.
A review of selected experimental works on the gamma-decay from the Giant and Pygmy Dipole Resonances is presented. The common feature of these experiments is that gamma-decay originates from ...dipole states populated using reactions induced by heavy ions. The focus is the investigation of dipole modes built on the ground and excited states. The major developments made during the years regarding the detection of high-energy gamma-rays are briefly discussed together with specific results concerning a chosen number of problems among those that were addressed by the theoretical works of P.F. Bortignon. They are: i) the dipole mode in the initial stages of reactions among heavy ions with different
N
/
Z
values for the target and projectile; ii) the problem of isospin symmetry in nuclei at finite temperature; iii) pygmy states far from stability; iv) the nature of the pygmy states.
The γ decay of the elusive narrow, near-threshold proton resonance in 11B was investigated at Laboratori Nazionali di Legnaro (INFN) in a particle-γ coincidence experiment, using the 6Li(6Li,pγ) ...fusion-evaporation reaction and the GALILEO-GALTRACE setup. No clear signature was found for a possible E1 decay to the 1/21−, first-excited state of 11B, predicted by the Shell Model Embedded in the Continuum (SMEC) with a branching of 0.98−69+167×10−3 with respect to the dominant particle-decaying modes. The statistical analysis of the γ-ray spectrum provided an average upper limit of 2.37×10−3 for this γ-ray branching, with a global significance of 5σ. On the other hand, by imposing a global confidence level of 3σ, a significant excess of counts was observed for E=γ9300(20) keV, corresponding to a resonance energy of 11429(20) keV (namely 200(20) keV above the proton separation energy of 11B) and a γ-ray branching of 1.12(35)×10−3. This result is compatible with the SMEC calculations, potentially supporting the existence of a near-threshold proton resonance in 11B.
Isomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon
238
U beam. Isomers ...reported in nuclei spanning a predicted prolate-oblate shape change boundary,
111
Zr (
E
=
283.1
keV;
τ
=
0.326
(
63
)
μ
s),
112
Nb (
E
=
44.2
keV;
τ
=
0.094
(
26
)
μ
s),
113
Nb (
E
=
135.4
keV;
τ
=
0.846
(
80
)
μ
s), and
115
Mo (
E
=
198.6
keV;
τ
=
63
(
4
)
μ
s), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from
γ
-ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for
N
>
70
in this region. In addition, isomers in
108
Nb,
109
Nb,
113
Tc,
117
Ru,
119
Ru,
120
Rh, and
122
Rh, not spanning the prolate-oblate transition discussed, are presented.
High energy gamma-rays from the 116Sn + 24Mg reaction at 23A MeV were measured using the MEDEA detector at LNS – INFN Catania. Combining this new data with previous measurements yields a detailed ...view of the quenching of the Giant Dipole Resonance as a function of excitation energy in nuclei of mass A in the range 120÷132. The transition towards the disappearance of the dipole strength, which occurs around 230 MeV excitation energy, appears to be remarkably sharp. Current phenomenological models give qualitative explanations for the quenching but cannot reproduce its detailed features.
Investigation of the properties of a 1″×1″ LaBr3:Ce scintillator Nicolini, R.; Camera, F.; Blasi, N. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2007, Volume:
582, Issue:
2
Journal Article
A natural silicon target was investigated in a
nat
Si(
γ
,
γ
′
) photon-scattering experiment with fully linearly-polarised, quasi-monochromatic
γ
rays in the entrance channel. The mean photon ...energies used were
⟨
E
γ
⟩
=
9.33, 9.77, 10.17, 10.55, 10.93, and 11.37 MeV, and the relative energy spread (full width at half maximum) of the incident beam was
Δ
E
γ
/
⟨
E
γ
⟩
≈
3.5–4 %. The observed angular distributions for the ground-state decay allow firm spin and parity assignments for several levels of the stable even-even silicon isotopes.
A novel technique for measuring the HPGe detector pulse shape as a function of the γ-ray interaction position inside the detector volume is presented. This technique is based on a specific pulse ...shape comparison procedure. Its main feature is that it allows to characterize the 3D position response of a HPGe segmented detector in a much shorter time as compared with the standard coincidence techniques. The method was first validated using a GEANT simulation of a 36-fold HPGe AGATA detector realized taking into account the effects of the electronic chain response and electrical noise on the calculated signal shape. This procedure was then applied to extract experimentally the position response of a non-segmented coaxial HPGe detector along the radial direction, using a 438
MBq
137Cs collimated γ-source. The results of this measurement show a dependence of the pulse shape as a function of γ-ray interaction radial coordinate consistent with that obtained with calculations. The signal acquisition rate reached using this characterization technique allows to realize a full scan of a large volume highly segmented HPGe detector in less than a week.
g factor of the 12+K-isomer in 174W Rocchini, M.; Nannini, A.; Benzoni, G. ...
The European physical journal. A, Hadrons and nuclei,
2020/11, Volume:
56, Issue:
11
Journal Article
Peer reviewed
The
g
factor of the
12
+
K
-isomer in
174
W has been measured by means of the time-differential perturbed angular distribution technique as
g
(
12
+
)
=
+
0.304
(
11
)
. In addition, the half-life of ...the isomer has been remeasured as
T
1
/
2
(
12
+
)
=
124
(
8
)
ns, in agreement with the literature value and confirming the anomalous hindrance
F
of the
E
2 transition to the
10
+
level of the ground state band with respect to the
γ
-tunnelling model prediction. The measured
g
factor has been compared with estimates based on experimental
g
factors from odd-mass isotopes in the same mass region and with Nilsson model calculations. The results establish unique features of the
12
+
K
-isomer in
174
W, which can possess a non-pure intrinsic configuration and/or can be characterised by values of the intrinsic quadrupole moment
Q
0
and the rotational
g
factor
g
R
significantly different with respect to the majority of
K
-isomers at mass
A
≈
180
.
The PreSPEC setup for high-resolution γ-ray spectroscopy using radioactive ion beams was employed for experimental campaigns in 2012 and 2014. The setup consisted of the state of the art Advanced ...GAmma Tracking Array (AGATA) and the High Energy γ deteCTOR (HECTOR+) positioned around a secondary target at the final focal plane of the GSI FRagment Separator (FRS) to perform in-beam γ-ray spectroscopy of exotic nuclei. The Lund York Cologne CAlorimeter (LYCCA) was used to identify the reaction products. In this paper we report on the trigger scheme used during the campaigns. The data-flow coupling between the Multi-Branch System (MBS) based Data AcQuisition (DAQ) used for FRS-LYCCA and the “Nouvelle Acquisition temps Réel Version 1.2 Avec Linux” (NARVAL) based acquisition system used for AGATA are also described.
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