Speaker
Description
Regions near closed shells in areas of the nuclear chart far from stability are very interesting from the point of view of nuclear structure, since a shell model description based on single-particle states can be challenged by collective effects. One of the most interesting regions is the one around the doubly-magic $^{78}$Ni nucleus, with $Z = 28$ and $N = 50$ [Taniuchi2019].
Odd-$A$ $N = 50$ isotones are particularly well suited to probe proton single-particle configurations outside the $^{78}$Ni core. The low-lying structure of the $^{79}$Cu to $^{89}$Y isotones, with $Z = 29$ to $Z = 39$, is expected to arise from the progressive filling of the proton $pf$ shell, involving the $f_{5/2}$, $p_{3/2}$ and $p_{1/2}$ single-particle orbitals and giving rise to low-lying $5/2^{-}$, $3/2^{-}$ and $1/2^{-}$ states. The occurrence of single-particle states experimentally observed as the ground state and low-lying first-excited states with dominant $p_{3/2}$ and $f_{5/2}$ configurations leads to magnetic dipole M1 $\Delta l = 2$ connecting transitions between them, which are $l$-forbidden in the extreme shell model picture [Sachs1951,Govil1964].
This is so because magnetic dipole isovector operator does not change the orbital angular momentum. Nonetheless, such transitions are still experimentally observed, although with rates that are generally much lower than those of allowed transitions, or even below the single-particle limit. It is expected that these transitions result from the breakdown of $l$-forbiddenness, influenced by other nuclear dynamic effects, such as core polarization [Horie1954] and meson exchange mechanisms [Andrejtscheff1981].
Therefore, the investigation of $l$-forbidden M1 transitions may provide insight into the role of these effects within the atomic nucleus.
In this work we aim at extending the systematics of M1 transitions to constrain the underlying proton configurations and try to clarify the origin of the $l$-forbidden M1 hindrance. To this end, two complementary experiments were performed at the ISOLDE (CERN) facility and ILL reactor in Grenoble, France. In the former the structure of $^{83}$As was investigated via the $\beta$ decay of $^{83}$Ga at the ISOLDE Decay Station, while in the second excited states in $^{85}$Br, $^{87}$Rb where populated in the $\beta$ decay $^{85}$Se and $^{87}$Kr, which were transported and analyzed using the LOHENGRIN spectrometer. Lifetimes of excited states in nuclei of interest investigated by fast-timing techniques.
The presentation will address the experimental methodologies and the data analysis procedures. The results will be discussed in the context of other available data for the region, completing the systematics from $^{81}$Ga to $^{87}$Rb where the dominant single-particle configurations are due to the $p_{3/2}$ and $f_{5/2}$ orbitals in the $28$-$50$ proton shell. Based on these measurements the systematics of $l$-forbidden M1 transitions in $N = 50$ isotones will be described.
Andrejtscheff1981] W. Andrejtscheff et al., Nuclear Physics A 351 (1981) 54.
[Govil1964] I.M. Govil and C.S. Kurana, Nuclear Physics 60 (1964) 666.
[Horie1954] Horie and Arima, Prog. Theor. Phys. 11 (1954) 509.
[Sachs1951] R.G. Sachs and M. Ross, Phys. Rev. 84 (1951) 379.
[Taniuchi2019] Taniuchi et al., Nature 569 (2019) 53.