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Back, B. B.

Publications and source records attributed to Back, B. B..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Nuclear rotation at the fission limit in Rf 254

A ground-state rotational band in the fissile nucleus Rf254 was observed for the first time. Levels up to spin 14 and excitation energy of 1.56 MeV were observed. The Rf254 nuclei were produced using the Pb206(Ti50,2n) fusion-evaporation reaction. It is the weakest reaction channel ever studied using in-beam γ-ray spectroscopic methods. The reaction products were separated from the beam in the Argonne gas-filled analyzer (AGFA). The Rf254 nuclei were implanted into a double-sided Si strip detector at the AGFA focal plane and tagged with subsequent ground-state spontaneous fission decays using temporal and spatial correlations. Prompt γ rays in coincidence with the Rf254 recoils were detected in the Gammasphere array of Ge detectors. In order to identify the ground-state rotational band in Rf254, a method for identifying rotational bands in low statistics γ-ray spectra was developed. The deduced Rf254 kinematic moment of inertia is smaller compared to neighboring even-even nuclei. This is most likely associated with a slightly lower quadrupole deformation and stronger pairing correlations in Rf254. The behavior of the moment of inertia as a function of rotational frequency is similar to that of the lighter N=150 isotones Fm250 and No252.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Initial tests of Accelerator Mass Spectrometry with the Argonne Gas-Filled Analyzer and the commissioning of the MONICA detector

As the scope of Accelerator Mass Spectrometry (AMS) expands, there is an increased need to extend the capability of isobaric separation to the medium-heavy mass region. Existing AMS facilities are limited in their ability to separate radioactive nuclei in the A = 100–200 range of interest from their neighboring stable isobars, as such measurements require higher energies than available in most facilities. ATLAS is one of the highest energy system used for AMS based experiments and has enabled isobaric discrimination for medium to heavy nuclides, notably via the Gas-Filled Magnet technique. Further, a preparatory experiment performed in November, 2019, successfully demonstrated isobaric separation of 92 Zr- 92 Mo using the Argonne Gas-Filled Analyzer (AGFA) with high magnetic rigidity. Since that time, MONICA, an eight-anode ionization chamber that measures both energy loss and position with two sets of split anodes, has been developed to aid in AMS experiments at AGFA and has undergone four commissioning runs at the Nuclear Science Laboratory at the University of Notre Dame utilizing Si, Fe/Ni, and Mn beams. This report presents the AGFA AMS run (November 2019) and the subsequent commissioning runs of the MONICA detector, including preliminary measurements on the long-lived isotopes 39 Ar (268 y) and for the first time on 42 Ar (33 y).

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Upgrade and Operation of the ATLAS Radiation Interlock System (ARIS)

ATLAS (the Argonne Tandem Linac Accelerator Sys-tem) is a superconducting heavy ion accelerator which can accelerate nearly all stable, and some unstable, iso-topes between hydrogen and uranium. Prompt radiation fields from gamma and or neutron are typically below 1 rem/hr at 30 cm, but are permitted up to 300 rem/hr at 30 cm. The original ATLAS Radiation Interlock System (ARIS), hereafter referred to as ARIS 1.0 was installed 30 years ago. While it has been a functional critical safe-ty system, its age has exposed the facility to high risk of temporary shutdown due to failure of obsolete compo-nents. Topics discussed will be architecture, hardware improvements, functional improvements, and operation permitting personnel access to areas with low levels of radiation.

Blomberg, B. R.↗
DOE OSTI2021

Solving the Puzzles of the Decay of the Heaviest Known Proton-Emitting Nucleus 185 Bi

Two long-standing puzzles in the decay of 185 Bi, the heaviest known proton-emitting nucleus are revisited. These are the non-observation of the 9/2 – state, which is the ground state of all heavier odd-A Bi isotopes, and the hindered nature of proton and α decays of its presumed 60-μs 1/2 + ground state. The 185 Bi nucleus has now been studied with the 95 Mo( 93 Nb; 3n) reaction in complementary experiments using the Fragment Mass Analyzer and Argonne Gas-Filled Analyzer at Argonne National Laboratory’s ATLAS facility. The experiments have established the existence of two states in 185 Bi; the short-lived T 1/2 = $2.8$$^{+2.3}_{–1.0}$ μs, proton- and α-decaying ground state, and a 58(2)–μs γ-decaying isomer, the half-life of which was previously attributed to the ground state. The reassignment of the ground-state lifetime results in a proton-decay spectroscopic factor close to unity and represents the only known example of a ground-state proton decay to a daughter nucleus ( 184 Pb) with a major shell closure. Furthermore, the data also demonstrate that the ordering of low- and high-spin states in 185 Bi is reversed relative to the heavier odd-A Bi isotopes, with the intruder-based 1/2 + configuration becoming the ground state, similar to the lightest At nuclides.

150 ≤ A ≤ 189↗
DOE OSTI2021

Configuration mixing in 28 Mg and the 26 Mg (t,p) 28 Mg reaction

We have studied the 26 Mg(t,p) 28 Mg reaction. The nucleus 28 Mg lies between 24 Mg and 32 Mg, the latter of which is central to the “island of inversion” characterized by low-lying neutron fp-shell configurations. The present results show that the second-excited 0 + state in 28 Mg contains large fp-shell occupation for the neutrons. The experiment was performed in inverse kinematics using the HELIcal Orbit Spectrometer (HELIOS) at Argonne National Laboratory. Shell-model calculations using the SDPF-MU interaction yielded level energies and two-neutron transfer amplitudes that were used in one-step distorted-wave Furthermore, born approximation calculations to provide theoretical predictions of the proton angular distributions. In many, but not all cases, the data were in good agreement with theoretical predictions.

