Fall 2017 Nuclear Physics Seminar


Seminars are Mondays 2:20 PM in Nielsen 304. Please contact Miguel Madurga for questions (mmadurga@utk.edu).


Aug. 28        Jeremy Bungaard           UTK

High Precision Measurements Using The NIFFTE fissionTPC

                                                        

Abstract: The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has developed the fission Time Projection Chamber (fissionTPC) to make new, high precision fission cross section measurements as called for by the Nuclear Energy and Stockpile Stewardship communities. The fissionTPC is annually deployed to the Los Alamos Neutron Science Center (LANSCE) where it operates in beam, colliding neutrons with heavy actinide tar- gets, inducing fission. This detector was developed at the Lawrence Livermore National Laboratory’s (LLNL) TPC lab, where it routinely measures spontaneous fission (SF) from radioactive sources, typically 252Cf and 244Cm, to characterize detector response, gauge performance, and evolve the design. An important aspect of these source mea- surements is to develop analyses and demonstrate the fissionTPC’s performance abilities/limitations by measuring the α/SF branching ratio of 252Cf; our method benchmarks the fissionTPC’s ability to deduce the α/SF branching ratio with sub-percent precision. The talk will discuss the need for improved fission measurements, details of how the detector operates to create 3D ionization tracks, and analysis methods developed for the 252Cf α/SF branching ratio measurements. The results from the NIFFTE collaboration’s recent publication (submitted June 2017 to PRC) of the neutron induced fission cross section ratio of 238U/235U will also be summarized.

                                


Sept. 11        Xesus Pereira              UTK

                                        

Study of transfer reactions induced by a 16C beam

                                                        

Recent experiments have evidenced the existence of new nuclear shell gaps at N=14 and N=16 in neutron-rich oxygen isotopes associated with the vanishing of the N=20 shell gap. However, in the neutron-rich carbon isotopes, the extent to which these gaps persist is unclear. In an effort to answer this question we have attempted to probe the low-lying level structure of 17C using the (d,p) transfer reaction to locate the single-particle orbitals involved in the formation of the N=14 and N=16 shell gaps.        

The experiment was carried out at the GANIL facility. A 16C beam at 17.2 AMeV produced by fragmentation was used to bombard a CD2 target. The light ejectiles were detected using the TIARA silicon strip array while a Si-Si-CsI telescope was placed at zero degrees to identify beam-like residues. In addition, four HPGe-EXOGAM clover detectors were used to measure the gamma-rays arising from 17C bound excited states.

The measured angular distributions confirm the spin and parity assignments of 3/2+, 1/2+ and 5/2+ for the ground and the first and second excited states located at 217 keV and 335 keV respectively. The spectroscopic factors deduced for these excited states indicate a large single particle strength, in agreement with shell model calculations. With a strong l = 0 valence neutron component and a low separation energy, the first excited state of 17C appears as a good one-neutron halo candidate.


Sept. 18        Gaute Hagen               ORNL

Coupled-cluster computations of atomic nuclei

This talk reviews recent progress in coupled-cluster computations of atomic nuclei based on state-of-the-art interactions from chiral effective-field-theory. An optimization of an interaction from chiral effective field theory to few-nucleon systems and oxygen isotopes yielded much improved binding energies and charge radii in light and medium-mass nuclei. The computation of the nucleus 48Ca showed that its neutron skin (difference between the radii of the neutron and proton distributions) is smaller than previously thought, and we made prediction for its dipole polarizability which has recently been measured. We performed coupled-cluster computations of neutron rich calcium isotopes complementing a recent measurement of charge radii in 49,51,52Ca obtained from laser spectroscopy experiments at ISOLDE, CERN. This experiment found an unexpectedly large charge radius for 52Ca that questions its magicity. We also predicted the 2+ state in 78Ni from a correlation with the 2+ state in 48Ca using chiral nucleon-nucleon and three-nucleon interactions. Our results confirm that 78Ni is doubly magic. It was also found that continuum effects play an important role in the level ordering of 79Ni. I will also show recent results for neutron deficient tin isotopes, where in particular we make predictions for the structure of 100Sn from first principles.


