Argonne National Laboratory

Helical Orbit Spectrometer (HELIOS)

Helical Orbit Spectrometer (HELIOS)

HELIOS is a charged-particle spectrometer designed to study reactions with radioactive ion beams. It offers excellent resolution, solid-angle acceptance and versatility, allowing its use in a broad range of experiments for nuclear structure determinations and nuclear astrophysics.

HELIOS is ideally suited to the study of single- and multi-nucleon transfer reactions in the inverse kinematics regime. Such an approach is necessary for nuclear structure studies with radioactive-ion beams, such as those from CARIBU and, in the near future, AIRIS.

The concept is based on a superconducting solenoidal spectrometer with uniform axial field. The target and detector are both on the solenoid axis in the field with the reaction products bent back to the axis. The target-to-detector distance and the energy of the particles translate to give the desired information of excitation energy and center-of-mass angle, both key properties for revealing nuclear structure information.

The HELIOS instrument has a number of attractive features:

  • Greatly improved effective resolution over conventional techniques
  • Large solid angle
  • Compact detectors and electronics
  • Easy particle identification based on the cyclotron period

HELIOS is well suited to experiments that probe the structure of the exotic nuclei that are currently of high interest:

  • Single-nucleon transfer reactions
  • Pair transfer
  • Inelastic scattering
  • Even knockout reactions

The device consists of a 0.9m bore, and 2.35m long 2.85-T superconducting solenoid (repurposed MRI magnet). Following a very successful implementation at ATLAS, other similar devices are under construction at the ISOLDE facility at CERN and plans are underway for one at the future FRIB facility, called SOLARIS.