Institute of research into the fundamental laws of the Universe

Fundamental Research Division

R3B

Reaction studies with Relativistic Radioactive Beams

FAIR complex with accelerators and experimental areas

Goals of the experiment

The R³B experiment is part of the FAIR project (Facility for Antiproton and Ion Research, http://www.gsi.de/fair) to be built at GSI (Darmstadt, Germany). The FAIR project gathers different physics around a common facility: exotic nuclei at low and high energy, hadronic physics with proton – antiproton collisions, relativistic heavy-ion collisions (a few 10 GeV per nucleon), plasma physics and atomic physics.

The international collaboration R³B (Reaction studies with Relativistic Radioactive Beams) has built a scientific program of experiments devoted to the physics of stable and radioactive beams at energies between 150 MeV and 1.5 GeV per nucleon: nuclear structure, reaction mechanisms, study of the fission of radioactive isotopes, reaction cross-sections of interest for astrophysics, etc. The production of the radioactive beams is done using the in-flight fragmentation: a primary stable beam reacts impinges on a production target and the projectile fragments are selected in mass and charge with a high-resolution magnetic spectrometer and focussed afterwards in order to form a secondary beam of well identified radioactive isotopes.

Among the nuclear physics facilities in Europe and around the world capable of producing exotic beams, GSI (Gesellschaft für SchwerIonenforschung) in Darmstadt, Germany, is at the moment the only accelerator where experiments using stable or radioactive beams at 1 GeV per nucleon can be performed. The use of such an energy domain is important in order to limit the influence of reaction mechanisms on the observables where nuclear-structure information and the reaction mechanisms are mixed. Indeed, the physics R³B will take fully advantage of the experience acquired by the scientific teams already working at GSI.

The reactions of secondary beams of rather high energy on fixed targets have developed their potential as exploratory tools to study the properties of atomic nuclei far from stability. These reactions have permitted, for example, the extraction of detailed spectroscopic data weakly influenced by reaction mechanisms, or to improve our understanding of the nuclear interaction through the study of the nuclear structure far from stability.

A major research focus of the LENA group at DPhN is the structure of exotic nuclei and, since more recently, the study of short-range correlations (SRC) in exotic nuclei. The GSI accelerator and, later, FAIR, are the ideal instruments to perform these kinds of measurements.

The energy (around 1 GeV per nucleon) necessary for these reactions as well as the high intensity of the secondary beams which will be available at the future FAIR/R³B facility create the necessity to build new experimental apparatus with improved performances: a central dipole magnet placed behind the reaction target providing a higher bending power and a larger angular aperture, faster detectors providing higher resolutions in the reconstruction of the kinematics at the reaction points. The construction of FAIR has been divided into different phases. The first one, called "phase 0", started in 2018 and will allow to perform experiments with the R3B device designed for FAIR and the beams delivered by the GSI accelerator, while waiting for the completion of the FAIR accelerator system.

R3B experimental set-up during phase 0

Context

The RIKEN and NSCL/FRIB accelerators are the main competitors of GSI/FAIR. The RIKEN radioactive beam production facility, already in operation, can deliver primary beams with energies of 350 MeV per nucleon, lower than at R3B. FRIB, the NSCL upgrade under construction, aims at energies of 300 MeV per nucleon.

Compared to SPIRAL2, one should rather talk about complementarity rather than competition. Indeed, the mechanisms for producing radioactive beams are very different between the two installations, which means that the beams available will also be different, since SPIRAL 2 will specialize in beams produced with uranium-238 fission fragments. Similarly, the energies of the SPIRAL 2 beams will be of the order of a few MeV per nucleon, compared to the hundreds of MeV of the beams that will be used at R3B.

IRFU contributions

IRFU has designed and built GLAD (GSI Large Acceptance Dipole) spectrometer, a key element of the R3B device which was delivered in 2015 and used since 2018. Moreover, the IRFU has designed and realized, in the framework of the ANR COCOTIER project, a liquid hydrogen target which was used for the first time in March 2021. This is an asset to perform experiments based on a quasifree scattering reaction, such as the study of short range correlations and several other experiments at the heart of the R³B program.

