Abstract
The structure of the
102
254
No
152
nucleus has been studied for more than 20 years: the last publications on its decay spectroscopy are from LBNL 1, GSI 2, JYFL 3 and ANL 4. Four decay ...schemes featuring two isomers have been published and are interpreted differently in terms of excitation energy and decay scheme of the 2
nd
isomer and configuration assignments of both K-isomers. These discrepancies have triggered new experiments including this one, performed with the GABRIELA 5, 6 array, at the focal plane of the SHELS 7 separator at the FLNR, Dubna. The first part of this proceeding will present the experimental setup and the analysis techniques used to reveal the electromagnetic decay of the known isomers in
254
No. The second part will focus on the new results obtained with more than 1 million
254
No nuclei implanted in the focal plane detector. In particular, the internal conversion electron spectrum observed in the decay of the 8
−
K-isomer has revealed the presence of a strong transition, most likely E0, suggesting low-lying shape coexistence in this nucleus as predicted in 8, 9. The γ-ray spectrum obtained from the decay of the short-lived 170
μ
s isomer has revealed new γ-ray lines putting in doubt the previous interpretations about this isomer decay.
Alpha-decay spectroscopy of $$^{257}$$Rf Hauschild, K.; Lopez-Martens, A.; Chakma, R. ...
The European physical journal. A, Hadrons and nuclei,
01/2022, Letnik:
58, Številka:
1
Journal Article
An experiment on the study of the
246
Fm spontaneous fission was conducted using the SHELS separator. The isotope was synthesized in the complete fusion reaction of
40
Ar beam ions and
208
Pb target ...nuclei. The neutron yields of
246
Fm spontaneous fission (
ν
¯
=
3.79
±
0.30
,
σ
ν
2
=
2.1
) were obtained using the SFiNx detector system. The multiplicity distribution of emitted prompt neutrons was restored using the Tikhonov method of statistical regularisation (
ν
¯
r
=
3.79
±
0.20
,
σ
ν
r
2
=
2.8
). The spontaneous fission branching ratio (
b
SF
=
0.061
±
0.005
) and the half-life (
T
1
/
2
=
1
.
50
-
0.07
+
0.08
s) of the isotope were determined. The experimental data were compared with scission point model predictions. An agreement was observed in the average number of neutrons per spontaneous fission process. However, the forms of the experimental and model prompt neutron multiplicity distributions differ significantly.
The new neutron-deficient isotope $^{249}$No was synthesized for the first
time in the fusion-evaporation reaction $^{204}$Pb($^{48}$Ca,3n)$^{249}$No.
After separation, using the kinematic separator ...SHELS, the new isotope was
identified with the GABRIELA detection system through genetic correlations with
the known daughter and granddaughter nuclei $^{245}$Fm and $^{241}$Cf. The
alpha-decay activity of $^{249}$No has an energy of 9129(22)$~$keV and
half-life 38.3(2.8) ms. An upper limit of 0.2\% was measured for the fission
branch of $^{249}$No. Based on the present data and recent information on the
decay properties of $^{253}$Rf and aided by Geant4 simulations, the ground
state of $^{249}$No is assigned the 5/2$^+$622 neutron configuration and a
partial decay scheme from $^{253}$Rf to $^{245}$Fm could be established. The
production cross-section was found to be $\sigma$(3n)=0.47(4) nb at a
mid-target beam energy of 225.4 MeV, which corresponds to the maximum of the
calculated excitation function. Correlations of the $^{249}$No alpha activity
with subsequent alpha decays of energy 7728(20) keV and half-life
$1.2_{-0.4}^{+1.0}$ min provided a firm measurement of the electron-capture or
$\beta^{+}$ branch of $^{245}$Fm to $^{245}$Es. The excitation function for the
1n, 2n and 3n evaporation channels was measured. In the case of the
2n-evaporation channel $^{250}$No, a strong variation of the ground state and
isomeric state populations as a function of bombarding energy could be
evidenced.
An analysis of recent experimental data J. Khuyagbaatar et al., Phys. Rev. C 104, L031303 (2021) has established the existence of two fissioning states in \(^{253}\)Rf: the ground state and a ...low-lying isomeric state, most likely involving the same neutron single-particle configurations as in the lighter isotone \(^{251}\)No. The ratio of fission half-lives measured in \(^{253}\)Rf was used to predict the fission properties of the 1/2\(^{+}\) isomeric state in \(^{251}\)No and draw conclusions as to the stability against fission of even lighter Rf systems. This paper focusses again on the fission properties of \(^{253}\)Rf and their impact on the stability of other neutron deficient isotopes, using new and improved data collected from two experiments performed at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. Two fission activities with half-lives of 52.8(4.4)\(~\mu\)s and 9.9(1.2) ms were measured in the case of \(^{253}\)Rf, confirming the results of J. Kkuyagbaatar et al. A third state, at much higher excitation energy, was also observed through the detection of its electromagnetic decay to the 52.8\(~\mu\)s state. This observation leads to the opposite quantum-configuration assignments for the fissioning states as compared to the ones established by J. Khuyagbaatar et al., namely that the higher-spin state has the shortest fission half-life. This inversion of the ratio of fission hindrances between the low and high-spin states is corroborated in the isotone \(^{251}\)No by the non observation of any substantial fission branch from the low-spin isomer.
Spontaneous fission of 246Fm Isaev, A V; Mukhin, R S; Andreev, A V ...
arXiv.org,
03/2022
Paper, Journal Article
Odprti dostop
An experiment on the study of the \(^{246}\)Fm spontaneous fission was conducted using the SHELS separator. The isotope was synthesized in the complete fusion reaction of \(^{40}\)Ar beam ions and ...\(^{208}\)Pb target nuclei. The neutron yields of \(^{246}\)Fm spontaneous fission (\(\overline{\nu} = 3.79\pm0.30\), \(\sigma^{2}_{\nu} = 2.1\)) were obtained using the SFiNx detector system. The multiplicity distribution of emitted prompt neutrons was restored using the Tikhonov method of statistical regularisation (\(\overline{\nu}_{r} = 3.79\pm0.20\), \(\sigma^{2}_{\nu r} = 2.8\)). The spontaneous fission branching ratio (\(b_{SF} = 0.061\pm0.005\)) and the half-life (\(T_{1/2} = 1.50^{+0.08}_{-0.07}\) s) of the isotope were determined. The experimental data were compared with scission point model predictions. Excellent convergence was observed in the average number of neutrons per spontaneous fission process. However, the forms of the experimental and model prompt neutron multiplicity distributions differ significantly.
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