acquisition software. On • Pedro Rodriguez is a Diagnostic Engineer II who Wednesday, there was a detector and electronics test joined the lab this week, under the line managemet in the A1900. An isotope-harvesting experiment began of Steve Lidia. Thursday morning and is collecting radioactive krypton • Alexander Nunham is a Controls Engineer who isotopes in the S2 vault. joined the lab this week. His line manager is Stephen Stanley. CIDER AND DONUTS NEXT • Andrew Slatin is a student employee who joined the TUESDAY MORNING lab this week. His line manager is Patrick Glennon. FRIB/NSCL leadership would like to recognize the continued efforts of laboratory staff operating NSCL and realizing FRIB by offering complimentary donuts, cider and coffee next Tuesday, October 6th @ 9:30 in the Atrium. Bagels will return October 13.

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EDITOR: ERIN O’DONNELL | 517-908-7198 | [email protected]

National Superconducting Cyclotron Laboratory Operation of NSCL as a national Michigan State University user facility is supported by the 640 S. Shaw Lane, East Lansing, Michigan 48824-1321 Experimental Nuclear Physics Program Phone 517-355-9671 www.nscl.msu.edu of the U.S. National Science Foundation