Overview of neutron–proton pairing Frauendorf, S.; Macchiavelli, A.O.
Progress in particle and nuclear physics,
09/2014, Letnik:
78, Številka:
C
Journal Article
Recenzirano
Odprti dostop
The role of neutron–proton pairing correlations on the structure of nuclei along the N=Z line is reviewed. Particular emphasis is placed on the competition between isovector (T=1) and isoscalar (T=0) ...pair fields. The expected properties of these systems, in terms of pairing collective motion, are assessed by different theoretical frameworks including schematic models, realistic Shell Model and mean field approaches. The results are contrasted with experimental data with the goal of establishing clear signals for the existence of neutron–proton (np) condensates. We will show that there is clear evidence for an isovector np condensate as expected from isospin invariance. However, and contrary to early expectations, a condensate of deuteron-like pairs appears quite elusive and pairing collectivity in the T=0 channel may only show in the form of a phonon. Arguments are presented for the use of direct reactions, adding or removing an np pair, as the most promising tool to provide a definite answer to this intriguing question.
We present expressions for the matrix elements of the spin–spin operator
S
n
·
S
p
in a variety of coupling schemes. These results are then applied to calculate the expectation value
⟨
S
n
·
S
p
⟩
in ...eigenstates of a schematic Hamiltonian describing neutrons and protons interacting in a single-
l
shell through a Surface Delta Interaction. The model allows us to trace
⟨
S
n
·
S
p
⟩
as a function of the competition between the isovector and isoscalar interaction strengths and the spin–orbit splitting of the
j
=
l
±
1
/
2
shells. We find negative
⟨
S
n
·
S
p
⟩
values in the ground state of all even–even
N
=
Z
nuclei, contrary to what has been observed in hadronic inelastic scattering at medium energies. We discuss the possible origin of this discrepancy and indicate directions for future theoretical and experimental studies related to neutron–proton spin–spin correlations.
The apparent splitting between orbitals that are spin-orbit partners can be substantially influenced by the effects of weak binding. In particular, such effects can account for the observed decrease ...in separation of the neutron 1p_{3/2} and 1p_{1/2} orbitals between the ^{41}Ca and ^{35}Si isotopes. This behavior has been the subject of recent experimental and theoretical works and cited as evidence for a proton "bubble" in ^{34}Si causing an explicit weakening of the spin-orbit interaction. The results reported here suggest that the change in the separation between the 1p_{3/2} and 1p_{1/2} partners occurs dominantly because of the behavior of the energies of these 1p neutron states near zero binding.
Here, absolute cross sections for the addition of s- and d-wave neutrons to 14C and 14N have been determined simultaneously via the (d,p) reaction at 10 MeV/u. The difference between the neutron and ...proton separation energies, ΔS, is around -20 MeV for the 14C+n system and +8 MeV for 14N+n. The population of the 1s1/2 and 0d5/2 orbitals for both systems is reduced by a factor of approximately 0.5 compared with the independent single-particle model, or about 0.6 when compared with the shell model. This finding strongly contrasts with results deduced from intermediate-energy knockout reactions between similar nuclei on targets of 9Be and 12C. The simultaneous technique used removes many systematic uncertainties.
The neutron-rich nucleus Ba-144 (t(1/2) = 11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for ...such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV Ba-144 beam on a 1.0-mg/cm(2) Pb-208 target. The measured value of the matrix element, < 3(1)(-)parallel to M(E3)parallel to 0(1)(+)> = 0.65((+17)(-23)) eb(3/2,) corresponds to a reduced Bd(E3) transition probability of 48((+25)(-34)) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.
.
We present an account of the current status of the theoretical treatment of inclusive (
d
,
p
) reactions in the breakup-fusion formalism, pointing to some applications and making the connection ...with current experimental capabilities. Three independent implementations of the reaction formalism have been recently developed, making use of different numerical strategies. The codes also originally relied on two different but equivalent representations, namely the prior (Udagawa-Tamura, UT) and the post (Ichimura-Austern-Vincent, IAV) representations. The different implementations have been benchmarked for the first time, and then applied to the Ca isotopic chain. The neutron-Ca propagator is described in the Dispersive Optical Model (DOM) framework, and the interplay between elastic breakup (EB) and non-elastic breakup (NEB) is studied for three Ca isotopes at two different bombarding energies. The accuracy of the description of different reaction observables is assessed by comparing with experimental data of (
d
,
p
) on
40,48
Ca. We discuss the predictions of the model for the extreme case of an isotope (
60
Ca) currently unavailable experimentally, though possibly available in future facilities (nominally within production reach at FRIB). We explore the use of (
d
,
p
) reactions as surrogates for
(
n
,
γ
)
processes, by using the formalism to describe the compound nucleus formation in a
(
d
,
p
γ
)
reaction as a function of excitation energy, spin, and parity. The subsequent decay is then computed within a Hauser-Feshbach formalism. Comparisons between the
(
d
,
p
γ
)
and
(
n
,
γ
)
induced gamma decay spectra are discussed to inform efforts to infer neutron captures from
(
d
,
p
γ
)
reactions. Finally, we identify areas of opportunity for future developments, and discuss a possible path toward a predictive reaction theory.
We propose a phenomenological approach to examine the role of short- and long-range nucleon-nucleon correlations in the quenching of single-particle strength in atomic nuclei and their evolution in ...asymmetric nuclei and neutron matter. These correlations are thought to be the reason for the quenching of spectroscopic factors observed in (e,e′p), (p,2p) and transfer reactions. We show that the recently observed increase of the high-momentum component of the protons in neutron-rich nuclei is consistent with the reduced proton spectroscopic factors. Our approach connects recent results on short-range correlations from high-energy electron scattering experiments with the quenching of spectroscopic factors and addresses for the first time quantitatively this intriguing question in nuclear physics, in particular regarding its isospin dependence. We also speculate about the nature of a quasi-proton (nuclear polaron) in neutron matter and its kinetic energy, an important quantity for the properties of neutron stars.
Two-Neutron Halo is Unveiled in F 29 Bagchi, S.; Kanungo, R.; Tanaka, Y. K. ...
Physical review letters,
06/2020, Letnik:
124, Številka:
22
Journal Article
Recenzirano
Odprti dostop
We report the measurement of reaction cross sections (σexR) of 27,29F with a carbon target at RIKEN. The unexpectedly large σexR and derived matter radius identify 29F as the heaviest two-neutron ...Borromean halo to date. The halo is attributed to neutrons occupying the 2p3/2 orbital, thereby vanishing the shell closure associated with the neutron number N=20. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of 27F but are challenged for 29F.
In an experiment performed at Lawrence Berkeley National Laboratory's 88-inch cyclotron, the isotope 244Md was produced in the 209Bi (40Ar , 5n) reaction. Decay properties of 244Md were measured at ...the focal plane of the Berkeley Gas-filled Separator, and the mass number assignment of A = 244 was confirmed with the apparatus for the identification of nuclide A . The isotope 244Md is reported to have one, possibly two, α -decaying states with α energies of 8.66(2) and 8.31(2) MeV and half-lives of 0.4 + 0.4 − 0.1 and ∼ 6 s , respectively. Additionally, first evidence of the α decay of 236Bk was observed and is reported.
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