Bellotti Ion Beam Facility at LNGS

• Bellotti Ion Beam Facility is an underground 3.5 MV ion accelerator designed for low-background nuclear physics and rare-event searches.
• It complements the existing LUNA 400 kV machine by offering enhanced beam control and ultra-quiet conditions via deep rock overburden.
• The facility features advanced beam optics, differential pumping, and sophisticated detector setups for precise nuclear astrophysics measurements.

The Bellotti Ion Beam Facility (BIBF) is an underground ion-beam installation at the Istituto Nazionale di Fisica Nucleare–Laboratori Nazionali del Gran Sasso (LNGS), recently installed in Hall B and named in honour of Prof. Bellotti. It is described as a major advance for studying nuclear processes in an underground environment, and as the first megavolt-class accelerator operating underground at Gran Sasso. In the LNGS nuclear-physics planning literature, it is based on a 3.5 MV “Inline” Singletron accelerator that complements the existing 400 kV LUNA electrostatic machine and has been optimized for the ultra-quiet background conditions provided by 3800 m w.e. of rock overburden (Buompane et al., 22 Dec 2025). In a proposed rare-event application, the same facility is presented as a tunable accelerator-based source for light scalar bosons, coupled to large-volume low-background detectors already operating or under construction at LNGS (Broggini et al., 3 Sep 2025).

1. Institutional setting and underground context

LNGS is characterized as one of the largest underground physics laboratory, with a peculiar environment suited for experiments in Astroparticle Physics, Nuclear Physics and Fundamental Symmetries. Within that setting, BIBF occupies a specific role: it extends the underground accelerator program beyond the 400 kV LUNA machine and broadens the accessible parameter space for direct low-background measurements. The Mid Term Plan workshop organized at LNGS framed the facility as part of a coordinated national strategy of the Nuclear Physics Division of the Istituto Nazionale di Fisica Nucleare, with explicit discussion of opportunities enabled by state-of-the-art detection systems and by synergy among LUNA, BIBF, ERNA at CIRCE, and other laboratories such as LNS and LNF (Buompane et al., 22 Dec 2025).

The underground environment is described through two complementary measures. One source emphasizes the 3800 m w.e. overburden that enables ultra-quiet operation, while another states that the accelerator hall sits beneath approximately 1 400 m of rock, yielding a cosmic-ray muon flux reduction by a factor of approximately $10^6$ relative to surface laboratories (Buompane et al., 22 Dec 2025, Broggini et al., 3 Sep 2025). A common simplification is to treat BIBF as merely a replacement for LUNA; the facility descriptions instead present it as a complement to LUNA, with dedicated solid-target and windowless gas-target stations and a program extending from direct nuclear astrophysics to detector calibration, applied-physics studies, fundamental-symmetry tests, and proposed beyond-the-Standard-Model searches.

2. Accelerator platform, beam production, and transport optics

In the nuclear-physics workshop report, BIBF is based on a 3.5 MV “Inline” Singletron accelerator manufactured by HVEE. The accelerator body includes an ion source at ground potential, a 3.5 MV high-voltage column, a first $90^\circ$ bending magnet for mass separation, a short switchyard where gas or solid targets may be installed, and a second $90^\circ$ analyzer feeding one of two target stations. In the light-scalar study, the facility is instead described as built around a 3.5 MV tandem accelerator originally developed in the LUNA program, connected to a straight 6 m transfer line with three $45^\circ$ analyzing magnets and two differential-pumping stages (Buompane et al., 22 Dec 2025, Broggini et al., 3 Sep 2025). This suggests that the literature is documenting different operational descriptions or beamline emphases rather than a single uniformly presented layout.

The ion species explicitly listed for the facility are $\mathrm{H}^+$, $\mathrm{H}_2^+$, $\mathrm{H}_3^+$, $\mathrm{He}^+$, and $\mathrm{C}^+$. Typical beam currents at the target entrance are up to 1 mA for protons, 0.5 mA for helium, and 0.15 mA for carbon. The accelerator delivers up to 3.5 MeV terminal voltage with a ripple of 30 V pp and long-term stability better than 20 V over days, while adjustable ion optics allow final beam energies between a few hundred keV per nucleon and 3.5 MeV per charge. The source and focusing elements are water-cooled or $\mathrm{LN}_2$ cooled to ensure temperature stability and beam emittance $90^\circ$0 (Buompane et al., 22 Dec 2025).

