The doubly magic 100Sn and 132Sn

Nuclei with similar number of protons and neutrons are particularly interesting because some of their properties such as deformation, or the effects of the proton-neutron interaction, are reinforced by the fact that neutrons and protons occupy the same orbitals. They are also interesting because it is here where pairs of mirror nuclei (nuclei where the number of neutrons and protons are exchanged) can be best studied. If isospin is a good quantum number in nuclei, then mirror nuclei should be identical. Reality tells us that they are very similar but not identical and it is through these small differences that isospin symmetry can be investigated. Among all possible N=Z nuclei, 100Sn occupies a central place because it is the heaviest N=Z doubly-magic nucleus known today. It was predicted that the decay of this nucleus should proceed dominantly to a single excited state and that it will be  very fast, even faster than the well-known superallowed Fermi transitions. This was indeed confirmed at GSI in 2012.
However, some ambiguities remained and needed further research. Firstly, the exact excitation energy of this state could not be determined unambiguously and secondly, the way it decays via γ-ray de-excitation is unclear.  Hoping to clarify both points, a follow-up experiment was carried out at RIKEN in 2014 with the EURICA array. Still, neither of the two ambiguities could be solved. Consequently, the IFIC group proposed in 2016 an  experiment at RIKEN aiming at a possible solution by means of DTAS and the total-absorption technique. This experiment can also yield improved information of the BGT, which is of great relevance for understanding the  quenching of the gA constant and in turn has relevant consequences for neutrinoless double-beta decays. This experiment can also prove if the state observed previously was indeed the only one, with important consequencess for the Shell Model calculations in this region. The experiment was partially performed at RIKEN. Due to technical issues with BigRIPS only 4 out of 10 approved days could be used. This partial experiment is the topic of our FPI-PhD J.A. Victoria (In progress). Because the statistics did not suffice for the 100Sn study a new proposal was presented at RIKEN and got 9 days granted. The realization of this experiment is expected to happen early 2024.

Another very interesting case is the other tin doubly-magic nucleus 132Sn. This nucleus does not lie far from stability and thus, the spectroscopic information is large. Nevertheless, some experiments such as transfer  reactions are still challenging at present RIB facilities. Therefore, information such as the location of the neutron single-particle i13/2 state is still unknown. It is also important to know how nuclei near 132Sn evolve as we increase the number of neutrons or  decrease the number of protons, namely, further from the stability. Moreover, these nuclei are also relevant for r-process nucleosynthesis, as explained in the following. When no experimental data are available, theoretical predictions are often used in r-process network calculations (see also previous section on r-process). So far, all theoretical calculations assume that, when states are populated in β-decay above the neutron separation energy (Sn) the subsequent de-excitation of these states proceeds through the emission of neutrons. Today we know that nuclear structure can have a strong influence in hindering neutron emission, which calls for further experimental efforts. Recently, a strong neutron-gamma competition has been observed above Sn both around 78Ni and in the region of 132Sn. The study of the disintegration of 133In in 133Sn provides excellent conditions to investigate relevant single-particle states and the way they decay in neutron-rich nuclei around 132Sn. The doubly-magic core nature of 132Sn makes the nuclear shell model rather reliable to determine structure and transition probabilities. A recent publication shows the investigation at ISOLDE of excited states in 133Sn populated in the β-decay of 133In. However, due to the low efficiency of the HPGe detectors used in that work, a
quantitative analysis of the neutron-gamma competition in the de-excitation of these states above Sn was not possible. In Task 2.1 of this NAKT proposal we plan to measure the beta decay of 133In with Lucrecia at ISOLDE, with the aim to quantify the neutron-gamma competition in the de-excitation of neutron-unbound states in 133Sn.

