The INFN Laboratori Nazionali di Frascati was established in 1954 to host an electro-synchrotron, the first particle accelerator built in Italy, and since then played a crucial role in the field of nuclear and particle physics and for the development of acceleration and detection technologies. Within this historical tradition, a workshop was organized at LNF in the framework of the Nuclear Physics Mid Term Plan Italy, an initiative of the Nuclear Physics Division of the Istituto Nazionale di Fisica Nucleare, to discuss the detection techniques employed in nuclear physics and to identify the open issues to be addressed to realize the scientific programs of the experiments foreseen in a midterm perspective. The present report summarizes the outcome of the discussions.

Isoscalar dipole transitions are a distinctive fingerprint of cluster structures. A 1^{-} resonance at 7.27(10) MeV, located just below the α-emission threshold, has been observed in the deuteron inelastic scattering reactions off ^{10}Be. The deformation lengths of the excited states in ^{10}Be below 9 MeV have been inferred from the differential cross sections using coupled channel calculations. This observed 1^{-} resonance has isoscalar characteristics and exhausts approximately 5%-15% of the isoscalar dipole energy-weighted sum rule, providing evidence for pronounced α cluster structure in ^{10}Be. The Gamow coupled channel approach supports this interpretation and suggests the near-threshold effect might be playing an important role in this excitation energy domain. The α+α+n+n four-body calculation reproduces the observed enhanced dipole strength, implying that the four-body cluster structure is essential to describe the 1^{-} states in ^{10}Be.

This white paper is the result of a collaboration by those that attended a workshop at the Facility for Rare Isotope Beams (FRIB), organized by the FRIB Theory Alliance (FRIB-TA), on Theoretical Justifications and Motivations for Early High-Profile FRIB Experiments. It covers a wide range of topics related to the science that will be explored at FRIB. After a brief introduction, the sections address: (II) Overview of theoretical methods, (III) Experimental capabilities, (IV) Structure, (V) Near-threshold Physics, (VI) Reaction mechanisms, (VII) Nuclear equations of state, (VIII) Nuclear astrophysics, (IX) Fundamental symmetries, and (X) Experimental design and uncertainty quantification.

Lying between 16O and 40Ca, the sd shell is well described by robust phenomenological and ab initio nuclear theories. In this work, however, we highlight an unexplained reduction in electric-quadrupole strength in the rare isotope 32Si, studied through sub-barrier Coulomb excitation. It is found that the oblate nature of the deformation is well reproduced, while the absolute scale of quadrupole deformation, however, is inhibited by approximately a factor of 2 compared to theoretical predictions. Through comparison with shell-model and ab initio calculations, we present a number of possible explanations for this inhibited E2 strength. By comparing the results of these calculations to multiple observables, we conclude that there is a reduced role for out-of-space excitations in 32Si, resulting in a reduction in the corrections normally applied to both models.

Two unexpected behaviors have been observed in heavy-ion fusion excitation functions at energies above the Coulomb barrier. The first behavior is observed in overlapping excitation spectra. Fusion excitation functions sigma(E) that have different entrance channels but fuse to the same compound nucleus appear to overlap in the energy domain above the barrier. The overlap emerges after scaling the center of mass energy of each excitation function by a constant scaling factor, SF. The second behaviour stems from the structure of the fusion excitation curve. Contrary to descriptions from coupled-channels or other model calculations, heavy-ion fusion excitation functions are not smooth near and above the Coulomb barrier. There appears to be weak but noticeable oscillations or structures within the excitation functions that can be observed clearly in the representation d(sigma E)/dE and in comparison with theoretical calculations sigma(E) - sigma(th)( E). Moreover, the corresponding d(sigma E)/dE spectra for systems that form the same compound nucleus also overlap well in this energy range, including their fine structures, but the uncertainty is large. It appears the two behaviors are correlated and the reasoning behind these behaviors are yet unknown, but may be due to the compound-channel effect.

