FRIB partners with Argonne National Laboratory to develop dual-mode solenoidal spectrometer for reaction studies with reaccelerated beams

By Ben Kay, Argonne National Laboratory

In January, FRIB partnered with Argonne National Laboratory to plan the development of SOLARIS, a dual-mode spectrometer for a broad range of reactions studies at FRIB using reaccelerated beams.

SOLARIS brings together two demonstrated technologies developed for the FRIB era over the last decade. One is a vacuum-mode device that operates like HELIOS, a solenoidal spectrometer with an on-axis silicon array, pioneered at Argonne. This technique has proven an efficient, versatile, and high-resolution approach to the study of direct nuclear reactions with moderately intense radioactive ion beams of greater than 10,000 ions per second. For the other mode, the Active Target Time Projection Chamber (AT-TPC) developed by the National Superconducting Cyclotron Laboratory over the same period as HELIOS, will be used in the bore of the SOLARIS solenoid. The AT-TPC brings many advantages that perfectly complement the vacuum mode of operation. These include the ability to be used with lower intensity beams, explore reactions with multiple final exit channels, determine excitation functions, use pure target gases, and it exhibits almost complete solid-angle coverage.

With this dual mode of operation, SOLARIS can take advantage of the full dynamic range of reaccelerated (ReA) beam facility in terms of beam energy, beam species/mass, and beam intensity. These capabilities enable a program of reaction studies with beams in the sub MeV per nucleon regime up to the highest energies available with present and future ReA configurations, as well as for intensities of around 100 ions per second up to intense, stable beams in the nano-ampere regime, and for all masses of the beam.

There has been a longstanding interest in developing an instrument like SOLARIS for FRIB, following promising results from HELIOS and concurrent development of active-target technologies. A dedicated working group has met at Low Energy Community meetings since 2010. It transpires that large-bore superconducting solenoids of the type used in medical magnetic resonance imaging, with typical fields around 2-4 T and outstanding uniformity, are ideal for the transport of light ions following nuclear reactions. In anticipation of a future device at FRIB, Argonne acquired a decommissioned 4-T MRI magnet in 2014. It had been warmed up and placed in storage, though it appeared viable for future use. Having a solenoid available considerably reduces the cost of a future project.

In 2017, a group of 50 scientists met to set about defining the best path forward for a HELIOS-like spectrometer at FRIB. It is out of this meeting that the dual-mode SOLARIS approach was born. A subsequent white paper and proposal led to funds to demonstrate the suitability of the 4-T solenoid for use as the SOLARIS magnet. A test of the solenoid carried out in early 2019. The magnet was cooled down, and the field successfully ramped up and down. The solenoid was transported to FRIB in summer 2019 and seated at the end of the ReA6 beamline.

With the plans put forth in early 2020, several significant milestones have been defined for SOLARIS. The first is set by the exciting opportunities made available by the operation of ReA in a standalone mode following the cessation of the coupled cyclotrons and before FRIB operations.

The community response was highly enthusiastic, with seven experiments conditionally approved, requesting both the use of the vacuum-mode and AT-TPC-mode of SOLARIS. The structures necessary to support the AT-TPC in the SOLARIS solenoid been completed, and in-field tests of this setup are likely to be carried later in 2020. The silicon array and data acquisition used in HELIOS at Argonne will be used to enable the experiments using the vacuum-mode to go ahead in this time frame. This approach was recently used to commission the ISOLDE Solenoidal Spectrometer at CERN, with great success. Further, it coincides with GRETINA being at ATLAS, alleviating the pressures on the use of HELIOS at ATLAS. The mechanical engineering of the vacuum chambers and related structures starts in the summer of 2020.

The second major milestone is to have SOLARIS available for use in year one of FRIB operations. This will be an evolution of the setup used in the ReA standalone campaign. The completion of SOLARIS will see it use dual silicon arrays in being used in vacuum mode. This will enable multiple channels to be measured in any given experiment, maximizing the physics output. SOLARIS will be designed to enable quick, reproducible switching between the vacuum mode and AT-TPC mode of operation.

In vacuum mode, universal mounting systems will be implemented to allow the addition of community-developed auxiliary instrumentation, such as gas-target systems, gas-jet target systems, gamma-ray detectors, recoil detections, and so on. Many such systems are being considered, and some demonstrated as prototypes at HELIOS and elsewhere. It is expected that SOLARIS will continuously evolve, as it is an inherently flexible system centered around the large superconducting solenoid. The flexibility and versatility should lead to a wealth of exciting physics opportunities at FRIB.

On behalf of those involved in SOLARIS, we would like to thank FRIB management and the DOE for providing a path forward for SOLARIS.

More information about SOLARIS can be found here or by contacting Ben Kay (