Nuclear Spectroscopy Group

Shape coexistence in neutron-deficient Hg, Pb, and Po nuclei near the N=104 midshell is considered well established, yet direct spectroscopic evidence remains limited. A key objective of the JYFL experimental programme has been to shed light on these phenomena using complementary in-beam spectroscopic techniques. Notably, the JUROGAM arrays enabled the first direct observation of a prolate structure in the beyond-midshell nucleus ¹⁸⁵Pb and an oblate non-yrast band in the midshell nucleus ¹⁸⁶Pb. The persistence of shape coexistence near the proton dripline was explored in ¹⁸⁰Pb, where prompt γ-rays from proton-unbound states were observed. It is noteworthy that, apart from the excited 0⁺ state in ¹⁸⁴Pb, excited states in Pb isotopes with N≤104 midshell have only been measured at JYFL. Collectivity has also been investigated in ^186,188,190Pb through plunger lifetime measurements. More recently, combined in-beam γ-ray and electron spectroscopy with the SAGE spectrometer led to a reassignment of the shapes of excited 0⁺ states in ¹⁸⁶Pb and the first direct measurement of three different deformations in atomic nucleus.

Experiments performed at JYFL have also led the way in in-beam spectroscopy of very heavy nuclei in the region of ²⁵⁴No. Systematic studies of the rotational bands and high-K structures in these nuclei have led to a wealth of information concerning the underlying shell structure and single-particle states around the Z=100 and N=152 deformed shell gaps. A major milestone was the in-beam study of ²⁴⁶Fm, the nucleus with the lowest production cross-section (~10 nb) investigated to date, conducted at JYFL. This was followed by a landmark experiment on the Z=104nucleus ²⁵⁶Rf, the heaviest nucleus studied in-beam so far and a key gateway to the superheavy element region. Subsequent studies of ²⁵¹Md,  ²⁵³No, and ²⁵⁵Lr were conducted using the SAGE spectrometer. 

Experimental studies of proton- and α-particle emission are examples of decay spectroscopy. Decay spectroscopy is suitable method for the probing of the fundamental properties (such as the half-life, decay energy, and mass) of the most exotic isotopes. The extreme selectivity of decay spectroscopy is achieved by correlating the aforementioned decay properties to those of the subsequently decaying daughter species. Therefore, decay spectroscopy studies are feasible with very low number of observations; only a few recorded events, even a single decay, permit reliable probing of nuclear structure at the extremes of the chart of nuclides. This makes decay spectroscopy an ideal tool for the identification of new isotopes, which are often produced with minuscule yields. With the data obtained in the decay-spectroscopy experiments one can probe, for example, (i) the strength of the nuclear shell closures, (ii) the location of the one and two proton driplines, and (iii) test the predictive power of the atomic mass models. Additionally, the proton-emission rate is sensitive not only to the decay energy, but also to shape of the parent nucleus, and to the angular momentum carried away by the emitted proton, hence, allowing the studies of the shape of the most exotic atomic nuclei.

As of today the two separators, RITU (Recoil-Ion Transport Unit) and MARA (Mass-Analyzing Recoil Apparatus), maintained by the group are routinely employed in various decay-spectroscopy experiments along the proton dripline. These experiments have yielded multiple scientific discoveries and breakthroughs. For example, as of 10.08.2026 the number of new isotopes discovered with RITU and MARA is at least 31 ranging from ¹¹⁶La to ²⁴⁰Es, and the number of new isomeric (metastable) excited states found is at least 88 ranging from ¹¹³Xe to ²⁵⁵Lr [1].

[1] https://frib.msu.edu/public-engagement/learning-resources-and-programs/…

The spectroscopy of N≈Z nuclei has long been of interest due to the wide range of unique phenomena observed in this region of the nuclear chart. These nuclei are also directly relevant to nucleosynthesis, particularly through their role in the rapid proton capture (rp) process via (p,γ) reactions. A distinctive feature of N≈Z nuclei is the potential for neutron–proton (np) pairing. While nucleon pairing is well understood away from the N=Z line, near this line both isovector (T=1) and isoscalar (T=0) np-pairing modes become increasingly significant. A longstanding question remains: how do these different pairing modes manifest in nuclear structure?

To address this, we have exploited the RBT technique to exotic, proton-rich, medium-mass nuclei, whose excited states are still poorly known. Since 2005, several successful RBT experiments have been conducted on nuclei near the N=Z line, including ⁷⁸Rb, ⁷⁸Y, ⁶⁶As, ^70,71Kr, and ⁶⁶Se. These studies have enabled systematic investigations of Coulomb Energy Differences and Triplet Energy Differences in this mass region.

The integration of MARA, JUROGAM 3, JYTube, and advanced focal-plane detectors represents a flagship setup for advancing these studies. Notably, the introduction of the APPA device has enabled lifetime measurements of N≈Z nuclei, providing crucial data to constrain observables such as spectroscopic quadrupole moments (Q_s​) obtained from Coulomb excitation experiments at radioactive ion beam facilities.