Single-neutron adding data was collected in order to determine the distribution of the single-neutron strength of the 0f_7/2 , 1p_3/2 , 1p_1/2 and 0f_5/2 orbitals outside of Z=16, N=18 , ^34 S. The ^34 S(d,p) ^35 S reaction has been measured at 8 MeV/u to investigate cross sections to excited states in ^35 S. Outgoing proton yields and momenta were analyzed by the Super-Enge Split-Pole Spectrograph in conjunction with the CeBrA demonstrator located at the John D. Fox Laboratory at Florida State University. Angular distributions were compared with Distorted Wave Born Approximation calculations in order to extract single-neutron spectroscopic overlaps. Spectroscopic overlaps and strengths were determined for states in ^35 S up through 6 MeV in excitation energy. Each orbital was observed to have fragmented strength where a single level carried the majority. The single-neutron centroids of the 0f_7/2 , 1p_3/2 , 1p_1/2 and 0f_5/2 orbitals were determined to be 2360^+90_-40 keV, 3280^+80_-50 keV, 4780^+60_-40 keV, and ≳ 7500 keV, respectively. A previous discrepancy in the literature with respect to the distribution of the neutron 1p_1/2 strength was resolved. The integration of the normalized spectroscopic strengths, up to 5.1 MeV in excitation energy, revealed fully-vacant occupancies for the 0f_7/2 , 1p_3/2 , and 1p_1/2 orbitals, as expected. The spacing in the single-neutron energies highlighted a reduction in the traditional N=28 shell-gap, relative to both the 1p spin-orbit energy difference ( N=32 ) and the lower limit on the N=34 shell spacing.

The spin-orbit splitting between neutron 1p orbitals at 33Si has been deduced using the single-neutron-adding (d,p) reaction in inverse kinematics with a beam of 32Si, a long-lived radioisotope. Reaction products were analyzed by the newly implemented SOLARIS spectrometer at the reaccelerated-beam facility at the National Superconducting Cyclotron Laboratory. The measurements show reasonable agreement with shell-model calculations that incorporate modern cross-shell interactions, but they contradict the prediction of proton density depletion based on relativistic mean-field theory. The evolution of the neutron 1p-shell orbitals is systematically studied using the present and existing data in the isotonic chains of N=17, 19, and 21. In each case, a smooth decrease in the separation of the 1p3/2-1p1/2 orbitals is seen as the respective p-orbitals approach zero binding, suggesting that the finite nuclear potential strongly influences the evolution of nuclear structure in this region.

The recent discovery and spectroscopic measurements of O27 and O28 suggests the disappearance of the N=20 shell structure in these neutron-rich oxygen isotopes. We measured one- and two-proton removal cross sections from F27 and Ne29, respectively, extracting spectroscopic factors and comparing them to shell model overlap functions coupled with eikonal reaction model calculations. The invariant mass technique was used to reconstruct the two-body (O24+n) and three-body (O24+2n) decay energies from knockout reactions of F27 (106.2 MeV/u) and Ne29 (112.8 MeV/u) beams impinging on a Be9 target. The one-proton removal from F27 strongly populated the ground state of O26 and the extracted cross section of 3.4−1.5+0.3 mb agrees with eikonal model calculations that are normalized by the shell model spectroscopic factors and account for the systematic reduction factor observed for single nucleon removal reactions within the models used. For the two-proton removal reaction from Ne29 an upper limit of 0.08 mb was extracted for populating states in O27 decaying though the ground state of O26. The measured upper limit for the population of the ground state of O26 in the two-proton removal reaction from Ne29 indicates a significant difference in the underlying nuclear structure of F27 and Ne29.Published by the American Physical Society2024

The ^{22}Mg(α,p)^{25}Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts. We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new ^{22}Mg(α,p)^{25}Al reaction rate is a factor of ∼4 higher than the previous direct measurement of this reaction within temperatures relevant for x-ray bursts, resulting in the ^{22}Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the (α,p) reaction.

An unexpected overlapping behavior has been observed for fusion excitation functions sigma(E) from different entrance channels that fuse to the same compound nucleus in energies above the barrier. The overlap appears when the center of mass energy of each excitation function is multiplied by a constant scaling factor. The scaling factors are determined by adjusting and are nearby the ratios of corresponding Coulomb barriers of each entrance channel. Moreover, the corresponding d(sigma E)/dE spectra also overlap well in this energy range, including their fine structures. The reasoning behind these behaviors are unknown. One possibility is the compound-channel effect, since the overlap behavior does not occur between collision systems leading to different compound nuclei.

We demonstrate a new technique for obtaining fission data for nuclei away from β stability. These types of data are pertinent to the astrophysical r process, crucial to a complete understanding of the origin of the heavy elements, and for developing a predictive model of fission. These data are also important considerations for terrestrial applications related to power generation and safeguarding. Experimentally, such data are scarce due to the difficulties in producing the actinide targets of interest. The solenoidal-spectrometer technique, commonly used to study nucleon-transfer reactions in inverse kinematics, has been applied to the case of transfer-induced fission as a means to deduce the fission-barrier height, among other variables. The fission-barrier height of ^{239}U has been determined via the ^{238}U(d,pf) reaction in inverse kinematics, the results of which are consistent with existing neutron-induced fission data indicating the validity of the technique.

