First SOLARIS experiment completed

11 June 2021

A team of FRIB scientific users have completed the first experiment using SOLARIS, the SOLenoid spectrometer Apparatus for ReactIon Studies, in the Active-Target Time-Projection Chamber (AT-TPC) mode. Argonne National Laboratory (ANL) developed SOLARIS, a dual-mode spectrometer for a broad range of reactions studies at FRIB using reaccelerated beams.

Ben Kay, an experimental nuclear-structure physicist at ANL, is the SOLARIS lead.

The dual modes of SOLARIS use either the AT-TPC or a silicon detector array. The design is optimized for the study of a broad range of direct reactions, such a nucleon transfer and inelastic scattering, at incident beam energies around the Coulomb barrier. It builds on two technologies developed in anticipation of FRIB, with a focus on achieving good Q-value resolution and efficiency. The AT-TPC mode is the first SOLARIS operating mode to be implemented.

Daniel Bazin, a senior physicist at FRIB, prepared AT-TPC for use. Clementine Santamaria, a research associate at FRIB, was the experiment spokesperson.  

The experimenters used an oxygen-16 beam from NSCL’s ReAccelerator facility ReA6 linear accelerator (linac) at 10 million electron-volts per nucleon (MeV/u). The AT-TPC was placed in the center of the SOLARIS solenoid, which was then ramped up to a field of 3 tesla (T). The goal of this experiment (E20020) is to search for a resonance in oxygen-16 located just above the 4 alphas separation energy of 14.4 MeV, and observe its decay into four alpha particles. This resonance would be the equivalent of the Hoyle state, an excited state of carbon-12 that is very close to the alpha-particle separation energy, the energy threshold about which the nucleus can emit an alpha particle (helium-4). Astronomer Fred Hoyle first proposed this state in 1954 to explain the production of carbon in stars.

The experimenters filled the AT-TPC with 700 Torr of pure helium gas used both as the reaction target and the detector medium. The team was interested in reaction events related to the inelastic excitation of oxygen-16 to the sought-after resonance, with subsequent decay into four alpha particles. The expected events of this process would exhibit five alpha particles. The fifth alpha is the “recoil” alpha from the target medium. Several such events were observed during the experiment.

The analysis of the data involved several steps, including identifying and separating the individual tracks, and reconstructing the momenta and angles. The team conducted a preliminary analysis during the experiment to verify the observation of elastic scattering events and validate the trigger conditions. They recorded more than 2 million reaction events during this run.

ReA6 is the upgraded ReAccelerator facility at NSCL and FRIB. It will provide broader opportunities for nuclear-physics experiments with higher beam energies than the previous ReA3.