A next generation experimental setup for studies of Reactions with Relativistic Radioactive Beams
The experimental setup for studies of reactions with relativistic radioactive beams R3B is part of the FAIR project. In the frame of the R3B international collaboration a versatile reaction setup with high efficiency, acceptance, and resolution for kinematically complete measurements of reactions with high-energy radioactive beams has been developed and constructed. The setup will be located in the High-Energy Cave (HEC) at the focal plane of the high-energy branch of the Super-FRS. The experimental configuration is based on a concept similar to the R3B reaction setup at GSI introducing substantial improvement with respect to resolution and an extended detection scheme, which comprises the additional detection of light (target-like) recoil particles and a high-resolution fragment spectrometer. The setup in HEC is adapted to the highest beam energies (corresponding to 20 Tm magnetic rigidity) provided by the Super-FRS capitalizing on the highest possible transmission of secondary beams. Parts of the systems described below have been commissioned and successfuly used in different experiemnts performed in Cave C at GSI.
The R3B setup covers experimental reaction studies with exotic nuclei far off stability, thus enabling a broad physics programme with rare-isotope beams with emphasis on nuclear structure and dynamics. Astrophysical aspects and technical applications are also concerned. In order to cover such a large physics programme several different detection subsystems as well as sophisticated DAQ system and data-analysis softwareare planned (see below). The work is performed inside the R3B collaboration which includes more than 50 different institutes from all over the world.
In the following, systems developped under the guidance of the R3B local group will be described in more details.
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 (see tabels below) will constitute a major step forward.
The final design described in the NeuLAND Technical Report is the result of several years of R&D studies. Different design concepts have been followed, including converter-based solutions, e.g., a detector based on steel converter plus charged-particle detection with resistive-plate chambers (RPC). The RPC concept has been ruled out mainly because of its insufficient multi-neutron detection capability. A fully active detector with calorimetric properties has turned out to be the best solution. Submodules have been tested with proton and electron beams resulting in time resolutions of better than the required σ(t) ≤ 150 ps, even with inexpensive photomultipliers. In accordance with simulations, improved timing properties have been obtained with optimized light guides.
The details of the technical realization including mechanics, readout electronics, tests, calibrations, as well as the construction procedure are summarized in the NeuLAND Technical Report, which was finalized in November 2011. The construction of the detector has started in spring 2013. During Oct. 2014 the NeuLAND demonstrator ( 1/6 of the full size detector) was successfully tested in GSI beam times. The mounting of NeuLAND double-planes is carried out at GSI with a team from contributing institutes guided by the R3B local group. Necessary quality controls have been established. Prior to the installation of NeuLAND at its final location in the R3B hall at the FAIR site, detector has been commissioned and used in experiments at Cave C.
More details can be found here.
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.
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.
- 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
The detector for light (target-like) particles is a substantial part of the R3B setup. It allows registration of recoils in coincidence with the heavy fragments, neutrons and the γ-rays. This set-up gives thus a unique possibility to study elastic, inelastic and quasi-free scattering, knockout and breakup reactions. The recoil particle detector provides precise tracking, vertex determination as well as energy and multiplicity measurement with high efficiency and acceptance. The latter two parameters are very important when dealing with radioactive beams. According to previous experience, for the case of (in)elastic scattering, the angular resolution (in centre-of-mass and laboratory system) should be around a few mrad and the resolution in excitation energy, E*, better than 1 MeV.
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 Eγ=15 MeV Lab system).
- γ sum energy: σ(Esum)/<Esum> < 10%.
- γ multiplicity: σ(Nγ)/<Nγ> < 10%
- γ energy resolution: ΔE/E = 3-5 %.
- LCP energy resolution: ΔEp/Ep < 3%.
Califa detector has been used in different experimens in the Cave C at GSI.
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.
More details can be found here.