The Gamma Spectroscopy Group at IFIC is devoted to the investigation of several aspects of the structure of atomic nuclei and the applications of Nuclear Physics to related fields.
The origin and formation of heavy elements (>Fe) in the universe is one of the main open questions in science today. Already in 1957, it became clear that essentially two different mechanisms are responsible for the existence of almost all the heavy elements in our environment: the slow (s-) and the rapid (r-) neutron capture processes. Nowadays the site and main features of the s-process mechanism are well understood and thus,
nucleosynthesis is used as a unique tool that serves to probe, explore and quantify the physical conditions of the stellar environment where heavy elements are synthesized. This information, in turn, becomes of great relevance to constrain the much more uncertain r-process, to benchmark state-of-the-art stellar models and as a diagnostic of galactic chemical evolution models.
From the experimental viewpoint, one of the most active fronts of research in this field is the study of radioactive nuclei, which act as branching points along the nucleosynthesis path. In these cases, the abundance pattern of the final stable isotopes surrounding the branching is determined by a competition between neutron capture and beta-decay. Thus, the final abundances reflect, on one side, the stellar conditions of temperature, neutron density and evolutionary time-scale and, on the other hand, the neutron-capture and beta-decay rates involved. Beta decay half-lives are generally known to a high level of accuracy. However, the neutron capture cross sections of these radioactive species are challenging to measure due to the difficulty to obtain sufficiently large amounts of sample material, to the ineluctable presence of contaminating isotopes and to the γ-ray background arising from the sample activity itself.
There are about 20 s-process branching nuclei with crucial astrophysical interest. One of the most emblematic cases in the field of nucleoysnthesis is 79Se(n,γ). Due to the thermal dependency of its beta-decay, the s-process path-split at 79Se becomes highly sensitive to the thermal conditions and evolutionary time scales of the s-process mechanism. The measurement of 79Se(n,γ) was one of the main objectives of the ERC Consolidator Grant HYMNS. To achieve this goal we implemented the novel i-TED concept, which required a significant effort in terms of technical developments. The experiment was successfully performed in 2022 at CERN n_TOF.
Another interesting example is 94Nb. This unstable isotope acts as a branching in the s-process flow, thus impacting the abundances of the light stable Mo-isotopes. The measurement of its neutron-capture cross section could help disentangling anomalies found in the 94-95Mo isotopes in SiC-grains in primitive meteorites. This experiment was successfully carried out at CERN n_TOF in 2022.
In 2024 we have focused our efforts in improving neutron-capture cross sections of stable isotopes like 146Nd and 209Bi. These two measurements are the main topic of two PhD Thesis by B. Gameiro and G. de la Fuente, respectively.
Another fascinating open question in nucleosynthesis concerns the different abundance patterns observed in the Ge-Mo region, which cannot be explained by s-process, r-process or any combination of them . The Light Element Primary Process (LEPP) was therefore invoked to account for such discrepancies. More recently , the so-called intermediate (i-) neutron capture process was proposed as a plausible explanation.
In this scenario, the 13C(α,n) source is strongly activated during the He-shell flashes in rapidly accreting white dwarfs (WDs) in close binary systems, yielding neutron densities of about 1015 n/cm3, which are intermediate between those of the s- and the r-environments. The signature of the i-process is clear both in stellar observations and in presolar grains. From an experimental standpoint, neutron capture cross sections of short-lived
radioactive nuclei become of pivotal importance to eventually validate the i-process hypothesis and benchmark its physical conditions. A recent sensitivity study indicates the key
role of the neutron-capture rates of about 10 radioactive isotopes. As we discuss in this reference, many of these nuclides could be produced and collected at RIB facilities, such as ISOLDE or MEDICIS, for subsequent investigation at the neighboring CERN n_TOF.
Although the envisaged sample sizes are not yet sufficient for a TOF measurement, several of them could be studied via cyclic neutron-activation at the new NEAR n_TOF station. The intended activation method is extraordinarily sensitive, allowing for the measurement of ng-size (10^14 atoms) sample amounts, like e.g. 147Pm (2.62 yr) at FZK. A customized moderator assembly at NEAR allows one to produce several quasi-Maxwellian distributions, fully covering the stellar energy range from a few keV up to about 100 keV. The IFIC-UPC groups have been actively involved in the design and commissioning of both the new generation spallation target and the NEAR station.
