High Rigidity Spectrometer (HRS)

The proposed High Rigidity Spectrometer (HRS) will substantially increase FRIB’s scientific reach and productivity. A New Era of Discovery: The 2023 Long Range Plan for Nuclear Science (PDF), by the Nuclear Science Advisory Committee (NSAC), a federally chartered advisory committee to the U.S. Department of Energy and National Science Foundation, identified HRS as a key instrument for FRIB.

HRS is proposed as the first major addition to FRIB’s experimental facilities. It consists of two segments: the High-Transmission Beam Line (HTBL) and the Spectrometer Section. The HTBL transports rare isotope beams from the Advanced Rare Isotope Separator (ARIS) fragment separator to the reaction target stationed at the entrance of the Spectrometer Section. ARIS delivers specific rare isotope beam (RIB) to the fragment separator focal plane. From there, RIBs are delivered to experimental areas including HRS, the decay station, and the S800 spectrograph.

At the Spectrometer Section, charged reaction products created at the target are analyzed. A wide variety of high-impact ancillary detector systems developed by the nuclear science community for experiments at FRIB will be used in combination with the HRS, such as the Gamma-Ray Energy Tracking Array (GRETA) and the Modular Neutron Array (MoNA-LISA).


The HRS project scope includes the design, procurement, installation, and commissioning of the technical elements including the magnets and concomitant  diagnostics, controls, power supplies, and cryogenic ancillary systems. HRS achieved Critical Decision 0 (CD-0) in November 2018 as part of the U.S. Department of Energy’s staged project approval process. CD-0 documents that a mission need, such as a scientific goal or a new capability, requiring material investment exists.

A Preliminary Design Report has been developed with input from the FRIB user community.

The U.S. Department of Energy Office of Science (DOE-SC) Office of Nuclear Physics approved Critical Decision 1 (CD-1: Approve Alternative Selection and Cost Range) for the High Rigidity Spectrometer project 16 September 2020.

Science HRS will enable

The key characteristic of the proposed HRS is its ability to accommodate rare-isotope beams up to magnetic rigidities of 8 Tesla meters (Tm). That matches the rigidities at which the rare-isotope production in the Advanced Rare Isotope Separator is optimized for isotopes across the chart of nuclei, even with an envisioned FRIB upgrade to 400 MeV/u, or FRIB400.

HRS will substantially increase FRIB’s scientific reach and productivity in nuclear structure and nuclear astrophysics research, tests of fundamental symmetry, and applications of isotopes. The gains in sensitivity are particularly strong for experiments with the most neutron-rich isotopes that have the highest potential for discovery.

The largest luminosity gains are achieved for the most neutron-rich species, including many of the nuclei that in the path of the astrophysical r process.


A collaboration of scientists (from 20 U.S. universities; Argonne National Laboratory, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and Oak Ridge National Laboratory; and scientists in the field of rare-isotope research from Europe, Japan, and Canada) developed the scientific case and conceptual design for HRS. Learn more about HRS at hrs.lbl.gov.

HRS information

An article about the HRS was published in Nuclear Physics News (volume 32, issue 4): The High Rigidity Spectrometer at the Facility for Rare Isotope Beams

A view of the interior of the High Rigidity Spectrometer.

An interior view of the MSU-funded High Rigidity Spectrometer and Rare Isotope Harvesting Vault building addition at FRIB.