Sept. 25        Rin Yokoyama             UTK

Shape evolution of neutron-rich midshell nuclei around A~160 studied by gamma-ray spectroscopy.

 

Study of the nuclear deformation of midshell nuclei is important to understand how the shell effect drive macroscopic shape of nuclei to have different shapes at different proton or neutron numbers. In neutron-rich A~160 region, onset of higher-order deformations such as octupole () or hexadecupole () deformations are predicted. Such exotic shapes of unstable nuclei are not well understood. However, those deformation can change single particle levels and consequently affect the decay property of the nuclei. Properties of the nuclei in this region is also important in astrophysical point of view because they are on the decay path of nuclei produced in r-process. I am going to present some of the results from isomer and beta-gamma spectroscopy experiments at a fragmentation facility, RIBF, at RIKEN Nishina Center in Japan and discuss on the various shapes of the nuclei in this region.

 


Oct. 2            Adam Matyja               UTK

Neutral meson, photo n and jet production in the ALICE experiment

The ALICE experiment is dedicated to studies of the quark-gluon plasma (QGP) state, which is created in heavy ion collisions. Both photons, neutral mesons and jets are excellent probes of QGP. The direct photons enable a test of perturbative QCD constraining parton distributions and fragmentation functions. Thermal photons provide the temperature of the medium. The medium-induced energy loss of particles can be investigated via the measurement of neutral meson spectra. A modification of the jet structure in medium compared to vacuum can also provide hints to the properties of QGP.

Both direct photons, neutral mesons and jets have been measured by the ALICE experiment at LHC. Results allow measurements of the spectra of particles or jets with high precision over the wide dynamical range. An overview of the recent results on photon, meson and jet physics from ALICE will be shown.


Oct. 9             B. Charles Rasco        ORNL

Who cares about 𝛃-decay?

Beta-decay has a long and interesting history. It influenced the original studies of the weak force and was among the first theoretical ideas to require the invention of a new particle. There are many current research areas connected with 𝛃-decay. One area of 𝛃-decay influence is on the calculations of the production of elements in the galaxy through the r process. A more practical area of research that involves 𝛃-decay is the study of the decay heat energy produced in nuclear reactors. This practical side of 𝛃-decay in turn influences fundamental physics since nuclear reactors are excellent sources of electron antineutrinos. There are many electron antineutrino experiments that use nuclear reactors as a source of antineutrinos. And in order to make precision electron antineutrino measurements the 𝛃-decay that occurs in a nuclear reactor must be precisely understood.Currently it is not precisely understood. 𝛃-decay is also a very useful and unique tool to gain insight into nuclear structure.

In this seminar I will discuss the basics of the weak interaction and its connection to 𝛃-decay. I will also discuss the other components of 𝛃-decay, such as 𝛃-delayed neutron emission and how they are measured. After the the background discussion, I will comment on the connection between 𝛃-decay and reactor physics, the r process and the generation of the heavier elements in the universe, some nuclear structure, and to the recent 𝛃-decay experiments the UTK and ORNL groups have performed. In short, I will answer: Who cares about 𝛃 decay?


Oct. 16         Jason Fry                     UVA

Nab: A Precision measurement of unpolarized neutron beta decay

The Nab collaboration proposes to measure the electron-neutrino correlation parameter a with a precision of δa/a = 10^-3 and the Fierz interference term b to δb = 3 × 10^-3 in unpolarized free neutron β decay. These results are expected to lead to a new, precise, independent determination of the ratio λ = GA/GV that will sensitively test CKM unitarity. A long asymmetric spectrometer guides the decay products to two large area silicon detectors in order to precisely determine the electron energy and proton time of flight. The Nab apparatus is under installation on the Fundamental Neutron Physics Beamline at the SNS at ORNL and commissioning will begin in the near future. An overview of the Nab experiment, systematics effects associated with spectrometer magnetic fields, and the first tests of the spectrometer will be presented.


