Functional specifications and conceptual design
The scientific goals for the HRS articulated by the user community in the HRS white paper have been translated into functional specifications. The functional specifications have been defined for three distinct operational modes of the Spectrometer Section of the HRS, and for the High Transmission Beam Line (HTBL) from the FRIB Fragment Separator to the Spectrometer Section. The three modes for the spectrometer section are referred to as the "High-resolution mode", the "Neutron invariant-mass mode", and the "Time-of-flight magnetic-rigidity mass-measurement mode". Details of the specification and conceptual design can be found in the HRS Conceptual Design Report, posted on the HRS Project site. A brief overview is given here.
Operational Modes of the HRS
The HRS has three basic operational modes meant for different categories of experiments. The specifications for each of these modes were deduced from the relevant experimental programs. The summary of these specification is provided in the table on the top-right. The conceptual design of the HRS described in the HRS Conceptual Design Report satisfies the requirements imposed by these three sets of specifications in a single layout, i.e., no hardware changes are required to switch from one operation mode to another mode. A separate set of specifications were deduced for the High Transmission Beam Line, which are displayed at the bottom-right.
High resolution mode - This mode is necessary for the largest fraction of the science program envisioned with the HRS. It allows for the particle identification of the heaviest nuclei (A=238), which requires a flight path of 25 m in the Spectrometer Section to achieve a mass resolving power of at least 400, with a time-of-flight resolution of 150 ps. In this mode, a relatively high momentum resolving power of 1500 with a beam spot size of 5 mm (FWHM) must be achieved with achromatic beam transport even without tracking the incoming beam. The specifications for the momentum acceptance (±2.5%) and solid angle coverage (15 msr) are sufficient for a large fraction of the experimental programs, including in-flight fission experiments that require a solid-angle coverage of 14.4 msr. The space around the target station is sufficient for the placement of all ancillary detector systems, including GRETA. The rigidity requirement is 8 Tm.
Neutron invariant-mass mode - This mode is necessary for experiments in which fast neutrons must be detected with a large solid angle coverage (32 msr) at forward angles, which is necessary for the neutron invariant-mass spectroscopy experiments, as well as heavy-ion collision experiment aimed at elucidating the Equation of State. In this mode, the specifications for mass and momentum resolving power of the Spectrometer Section of the HRS are less stringent than for the high-resolution mode (momentum resolving power of 290 and mass resolving power of 220, respectively), but the momentum acceptance specification is larger (±5%). A key specification for this mode is the 32-msr solid-angle coverage for the neutrons at forward angles. A neutron flight-path length of up to 15 m must be accommodated. Less space for the placement of ancillary detectors is needed, but the use of GRETA with its most forward detector ring removed should be possible (at least 90 cm space up- and downstream of the target). To insert a large Time Projection Chamber for heavy-ion collision, a dipole magnet with a gap size of at least 60 cm required.
Time-of-flight magnetic rigidity mass-measurement mode - This mode is specific for the ToF-Bρ mass measurements. The Reconfigured A1900 of the ARIS Fragment Separator, the HTBL, and the Spectrometer Section of the HRS must serve to create a very long flight path of at least 90 m. From the ion-optical point of view, the Spectrometer Section is operated in the same way as in the high-resolution mode, whereas the HTBL is operated such that these two are dispersion-matched and the entire system is doubly-achromatic. By using dedicated detector systems for the ToF measurements, resolutions of better than 30 ps must be achieved to perform mass measurements with a mass resolving power of 104. A momentum resolving power of better than 104 is required, but since there are no reactions that are being studied there are no stringent specifications on the momentum and angular acceptance of the Spectrometer Section.
The three operational modes of the HRS are displayed in the figures below. For details of these modes and their operational characteristics, please refer to the HRS Conceptual Design Report.
The three operational modes of the HRS: (left) High-resolution mode, (middle) the Neutron invariant-mass mode, and (right) the Time-of-flight magnetic-rigidity mass-measurement mode.