Contacts

Scientific leader of COCOTIER:

R³B-GLAD project manager: Bernard Gastineau

Site de R3B

#2108 - Last update : 04/08 2021

COCOTIER

Liquid hydrogen target for Short-Range Correlations at R3B

COCOTIER target and cryostat.

Scientific challenges and project framework

The COCOTIER project (for short-range COrrelations and Isotopic spin at R3B - COrrélations à COurte porTée et spin IsotopiquE à R³B) aims at studying short-range correlations in exotic nuclei produced by fragmentation at the radioactive ion beam factory GSI and, later on, FAIR. Short-range correlations formed by the combination of the intermediate-range attraction with the short-range repulsive term of the nucleon-nucleon potential pairs nucleons in a very compact spatial configuration (1-1.5 fm) with a large relative momentum. That is the reason why those correlations may provide a way to probe in the laboratory the short-range part of the nucleon-nucleon interaction.

Those correlations were studied only in the region of the stable nuclei with values of the asymetry ratio between neutron and proton numbers (N/Z) close to 1 (N/Z~1-1.5). Those studies were mainly carried out with a series of experiments done at the electron accelerator of Jefferson Lab, JLab (United States). These measurements are limited to stable nuclei, and they were obtained with a low statistics. The objective of the COCOTIER project is to overcome these two limitations by performing measurements at high luminosity in inverse kinematics. The study of the short-range correlations in ¹⁶C (and ¹²C as a reference) will be the goal of the first experiment at the R³B facility, scheduled in May 2022.

The figure adapted from Nature 560, 617 (2018) shows how our measurement will allow for the first time to compare nuclei with similar mass but very different N/Z asymmetry (12,16C)

Detection system of R3B at GSI.

Technical details

For the study of these correlations, we use the mechanism of quasi free scattering in inverse kinematics: the exotic ion beam is sent on a proton target, and all the reaction products are subsequently detected. It is then possible to count the pair numbers, and to deduce the momentum of the nucleons forming the pair. To perform this experiment, a liquid Hydrogen target has beeen built at IRFU. It will be coupled to the R³B detection system. This system combines a trajectograph (Silicon tracker), the CALIFA calorimeter, the NeuLAND neutron detector, and the analysis system of the fragment residue and of the charged particles, which are emitted at forward angles and deviated by the magnetic field of GLAD, a very large acceptance dipolar magnet built at IRFU.

Project schedule

The COCOTIER project is funded with an ANR (project PI: IRFU, A. Corsi) starting from October 1^st, 2017. The Liquid H₂ target has been built at IRFU and installed at GSI end 2019. The first experiment with the COCOTIER target was performed in March 2022. Two other experiments, including the measurement of Short Range Correlations, were approved by the GSI Program Advisory Committee in 2020 and are currently planned for 2022.

Contact : IRFU, DPhN

#4181 - Last update : 05/03 2021

R3B-GLAD (English)

GSI Large Acceptance Dipole

FicheR3BGLAD.doc

Physics/Science:

Magnets & accelerators / superconducting magnets

Introduction

Within the detection set-up of the future R³B experimental hall, the large aperture superconducting spectrometer GLAD (GSI Large Acceptance Dipole) will play a central role. This magnet is being designed in CEA-Saclay, DAPNIA, and will be built there with the help of subcontracting industrial companies. GLAD will be used by all the experiments which will be performed by the R³B collaboration, whether these experiments will be aiming at studying the structure of exotic nuclei or the reaction mechanisms (e.g. in spallation reactions).

The preliminary design study of the GLAD spectrometer was performed in 2001 and 2002 and was funded within the 5^th framework programme of the European Union in order to cope with the ambitious specifications of such a magnet (large aperture, combined with a rather high field integral of roughly 5 T.m and a very low fringe field of 20 mT and below at 30 cm of the magnet entrance). An efficient separation of the tracks of the heavy fragments and of the protons is also ensured with the high bending power of the magnet.

