Lifetimes of low-lying excited states in the νi13/2+ bands of the neutron-deficient osmium isotopes 169,171,173Os have been measured for the first time using the recoil-distance Doppler shift and ...recoil-isomer tagging techniques. An unusually low value is observed for the ratio B(E2;21/2+→17/2+)/B(E2;17/2+→13/2+) in 169Os, similar to the “anomalously” low values of the ratio B(E2;41+→21+)/B(E2;21+→0gs+) previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighbouring 168,170Os. Furthermore, the evolution of B(E2;21/2+→17/2+)/B(E2;17/2+→13/2+) with increasing neutron number in the odd-mass isotopic chain 169,171,173Os is observed to follow the same trend as observed previously in the even-even Os isotopes. These findings indicate that the possible quantum phase transition from a seniority conserving structure to a collective regime as a function of neutron number suggested for the even-even systems is maintained in these odd-mass osmium nuclei, with the odd valence neutron merely acting as a “spectator”. As for the even-even nuclei, the phenomenon is highly unexpected for nuclei that are not situated near closed shells.
Fission fragment atomic number measurements using Bragg detectors Sosnin, N.V.; Smith, A.G.; Wright, T. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2020, Letnik:
957
Journal Article
Recenzirano
Odprti dostop
A Bragg detector with a Frisch grid and 1 metre-long time-of-flight section with microchannel plate assemblies was characterized at the Lohengrin fission fragment separator at the Institut ...Laue-Langevin (ILL) by measuring 235U fission fragments of selected masses and energies at the Lohengrin focal plane. The aim of the measurements was to investigate the response of Bragg detectors to differing nuclear charge states. Energy-loss and a signal-risetime dependent parameters were defined and extracted from the accumulated signal pulse shapes, and their behaviour as a function of fragment velocity was investigated. The experimental results are compared to a SRIM-2013 simulation. Measurements of fission fragment ranges in isobutane are presented. Average proton numbers for measured fragment masses were identified and could be resolved by the parameters, which exhibited monotonic variation with charge. The results differed from SRIM both in trends and in values, with SRIM simulations exhibiting non-monotonic behaviour with proton number.
A new method of extracting the γ-ray intensities necessary to perform lifetime measurements using the differential decay curve method (DDCM) is presented in this work, the unresolved Doppler-shifted ...components method (UDCM). The UDCM allows for a DDCM analysis to be performed using a γ-ray transition for which the fully Doppler-shifted and degraded components are unresolvable in energy and so are detected as a single peak. This technique was used to measure the known lifetime of the yrast 21+ state in 50Mn with a depopulating transition that does not have resolvable fully Doppler-shifted and degraded components. The lifetime measured through applying the UDCM was consistent with the standard DDCM measurement of the 21+ state. Use of the UDCM allows for DDCM lifetime measurements to be made using transitions of smaller γ-ray energies, smaller recoil velocities and, in some cases, with a smaller uncertainty. In contrast to a standard DDCM analysis, a UDCM analysis is also independent of the widths of the fully Doppler-shifted and degraded components and as a result they do not need to be determined.
The concept, design and prototype developments for the Lund–York–Cologne CAlorimeter (LYCCA) is presented. LYCCA is a modular device for the NUclear STructure, Astrophysics and Reactions (NUSTAR) ...science pillar of the Facility for Antiproton and Ion Research (FAIR) at Darmstadt, Germany. LYCCA is designed to discriminate heavy ions produced in nuclear reactions induced by relativistic radioactive ion beams. Measurements of energy loss, total energy, and time-of-flight allow the derivation of proton number, Z, and mass number, A, of the reaction products. LYCCA-inherent tracking of the flight paths of the reaction products enables coincident HIgh-resolution in-beam γ-ray SPECtroscopy (HISPEC) of atomic nuclei far from the line of β-stability.
Photo-response of the N=Z nucleus 24Mg Deary, J.; Scheck, M.; Schwengner, R. ...
