R3B - Reactions with Relativistic Radioactive Beams

Overview

The R³B (Reactions with Relativistic Radioactive Beams) scientific collaboration is an international sub-collaboration of the FAIR project Pillar NUSTAR, bringing together more than 200 participating scientists from over 20 countries. The collaboration is dedicated to the planning, development, and realization of the R³B experimental setup at FAIR, as well as the analysis and interpretation of scientific data obtained with the setup.

Scientific Focus

The scientific program of the R³B collaborationfocuses on utilizing nuclear reactions to explore the static and dynamic properties of exotic nuclei far from stability. Key research pillars include:

Spectroscopic studies of very exotic nuclei at and beyond the proton and neutron drip lines
Equation-of-state studies of neutron-rich nuclear matter
Nucleon correlations, probing both long- and short-range correlations in nuclei
Nuclear astrophysics, determining key reaction rates
Fission & large deformations, investigating nuclear structure and dynamics
Hypernuclear physics, studying hypernuclei under extreme conditions

To deliver this diverse physics program, our state-of-the-art experimental setup is built around GLAD, a large-acceptance, superconducting dipole magnet.

Experimental Setup

The R³B setup provides high-efficiency, high-resolution, and large-acceptance capabilities for kinematically complete measurements of reactions with high-energy radioactive beams.

Located in the High-Energy Cave (HEC) at the focal plane of the Super-FRS high-energy branch, the setup is optimized for high magnetic rigidities (up to 20 Tm) to capitalize on maximum secondary beam transmission. Key subsystems have already been commissioned and successfully operated in Cave C at GSI during the FAIR construction phase.

The R³B setup comprises of several different detection subsystems as well as sophisticated DAQ system and data-analysis software.

The full R3B set-up in the high-energy cave at FAIR. The copyright "© GSI/FAIR, Zeitrausch."

Detection subsystems

(NeuLAND, Large-acceptance dipole, Active target, Proton tracking detector, Heavy-ion tracking detectors, Target-recoil detector, Gamma spectrometer, Low-energy neutron detector, High-resolution spectrometer)

Neutron ToF Spectrometer

NeuLAND (new Large-Area Neutron Detector) is the next-generation neutron detector designed for R³B which meets all requirements defined by the ambitious physics program proposed for the R³B facility. NeuLAND features a high detection efficiency, a high resolution, and a large multi-neutron-hit resolving power. This is achieved by a highly granular design of plastic scintillators, avoiding insensitive converter material. The detector consist of 3000 individual submodules with a size of 5x5x250 cm³, arranged in 30 double planes with 100 submodules providing an active face size of 250x250 cm² and a total depth of 3 m. NeuLAND can be divided into two detectors for special applications and will be placed at different distances from the target, in order to meet specific experimental demands. A momentum resolution of Δp/p of 10^-3 similar to that for the charged particles is desired, resulting in resolution requirements for the time of flight of σ(t) < 150 ps and a position resolution of σ(x,y,z) ≈ 1.5 cm for given flight paths in the range from 10 to 35 m. For an experiment on a medium mass nucleus at about 500 MeV/nucleon, invariant-mass resolutions of about ΔE =20 keV at 200 keV above the neutron threshold (ΔE=30 keV at 1 MeV respectively) will be reached using the maximum flight path. Apart from the excellent energy resolution of NeuLAND, the enhanced multi-neutron recognition capability with an efficiency of up to ~50% for a reconstructed five-neutron event at 1 GeV will constitute a major step forward.

Large-acceptance dipole

For the variety of experiments to be performed within the R3B project, a zero-degree superconducting dipole magnet has been designed and constructed. The main parameters of the spectrometer are: (i) A large vertical gap providing an angular acceptance of ±80 mrad for neutrons; (ii) A maximum bending angle of 40°, ensuring an acceptance close to 100% even for experiments with very different magnetic rigidities of the beam and the fragments; (iii) A high field integral of about 5 Tm, which allows a bending angle of 18° for a 15 Tm beam (e.g. 1 GeV/nucleon ¹³²Sn or 500 MeV/nucleon ⁸He) or 14° for 20 Tm beams, the maximum rigidity provided by the Super-FRS. A momentum resolution Δp/p of around 10^-3 can be achieved by tracking the particles with high resolution (see below). The design includes four superconducting coils which are tilted to match the required acceptance angle for the particles of interest. The side coils are optimized to reduce the fringe field, and guarantee a low magnetic field in the target region, where detectors have to be placed. The magnet has been has been delivered in November 2015 to GSI. In February 2016 together with its large vacuum chamber the magnet has been moved into Cave C, where it was used in experiments until move t the HEC at the FAIR.