Goals of the project:

GLAD is being designed in order to reach four main goals:

- A bending power of 4.8 T.m which permits to bend to 18 deg fragments of ¹³²Sn⁵⁰⁺ whose kinetic energy is 1 GeV per nucleon

- A large angular aperture, both horizontal and vertical, for light charged particles, nuclear fragments as well as for neutrons whose trajectories are not modified by the magnetic field,

- A large momentum acceptance to allow for the simultaneous detection of protons and heavy beam residues of the same kinetic energy per nucleon produced at the target point in front of GLAD

- Insure a low fringe field, with emphasis on the fringe field in the target region (entrance of the magnet) in order to make possible the use of detectors around the target which are sensitive to the presence of magnetic fields such as photomultipliers

The characteristics of GLAD will make feasible experiments which require the coincidence measurement of light particles and heavy fragments which have a low kinetic energy in the centre-of-mass frame of the projectile. Such particles and fragments are essentially particles from the de-excitation of the projectiles which have undergone a nuclear reaction in the target. This magnet is therefore very well suited for the study of spallation in coincidence and inverse kinematics as well as for the study of the fission of nuclei from Coulomb excitation. Such a coincidence measurement will allow for the reconstruction of the excited states of the projectile during the nuclear reaction in the target. This will give therefore access to information on the nuclear structure of the projectile.

Localisation:

The GLAD magnet will be used in the R³B hall of the FAIR facility in GSI, Darmstadt, Germany.

The integration of the superconducting coils inside the cold mass, the instrumentation of the device as well as the integration of the dipole magnet inside its cryostat will be performed in Saclay with the help of subcontractors from the industry (2010). Tests will be performed in Saclay as well as on the GSI / FAIR site after it will have been sent there from Saclay (2011).

Contacts

Jean-Eric DUCRET

Bernard GASTINEAU

The funding of the construction of the R³B-GLAD dipole magnet is provided by the European Union within the 6^th framework programme, GSI and the DAPNIA. The DAPNIA project team on this magnet is fully in charge of its design and of its construction.

The R³B international collaboration gathers around 150 physicists from 50 different research institutes and universities around the world (Europe, India, China, Northern America and Russia).

The R³B-GLAD project aims at designing and building in less than 5 years a magnet whose weight will be approximately 50 tons, width 7 m, height 4 m and length 3.5 m. The energy stored in the 4.5 tons of superconducting niobium-titanium (NbTi) cable will be roughly 24 MJ.

The specifications of the experimental parameters for the dipole magnet (bending power, fringe field, etc) have been given by the R³B collaboration. The DAPNIA is in charge of the design, of the construction and of the tests of the magnet in Saclay. After these tests, GLAD will be shipped to GSI in Germany for installation, testing and experiments. The FAIR facility will have to provide, for a proper functioning of the magnet, all the cryogenics (liquid helium and nitrogen) as well as the power supply and the entire local infrastructure.

The collaboration of GSI / FAIR staff members is foreseen as early in the construction of the magnet as the dipole integration in the cryostat. They will also take part in the tests in Saclay.

o The SACM is the division in charge of project, from the conceptual design to the construction and the tests, including in particular the detailed design study with the original magnet design, the superconducting cable which is the base of the magnetic coils, the cryogenics and the magnet cooling working on the thermo siphon concept and the entire magnet protection scheme,

- SIS

o The SIS is in charge of the mechanical design of the magnet and the cold mass, whose major difficulty of construction is to ensure the mechanical stability of the whole magnet and compensate for the efforts on the coils provided by the magnetic field, of the order of 400 tons/m,

- SPhN

o The SPhN who ensures the scientific responsibility of the project provides the link between the R³B collaboration and the project team and helps the technical staff to translate into technical parameters the parameters provided by the physics to be performed within R³B and with the GLAD magnet

The DAPNIA in CEA-Saclay is the only research institution working on the realisation of the GLAD magnet.

View of the cold mass (cryostat + magnetic coils) of R3B-GLAD

Magnetic field map of GLAD. The red lines are different tracks of particles in the magnetic field volume.

