.
An overview of relevant results on the study of 1
−
states focusing on their excitation with nuclear probes is given. Results obtained for the
90
Zr ,
124
Sn , and
208
Pb nuclei using the (
17
O,
...17
O′γ) reaction are compared with available data obtained with the (γ, γ′, and (p, p′) reactions. These comparisons allow to learn on the nature of the populated states, particularly the E1 states, whose isospin character is presently poorly known. The DWBA description of the data is discussed in terms of different form factors, standard collective form factor and form factors obtained by folding microscopically calculated transition densities. The relevant aspects related to the used theoretical approach are also presented. The main objective of the analyses is the extraction of the values of the fraction of the energy weighted sum rule strength for the isoscalar dipole excitation. For completeness, in all cases, the DWBA analysis was made also for the excitations of 2
+
and 3
−
states.
.
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 isospin mixing was deduced in the compound nucleus Zr-80 at an excitation energy of E* = 54 MeV from the gamma decay of the giant dipole resonance. The reaction Ca-40 + Ca-40 at E-beam = 136 MeV ...was used to form the compound nucleus in the isospin I = 0 channel, while the reaction Cl-37 + Ca-44 at E-beam = 95 MeV was used as the reference reaction. The. rays were detected with the AGATA demonstrator array coupled with LaBr3:Ce detectors. The temperature dependence of the isospin mixing was obtained and the zero-temperature value deduced. The isospin-symmetry-breaking correction delta(C) used for the Fermi superallowed transitions was extracted and found to be consistent with beta-decay data.
The properties of pygmy dipole states in Pb-208 were investigated using the Pb-208(O-17, O-17'gamma) reaction at 340 MeV and measuring the gamma decay with high resolution with the AGATA demonstrator ...array. Cross sections and angular distributions of the emitted gamma rays and of the scattered particles were measured. The results are compared with (gamma, gamma') and (p, p') data. The data analysis with the distorted wave Born approximation approach gives a good description of the elastic scattering and of the inelastic excitation of the 2(+) and 3(-) states. For the dipole transitions a form factor obtained by folding a microscopically calculated transition density was used for the first time. This has allowed us to extract the isoscalar component of the 1(-) excited states from 4 to 8 MeV.
We demonstrate the possibility to extract nuclear state femtosecond lifetimes from two-step
γ
ray cascades measured with a Ge-detector array. The technique is based on measuring the Doppler shift of ...a
γ
ray, caused by the recoil of a preceding
γ
ray emission. Since the two
γ
rays are populating/de-populating the same state they form a start/stop signal, the delay of which is compared to the slowing down motion of the nucleus within the target material. A multi-detector array combined with digital acquisition electronics in list mode, allows to measure several angular combinations and two-step cascades efficiently and simultaneously within one single experiment. The concept was demonstrated with the FIPPS array for the
35
Cl(n,
γ
)
36
Cl
reaction, where we obtained good agreement with literature values showing the validity of the method.
The gamma decay from the high-lying states of Ce-140 excited via inelastic scattering of O-17 at a bombarding energy of 340 MeV was measured using the high-resolution AGATA-demonstrator array in ...coincidence with scattered ions detected in two segmented Delta E-E silicon detectors. Angular distributions of scattered ions and emitted gamma rays were measured, as well as their differential cross sections. The excitation of 1(-) states below the neutron separation energy is similar to the one obtained in reactions with the alpha isoscalar probe. The comparison between the experimental differential cross sections and the corresponding predictions using the distorted-wave Born approximation allowed us to extract the isoscalar component of identified 1(-) pygmy states. For this analysis the form factor obtained by folding microscopically calculated transition densities and optical potentials was used.
Abstract
The structure of the
11
/
2
1
+
state in
131
Sb was investigated at the LOHENGRIN spectrometer of Institut Laue-Langevin via neutron-induced fission of
235
U and lifetime measurements, ...yielding
T
1
/
2
=
3
(
2
)
, at the edge of the sensitivity of the experimental method. This first result for the
11
/
2
1
+
state half-life in neutron-rich Sb isotopes provides a quadrupole reduced transition probability to the ground state of
B
(
E
2
)
=
1.4
−
0.6
+
1.5
W.u., very close to the
B
(
E
2
;
2
1
+
→
0
1
+
)
in
130
Sn. Realistic shell-model calculations reproduce well both the experimental level scheme of
131
Sb and the
B
(
E
2
;
11
/
2
1
+
→
7
/
2
1
+
)
value, indicting a dominant
2
+
(
130
Sn
)
⊗
π
g
7
/
2
core-proton coupled configuration for the
11
/
2
1
+
state.
AGATA is a modern
γ
-ray spectrometer for in-beam nuclear structure studies, based on
γ
-ray tracking. Since more than a decade, it has been operated performing experimental physics campaigns in ...different international laboratories (LNL, GSI, GANIL). This paper reviews the obtained results concerning the performances of
γ
-ray tracking in AGATA and associated algorithms. We discuss
γ
-ray tracking and algorithms developed for AGATA. Then, we present performance results in terms of efficiency and peak-to-total for AGATA. The importance of the high effective angular resolution of
γ
-ray tracking arrays is emphasised, e.g. with respect to Doppler correction. Finally, we briefly touch upon the subject of
γ
-ray imaging and its connection to
γ
-ray tracking.
Lifetime measurements in 131Sb at LOHENGRIN Bottoni, S.; Gamba, E. R.; De Gregorio, G. ...
Journal of physics. Conference series,
09/2023, Letnik:
2586, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The neutron-rich 131Sb nucleus was studied at Institut Laue-Langevin, with the LOHENGRIN spectrometer, via neutron-induced fission of 235U and lifetime measurements using LaBr3(Ce) detectors. The ...lifetimes of the 11/21+ and 19/21+ states were measured with the generalized centroid difference and the decay slope methods, yielding T½=3(2) ps and T½=1347(171) ps, respectively. The measured value for the 11/21+ is at the limit of the fast-timing technique. The level scheme of 131Sb and the decay properties of the 11/21+ state were described by realistic shell-model calculations, which reproduce well both the experimental spin and energy sequences of 131Sb and the measured B(E2; 11/21+→7/21+) value, pointing to an almost pure 2+(130Sn) ⊗ πg7/2, core-proton coupled configuration for the 11/21+ state.
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