More details can be found here.

Active target

For some of the reactions to be studied the most interesting information is concentrated in the region of very low momentum transfer, and therefore demands for high resolution detection of very slow target-like recoil particles. A very promising solution to such experimental challenges is the use of an active target detector, where the gas constitutes both the target and the detection medium. Such a detector may have a very high efficiency, very low particle detection thresholds, and may use different gases, which can therefore be chosen to best suit the experiment purpose.

The physics case addressed within R³B with the active target concept is quite similar to that of the EXL project, but the two different projects are well complementary. The EXL project will allow to reach high luminosities for all cases where the nuclear lifetime of the exotic beam particles is sufficiently long (> 500 ms) to allow for beam preparation in the CR/NESR rings and for continuous accumulation and stagging. Keeping in mind that the rate capability of the active target technique will be limited, especially for very heavy projectiles, to about 10⁵ s^–1 or less, luminosities of the order of 10²⁸ cm^–2 sec^-1, as expected for EXL, will not be reachable with external active targets. On the other hand, there exist many cases of exotic beams of high interest for nuclear structure and nuclear astrophysics, which are located far outside the valley of stability, and consequently have half-lives much shorter than 1 s, and relatively low production rates considerably below 10⁵ s^–1. For such cases the technique of active targets is presently considered as the only way to perform high resolution experiments at low momentum transfer with sufficient luminosities to extract the cross sections of interest.

The R³B active target detector is a relatively novel gas detector concept where the gas constitutes both the target and the detection medium. Mostly, inverse reactions will be used where the target gas is p, d, ³He or ⁴He, either pure or mixed with standard detection gases such as isobutane C₄H₁₀. The recoil particles have mostly very low energy for the most important centre of mass angles near zero degree in quasielastic reactions. The identification and good resolution of such low energy particles together with high efficiency is most uniquely realized in a gas detector. The working principle of such detectors as the one used at GSI, IKAR, and the one tested and used at Ganil, Maya, can be characterized as Charge Projection Chamber by analogy with the Time Projection Chambers (TPC) used in Particle Physics.

More information can be found in a dedicated Technical Design Report.

Proton tracking detectors

For the tracking of protons and light charged particles with beam-like velocities behind the GLAD magnet a Proton-Arm Spectrometer (PAS) based on scintillation fibers is foreseen. This concept offers several advantages: An easy integration into the vacuum since no gas-based detection systems are used, a high particle rate capability and insensitivity to magnetic fields, as well as the possibility to detect heavier fragments.

The PAS setup consists of two fiber detectors located behind the GLAD. They have vertical fibers and provide x-position information necessary to determine the deflection angle of the protons and other light charged particles in the magnetic field.The first detector has an aperture of 195x100 cm², while the second has an aperture of 247x100cm². The setup is completed by a time-of-flight wall (PASyToF) which serves as a trigger detector and also measures the y-position of the protons. A rough energy measurement will also confirm that the measured particle is a proton and not a heavier fragment.

The geometry and the operational parameters are optimized to detect minimum ionizing particles with an efficiency of greater than 97%, a spatial resolution of better than 0.25 mm (σ), and angular resolution of 0.25 mrad (σ). These design goals have been confirmed during several prototype tests performed at different facilities.

More detailed information on the PAS detector can be found here.

Heavy-ion tracking detectors and velocity measurements

For the large variety of different experiments proposed for the R³B setup, various tracking-detector systems with different demands are required.

