Nuclear matter

The Nuclear Matter team studies the structure of atomic nuclei through the detection of gamma photons emitted by them. This research allows us to better understand the matter that surrounds us as well as the exotic elements involved in particular in certain astrophysical phenomena.

At the heart of the matter that surrounds us, atomic nuclei are microscopic but complex objects. The nucleons that make them up are linked by the strong fundamental nuclear interaction, and follow the laws of quantum physics. Thus, a nucleus can be in different states depending on the distribution and movement of the nucleons. Beyond the fundamental state – of lower energy – there is a multitude of excited states. The properties of these states depend on the strong interaction, and how a composite system reacts to it.

We use the detection of gamma radiation – much more energetic than visible light – emitted when a nucleus de-excites: this experimental technique allows us to study the states linked by such transitions. In this context, we are involved in the development of new, more efficient and high-resolution gamma detectors. These advances are essential for the study of so-called exotic nuclei: these nuclei, which are very unstable, can only be produced in small quantities. Although they are absent on Earth, they are involved in astrophysical processes, notably supernovae, which have enabled the production of the nuclei that make up ordinary matter. The study of these nucleosynthesis processes is also part of our activities.

The core of our research activity lies in the analysis of data, mainly from gamma radiation multi-detectors installed in international infrastructures.
Two major axes are privileged:

• Characterization of extreme states of the atomic nucleus
• Understanding the mechanisms of nucleosynthesis of elements

The analyses carried out are complex and are accompanied by research and development of efficient and innovative methods.

• Development of analytical methods

Characterization of extreme states of the atomic nucleus

The atomic nucleus is a many-body quantum system composed of protons and neutrons whose composition can be varied by moving along the nucleus map. It is therefore a perfect laboratory for studying nuclear interaction. Our knowledge on this subject was first established from experimental studies on stable nuclei. But as we move away from the nuclear valley, new phenomena can appear and allow us to probe nuclear matter in its extreme conditions in spin, isospin and excitation energy. These so-called “exotic” nuclei are generally deformed (the spatial distribution of nucleons is no longer spherical), and make it possible to investigate the effects of layers associated with degrees of freedom of vibration and/or rotation of the nucleus surface. Theoretical models including degrees of freedom of deformation very quickly suggested that these layer effects were strongly correlated with the shape a nucleus can take.

High-resolution gamma-ray spectroscopy is one of the different experimental approaches for accessing the nuclear properties of these nuclei. The group is involved in many major international projects in this field such as the European project AGATA. Such detectors, coupled with facilities delivering beams of accelerated nuclei (GANIL, LNL, GSI…), make it possible to probe the properties of nuclei not naturally present on Earth, artificially created during nuclear collisions and whose lifetime is a few fractions of seconds. The measurement of the energy of gamma radiation emitted during the de-excitation of these nuclei allows us to trace their properties, knowledge of which is essential to improve theoretical models.

Mechanisms for nucleosynthesis of elements

Different nucleosynthesis processes have formed the atomic nuclei that make up the matter that can be observed in the solar system, the Galaxy and the Universe. At the end of the first few minutes after the Big Bang, the nuclei formed by primordial nucleosynthesis are largely hydrogen and helium-4 nuclei, along with a few other light nuclei. Beyond that, the existence of nuclei such as carbon, oxygen, iron, etc., is almost entirely due to the formation and evolution of stars of different kinds. The most massive among them accomplish in their core fusions of nuclei leading to the iron region (nuclei with the highest binding energy). It then becomes impossible to produce energy by nuclear fusion, and the star collapses on itself: an explosion called type II supernova (or core-collapse supernova) occurs. The outer layers bounce off the compact core and their ejection disperses the elements produced by the star into the interstellar medium. It is not only the nuclei produced by fusion, but also heavier nuclei, beyond the iron region, that were formed by different processes during the star’s life and during its explosion. Other events lead to the production and distribution of formed nuclei in stars: the coalescence of neutron stars (cooled cores of type II supernovae), or explosions that occur in binary systems as a result of the flow of matter from one star to another.

Our research activities in nucleosynthesis focus on explosive processes, which have the particularity of involving exotic nuclei. The short life span of these nuclei makes their study difficult, a very large part of them is unknown experimentally and poorly known theoretically. We are particularly interested in p-process nucleosynthesis, which describes how the stable nuclei occupying the proton-rich side of the stability valley (which is the set of nuclei whose number of protons and neutrons equilibrates to form a stable system) could appear. Scenarios describing this process involve tens of thousands of reactions, the majority of which are beyond experimental reach. The theoretical models used to model them, however, need to be adjusted using experimental measurements on crucial reactions. In this framework, we are involved in measurement campaigns to improve the knowledge of the effective cross sections and nuclear parameters that allow us to calculate them from Hauser Feschbach’s statistical model: optical potentials, level densities, gamma decay forces. Concerning the measurement of effective cross sections, we are using our expertise in gamma detection to develop so-called “in-beam” approaches, which make it possible to detect the nucleus resulting from a reaction at the time of its production, thanks to its de-excitation by gamma radiation. Two techniques are being studied: the sum peak method, based on calorimetry (detection of all the radiation emitted by a nucleus), and the angular distribution method, based on spectroscopy (identification of the lines emitted and measurement of their intensity according the angle of emission).