20 ≤ A ≤ 38↗
DOE OSTI2020

Quadrupole and octupole collectivity in 143 Ba

The neutron-rich barium nuclei have been the subject of intense interest due to the enhanced octupole correlations they are predicted to exhibit. The observation of enhanced octupole collectivity in 144,146 Ba as measured in sub-barrier Coulomb excitation, consistent with static octupole deformation, has further heightened this interest. In the present work, these studies are extended to the neighboring odd-mass 143 Ba to investigate the interplay between single-particle and collective octupole degrees of freedom. A new measurement of the first 9/2 – -state lifetime is also presented. Reflection-Asymmetric Triaxial Particle Rotor Model calculations indicate that the negative-parity bands in 143 Ba can be understood as a decoupled structure of νh 9/2 parentage, while the positiveparity bands are built on a decoupled octupole phonon. Here, no evidence for E3 excitation is observed in this work, but an upper limit is placed on the E3 matrix element to the lowest octupole band.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Experimental study of the low-lying negative-parity states in 11 Be using the 12 B( d , 3 He) 11 Be reaction

In this work, low-lying negative-parity states in 11 Be having dominant p-wave neutron configurations were studied using the 12 B(d, 3 He) 11 Be proton-removal reaction in inverse kinematics. The 1/2 $\overset{-}{1}$ state at 0.32 MeV, the 3/2 $\overset{-}{1}$ state at 2.56 MeV and one or both of the states including the 5/2 $\overset{-}{1}$ level at 3.89 MeV and the 3/2 $\overset{-}{2}$ level at 3.96 MeV were populated in the present reaction. Spectroscopic factors were determined from the differential cross sections using a distorted wave Born approximation method. The p-wave proton removal strengths were well described by the shell model calculations while the Nilsson model calculation underestimates the spectroscopic factors for the higher excited states. Results from both variational Monte Carlo and no-core shell model calculations were also compared with the experimental observations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Reproducing heavy-ion fusion cross sections at extreme sub-barrier energies with a simple formula: Analysis of fusion hindrance with a Gaussian barrier distribution

Heavy-ion fusion hindrance occurs at extreme sub-barrier energies. This behavior is well reproduced with a simple cross section formula, which was developed by Siwek-Wilczynska et al., based on a single-Gaussian distribution of fusion barrier heights, before the discovery of the hindrance phenomenon. This expression has not yet been widely used and referenced in the literature. An analysis by using this simple formula is presented for 29 systems, from 16 O + 18 O to 64 Ni + 124 Sn , all being measured down to less than 10μb. The agreement with the data is even better than the ones from sophisticated Coupled-channels calculations. This simple expression also applies to fusion reactions in lighter systems. Here, the three parameters contained in this formula vary in a relatively smooth fashion over the whole mass range, and can be used to extrapolate cross sections or to obtain an estimate of the excitation function for systems which have not been measured. Extensions and restrictions of this method are also discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Probing the Single-Particle Character of Rotational States in F 19 Using a Short-Lived Isomeric Beam

A beam containing a substantial component of both the J π = 5 + , T 1 / 2 = 162 ns isomeric state of F 18 and its 1 + , 109.77-min ground state is utilized to study members of the ground-state rotational band in F 19 through the neutron transfer reaction ( d , p ) in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13 / 2 + band-terminating state. The agreement between shell-model calculations using an interaction constructed within the s d shell, and our experimental results reinforces the idea of a single-particle–collective duality in the descriptions of the structure of atomic nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Reaction rate for carbon burning in massive stars

Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for 12 C+ 12 C fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of 12 C+ 12 C fusion cross sections where these backgrounds have been minimized. In conclusion, it is found that the astrophysical S factor exhibits a maximum around E cm =3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Experimental Measures of Fission Time Scales

The nuclear fission process involves a drastic rearrangement of the nuclear matter of the fissioning system as it undergoes shape changes from a single, compact nucleus to two separated fission fragments. Understanding the dynamics of this process has been the focus of much research for decades, both theoretical and experimental, using a number of different approaches. Early on, Kramers [1] proposed that the fission rate is not well described by the statistical counting of transition states as proposed by Bohr and Wheeler [2] but that the friction or viscosity of the nuclear matter can play a substantial role in slowing down the rate by large factors. Aside from the theoretical estimates of fission time scales, a number of different approaches have been used to obtain experimental information on this quantity. Although the divergence of results obtained by the different experimental approaches have been known for years, these discrepancies have not yet been resolved and they have recently received renewed attention [3]. In this paper I will recount some of the experimental approaches to estimate the fission time scales and discuss their respective strengths and weaknesses without coming close to provide a resolution of the discrepancies.

Back, B. B.↗
DOE OSTI2016

Participant and spectator scaling of spectator fragments in Au + Au and Cu + Cu collisions at s N N = 19.6 and 22.4 GeV

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to nitrogen (Z = 7), are measured in PHOBOS. These fragments are observed in Au+Au ( s N N = 19.6 GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity (η). The dominant multiply-charged fragment is the tightly bound helium (α), with lithium, beryllium, and boron all clearly seen as a function of collision centrality and pseudorapidity. In this paper, we observe that in Cu+Cu collisions, it becomes much more favorable for the α fragments to be released than lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. Finally, a detailed comparison of the shapes for α and lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