The scalar-boson proposal gives a more run-specific operating picture for proton mode: beam energies from threshold up to 3.5 MeV, a typical physics run at $90^\circ$1 MeV, terminal-voltage variation in steps of about 1–2 keV over the 0.1–3.5 MeV range, maximum DC proton current $90^\circ$2 mA with nominal 0.5–1.0 mA in extended runs, energy stability better than 1 keV over multi-hour runs, energy spread $90^\circ$3, and an essentially continuous-wave duty cycle with scheduled interruptions only for beam tuning or maintenance, corresponding to less than 5% downtime (Broggini et al., 3 Sep 2025). Beam transport downstream of the bending system employs quadrupole doublets and triplets to tailor the beam spot on target to diameters down to 1–2 mm, with Faraday cups, beam-profile monitors, optical viewers, and collimators supporting reproducible steering and alignment (Buompane et al., 22 Dec 2025).

3. Target stations, diagnostics, and detector infrastructure

The facility is organized around two beam lines and two target environments. Solid-target and windowless gas-target stations are installed on these beam lines, and the gas-target station can be configured either for nuclear-astrophysics experiments or for detector-calibration and applied-physics studies. Upstream beamline vacuum is maintained at $90^\circ$4 mbar, while target chambers operate at $90^\circ$5 mbar or, for the gas target, at mbar-level pressures in the interaction region with differentially pumped stages. In the scalar-search configuration, the beamline is housed in a stainless-steel vacuum tank with outer dimensions $90^\circ$6, the target chamber has diameter 50 cm and length 60 cm, and the beamline and chamber reach approximately $90^\circ$7 mbar and $90^\circ$8 mbar under beam, respectively (Buompane et al., 22 Dec 2025, Broggini et al., 3 Sep 2025).

At the solid-target station, the beam stop can be either an $90^\circ$9-cooled copper-strip calorimetric beam stop or a water-cooled Faraday cup. A movable flange can host HPGe detectors with 100–140% relative efficiency, plastic and BGO anti-Compton shields, or large-area Si drift-detector arrays consisting of 64 channels of 1 mm-thick SDDs with 190 eV FWHM at 8 keV. The gas-target station includes a windowless, differentially pumped He or $90^\circ$0 gas cell, an internal beam calorimeter for current measurement, an array of eighteen $90^\circ$1 counters embedded in polyethylene moderators with approximately 40% neutron-detection efficiency, and three HPGe detectors in close geometry for prompt $90^\circ$2-ray capture studies. A separate BGO array of six 28 cm-long prisms forms a segmented $90^\circ$3 calorimeter with 11% energy resolution at 1.33 MeV, explicitly intended for radiative-capture measurements (Buompane et al., 22 Dec 2025).

The facility also incorporates low-background signal-processing and shielding subsystems. Fast digitizers at 3.2 GS/s coupled to AI-based classifiers distinguish single-site from multi-site and noise pulses in HPGe and BEGe detectors, suppressing electronic and environmental backgrounds at the few-counts-per-day level. Multi-tier passive shields of 25 cm OFHC lead plus 1 cm electrolytic copper, enclosed in nitrogen-flushed boxes, reduce $90^\circ$4 background by approximately $90^\circ$5 below 2 MeV, while polyethylene layers reduce environmental neutrons (Buompane et al., 22 Dec 2025). In the scalar-boson proposal, local passive shielding around the beamline is specified as 30 cm of low-activity lead and 5 cm of borated polyethylene, with the vacuum tank also serving as a Faraday cage for electromagnetic isolation (Broggini et al., 3 Sep 2025). This suggests a program-dependent shielding configuration superposed on the core underground low-background environment.

A specialized thick-target configuration is described for proton-induced rare-event production. In that setup, the proton beam impinges on a solid, self-supporting $\mathrm{TaF}_