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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Terminating states in the positive-parity structures of As67

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus 67As have been studied by using the 40Ca(36Ar,2αp)67As reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson–Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states.

59 ≤ A ≤ 89↗
DOE OSTI2016

Characterization of a gamma-ray tracking array: A comparison of GRETINA and Gammasphere using a 60 Co source

In this paper, we provide a formalism for the characterization of the tracking arrays with emphasis on the proper corrections required to extract their photopeak efficiencies and peak-to-total ratios. The methods are first applied to Gammasphere, a well characterized 4pi array based on the principle of Compton suppression, and subsequently to GRETINA. The tracking efficiencies are then discussed and some guidelines as to what clustering angle to use in the tracking algorithm are presented. It was possible, using GEANT4 simulations, to scale the measured efficiencies up to the expected values for the full 4pi implementation of GRETA.

efficiency measurements↗
DOE OSTI2014

Photon strength and the low-energy enhancement

Several measurements in medium mass nuclei have reported a low-energy enhancement in the photon strength function. Although, much effort has been invested in unraveling the mysteries of this effect, its physical origin is still not conclusively understood. Here, a completely model-independent experimental approach to investigate the existence of this enhancement is presented. The experiment was designed to study statistical feeding from the quasi-continuum (below the neutron separation energy) to individual low-lying discrete levels in {sup 95}Mo produced in the (d, p) reaction. A key aspect to successfully study gamma decay from the region of high-level density is the detection and extraction of correlated particle-gamma-gamma events which was accomplished using an array of Clover HPGe detectors and large area annular silicon detectors. The entrance channel excitation energy into the residual nucleus produced in the reaction was inferred from the detected proton energies in the silicon detectors. Gating on gamma-transitions originating from low-lying discrete levels specifies the state fed by statistical gamma-rays. Any particle-gamma-gamma event in combination with specific energy sum requirements ensures a clean and unambiguous determination of the initial and final state of the observed gamma rays. With these requirements the statistical feeding to individual discrete levels is extracted on an event-by-event basis. The results are presented and compared to {sup 95}Mo photon strength function data measured at the University of Oslo.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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 OSTI2016

Rotational band structure in Mg 32

There is significant evidence supporting the existence of deformed ground states within the neutron-rich N ≈ 20 neon, sodium, and magnesium isotopes that make up what is commonly called the “island of inversion.” However, the rotational band structures, which are a characteristic fingerprint of a rigid nonspherical shape, have yet to be observed. Here, in this work, we report on a measurement and analysis of the yrast (lowest lying) rotational band in 32 Mg up to spin I = 6 + produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ-ray tracking detector array, GRETINA (γ-ray energy tracking in-beam nuclear array). Large-scale shell-model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Finally, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked-shell-model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Strongly coupled rotational band in Mg 33

The “island of inversion” at N≈20 for the neon, sodium, and magnesium isotopes has long been an area of interest both experimentally and theoretically due to the subtle competition between 0p-0h and np-nh configurations leading to deformed shapes. However, the presence of rotational band structures, which are fingerprints of deformed shapes, have only recently been observed in this region. In this work, we report on a measurement of the low-lying level structure of Mg33 populated by a two-stage projectile fragmentation reaction and studied with the Gamma Ray Energy Tracking In-Beam Nuclear Array (GRETINA). The experimental level energies, ground-state magnetic moment, intrinsic quadrupole moment, and γ-ray intensities show good agreement with the strong-coupling limit of a rotational model.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Examination of the low-energy enhancement of the γ -ray strength function of Fe 56

A model-independent technique was used to determine the gamma-ray strength function (gamma SF) of Fe-56 down to gamma-ray energies less than 1 MeV for the first time with GRETINA using the (p, p') reaction at 16MeV. No difference was observed in the energy dependence of the gamma SF built on 2(+) and 4(+) final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

Jones, M. D.↗
DOE OSTI2018

Intruder configurations of excited states in the neutron-rich isotopes P 33 and P 34

Here, excited states in the neutron-rich isotopes P 33 and P 34 were populated by the O 18 + O 18 fusion-evaporation reaction at E lab = 24 MeV. The Gammasphere array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus S 36 . The use of Microball enabled the selection of the proton emission channel. It also helped in determining the exact position and energy of the emitted proton; this was later employed in kinematic Doppler corrections. 16 new transitions and 13 new states were observed in P 33 and 21 γ rays and 20 energy levels were observed in P 34 for the first time. The nearly 4 π geometry of Gammasphere allowed the measurement of γ -ray angular distributions leading to spin assignments for many states. The experimental observations for both isotopes were interpreted with the help of shell-model calculations using the (0+1) ℏ ω PSDPF interaction. The calculations accounted for both the 0p-0h and 1p-1h states reasonably well and indicated that 2p-2h excitations might dominate the higher-spin configurations in both P 33 and P 34 .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Lifetime Measurement of the 2{sub 1}{sup +} State in {sup 20}C

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2{sub 1}{sup +} state in the near-dripline nucleus {sup 20}C. The deduced value of {tau}{sub 2{sub 1}{sup +}}=9.8{+-}2.8(stat){sub -1.1}{sup +0.5}(syst) ps gives a reduced transition probability of B(E2;2{sub 1}{sup +}{yields}0{sub g.s.}{sup +})=7.5{sub -1.7}{sup +3.0}(stat){sub -0.4}{sup +1.0}(syst) e{sup 2} fm{sup 4} in good agreement with a shell model calculation using isospin-dependent effective charges.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

First Spectroscopy of the Near Drip-line Nucleus Mg 40

One of the most exotic light neutron-rich nuclei currently accessible for experimental study is 40 Mg , which lies at the intersection of the nucleon magic number N=28 and the neutron drip line. Low-lying excited states of 40 Mg have been studied for the first time following a one-proton removal reaction from 41 Al, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 γ-ray array and the ZeroDegree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the Z=12, N ≥ 20 Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Effect of Alkyl Chain Branching Point on 3D Crystallinity in High N‐Type Mobility Indolonaphthyridine Polymers

Abstract Herein, this study investigates the impact of branching‐point‐extended alkyl chains on the charge transport properties of three ultrahigh n‐type mobility conjugated polymers. Using grazing incidence wide‐angle X‐ray scattering, analysis of the crystallinity of the series shows that while π–π interactions are increased for all three polymers as expected, the impact of the side‐chain engineering on polymer backbone crystallinity is unique to each polymer and correlates to the observed changes in charge transport. With the three polymers exhibiting n‐type mobilities between 0.63 and 1.04 cm 2 V −1 s −1 , these results ratify that the indolonaphthyridine building block has an unprecedented intrinsic ability to furnish high‐performance n‐type organic semiconductors.

Fallon, Kealan J.↗