Development of analytical methods

The nuclear reactions used in our research activities produce large amounts of data that need to be processed to extract the rarest events. The chain of transformation of raw data is also complex: mastering this entire chain is essential to ensure that the interpretations given in publications are sound.

In this context, we are working to develop centres of expertise within our group and in collaboration with the Institute’s support services, centres of expertise. The first concerns simulation, an essential tool for building the detectors of the future, understanding and optimising their response function, but also for mastering complex analyses. The implementation and development of physics-generating computer programs is also part of this area. GEANT4, developed by CERN, is the main environment that enables us to carry out these tasks. So far, notably via the SToGS tool, based on GEANT4, which we have developed, we have been able to, for example:

• constrain the design of the PARIS gamma calorimeter
• Perform realistic simulations of experiments with AGATA
• Prepare measurement campaigns for nuclear astrophysics

The second pole is focused on all issues related to data analysis: processing methods, storage, tools, algorithms. Numerous software bricks have been developed, mainly using the object-oriented language C++ and the ROOT environment, to enable the processing of data, in particular from the AGATA spectrometer. These bricks are all projects based on an infrastructure hosted on a git server. Of course, when they are linked to responsibilities within international collaborations, these software bricks are updated to take into account hardware and software evolutions which allow to speed up processing times or to make an algorithm more efficient. The development of algorithms, which can run in such software bricks, is an integral part of our activities. These studies have enabled us, for example, to:

• Implement fuzzy logic to automatically calibrate sensors
• Using graph theory to manipulate level diagrams
• Implement Machine Learning techniques, neural networks, to perform gamma-neutron discrimination in the NEDA neutron detector modules.

General presentation

AGATA (for “Advanced GAmma-ray Tracking Array”) is a European project in which France is one of the main actors. The ambition is to build, maintain, improve and operate a new type of gamma radiation detector (photons). The experimental challenge consists of:

• Collecting precisely (in position ~5mm and energy ~0.1%) the impacts left in the detector by the successive interactions of all the radiation emitted by a nuclear reaction
• Reconstructing from these points the real trajectories left by each photon

The basic brick is a hyper-pure Germanium semiconductor crystal (cylinder of about 10cm by 10cm) electrically segmented into 36 segments (6 wafers, 6 sectors), each segment being therefore sensitive to only part of the active volume. A thirty-seventh signal is generated by the entire active volume. The signals are all digitized (at a frequency of 100Mhz) and transferred to computer farms for processing.

• A first algorithm, called PSA for “Pulse Shape Analysis”, digitizes the 37 signals to extract a list of interaction points for each crystal
• A second, the “tracking” algorithm, is based on all the points in all the crystals to reconstruct the trajectories of the different gamma rays

The detector has been gradually expanded since 2009 to cover more and more space around the target where the reactions take place. This process is accompanied by phases of renewal of the detector’s components, whether hardware (electronics, mechanics, etc.) or software (acquisition system, data analysis and processing programs, etc.). The detector, which is nomadic, travels to the largest nuclear physics accelerators. It began its life cycle at Legnaro in Italy, then went through GSI (Darmstadt, Germany) and the GANIL (Grand Accélérateur National d’Ions Lourds) located in Caen, France.

AGATA in IP2I

IP2I’s Nuclear Matter group has been involved in the AGATA project since the beginning, well before the first data acquisitions: it has several responsibilities in this international project.

The main activity carried out is the development, implementation and improvement of software bricks for the AGATA detector but also for the associated detectors which are different from one experimental site to another. Our group has thus made a significant contribution:

• To AGATA’s Technical Design Report by carrying out realistic simulations showing the high gain compared to the spectrometers of the previous generation
• By implementing the software infrastructure (git, cmake, continuous integration…) to manage the building bricks for online and offline data processing
• By developing software interfaces for data integration and algorithms [Gw/ADF], whether in the acquisition environment or off-line
• By developing new bricks for data processing and analysis. Ex:
  • GammaWare: general software suite for gamma spectroscopy
  • GANPRO: software bricks for GANIL data processing
  • AGASPY: online data monitoring
  • Gw/Cubix: graphical interface for data analysis

All these software bricks are hosted in the gitlab infrastructure of the IN2P3 computing center.

The data attached to the detector, in addition to the data collected during an experiment, must be accessible at all times and kept over the life cycle of the detector and beyond. Our group is involved at two levels in this issue.

1. We have set up, with the support of the IP2I IT department, a database for tracking the hardware components of the detector. The most important parts are labeled with a barcode that allows to find, through a graphical interface developed at IP2I, a set of current or past information, which allows to travel through the history. In this way, movements from one site to another are managed in the same way as, for example, parcels delivered by major trading platforms. The results of tests, repairs, composition/decomposition of parts of complex objects, all relevant information (the system is extensible) are recorded and stored over time.
2. The data produced by the detector can be massive (several tera bytes for an experiment of a week’s length). This data is transferred from the production site (where the detector is used) to two large computer centres for hostage. The first, CCIN2P3, is located in Villeurbanne (Lyon), France, and the second is in Bologna, Italy. Implementing the tools for transferring this data to and from the storage centres is one of the tasks assumed by our research group.