INSTRUMENTATION (ONGLET)

Specificities of the project & technical description

Duration: approximately 5 years

Dimensions: Length = 3.5 m; Width = 7 m; Height = 4 m

Total weight = 50 tons; Cold mass weight = 20 tons

Superconducting cable: NbTi, Length = 16 km, weight = 4.5 tons

Current density < 80 A/mm²

Current in the coils = 3700 A

Magnetic strength on the cable = 300 to 400 tons/m

Stored energy = 24 MJ

Field integral = 4.8 T.m

The protection system of the magnet is relying on an active detection of the transitions of the cable to the resistive phase followed by a discharge of the magnet on an external resistor. The coils are pre-constrained in an aluminium alloy casing in order to compensate for the relatively high efforts on the coils provided by the magnetic field (300 to 400 tons per meter).

The magnet cooling is provided by conduction of heat through the winding to a two-phase helium flow working in thermo siphon.

The cryostat, the helium phase separator and the thermal screens will be placed inside a conical and elliptical vacuum chamber (~ 7 m x 4 m at the exit)

In July 2001, the technical design report was validated by an international review committee of experts. In 2003, the proposal for a CNI contract including the construction of GLAD was given to the European Commission for a funding within the 6^th framework programme while the final design studies went on. The funding was accepted at the end of 2005 and the project could start officially in the beginning of 2006.

The SACM is in charge of the management of this project which necessitates a contribution of the SIS. The SPhN is in charge of the scientific management of the project and makes the link between the R³B collaboration and the project team.

Based on the innovative concept of active shielding, the R³B-GLAD magnet is made of four superconducting coils (“Tigra-Trace” design: TIlted GRAded Trapezoidal RACEtrack). The two main coils are shaped in racetrack widening from the entrance to the exit of the magnet and are tilted vertically. The side coils, which contribute both to the main dipole field and to the magnetic shielding, are turned horizontally with respect to the magnetic symmetry axis. Such geometry allows for reducing the useful magnetic volume by putting the coils as close as possible from the particle and fragment track envelops, and, therefore, of the stored energy in the magnet which is an essential parameter on the price of such a device. The necessary angular aperture is hence provided while, at the same time, keeping a low fringe field at the entrance of the magnet, in the target region.

The number of turns in the main coils is gradually increasing from the entrance to the exit. This results in a rather flat plateau for the magnetic field inside the magnetic volume at 2.7 T while keeping the field value on the surface of the coil below 6.5 T. The side coils have also the same gradation in the number of turns and have been shaped in order to decrease rapidly the field outside the magnet (on the sides) and to provide a low fringe field in the target region which follows the specifications from the physics and the experiments (below 20 mT).

The apparent current density in the cable is kept below 80 A/mm². Such a design gives a rather compact magnet with a stored energy of approximately 24 MJ.

The active shielding concept allows building a fully superconducting magnet which provides a full linearity between the current in the coils and the field map of the magnet while keeping a rather low fringe field.

Longitudinal profile of the vertical component of the GLAD magnetic field along the symmetry axis of the magnet

#2053 - Last update : 09/29 2017

R3B-TPC (English)

DESCRIPTION OF THE PROJECT

Physics and programmes:

Innovation for detection systems/Detector developments

Nuclear matter in its extreme states / Exotic nuclei

Goals of the project

The construction of a new multi-track charged particle detector is necessary for the experiments which will be performed in the R³B hall of the future FAIR facility and which will require the detection of multi-charged particle final states after the magnetic analysis provided by the future GLAD spectrometer such as spallation, fission or proton or alpha decay reactions.

The spallation group of the Service de Physique Nucléaire (SPhN) is involved in two projects of experiments for R3B which will need the detection of such final states:

- the study of heavy nuclei spallation, in coincidence and inverse kinematics, such as lead-208 (²⁰⁸Pb) or uranium-238 (²³⁸U),

- The study in coincidence of fission of minor actinides which have undergone coulomb excitation in the electrostatic field of a heavy nucleus.

The aim of such a multi-track detector is three-fold:

- To detect efficiently all types of charged fragments and particles, from proton to uranium, i.e. on a dynamic range of the ionisation signals of 1 : 10⁴,

- To identify unambiguously the charge of each particle or fragment (at 1 GeV per nucleon kinetic energy, the nuclear fragments are completely stripped so that their electrical charges, which can be measured by detectors, are their atomic number, i.e. the charges of the nuclei),

- To determine precisely the geometry of the particle tracks in 3D (two angles and three coordinates) in order to allow the determination of their masses and the reconstruction of their kinematics at the reaction point after the numerical inversion of the magnetic transport within GLAD.

