45 research outputs found
Approximate solution of the pairing Hamiltonian in the Berggren basis
We derive the approximate solution for the pairing Hamiltonian in the
Berggren ensemble of single particle states including bound, resonance and
non-resonant scattering states. We show that this solution is reliable in the
limit of a weak pairing interaction
Magnetic dipole -ray strength functions of heavy nuclei in the configuration-interaction shell model
A low-energy enhancement (LEE) has been observed in the deexcitation
-ray strength function (SF) of compound nuclei. The LEE has
been a subject of intense experimental and theoretical interest since its
discovery, and, if the LEE persists in heavy neutron-rich nuclei, it would have
significant effects on calculations of r-process nucleosynthesis. Standard
configuration-interaction (CI) shell-model calculations in medium-mass nuclei
have attributed the LEE to the magnetic dipole SF but such calculations
are computationally intractable in heavy nuclei. We review a combination of
beyond-mean-field many-body methods within the framework of the CI shell model
that enables the calculation of SF in heavy nuclei, and discuss the
recent theoretical identification of a LEE in the magnetic dipole SF of
lanthanide isotopes.Comment: 8 pages, 5 figure
Description of Be, Li and Be nuclei within the Gamow Shell Model
In this work we study spectra of Be, Li, Be and elastic
scattering cross sections He(He, He)He, He(H,
H)He within the Gamow shell model (GSM) in the coupled-channel
formulation (GSM-CC). The evolution of channel amplitudes and the alignment of
the many-body state with the decay channel in the vicinity of the channel
threshold is studied for selected states. The GSM-CC in multi-mass partition
formulation applied to a translationally invariant Hamiltonian with an
effective finite-range two-body interaction reproduce well the spectra of
Be, Li, Be and elastic scattering reactions: He(He,
He)He, He(H, H)He. Detailed analysis of the dependence
of reaction channel amplitudes on the distance from the particle decay
threshold allowed to demonstrate the alignment of the wave function in the
vicinity of the decay threshold. This analysis also demonstrates the appearance
of clustering in the GSM-CC wave function in the vicinity of the cluster decay
threshold. We demonstrated that GSM formulated in the basis of reaction
channels including both cluster and proton/neutron channels allows to describe
both the spectra of nuclei with low-energy cluster thresholds and the
low-energy elastic scattering reactions with proton, H, and He
projectiles. Studying dependence of the reaction channel amplitude in a
many-body state on distance from the threshold, we showed an evolution of the
He, He clustering with increasing separation energy from the cluster
decay threshold and demonstrated a mechanism of the alignment of many-body wave
function with the decay threshold, i.e. the microscopic reorganization of the
wave function in the vicinity of the cluster decay threshold which leads to the
appearance of clustering in this state.Comment: 20 pages, 12 figure
Machine learning approach to pattern recognition in nuclear dynamics from the ab initio symmetry-adapted no-core shell model
A novel machine learning approach is used to provide further insight into
atomic nuclei and to detect orderly patterns amidst a vast data of large-scale
calculations. The method utilizes a neural network that is trained on ab initio
results from the symmetry-adapted no-core shell model (SA-NCSM) for light
nuclei. We show that the SA-NCSM, which expands ab initio applications up to
medium-mass nuclei by using dominant symmetries of nuclear dynamics, can reach
heavier nuclei when coupled with the machine learning approach. In particular,
we find that a neural network trained on probability amplitudes for -and
-shell nuclear wave functions not only predicts dominant configurations for
heavier nuclei but in addition, when tested for the Ne ground state, it
accurately reproduces the probability distribution. The nonnegligible
configurations predicted by the network provide an important input to the
SA-NCSM for reducing ultra-large model spaces to manageable sizes that can be,
in turn, utilized in SA-NCSM calculations to obtain accurate observables. The
neural network is capable of describing nuclear deformation and is used to
track the shape evolution along the Mg isotopic chain, suggesting a
shape-coexistence that is more pronounced toward the very neutron-rich
isotopes. We provide first descriptions of the structure and deformation of
Si and Mg of interest to x-ray burst nucleosynthesis, and even of
the extremely heavy nuclei such as Er and U, that build
upon first principles considerations.Comment: 10 pages, 9 figure
Ab initio translationally invariant nucleon-nucleus optical potentials
We combine the \textit{ab initio} symmetry-adapted no-core shell model
(SA-NCSM) with the single-particle Green's function approach to construct
optical potentials rooted in first principles. Specifically, we show that total
cross sections and phase shifts for neutron elastic scattering from a He
target with projectile energies between 0.5 and 10 MeV closely reproduce the
experiment. In addition, we discuss an important new development that resolves
a long-standing issue with spurious center-of-mass motion in the Green's
function formalism for many-body approaches. The new development opens the path
for first-principle predictions of cross sections for elastic scattering of
single-nucleon projectiles, nucleon capture and deuteron breakup reactions,
feasible for a broad range of open-shell spherical and deformed nuclei in the
SA-NCSM approach.Comment: 19 pages, 11 figures, to be submitted to Physical Review
Ab initio single-neutron spectroscopic overlaps in lithium isotopes
We calculate single-neutron spectroscopic overlaps for lithium isotopes in
the framework of the \textit{ab initio} symmetry-adapted no-core shell model.