Oct. 23         Aaron Sprow                UKY

A Detection System for Precision Beta Decay Studies

Precision neutron beta decay experiments serve well as a test of the Standard Model, and act as a sensitive probe for Beyond the Standard Model (BSM) physics. Next generation experiments such as Nab at the Spallation Neutron Source (SNS) and UCNB at the Los Alamos Neutron Science Center (LANSCe) require the coincident detection of the electron and proton to infer information about the undetected antineutrino from free neutron decay. To accomplish the experimental goals, a system with near 100% efficiency, linearity at the 1E-5 level, and high precision timing is necessary. A novel silicon detector architecture able to directly measure the electron and proton in the same detector that meets these requirements has been developed and currently is undergoing testing. I will present a discussion of the detection and data acquisition systems, as well as the ongoing efforts to understand the systematics associated with each. Additionally, I will address the results from preliminary UCNB efforts.


Nov. 1           Leendert Hayen           KU Leuven                   (special Wednesday seminar)

Taming the nuclear beta decay Hydra: One theoretical head at a time

The study of nuclear beta decay has been at the forefront of our current understanding of the physical landscape, and continues to play an essential role in the search for beyond Standard Model physics. In order to separate the wheat from the chaff of the myriad possible theoretical extensions, a reliable estimate of the Standard Model contribution is indispensable. Recently, the description of the allowed beta spectrum shape was revisited and extended in order to tackle these challenges. Besides the study of the fundamental nature of the weak interaction, the beta spectrum shape is an essential ingredient in several outstanding problems in particle physics, such as the reactor antineutrino anomaly. We will provide an overview of the current state-of-the-art and its challenges, and discuss its implications on the reactor anomaly.


Nov. 6           Jonas Braun                 TU Darmstadt

Electromagnetic Properties of One-Neutron Halo Nuclei in Halo

                                        

We exploit the separation of scales in weakly-bound nuclei to compute E2 transitions and elec- tromagnetic form factors in a Halo EFT framework. The relevant degrees of freedom are the core and the halo neutron. The EFT expansion is carried out in powers of Rcore/Rhalo, where Rcore and Rhalo denote the length scales of the core and halo, respectively. We propose a power-counting scenario for weakly-bound states in a one-neutron Halo EFT and discuss its implications for higher partial-wave bound states in terms of universality. Electromagnetic interactions are included via minimal substitution in the Lagrangian. We demonstrate that, depending on the observable and respective partial wave, additional local gauge-invariant operators contribute in LO, NLO and higher orders. In order to make numerical predictions, obtained correlations between electric observables in Halo EFT are combined with experimental and ab initio data.

                                


Nov. 13         Yuri Efremenko            UTK

        

First result from the COHERENT experiment

COHERENT collaboration just published first result of discovery of Coherent Elastic Neutrino Nucleus Scattering (CEvNS). This process has been predicted theoretically in 1973 but so far elude detection. Combination of the unique neutrino source SNS (ORNL) and progress in low threshold detectors let COHERENT collaboration unambiguously detect this process in a first time with 6.6 sigma confidence level. I will describe experimental efforts which lead to this measurement and will disscuss why this process is interesting for both nuclear and particle physics. I will outline plans for a future experimental efforts.


Nov. 20         Luis Istand                   UTK

Potassium-Based Halide Scintillators with High Energy Resolution

Our goal is to discover and develop low-cost, effective new scintillators that can be used for the detection of illicit radioactive materials. In order to unambiguously identify the specific gamma-ray signatures of radioactive elements, scintillator materials with energy resolution approaching 2% at 662 keV are required. Currently available radiation sensors have either inadequate energy resolution (NaI:Tl), unacceptably high cost (LaBr3:Ce, CZT and HPGe). In this work, we present a summary of the crystal growth and scintillation properties of new Eu2+ doped ternary scintillators that belong to the KA2X5 and K2AX4 compositional family (A = Sr, Ba; X = I, Br). These scintillators have light yields up to ~95,000 photons/MeV and energy resolution as good as 2.2% at 662 keV. Using the vertical Bridgman method, we have demonstrated that uniform KSr2X5:Eu single crystals can be successfully grown at much faster pulling rates (up to 7 mm/h) than other high performing scintillators. The effect of the intrinsic radioactivity due to 40K on their performance in nuclear security applications was evaluated.


Nov. 27         James Matta                ORNL


Dec. 4          (Grad. Students talk)