This three-fold aim must be reached in order to have a complete reconstruction, event by event, of the nuclear reaction kinematics at the target point. Such a reconstruction is the common goal of all the experiments to be performed within the R³B collaboration even though the nuclear physics domains explored there will be relatively different. This reconstruction will permit to determine the characteristics of the excited nucleus which is the intermediate state between the initial state (the projectile and the target nucleus) and the final state, after the complete de-excitation of the system.

The construction of a new detector is made necessary by the planned characteristics of the future GLAD magnet. In fact, its characteristics (angular aperture, momentum acceptance, bending power and physical dimensions of the device) will produce larger envelopes of the particle tracks at the exit of the magnet, with larger angles with respect to the GLAD magnet symmetry axis, as compared with the existing facility of the ALADIN magnet (GSI, Cave C). The detector used now by the spallation group of SPhN, which is the time projection chamber (TPC) MUSIC 4 (Link to KP3 web site) will indeed not be appropriate for the future experiments in the R³B hall.

The concept of time projection chamber was chosen, as for MUSIC 4, because this type of detector is relatively cheap to build as compared to other detectors. Furthermore, its performances will be within the requirements of the future experiments of R3B without major R&D efforts and can be easily adapted to different experimental conditions, for both low and high multiplicities in the final states.

Size of the project

- Investment: around 600 k€ over the duration of the project (4 to 5 years)

- Staff: roughly 15 person.year over the duration of the project

CONTACTS

Scientific leader : Jean-eric DUCRET
TPC design study leader : Philippe LEGOU

INSTRUMENTATION

Instrument

time projection chamber (TPC)

Technical description

o Active volume: H = 90 cm, L = 120 cm, W = 120 cm (entrance) & 250 cm (exit)

o Vertical drift, divided into two parts by a thin cathode foil which will generate the electrostatic field needed for the primary electron drift with the aim of getting a better position resolution on the reconstruction of the horizontal coordinates of the tracks (the horizontal plane is the dispersive plane of the GLAD magnet)

o Gaseous amplification with Micromegas (micro-meshes placed at roughly 100 µm from the charge collection plane)

o Charge collection performed with pads of 1 cm width and around 13 cm length in 9 rows in the longitudinal direction

o Number of pads: around 5 000

o Dynamics of the primary signal: 1 : 10⁴ (in the energy range where the TPC will be used, the gas ionisation by charged ions occurs at constant ion velocity; This means that the ionisation signal amplitude depends essentially on the square of the incident ion charge according to the Bethe-Bloch formula

o This large dynamics will be addressed by two types of amplification : 5 rows of pads will be equipped with large amplification (for the detection of light fragments) and 4 with a low amplification (for the heavy fragments); This will reduce the contraints on the coding electronics

- A specific study is going on in order to:

o Determine the influence of the micro-mesh transparency

o Produce a concept of an electrostatic configuration aiming at reducing the rate of positive ions drifting back into the active volume of the detector from the amplification sites ; In fact these ions whose mobility is very small, are accumulating within the active volume and are reducing the performances of this type of detector, particularly as far as resolution is concerned

- The electronic amplification chosen is discrete electronics. This choice allows for the decoupling of the amplification function and the coding function. It is made possible by the rather limited number of readout channels and will permit a technological evolution of the detector possibly required by future experiments. With the discrete electronics, we will be able to cope with the large primary signal dynamics while providing at the same time:

o Fast signals which will limit pile-up effects and will allow for working at higher beam intensities (the beam traverses the detector in this kind of experiments)

o Low detection thresholds

- Data acquisition counting rates: 10³ events per second, maximum

- Automated control of the detector and the gas purity on which depend critically its homogeneity, its efficiency and its precision.

Principle scheme of the TPC

#2109 - Last update : 09/29 2017

• Structure of nuclear matter › Atomic nucleus

• The Nuclear Physics Division • Accelerators, Cryogenics and Magnetism Division (DACM)