Research

The reactions that burn hydrogen in the CNO cycle have cross sections that fall exponentially towards stellar energies — at the Gamow window they can reach the picobarn scale, which at the surface of the Earth is indistinguishable from cosmic-ray background. LUNA's answer is 1400 m of dolomite overburden, which suppresses the muon flux by six orders of magnitude and makes the measurement possible.

I led the measurement and analysis of ¹²C + p and ¹³C + p radiative capture at LUNA and at Felsenkeller, which fixed the proton-capture rates on both carbon isotopes and with them the astrophysical ¹²C/¹³C ratio. I now lead the ¹⁹F + p working group at the new Bellotti Ion Beam Facility, the 3.5 MV accelerator installed underground at Gran Sasso.

In parallel I am the collaboration's DAQ Coordinator: the acquisition and slow-control infrastructure of every campaign, and of the MV facility, is designed and operated by me. It is what lets shifts be taken remotely, which for an experiment a kilometre underground is not a small thing.

AZURE2 is where a set of measured points becomes a cross section: a multi-level, multi-channel R-matrix model fitted to data across every channel that shares a compound nucleus, then extrapolated to the energies that stellar burning actually samples. It is used across academia, national laboratories and the IAEA.

Since taking over as maintainer I have worked on the parts that decide whether a modern analysis is feasible at all: analytic gradients and the residual Jacobian computed inside the forward model, so a fit or an MCMC no longer pays a factor of a few hundred for finite differences; a tabulate-once reaction-rate integrator; polarization observables; identical-particle scattering. Alongside that, the GUI, the documentation, and support and training for the user community.

The evaluations section of this site runs that same code, on a server, on real published analyses.

The 6.79 MeV state in ¹⁵O sits just above the proton threshold and dominates the low-energy behaviour of ¹⁴N(p,γ)¹⁵O — the slowest step of the CNO cycle, and therefore the reaction that sets the cycle's rate and the CNO neutrino flux. Its lifetime enters that extrapolation directly, and it is short enough that only a γ-ray tracking array can reach it.

I am spokesperson of the measurement performed with AGATA at Legnaro, and led the full analysis pipeline: Doppler correction, kinematic reconstruction, Geant4 simulation and the Bayesian extraction of the lifetime itself. The analysis is complete and the manuscript is under review at Nature Physics.

Every laboratory measurement of a low-energy cross section is made with atoms, not bare nuclei, and the electron cloud screens the Coulomb barrier. The enhancement it produces grows as the energy falls — exactly where astrophysics wants the number — and measured screening potentials have persistently come out larger than theory allows.

At CRYRING the ¹⁵N + p reaction is studied in inverse kinematics with a stored, cooled beam, which changes the atomic configuration of the collision and so attacks the discrepancy from a direction a normal target experiment cannot.