Although the general ideas were laid out early [BUR57], the astrophysical site and conditions for the r-process continue to be a challenge [ARC23]. Core Collapse Supernovae (CCSN) were favorite candidates since the beginning. However, astrophysical models do not easily meet the required conditions to reach the heaviest nuclei. On the other hand, neutron star mergers (NSM), although a plausible site, are thought to be too rare, in particular in the early moments of the universe, to explain the observed galactic chemical evolution. A spectacular turn of the situation happened in 2017 with the observation of gravitational waves (GW170817) emitted
by a NSM [ABB17] followed by the observation of electromagnetic emission [PIA17], which carry the fingerprint of heavy element synthesis [MET10], and the identification of freshly synthesized Sr [WAT19]. Thus, recent advances in observations [FRE18] open an avenue for more focused investigations combining astrophysics and nuclear physics in the coming years. The fingerprint of the r-process history in the universe is the abundance pattern of the different elements and their isotopes in stars, which can be compared with calculations. The latter depend both on astrophysical and nuclear physics parameters. From the viewpoint of the nuclear physics input data the situation is challenging. On the one hand, we need to create in the laboratory the very exotic nuclei produced in such stellar explosions. On the other hand, we need to use selective and sensitive devices to measure the tiny amount of nuclei produced. Consequently, a large part of the information needed is still missing. Therefore, one of our goals in this NAKT project is to extend current data availability in order to constrain calculations and pin down the details and nature of the astrophysical processes.
Two key quantities are half-lives T1/2 and beta-delayed neutron emission probabilities Pn. T1/2 defines the initial abundances and the speed of the process. Pn modifies the decay path towards stability and provides extra neutrons for late captures. One of the goals of our research is to provide direct experimental information on both for nuclei never studied experimentally before. In previous projects we initiated and led a campaign of measurements exploiting the unprecedented intensities at the RIBF facility of the RIKEN Nishina Center within the BRIKEN international collaboration. This task force delivered for the first time a large set of half-lives and Pn values for the most neutron-rich isotopes experimentally accessible today along the r-process path. The idea arose from our previous successful experiments with the BELEN neutron counter at GSI, developed within the FAIR/NUSTAR/DESPEC experiment by the UPC-IFIC groups. The new BRIKEN detector, the largest worldwide with 140 3He tubes, was designed at UPC and successfully commissioned at RIKEN in 2016. Six experiments were granted by the RIKEN PAC and four of them led by IFIC and UPC. The experiment around 78Ni delivered the PhD of Alvaro Tolosa (see section on PhD Thesis of our group). Another experiment was made in the mass region A~100 and another experiment in the neutron-rich rare-earth nuclei was completed in 2021 and it is the main topic of the PhD Thesis of Max Pallás (IFIC-UPC).
Nucleosynthesis in explosive hydrogen burning at high temperatures (T > 10^8 K) is characterized mainly by the rapid proton capture (rp-) process. Discussions of the possible scenarios can be found in literature, where Type I X-ray bursts (XRBs) are suggested as possible sites for the rp-process. These explosions are produced in binary systems in which a neutron star accretes hydrogen-rich material from a low-mass companion star, typically a Main Sequence or a Red-Giant star. Thermonuclear ignition takes place in semi-degenerate conditions, when the temperature and density in the accreted envelope become high enough to allow for a breakout from the hot CNO cycle. Nucleosynthesis eventually proceeds near the proton drip-line via the rp-process. Type I XRBs are characterized by Tpeak = 1-3 GK and ρ = 10^6-7 g/cm3. The luminosity curves associated with these explosions represent the main physical observables to which XRB-model predictions can be compared. Thus, an accurate knowledge of the weak-decay rates (beta+/EC-decay rates) of the waiting-point nuclei and their neighbors is of paramount importance for the performance of detailed model calculations that can reproduce and explain the physical observables of the XRBs. However, we are far from having an accurate knowledge of the weak-decay rates close to the proton drip-line. A TAGS experiment at CERN ISOLDE was recently performed to measure the beta strength of 64-66Ge and 64,65Ga isotopes. These nuclei directly lie in the path of the rp-process and their beta-decay rates have been pointed out as some of the main contributors to total weak-decay rate.
Meanwhile, progress is being made in the analysis of 64Ge and a new experiment proposal was approved to measure the neighbors 68Se and daughter. Along this line a new experiment has been recently approved at GSI
with the DTAS detector in the framework of the DESPEC collaboration. This experiment will also help to push the limit of our knowledge both on nuclear structure and astrophysics aspects around the 84Mo region (G-22-00101, B. Cederwall, A. Algora, et al.).