We report the associated neutron-nucleus asymptotic normalization coefficients
(ANCs) and spectroscopic factors (SFs) that are important ingredients in many
reaction cross section calculations. While spectroscopic factors have been
traditionally extracted from experimental cross sections, their sensitivity on
the type of reactions, energy, and the underlying models point to the need for
determining SF from first-principle structure considerations. As illustrative
examples, we present Li+n, Li+n, and Li+n, and we show that the
results are in a good agreement with those of other \textit{ab initio} methods,
where available, including the quantum Monte Carlo approach. We compare ANCs
and SFs to available experimentally deduced values, with a view toward
expanding this study to heavier nuclei and to extracting inter-cluster
effective interactions for input into analyses of existing and future
experimental data.Comment: 10 pages, 8 figure
A Drosophila screen identifies NKCC1 as a modifier of NGLY1 deficiency
N-Glycanase 1 (NGLY1) is a cytoplasmic deglycosylating enzyme. Loss-of-function mutations in the NGLY1 gene cause NGLY1 deficiency, which is characterized by developmental delay, seizures, and a lack of sweat and tears. To model the phenotypic variability observed among patients, we crossed a Drosophila model of NGLY1 deficiency onto a panel of genetically diverse strains. The resulting progeny showed a phenotypic spectrum from 0 to 100% lethality. Association analysis on the lethality phenotype, as well as an evolutionary rate covariation analysis, generated lists of modifying genes, providing insight into NGLY1 function and disease. The top association hit was Ncc69 (human NKCC1/2), a conserved ion transporter. Analyses in NGLY1-/- mouse cells demonstrated that NKCC1 has an altered average molecular weight and reduced function. The misregulation of this ion transporter may explain the observed defects in secretory epithelium function in NGLY1 deficiency patients
Optical potentials for the rare-isotope beam era
We review recent progress and motivate the need for further developments in
nuclear optical potentials that are widely used in the theoretical analysis of
nucleon elastic scattering and reaction cross sections. In regions of the
nuclear chart away from stability, which represent a frontier in nuclear
science over the coming decade and which will be probed at new rare-isotope
beam facilities worldwide, there is a targeted need to quantify and reduce
theoretical reaction model uncertainties, especially with respect to nuclear
optical potentials. We first describe the primary physics motivations for an
improved description of nuclear reactions involving short-lived isotopes,
focusing on its benefits for fundamental science discoveries and applications
to medicine, energy, and security. We then outline the various methods in use
today to build optical potentials starting from phenomenological, microscopic,
and ab initio methods, highlighting in particular the strengths and weaknesses
of each approach. We then discuss publicly-available tools and resources
facilitating the propagation of recent progresses in the field to
practitioners. Finally, we provide a set of open challenges and recommendations
for the field to advance the fundamental science goals of nuclear reaction
studies in the rare-isotope beam era.Comment: This paper is the outcome of the Facility for Rare Isotope Beams
Theory Alliance (FRIB - TA) topical program "Optical Potentials in Nuclear
Physics" held in March 2022 at FRIB. Its content is non-exhaustive, was
chosen by the participants and reflects their efforts related to optical
potential