The data processing and analysis tools developed by our laboratory are disseminated to the whole community during international schools in which we regularly participate or even organize.

Of course, we are also privileged users of our tools, which allows us to quickly analyze and publish our results. See for example:

• A publication for the general public
• Two recent scientific publications of the group: 96Kr and 81Ga

Our action is also oriented towards the improvement of systems and it is quite naturally that our group is oriented towards the technologies of the future such as those carried by Artificial Intelligence and more precisely Machine Learning technologies.

More specific tasks are also carried out for the benefit of the project by the technical services of IP2I, such as the reaction chamber shown in figures 1 and 2.

Reaction chamber for the AGATA-NEDA-DIAMANT campaign (GANIL, Caen – 2018) carried out by IP2I’s mechanical department. On the left: design; on the right: realization. High-stress project: light charged particle detector DIAMANT located deep inside the chamber and use of a carbon fiber support for the NEDA neutron detector. – © AGATA-IP2I

For more information

AGATA France website: http://agata.in2p3.fr
AGATA official website: https://www.agata.org/
Official website of the GRETINA collaboration (projet similar to AGATA in the US) : http://gretina.lbl.gov/

General presentation

PARIS (for « Photon Array for studies with Radioactive Ion and Stable beams ») is an international collaboration in which France (CNRS-IN2P3 and CEA) is one of the main contributors. The research and development phase were made possible thanks to funding from ANR (ANR PROVA). CThis detector, even if it is nomadic in design, is linked to the GANIL/SPIRAL2 accelerator project which, right from its development, motivated the construction of a new generation gamma calorimeter for low energy nuclear physics. The goal established on the basis of several letters of intent filed within the framework of GANIL/SPIRAL2, was to build a gamma calorimeter of high efficiency (over a range of up to a few tens of MeV), with high granularity while having an optimal resolution in energy and time.

The choice went to innovative scintillators (LaBr3 or CeBr3) produced by Saint-Gobain and then by SCIONIX. These recent materials guarantee efficiency and resolution for low energy gamma radiation (up to a few MeV). In order to increase the efficiency for higher energy gamma radiation, these materials have been optically coupled with a more standard scintillator (NaI) as shown in the following figure (left). The blue part (LaBr3 or CeBr3) is a cube with a 2-inches edge, followed by the NaI part in red with a depth of 6 inches. In green is the simulated trace of gamma radiation depositing energy (at the break points of the trace) in both layers.

The whole is read by a single photomultiplier (located at the end of the NaI part) as shown in the figure (photograph in the centre). The light signals (visible photons) generated by the gamma radiation energy deposits are collected there and converted into an electrical signal. The overall signal must therefore be deconvolved to separate the LaBr3 (or CeBr3) component from the NaI component. This is possible because these materials have different scintillation times. These basic building blocks are usually grouped into compact structures (3×3 matrices) as shown in the figure (right).

This modularity allows to diversify the assemblies (see next paragraph) in order to meet the different experimental requirements included in the specifications of the PARIS multi-detector. The goal of the collaboration is to obtain ultimately more than two hundred detection units.

PARIS at IP2I

IP2I has been involved in the PARIS project from the beginning, notably in the drafting of the specifications and through the simulations carried out to support the entire R&D phase. Our responsibilities on the international (“management board”) and national (ANR PROVA) scene logically concern the implementation of simulations.

The first step was the development of a simulation environment (based on GEANT4) in order to carry out the studies necessary for the design phase. The solution we developed has since led to a more complete tool, SToGS (« Simulation Toolkit fOr Gamma-ray Spectroscopy »), which aims to cover all gamma radiation detection needs, both in the design and operational phases.

The main contributions during the design phase have been:

• to demonstrate that a detector based on the concept of two optically coupled scintillator layers could meet the specifications.
• to set acceptable limits on the design of the multi-detector and the base bricks.
• to establish the theoretical response functions of the detector.
• to study non-linear effects of collection of the light signal in the LaBr3 and NaI optical coupling.

On this last point, measurements carried out by the other French laboratories involved in PARIS (Orsay, Strasbourg, Caen) have shown a dependence of the signal collected by the photomultiplier on the depth at which the gamma radiation deposits energy. This is illustrated in the following figure (left part) which shows how a source collimated and moved along the detector makes it possible to study the response function of the element as a function of the depth of interaction.

When gamma radiation deposits energy, this one is converted by the scintillator into a large number of isotropically emitted visible light photons (shown in the central figure): these are deployed in the module and are then reflected by the reflective envelope that surrounds almost the entire detector (central and right figure) before being absorbed – for those who manage to do so – by the photocathode of the photomultiplier. Thanks to the simulations carried out we were able to explain the non-linear effects observed. We have also been able to understand the loss of intrinsic resolution of LaBr3 (from 3% to 4%): the difference in optical index between the two materials traps the light that struggles to pass the separation surface from the LaBr3 part to the NaI part. This work has been the subject of several master degree training period which can be found here.