New K isomers in the actinide nucleus californium-248 (248Cf)

R. Orlandi1), H. Makii1), K. Nishio1), K. Hirose1), M. Asai1), K. Tsukada1), T.K. Sato1),
Y. Ito1), F. Suzaki1), Y. Nagame1), A.N. Andreyev1,2), E. Ideguchi3), N. Aoi3),
T.T. Pham3), S. Yan4), Y. Shen4), B. Gao5), G. Li5)1): Res. Gr. for Exotic Heavy-Element Nuclear Science, JAEA 2): Univ. of York 3): Osaka Univ.
4): China Inst. of Atomic Energy 5): Inst. of Modern Physics, Chinese Academy of Sciences

One of the major quests of nuclear physics is the search forlong-lived superheavy elements (SHEs). They are believed toexist in proximity of the next spherical shell gaps, predicted atneutron number N=184 and proton number Z=114 (or 120, or126), in the so-called “Island of Stability” (IoS). The location ofthe IoS is however still not known. Current predictions could beimproved by extending our knowledge of the proton and neutron
orbits in heavy nuclei, in particular by studying deformed
actinides near Z=100 and N=152. The reason is that part of theorbits associated with the IoS are lowered in energy by nuclear
deformation and come near the ground state in heavy actinides.
Knowledge of the nucleon orbits in the actinide region will
therefore permit reliable extrapolations to the IoS. Furthermore,neutron-rich actinides can be produced in sufficiently large
quantities to study their γ-ray spectrum, that is the source ofinformation to deduce the spin and energy of the underlying
nucleon orbits. With this aim, we developed a technique that cangenerate in the same experiment several actinides, not measured
till now, and investigate their nuclear structure.
In our method, several actinides are generated using multinucleon transfer (MNT) reactions. In the reaction, when anactinide target is irradiated by a heavy-ion beam, neutrons andprotons are exchanged in various patterns between the colliding
nuclei, resulting in the production of several isotopes (Fig. 1). It
is therefore necessary to determine what nuclei are produced ineach reaction event. In our setup, we detect and identify the lightejectiles, and from reaction kinematics determine the mass andproton number of the complementary heavy nucleus. To achievethis, we developed an array of Si telescope detectors, where theejectiles implant and are discriminated on an event-by-event
basis from their partial vs. total energy loss properties.
Immediately after the reaction, the nuclei produced are in ahighly excited state, and from the kinetic energy of the ejectile
we also determine the excitation of the compound nucleus witha very good resolution of about 250 keV. The excited actinides
in turn de-excite by emitting γ-ray photons, which are detectedby an array of Ge detectors and LaBr3 scintillators. The Ge
detectors are used for their excellent energy resolution (~2 keV);
the LaBr3 scintillators time resolution (~2 ns) permits instead tomeasure half-lives of decays in the nanosecond range.

Fig. 1 Schematic diagram of the reaction mechanism and method ofidentification. In this example, nuclei are generated in the MNTreaction of a 18O beam colliding onto 249Cf. In each reaction event,the heavy compound nucleus produced is determined by detecting
and identifying the complementary light ejectile in the Si array.

An example of the results obtained with this setup is the γ-ray
spectrum of californium-248 (248Cf, Z=98, N=150) [1]. 248Cf
was produced at the JAEA Tandem accelerator, by irradiating a249Cf target with a beam of 18O. The γ-ray spectrum of 248Cf,shown in Fig. 2, was obtained by selecting those events inwhich 19O was identified in the Si array. New γ rays were foundand placed in the level scheme. The most significant aspect inthe structure of 248Cf is the presence, at low excitation energy, oflong-lived excited states, called K isomers. K is a quantum
number that reflects the orientation of the nuclear spin withrespect to the deformation of the nucleus, and corresponds to theprojection of the spin onto the symmetry axis (see Fig. 2). Whena γ decay involves states with different K, the transition ishindered, which results in the long isomer half-life. Unlike theground-state, where protons and neutrons are coupled in pairs tospin 0, the spin of low-lying K isomers arises from the spin of
only two nucleons in outer orbits. The spin and energy of K
isomers thus permit to identify the orbits of the nucleons
involved. In 248Cf, we measured for the first time the half-life ofa state at 592 keV and confirmed that it is a K=2 isomer, witht1/2=4.6(1) ns. Furthermore, we found evidence of an additional
K isomer lying at 0.9(3) MeV, with K≥5 and t1/2≥150 ns.
Although further experiments are needed to measure its half-life,
the lower limit determined in our work already points to themost likely nucleon configuration. K isomers are particularly
important in the search for SHEs, since sometimes their halflives can be even longer than that of the ground state [2]. Thisimplies that some SHEs may be easier to discover as K isomers.
In the experiment, several actinide isotopes were produced,
and the ongoing analysis reveals new γ rays in other isotopes ofeinsteinium (Z=99) and fermium (Z=100). In future, usingheavier ion beams, we aim to extend this method to study thenuclear structure of trans-actinides and SHEs.

Fig. 2 (Left) γ-ray spectrum of 248Cf obtained by selecting events inwhich 19O was detected. (Right) Partial level scheme of 248Cf, that
includes γ-ray transitions labelled in the γ-ray spectrum. The simple
diagrams on top show the differences between the ground state bandand K isomers. The energy of the 0.9(3)-MeV K-isomer state wasdeduced from the excitation energy measured in the Si array.

References[1] R. Orlandi et al., Phys. Rev. C 106, 064302 (2022).
[2] H.M. David et al., Phys. Rev. Lett. 115, 132502 (2015).