The neutron -rich strontium, zirconium, and molybdenum nuclei have been observed to undergo a dramatic evolution, becoming strongly deformed around N = 60, sometimes interpreted as a quantum phase transition between "normal" and intruder configurations. Key to understanding this evolution is to understand the configurations in isolation, in regions where interference can be neglected. A deformed coexisting configuration is inferred from the presence of a 0 2 state which decreases in excitation energy with increasing neutron number, becoming the first -excited state at 98Mo. We present here the results of a low -energy Coulomb -excitation measurement of the nucleus 96Mo, extracting B(E2) values and quadrupole moments. It is found that, while the B(E2) values agree with those found in the literature, there is a significant disagreement with literature spectroscopic quadrupole moments. The results are compared with shell -model calculations using a 88Sr core with good agreement found, likely indicating that intruder structures do not significantly impact the ground -state structure, in contrast with the heavier molybdenum isotopes.

Detailed γ-ray spectroscopy of the exotic neon isotope ^28Ne has been performed for the first time using the one-neutron removal reaction from ^29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for ^28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

Information on the $^{38}$S level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam $γ$-ray spectroscopy. Prompt $γ$-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling $^{38}$S residues were selected by the Fragment Mass Analayzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of $^{38}$S residues and identify any associated $γ$-ray transitions. The new level information, including the extension of the even-spin yrast sequence through $J^π = 8^{(+)}$, was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-$Z$ $N=22$ isotones from $Z=14$ to $20$, and for $^{36}$Si-$^{38}$S in particular, was also presented with an emphasis on the role and influence of the neutron $1p_{3/2}$ orbital on the structure in the region.

The ^{22}Mg(α,p)^{25}Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts. We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new ^{22}Mg(α,p)^{25}Al reaction rate is a factor of ∼4 higher than the previous direct measurement of this reaction within temperatures relevant for x-ray bursts, resulting in the ^{22}Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the (α,p) reaction.

The neutron-rich strontium, zirconium, and molybdenum nuclei have been observed to undergo a dramatic evolution, becoming strongly deformed around $N=60$, sometimes interpreted as a quantum phase transition between ``normal'' and intruder configurations. Key to understanding this evolution is to understand the configurations in isolation, in regions where interference can be neglected. A deformed coexisting configuration is inferred from the presence of a ${0}_{2}^{+}$ state which decreases in excitation energy with increasing neutron number, becoming the first-excited state at $^{98}\mathrm{Mo}$. We present here the results of a low-energy Coulomb-excitation measurement of the nucleus $^{96}\mathrm{Mo}$, extracting $B(E2)$ values and quadrupole moments. It is found that, while the $B(E2)$ values agree with those found in the literature, there is a significant disagreement with literature spectroscopic quadrupole moments. The results are compared with shell-model calculations using a $^{88}\mathrm{Sr}$ core with good agreement found, likely indicating that intruder structures do not significantly impact the ground-state structure, in contrast with the heavier molybdenum isotopes.

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton ( N / Z ) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N / Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10 −21 s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28 O and 27 O through their decay into 24 O and four and three neutrons, respectively. The 28 O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers 1 , 2 , it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both 27 O and 28 O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28 O from a 29 F beam is consistent with it not exhibiting a closed N = 20 shell structure.

The $^{22}$Mg($\alpha$,$p$)$^{25}$Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts (XRBs). We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new $^{22}$Mg($\alpha$,$p$)$^{25}$Al reaction rate is a factor of $\sim$4 higher than the previous direct measurement of this reaction within temperatures relevant for XRBs, resulting in the $^{22}$Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the ($\alpha,p$) reaction

The weak r process in neutrino-driven winds following a core-collapse supernova is thought to contribute to the cosmic abundances of the first r-process peak elements between Se and Ag. Sensitivity studies have found that the early nucleosynthesis in the weak r process is primarily driven by (alpha, xn) reactions due to the high temperatures, and that current nuclear physics uncertainties in the (alpha, xn) rates result in significant uncertainties of the calculated abundances. The weak r-process path proceeds several nuclei away from stability where (alpha, xn) reaction cross sections have not yet been measured. In this paper we report the( 100)Mo(alpha, xn) cross section (between 8.9 and 13.2 MeV in the center of mass, corresponding to 3.5-6.8 GK) in inverse kinematics using the Multi-Sampling Ionization Chamber (MUSIC) detector at the Argonne Tandem Linac Accelerator System (ATLAS) facility. With this first measurement of the Mo-100(alpha, xn) cross section, we have demonstrated the ability of MUSIC to measure (alpha, xn) cross sections for A up to 100, therefore paving the way for further measurements with radioactive beams at ATLAS or the Facility for Rare Isotope Beams.