Science with the HRS
The scientific case for a high-rigidity spectrograph at FRIB has been spelled out in the HRS White Paper, submitted as input to the 2014-2015 Nuclear Science Advisory Committee (NSAC) Long Range Plan. The program envisioned in this white paper and further developed by the FRIB User Community through the HRS working group since then, addresses a broad range of exciting and important scientific questions, illustrated below.
The Facility for rare Isotope Beams (FRIB) will be the world’s premier rare-isotope beam facility producing a majority (~80%) of the isotopes predicted to exist. This unprecedented discovery potential can be realized by implementing state-of-the-art experimental equipment that can study these isotopes at the highest rates produced. Eleven of the 17 NSAC RIB Taskforce benchmarks, which were introduced to characterize the scientific research of a rare-isotope facility, require the use of fast beams at FRIB. All of these programs require the use of a magnetic spectrometer, but the rigidity of the existing devices (S800 spectrograph and Sweeper magnet) is limited to 4 Tm. The High Rigidity Spectrometer (HRS), with a magnetic rigidity of up to 8 Tm, will substantially increase FRIB’s scientific reach and productivity and addresses the overarching intellectual challenges from the 2015 NSAC Long Range Plan and the NRC Decadal Study. The opportunities with the HRS were originally detailed in a white paper for the HRS.
FRIB will allow users to answer the overarching questions from the NSAC 2007/15 Long Range Plan and the latest NRC Decadal Study, and thereby supporting the DOE Nuclear Physics Mission. The NSAC RIB TF developed 17 benchmarks to test facility capability to address these questions. Meeting these benchmarks has driven the technical scope and specifications for FRIB. The benchmarks also guide the equipment development necessary for the program.
The 8 Tm bending power of the proposed HRS matches the rigidities at which rare isotopes will be produced at FRIB, even with the envisioned FRIB upgrade to 400 MeV/u. This will enable the most sensitive experiments across the entire chart of nuclei, thereby enabling experiments with the most neutron-rich nuclei available at FRIB. In combination with the ability to use thicker reaction targets at the higher rigidity, gain factors in luminosity of 2 to 100 will be achieved for over 90% of experiments with neutron-rich isotopes, as shown in the figure below. These gains are over what will be possible with existing spectrometers at NSCL (S800 and Sweeper) that have a maximum rigidity of 4 Tm. The largest luminosity gains are achieved for the most neutron-rich species, including those in the path of the astrophysical r-process for which gains in luminosity by a factors of 5-20 can be achieved.
Luminosity gains that can be achieved with the High Rigidity Spectrometer in comparison with performing experiments with the existing S800 Spectrograph. The colors indicate the gain factors and the black contours indicate the estimated FRIB rare isotope beam production rates. The white lines indicate the astrophysical r-process path.