The European physical journal. A, Hadrons and nuclei,
7/9, Letnik:
59, Številka:
9
Journal Article
Recenzirano
Odprti dostop
The electric
E
1 and magnetic
M
1 dipole responses of the
N
=
Z
nucleus
24
Mg were investigated in an inelastic photon scattering experiment. The 13.0 MeV electrons, which were used to produce the ...unpolarised bremsstrahlung in the entrance channel of the
24
Mg(
γ
,
γ
′
) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one
J
π
=
1
-
, four
J
π
=
1
+
, and six
J
π
=
2
+
states in
24
Mg. De-excitation
γ
rays were detected using the four high-purity germanium detectors of the
γ
ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total
B
(
M
1
)
↑
=
2.7
(
3
)
μ
N
2
is observed, but this
N
=
Z
nucleus exhibits only marginal
E
1 strength of less than
∑
B
(
E
1
)
↑
≤
0.61
×
10
-
3
e
2
fm
2
. The
B
(
Π
1
,
1
i
π
→
2
1
+
)
/
B
(
Π
1
,
1
i
π
→
0
gs
+
)
branching ratios in combination with the expected results from the Alaga rules demonstrate that
K
is a good approximative quantum number for
24
Mg. The use of the known
ρ
2
(
E
0
,
0
2
+
→
0
gs
+
)
strength and the measured
B
(
M
1
,
1
+
→
0
2
+
)
/
B
(
M
1
,
1
+
→
0
gs
+
)
branching ratio of the 10.712 MeV
1
+
level allows, in a two-state mixing model, an extraction of the difference
Δ
β
2
2
between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV
0
2
+
level.
Fast-timing measurements at the focal plane of a separator can suffer from poor timing resolution. This is due to the variations in time-of-flight (ToF) for photons travelling to a given detector, ...which arise from the changes in the implantation positions of the recoil nuclei emitting the γ rays of interest. In order to minimise these effects on timing measurements, a procedure is presented that improves fast-timing data by performing ToF corrections on an event-by-event basis. This method was used to correct data collected with an array of eight LaBr3 detectors, which detected γ rays from spatially distributed 138Gd recoil-implants at the focal plane of the Recoil-Ion-Transport-Unit (RITU) spectrometer. The Generalised Centroid Difference (GCD) method was used to extract a lifetime from data in conjunction with a new procedure to calibrate the time walk. The lifetime of the first 2+ state in 138Gd, populated by the decay of the Kπ=8− isomeric state, was measured to be 229(24) ps using the ToF-corrected data, which is consistent within three standard deviations to the literature value. The results together with Monte-Carlo simulations show that the ToF correction procedure reduced the uncertainty in the measured lifetimes by 3 % in the case of the spatially distributed nuclei at the focal plane of RITU. However, ∼12 % has been estimated for a similar experiment when using a larger focal plane i.e. the Super-FRS at the FAIR facility.
Abstract
Excited states in the extremely neutron-deficient nucleus
$$^{172}{\mathrm {Pt}}$$
172
Pt
were populated via
$$^{96}\mathrm {Ru}(^{78}\mathrm {Kr},2p)$$
96
Ru
(
78
Kr
,
2
p
)
and
...$$^{92}\mathrm {Mo}(^{83}\mathrm {Kr},3n)$$
92
Mo
(
83
Kr
,
3
n
)
reactions. The level scheme has been extended up to an excitation energy of
$$\approx 5~\hbox {MeV}$$
≈
5
MeV
and tentative spin-parity assignments up to
$$I^\pi = 18^+$$
I
π
=
18
+
. Linear polarization and angular distribution measurements were used to determine the electromagnetic E1 character of the dipole transitions connecting the positive-parity ground-state band with an excited side-band, firmly establishing it as a negative-parity band. The lowest member of this negative-parity structure was firmly assigned spin-parity
$$3^-$$
3
-
. In addition, we observed an E3 transition from this
$$3^-$$
3
-
state to the ground state, providing direct evidence for octupole collectivity in
$$^{172}{\mathrm {Pt}}$$
172
Pt
. Large-scale shell model (LSSM) and total Routhian surface (TRS) calculations have been performed, supporting the interpretation of the
$$3^-$$
3
-
state as a collective octupole-vibrational state.
The electromagnetic transition probabilities of the yrast 2+ states in the midshell Te isotopes, two protons above the closed shell at Sn, are of great importance for the understanding of nuclear ...collectivity in these isotopes and the role played by the neutron-proton interactions and cross-shell excitations. However, the large uncertainty of the experimental data for the midshell nucleus 118Te and the missing data for 116Te make it difficult to pin down the general trend of the evolution of transition probabilities as a function of the neutron number. In this work, the lifetime of the yrast 2+ state in 118Te was measured, with the aim of reducing the uncertainty of the previous measurement. The result is τ2+ = 7.46(19) ps. In addition, the lifetime of the 4+ state was measured to be τ4+ = 4.25(23) ps. The experimental transition rates are extracted from the measured lifetimes and compared with systematic large-scale shell-model calculations. The trend of the B(E2; 0+ → 2+) values in the midshell area is in good agreement with the calculations and the calculated B4/2 ratio provide evidence for 118Te as a near perfect harmonic vibrator.
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