In most cases experiments using exotic beams have to include a twofold setup for identifying particles and their kinematic properties before and after the secondary reaction target. The incoming beam will be tracked (starting from the dispersive focus at the Super-FRS) in order to determine beam momentum, angle and position of incidence on the target, and, similarly, for the spectrometer after the target. The velocity of the incoming beam as well as of the fragments has to be determined with a resolution of Δβ/β<10^-3 in order to achieve unique isotope identification after the target also for heavy beams. Using 40 m distance between the dispersive focus at the Super-FRS and the reaction target the requirements for time-of-flight (TOF) detectors are quite moderate (σ(t) < 100 ps) in this case. Also moderate position resolution of Δx=3 mm would be sufficient. The detectors should be able to cope with rates of 10⁷ to 10⁸ s^-1 heavy ions at the dispersive focus of the Super-FRS. For the large-acceptance mode, large-area detectors are placed behind the GLAD magnet. For detection of light charged particles Proton-Arm Spectometer (PAS) will be used, see section on proton tracking. For heavy ions, depending on the reaction studied and detector position, different detector sizes have to be considered.

In view of the high momentum resolution mode (Δp/p < 10^-4 (σ)) needed for several experiments, the situation is quite different. The position resolution of all tracking devices has to be better than Δx < 0.5 mm. Especially the vertex and angle reconstruction at the secondary target will need much better resolution of Δx ~ 0.1 mm. The angular scattering and momentum spread introduced by the tracking detectors in the target region have to be small compared to these numbers while the detectors after the spectrometer could use a more massive layout. Also the expected γ-background limits the material budget of the detectors in the target area. For the R³B project being a multi-purpose setup for very different branches of experiments the development of position sensitive time-of-flight detectors with good resolution for tracking of exotic heavy ions using minimized mass density in order to suppress parasitic reactions and multiple scattering efficiently is mandatory. Different types of detectors are being or have been developed for tracking of heavy ions according to the demands of the different experiments

Si-strip detectors will be used for coincident detection of fragments and protons after the target as well as behind the large-acceptance magnet, where an array of Si-strip will be installed. For more details, see here.
Different scintillating fiber detectors are foreseen for the position measurements in front of and behind the magnet.
Next-generation time-of-flight wall TOFD is designed for high-precission time-of-flight and nuclear-charge measurements of nuclei up to U region at coutning rates up to 1 MHz. Similar detector with high-precission time-of-fight measurements but without information on nuclear charge is designed for studies of fission fragments.
For experiments at lower energies and for lighter ions (e.g. measurements of (γ,n) cross sections at very low relative energy of astrophysics relevance) low-mass channel-plate detectors (with less resolution) are foreseen.

Target-recoil detector

This detector provides a unique possibility to study fast (several tens of MeV/nucleon) protons, deuterons and alphas produced in (p,2p), (p,pd) and (p,pα) reactions. The precise angular reconstruction delivered by this detector enables an accurate reaction-vertex reconstruction, while - when it is combined with the total kinetic energy measurement by CALIFA - it also enables a four-momentum measurement of the light charged particles, which in turn can be used to reconstruct the crucial missing-mass quantity in a reaction.

The required angular resolution for the target-recoil tracker is 1 mrad (σ) for protons of hundreds-of-MeV and an efficiency of at least 65 % for detecting simultaneously the two protons that emerge from (p,2p) reactions t at several hundred MeV per nucleon and impinging on thin proton-rich target placed at the nominal target position. For developing this detector, the pixel-based ALPIDE sensor technology has been used.

More detailed information on this detector are available in this Technical Desig Report.

Gamma spectrometer

The Califa calorimeter is used for the study of most of the physical cases presented in the R³B Technical Proposal, though the requirements differ significantly from one experiment to the other. In some cases it is the γ−ray sum energy that is required, while in others the detector has to be able to provide γ-ray multiplicities and individual γ−ray energies for spectroscopic purposes. A key requirement of the detector is that it also has to act as the calorimeter for the target recoil. Hence, the detector has to stop and measure the total energy of high-energy light charged particles, e.g. protons up to 300 MeV, with good energy resolution. The main properties of Califa, high efficiency and good angular resolution, are imposed by the very particular kinematics of energetic γ-rays emitted by sources moving with relativistic velocities and by the typically low intensities of the secondary beams involved. The main requirements are listed below:

Total absorption efficiency: 80% (up to E_γ=15 MeV Lab system).
γ sum energy: σ(E_sum)/<E_sum> < 10%.
γ multiplicity: σ(N_γ)/<N_γ> < 10%
γ energy resolution: ΔE/E = 3-5 %.
LCP energy resolution: ΔE_p/E_p < 3%.

