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High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions

Periodic Reporting for period 4 - HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions)

Okres sprawozdawczy: 2020-12-01 do 2022-05-31

The main scientific challenge addressed in the HYMNS ERC project is the understanding of the origin of the heavy elements in the Universe. HYMNS tackles this question from an experimental perspective, aiming at reproducing and measuring in the laboratory some specific nuclear reactions that take place inside stars. These experiments, in combination with stellar models and meteorites analysis, can provide unique information on the physical conditions inside red-giant stars.
The main societal interest is to provide insight on the origin of the basic building blocks of nature, namely the chemical elements, from which everything around us is made from, including ourselves. In addition, the instruments developed in this project can be of interest for medical applications, particularly for the use of high-energy proton and ion beams to treat cancer tumors, the so-called hadron-therapy.
One of the main technical objectives in this project was to develop a novel detection system for this type of experimental studies, so-called i-TED, which could provide a significant improvement in detection sensitivity, when compared to state-of-the-art detection systems. This was necessary in order to access, for the first time, this type of reactions on some specific radioactive nuclei which, thus far, could not be measured owing to limitations in detection techniques.
The developed i-TED system comprises an array of four Compton gamma-ray imagers. Its background suppression capability was demonstrated by means of dedicated measurements performed at the CERN n_TOF facility. In addition, these measurements were used also to develop Machine-Learning based analysis algorithms, which allow one to maximize the background rejection capability of i-TED, while achieving a high performance in terms of detection efficiency for the neutron-induced reactions of interest. Once validated and calibrated, the developed i-TED system was applied to the measurement of the stellar neutron capture rate of 79Se, the main scientific goal of the HYMNS project. 79Se is a relevant isotope for stellar nucleosynthesis studies, because it is a radioactive nucleus whose half-life changes with the thermal conditions of the stellar environment. Therefore, neutron-capture on this radioactive nucleus can be used as a sort of thermometer to probe the temperature conditions inside red-giant stars. After the data has been fully analyzed and the cross-section for this reaction determined, it will become possible to constrain the thermal conditions in massive stars (M>8Msun), particularly during core He-burning and shell C-burning, which are the last two evolutionary stages before this type of stars explode as supernovae.
A large number of the project results are related to the technical development of the i-TED system, initially proposed in C. Domingo-Pardo, “i-TED: A novel concept for high-sensitivity (n,γ) cross-section measurements”, Nuclear Inst. and Methods A, Volume 825, p. 78-86 (2016).
In order to develop this innovative and complex apparatus, within the HYMNS project we had to perform first a series of technical developments. One such development consisted of large-area position-sensitive radiation detectors with silicon-photomultiplier readout, a work that we published in P. Olleros et al., “On the performance of large monolithic LaCl3(Ce) crystals coupled to pixelated silicon photosensors”, Journal of Instrumentation, Vol 13, Issue 03, pp. P03014 (2018). This type of detector was necessary in order to attain the high detection efficiency required for this type of experiments, while preserving a high energy- and position-resolution for applying the proposed Compton imaging methodology. A second technical achievement was the development of advanced position-reconstruction techniques, which enabled a high spatial resolution, of few mm, for the gamma-ray hit 3D-location in the large scintillation crystal. The results of this work were reported in V. Babiano, et al., “γ-Ray position reconstruction in large monolithic LaCl3(Ce) crystals with SiPM readout”, Nuclear Inst. and Methods A, Vol 931, p. 1-22 (2019) and in J. Balibrea-Correa et al., “Machine Learning aided 3D-position reconstruction in large LaCl3 crystals” Nuclear Inst. and Methods A, Vol 1001, id. 165249 (2021).
After these technical achievements, we were able to develop the first i-TED demonstrator, a work which was published in V. Babiano et al., “First i-TED demonstrator: A Compton imager with Dynamic Electronic Collimation” Nuclear Inst. and Methods A, Vol 953, article id. 163228 (2020). Subsequently, we applied this i-TED demonstrator for the first time in time-of-flight neutron-capture experiments, at the CERN n_TOF facility, in order to demonstrate the capability to reject background events by means of the imaging approach proposed in this project. The successful results of this study were reported in V. Babiano-Suarez et al., “Imaging neutron capture cross sections: i-TED proof-of-concept and future prospects based on Machine-Learning techniques”, The European Physical Journal A, Vol 57, Issue 6, id.197 (2021). These results were also disseminated by the HYMNS research team at international conferences, such as IEEE Nuclear Science and Medical Imaging Symposium, UK, 2019; International Conference on Nuclear Astrophysics IX, Germany, 2019; Nuclei in the Cosmos XVI, China 2021; IEEE Nuclear Science and Medical Imaging Symposium, Japan, 2021.
In an effort to exploit and transfer to society the developments of this project, a study was carried out to evaluate the applicability of the developed i-TED detection system to medical physics. In particular, the treatment of cancer tumors via the modern hadron-therapy technique requires an accurate monitoring of the ion range in the patient tissue. As we describe in J. Lerendegui-Marco et al., “Towards Machine Learning aided real-time range imaging in proton therapy”, Nature Scientific Reports, 12, 2735 (2022), both the hardware and software algorithms developed in the framework of HYMNS can help to improve these treatments by means of real-time range monitoring.
Once the i-TED detection system was fully developed, the 79Se neutron-capture experiment was carried out by the HYMNS research team in 2022 at the CERN n_TOF facility. Online monitoring allowed us to assess the quality of the data taken with i-TED, and preliminary results were presented recently at the Astrophysics with Radioactive Isotopes Conference, Budapest, June 2022. The final data analysis and astrophysical interpretation will be published in the near future.
From the technical point of view, the i-TED system developed in this project represents a significant step forward in the field of neutron-capture cross-section measurements with the time-of-flight technique, because it has allowed us to implement innovative strategies for background reduction. This has led to a situation where we can afford to measure, for the first time, isotopes that were out of experimental access before. This is the case of the 79Se experiment described below.
From the scientific standpoint we have been able to measure, for the first time, the neutron capture cross section of 79Se, which is of high relevance for stellar nucleosynthesis studies. It is expected that, once the data analysis has been finalized, these results will help to put strong constraints on the thermal conditions during the last two evolutionary stages of massive stars.
Nobel laurates Sheldon Lee Glashow and Barry Barish visit the HYMNS laboratory at the HI
i-TED system under calibration at CERN n_TOF
Measuring the neutron sensitivity of one i-TED module at CNA-Seville
i-TED fully integrated at n_TOF EAR1
Exploring the applicability of i-TED for ion-range monitoring at HIT Heidelberg
i-TED alignment for the 79Se(n,g) measurement
Assembly of the i-TED system at CERN n_TOF
i-TED fully installed and aligned at n_TOF EAR1
Four i-TED imagers assembled at the HYMNS laboratory of the HI
HYMNS team with the i-TED detector at CERN n_TOF