Shape of neutron-deficient Hg isotopes

The neutron-deficient Hg isotopes are located in a region of the nuclear chart with particular interest for shape coexistence and shape effects. Hg isotopes show an odd-even staggering in the variations of the mean square nuclear radii, which was unique until very recently. This phenomenon has attracted considerable attention, and very recently the nuclear radii measurements have been extended to the lightest accessible Hg isotopes  at CERN ISOLDE to establish the limits of the staggering. At this facility, we have performed a TAGS experiment to measure the beta strength of 182,184,186Hg isotopes. The goal of this measurement was to determine the shape of the β-decaying nuclei from a comparison of measured and calculated β-strengths. These measurements can provide complementary shape information to the one inferred from variations in nuclear radii. The β-decay of 186Hg has been analyzed recently, showing that it is also a very special case from the perspective of β-decay. The work, published in Phys. Lett. B, shows that the β-decay shape information obtained from the  interpretation of the experimental data does not coincide with the shape inferred from radii measurements. As a follow up, we have recently submitted a new proposal to study the β-decay of the neutron deficient odd Hg isotopes at ISOLDE with Lucrecia. The experiment has been granted 14 shifts of beamtime. This will allow us to study for the very first time the β-decay of shape isomers in the same isotope and further validate theoretical calculations in the region used for interpreting the 186Hg data. Calculations show a clear dependence of the β-strength on the shape of the beta decaying isomers, that combined with our measurements can provide additional information on the shape of the ground state of these interesting nuclei.

Neutrino properties

The weak interaction is one of the four known fundamental interactions in nature and it is at the core of the beta decay process. Thus, β-decay can be used as a research tool for nuclear structure, nuclear astrophysics and many practical applications, but at the same time, it is a unique tool to investigate the nature of the weak interaction itself. Nuclear reactors are the most powerful sources of electron antineutrinos that humans have created,  as every fission is followed on average by six β-decays. Thus, reactors can produce copious amounts of antineutrinos that have been used to investigate neutrino oscillation phenomena. Recently, the three experiments  double Chooz, RENO and Daya Bay, were able to measure with precision the mixing parameter theta13. For this, an accurate knowledge of the reactor antineutrino spectrum and flux is required, since this determines essentially the detector efficiency. Until recently this information was coming from the analysis of β-spectrum measurements performed at ILL. The antineutrino spectrum was obtained from the measurements by means of a conversion procedure applied to each decay branch. Thus, it depends on the nuclear structure of the fission products (FP) that is largely unknown and therefore, approximations must be introduced. A surprise came in 2011 when a very careful revision of the conversion procedure revealed a deficit of 6% in the number of antineutrinos detected at short baseline experiments. This phenomenon was named the reactor neutrino anomaly. The impact of this new evaluation in the neutrino community was large, since one of the possible explanations to the discrepancy is the existence of additional families of sterile (non-interacting) neutrinos. Another surprise came later when the statistics accumulated in the three-neutrino oscillation experiments showed that the shape of the spectrum could not be entirely reproduced, in particular there is a spectrum distortion centered around 6 MeV, which is still not understood. These features of the antineutrino spectrum can point to a problem in the conversion procedure (or in the original ILL data), and possible alternatives should be explored. To this aim in 2009 we started a collaboration with the Subatech-Nantes group to investigate the reconstruction of measured beta/antineutrino spectra using the direct summation method. Our idea was to use the TAGS technique that provides accurate β-intensity distributions to reconstruct antineutrino spectra of individual FPs. In this approach, the combination of the data of all FP in the reactor inventory, weighted by their corresponding fission yields, should give the correct spectrum. With this goal in mind, the most important FPs contributing to the antineutrino flux were identified and the most relevant ones were measured at the IGISOL-Jyvaskyla. In those experiments high purity isotopic beams were employed. Some of the results from these experiments have been already published in high impact journals. Another very important recent result is that the combination of all our related TAS measurements have notably improved the antineutrino summation method, which now has become a valid option to calculate the primary antineutrino spectrum for reactors [EST19]. This has reduced the discrepancy in the reproduction of the antineutrino flux to approximately 2%, thus questioning the reactor anomaly itself. This result is in line with other works, which is of great relevance. However, the problem is far from being fully solved yet. A new proposal was granted at IGISOL to continue measuring the next group of relevant contributors in order to further improve the overall quality of the summation calculations. This  experiment was performed and its analysis is in progress.