High granularity of Califa detector as well as large geometrical coverage and minimum dead layers between the crystals garantee high efficiency and high resolution for detecting γ rays and light particles scattered at large angles around the target.

More details on Califa calorimeter can be found in two dedicated Technical Design Reports: Califa Barrel and Califa Endcap.

Low-energy neutron detector

Spin-isospin giant resonances can be strongly excited in (p,n) reaction using inverse kinematics. One can use at the same time thick targets, without disturbing the energy spectrum of the recoiled neutrons, which has an importance in experiments with rare isotope beams. For such type of studies, a low-energy neutron detector around secondary target is needed. The energy of the neutrons will be measured with resolution better than 10 % and the scattering angle in laboratory frame with better than 1° angular resolution. We require also high efficiency for the spectrometer. The best candidate for such a setup is a time-of-flight (TOF) ball consisting of plastic scintillators.

High-resolution spectrometer

The tracking of heavy ion trajectories through the magnetic field of the large-acceptance dipole provides a Bρ measurement with a precision of ΔBρ/Bρ~10^-3 (sigma), as discussed above. In conjunction with energy-loss and time-of-flight measurements fragments are uniquely identified also in case of heavy nuclei. On the contrary, for a precise measurement of the momentum of recoiling fragments produced in reactions an additional momentum analysis is required. In case of high beam energies and medium-heavy to heavy nuclei a precision of the momentum measurement Δp/p in the order of 10^-4 (sigma) is needed.

The proposed solution is based on a spectrometer placed behind the large-acceptance dipole. The spectrometer will be placed at an angle of 18º, which is the maximum bending angle of the dipole (5 Tm field integral) for a beam with magnetic rigidity of 15 Tm, and will be operated as a zero-degree spectrometer. The focus at the R³B target is an achromatic focus with a beam spot size of around 2×2 cm², as in the usual operation mode. The momentum resolution will be obtained by tracking the particles through the spectrometer. The position at the focal plane will thus not be directly related to the momentum transfer in the reaction but rather reflect the momentum spread of the incoming beam. The spectrometer will have an acceptance of ±2.5% in momentum and ±40(±20) mrad in the (non)dispersive direction, respectively. The maximum rigidity of the Super-FRS amounts to 20 Tm in order to allow separation of fully stripped ions also for heavy nuclei. Considering energy-loss in two degraders at the pre-separator and at the mid-focal plane of the main separator, the maximum rigidity of the spectrometer will be limited to15 Tm.

The spectrometer will be installed in the Phase II of the R³B project, i.e. the R³B experiment would run during the first two years without the spectrometer.

DAQ

One of the main challenges of the R³B DAQ is to provide a maximum interoperability of the different detector systems and their associated DAQ sub-systems. To this goal, the data acquisition will be based on MBS – a distributed data acquisition system that provides a generalized multi-processor environment, suitable for the readout, control and data storage of heterogeneous setups based on common dead-time or time stamps. The present data acquisition and controls system of R³B/Cave-C serves as precursor and basis for developments for the later R³B experiment. Developments address the automated handling of large amounts of channels in view of e.g. pedestal determination and quality control, combining data acquisition and controls tasks. The distributed data acquisition system is supposed to run in parts in a time-synchronized distributed environment based on BuTiS and White Rabbit with timestamps with pico-second precision and 100ps/km accuracy.

The specific front-end electronics together with its digitization part is especially adapted to the individual detector system. The control, trigger and data flow are subsequently interfaced to the common DAQ system. We provide the necessary trigger types, like data, calibration and synchronization triggers together with the necessary dead-time locking in the frontends. For this we also work on integrated logic implementations e.g. TRLOII on the Vulom logic module.