M. W. Ahmed

Unambiguous identification of the second 2+ state in C12 and the structure of the hoyle state

The second J(π)=2+ state of 12C, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the 12C(γ,α0)(8)Be reaction. The alpha particles produced by the photodisintegration of 12C were detected using an optical time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2+ state in 12C at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:2(2)+→0(1)+)=0.73(13)e(2)  fm(4) [or 0.45(8) W.u.]. The Hoyle state and its rotational 2+ state that are more extended than the ground state of 12C presents a challenge and constraints for models attempting to reveal the nature of three alpha-particle states in 12C. Specifically, it challenges the ab initio lattice effective field theory calculations that predict similar rms radii for the ground state and the Hoyle state.

The HIγS Facility: A Free-Electron Laser Generated Gamma-Ray Beam for Research in Nuclear Physics

The High Intensity Gamma-ray Source (HIγS) is a joint project between Triangle Universities Nuclear Laboratory (TUNL) and the Duke Free Electron Laser Laboratory (DFELL). This facility utilizes intra-cavity back-scattering of the FEL photons in order to produce a γ-flux enhancement of approximately 103 over the existing sources. At present, gamma-ray beams with energies ranging from 2 to 50 MeV are available with intensities of 105–107 γ/s, energy spreads of 1% or better (with lower intensities), and 100% linear polarization. An upgrade is presently underway which will allow for the production of γ rays up to an energy of about 225 MeV having intensities in excess of 108 γ/sec. The primary component of the upgrade is a 1.2 GeV booster-injector which will provide for efficient injection at any chosen operating energy of the storage ring from 300 MeV to 1.2 GeV. In addition, an upgrade of the present OK-4 FEL to a helical undulator system (OK-5) is underway. This new system has many advantages over the pres...

An optical readout TPC (O-TPC) for studies in nuclear astrophysics with gamma-ray beams at HIγS1

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO2(80%) + N2(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HI?S) facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm3. Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N2 emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The events time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data. The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and 12C particles from the dissociation of 16O and of three alpha-particles from the dissociation of 12C have been measured during initial in-beam test experiments performed at the HI?S facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.

Chiral Dynamics in Photopion Physics: Theory, Experiment, and Future Studies at the HIγS Facility

We review photopion experiments on the nucleon in the near-threshold region. We compare the results with the predictions of the low-energy theorems of quantum chromodynamics calculated using chiral perturbation theory (ChPT), which is based on the spontaneous breaking of chiral symmetry as well as explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult to perform because the cross sections are small. Nevertheless, the field is now mature in terms of accuracy and sensitivity. We also discuss the accomplishments and limitations of past measurements, as well as future planned experiments at Mainz and HIγS. The technical developments required for the HIγS facility are emphasized. Finally, we show that future experiments will provide even more accurate tests of ChPT and will be sensitive to isospin-breaking dynamics due to the mass difference between the up and down quarks.

Parity measurements of nuclear dipole excitations using FEL-generated γ-rays at HIγS☆

First Nuclear Resonance Fluorescence (NRF) experiments were performed at the storage ring FEL-driven High Intensity Gamma Source (HIgS) at the DFELL. Azimuthal NRF intensity ratios were measured around the polarized HIgS beam. Electric character was deduced for 18 dipole excitations in 138 Ba [1]. The measurements demonstrate the superior performance of the HIgS facility in making such measurements. We report here on the performance of this setup. r 2002 Elsevier Science B.V. All rights reserved.

Nuclear Structure Physics With A Free Electron Laser

First Nuclear Resonance Fluorescence (NRF) experiments were performed at the storage ring FEL‐driven High Intensity γ‐ray Source (HIγS) at the DFELL. Sharp NRF γ‐lines were observed from 11B, 32S, 88Sr, and 138Ba nuclei. Azimuthal NRF intensity ratios were measured around the polarized HIγS beam. Electric character was thereby deduced for 18 dipole excitations in 138Ba in the energy range between 5.5 and 6.5 MeV [1] and for two dipole excitations in 88Sr at 4.742 MeV and 7.535 MeV excitation energy [2]. These initial experiments demonstrate the superior performance of the HIγS facility in making such measurements [3].

