LIfetime measurements with Solid Active targets

Informations projet

LISA

N° de convention de subvention: 101001561

DOI

10.3030/101001561

Date de signature de la CE 16 Février 2021

Date de début 1 Juin 2021

Date de fin 31 Janvier 2027

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 996 250,00

Contribution de l’UE

€ 1 996 250,00

Coordonné par

GSI HELMHOLTZZENTRUM FUR SCHWERIONENFORSCHUNG GMBH
Germany

Periodic Reporting for period 3 - LISA (LIfetime measurements with Solid Active targets)

Période du rapport: 2023-02-01 au 2024-07-31

Résumé du contexte et des objectifs généraux du projet

Within the LISA project, I want to address the question of the origin of collective motion in exotic atomic nuclei. The task requires the development of a new technique to study transition rates, or nuclear matrix elements, which quantify such collective behavior. In order to go beyond the state-of-art, I am developing a new active target system for in-beam gamma-ray spectroscopic experiments using fast, rare isotope beams. This active target will overcome present limitations of in-beam gamma-ray spectroscopy experiments with fast beams, where the resolution for transitions after Doppler correction is dominated by the energy loss in the target. Thus, even with the current state-of-the-art gamma-ray detectors, the performance is limited by the reaction target.
The new LISA device will consist of diamond crystals which, act at the same time, as a target inducing a nuclear reaction and as a detector which measures the characteristics of the beam and ejectiles, and thus the reaction. The accurate measurement of the energy loss of particles traversing the detectors allows for the unique use of a layered target, where each layer of diamonds will discriminate the nuclear charge of the traversing ions. The combined information from the layers of the active target, analyzed with trained neural networks, gives the reaction vertex location. This will allow for precise Doppler correction, even at velocity of 50% of the speed of light, and therefore enable high quality measurements of even the most exotic nuclei.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

Since the start of the project, a 2x2x2 prototype LISA array has been constructed, tested, and characterized in beam. This is the first implementation of a solid active target. The array of eight diamond detectors has been constructed from bare electronic grade chemical vapor deposition grown single crystals. After etching, electrodes were deposited, and the detectors were arranged in two layers, of each four detectors.
A dedicated vacuum chamber, signal feed-through, and precision motorized actuator system to position detectors has been designed, and except for the latter construction is finished. For the automatic positioning, the design is finished and components are ordered.
A data acquisition system has been set up. It consists of CSTA2 preamplifiers coupled to FEBEX digitizer cards. This combination gives excellent energy resolution, less than the 1% required by the physics program for the heaviest nuclei.
The detectors and the data acquisition system have undergone extensive testing with an alpha source, where an energy resolution of 0.7% was achieved.
In November and December of 2023, a single detector was tested with a heavy ion beam at GSI. The opportunity arose during the engineering accelerator beam-time and therefore compromises on the experimental setup and conditions (beam species and energy) had to be made. Nevertheless, important observations could be made. The energy loss of 238U fragments as well as fission products were identified with the diamond detector. The energy resolution allowed for unique identification of fragments over a large range of proton numbers.
The prototype was tested and characterized in a test experiment using a 132Xe beam at the HIMAC facility in January 2024. In this test, the first measurements of layer coincidences, particle identification in two layers, as well as first indications for reactions on the diamond detector itself were identified. The data analysis of both experiments has just begun.
In parallel, a GEANT4 based simulation framework is setup and first simulations are used to develop, test, and train the analysis based on neural networks. Accuracies of 90% have been reached and algorithm refinement is ongoing.
Three physics experiments devoted to study the evolution of collectivity and coherence in exotic nuclei have been conduced as preparatory studies for LISA.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

In the second half of the project, we have two dedicated beam times scheduled. The main objective is to build LISA as a full 5x5x5 array with 125 diamond detectors. For this some technical developments are needed.
The effect and robustness of the metallization on the detector surface as well as the high-voltage applied is under further investigation to improve the raw detector performance.
A dedicated multi-channel preamplifier is currently being developed and first tests are envisioned in the coming month. The goal is to develop a scalable electronics chain which allows for the easy instrumentation of many channels.
The new dedicated LISA preamplifier will have a variable gain factor and a compact design for the implementation inside the AGATA target area. Special attention to the power consumption and heat dissipation is given.
In the future, we plan to obtain reaction type and vertex recognition by automated machine learning techniques. The experimental data collected in 2023 will be used to train and test these algorithms and to further improve the accuracy of correctly identifying the reaction layers.
For this task, as well as the planning of test and physics experiments, we are developing a GEANT4 based Monte-Carlo simulations software package. This code will allow to study accurate reproduction of the LISA detector and provide test and training input for neural networks. These will be applied to the previous and upcoming experiments.
First physics experiments are planned as well. Proposals to the RIBF NP-PAC are under preparation.
Due to the delay of FAIR and therefore the AGATA physics campaign at FAIR, which was envisioned for 2025 at the time of writing of the proposal, physics experiments with the full LISA array and AGATA will have to wait until after the project finishes.

Digitized wave forms of heavy ions impinging at ~800 MeV/nucleon onto a single-crystalline diamond