Compton scattering of polarized {gamma} rays by {sup 16}O for E{sub {gamma}}=25-40 MeV

Measurements of the polarization asymmetries {sigma}({theta}{sub c.m.}) and {sigma}(E{sub {gamma}}) of Compton scattering by {sup 16}O have been performed in the energy range of 25-40 MeV over a range of scattering angles between 90 and 150 deg. . An analysis of the present data combined with previous results indicates that significant, narrow concentrations of E2 strength are not present below an excitation energy of 40 MeV. The existence, however, of a broad isovector giant quadrupole resonance exhausting a large percentage of the isovector energy weighted sum rule is not ruled out by the combined data. Additionally, the present data are insensitive to modifications to the free values of the nucleon polarizabilities, but cannot rule them out.

Measurement of the doubly-polarized He3→(γ→,n)pp reaction at 16.5 MeV and its implications for the GDH sum rule

G. Laskaris, 2, ∗ X. Yan, 2 J. M. Mueller, 2, † W. R. Zimmerman, 2 W. Xiong, 2 M. W. Ahmed, 2, 3 T. Averett, P.-H. Chu, 2, ‡ A. Deltuva, C. Flower, 2 A. C. Fonseca, H. Gao, 2 J. Golak, J. N. Heideman, 8 H. J. Karwowski, M. Meziane, 2 P. U. Sauer, R. Skibiński, I. I. Strakovsky, H. R. Weller, 2 H. Wita la, and Y. K. Wu 2 Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA Department of Physics, Duke University, Durham, North Carolina 27708, USA Department of Mathematics and Physics, North Carolina Central University, Durham, North Carolina 27707, USA College of William and Mary, Williamsburg, Virginia 23187, USA Institute of Theoretical Physics and Astronomy, Vilnius University, LT-01108 Vilnius, Lithuania Centro de F́ısica Nuclear da Universidade de Lisboa, P-1649-003 Lisboa, Portugal M. Smoluchowski Institute of Physics, Jagiellonian University, PL-30348 Kraków, Poland Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA Institut für Theoretische Physik, Leibniz Universität Hannover, D-30167 Hannover, Germany Department of Physics, The George Washington University, Washington DC 20052, USA (Dated: June 2, 2015)

Nuclear resonance fluorescence using different photon sources

Nuclear resonance fluorescence (NRF) is a photon-based active interrogation approach that provides isotope-specific signatures that can be used to detect and characterize samples. As NRF systems are designed to address specific applications, an obvious first question to address is the type of photon source to be employed for the application. Our collaboration has conducted a series of NRF measurements using different photon sources to begin to examine this issue. The measurements were designed to be as similar as possible to facilitate a straight-forward comparison of the different sources. Measurements were conducted with a high-duty factor electron accelerator using bremsstrahlung photons, with a pulsed linear accelerator using bremsstrahlung photons, and with a narrow bandwidth photon source using Compton backscattered photons. We present our observations on the advantages and disadvantages of each photon source type. Issues such as signal rate, the signal-to-noise ratio, and absorbed dose are discussed.

Astrophysical S factor for the {sup 7}Li(d,n{sub 0}){sup 8}Be and {sup 7}Li(d,n{sub 1}){sup 8}Be reactions

The absolute astrophysical S factor and cross section for the {sup 7}Li(d,n{sub 0}){sup 8}Be and {sup 7}Li(d,n{sub 1}){sup 8}Be reactions have been determined using deuteron beams with energies between 45 and 80 keV. The slope of the S factor is consistent with zero in the n{sub 0} case but is slightly negative in the n{sub 1} case. The S factor for the sum of both neutron groups at c.m. energies below 70 keV is S(E)=5400({+-}1500)-37({+-}21)E keV b, where E is the c.m. energy in keV.