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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions)

Teaser

Summary

The origin of the chemical elements in the Universe is one of the main open questions in science. Elements heavier than iron, like cerium, samarium, tungsten or lead, play a pivotal role, not only for the technology-driven transformational change of our society, but even for explaining the origin of life on Earth in its most primitive forms (see e.g. B. Schoepp-Cothenet et al., Nature, Scientific Reports volume 2, Article number: 263 (2012)). The recent measurement of a gravitational wave in conjunction with the light (electromagnetic spectrum) from a colliding binary system of neutron stars (S.J. Smart et al., Nature volume 551, pages 75â€“79 (2017)) represents an important step forward towards a quantitative understanding about the origin of the elements in the Universe. It is estimated that about half of the heavy nuclei arise from such explosive environments. The other half is thought to be synthesized on a very long time-scale along the life and evolution of stars, such as red-giants, by means of a sequence of nuclear reactions, namely neutron capture and beta decay reactions. This process of nucleosynthesis is called slow-neutron capture (s-) process. Nowadays, one of the overall objectives in this field of research is to confront measurements of specific nuclear reactions, like those taking place inside the stars, with observed isotopic and elemental abundances in order to gain a comprehensive and detailed understanding about the fundamental physical aspects of stellar structure, dynamics and evolution.
There are several radioactive isotopes of key relevance for the study of s-process nucleosynthesis which have not been measured yet. Their first measurement would allow one to uncover valuable and accurate information about the physical conditions inside of the stars, the chronology of our Earth and the Milky-Way Galaxy, or even gain experimentally guided insight about the characteristics of the first generations of stars in the Universe.
One objective of HYMNS is to develop a new apparatus, with enhanced sensitivity with respect to state-of-the-art detectors, which enables the measurement of stellar neutron-reaction rates in the laboratory using very small or radioactive samples. This novel detection system is named total-energy detector with imaging capability (i-TED) and it has been conceived for measuring both the energy and the trajectory of the gamma-rays arising from such reactions, thus enabling a superior level of background discrimination with respect to existing detection systems.
HYMNS is structured to tackle the first measurements of key s-process branching nuclei over the stellar energy range of interest. The first application in the framework of this ERC Grant will be to determine the neutron capture cross section of 79Se, which will provide the most stringent constraint for the thermal conditions and their time-dependence in state-of-the-art evolution models of massive stars.

Work performed

The main results achieved so far are related to the development of the i-TED apparatus itself. An i-TED demonstrator has been assembled and calibrated at the HYMNS laboratory of the HI. This demonstrator has been validated under real experimental conditions at the CERN neutron-beam facility n_TOF. Thus, a significant part of work has been devoted to the development of the individual position-sensitive detectors (PSDs) involved in i-TED. Each PSD consists of a large monolithic scintillation crystal optically coupled to a pixelated photosensor (SiPM). An important objective of this part was to achieve a good energy resolution for the detection of gamma-rays in the astrophysical energy range of interest, while preserving a good position resolution. In HYMNS we have been able to achieve an energy resolution, which is comparable or better than the one obtained with (non-pixelated) mono-cathode photosensors, such as photomultiplier tubes [see arXiv:1801.05059].
The SiPM pixelation is a crucial aspect for the development of i-TED because it ultimately enables the possibility to gain information on the incoming direction of the detected gamma-rays. We have developed algorithms, which allow us to achieve a position-reconstruction accuracy that is beyond the state of the art for this kind of radiation detectors (see [arXiv:1811.05469]).
Once the individual detection modules were developed and calibrated, we assembled an i-TED demonstrator aiming at technical validation and proof-of-principle measurements using the neutron beam available at the CERN n_TOF facility. The proof-of-concept experiment was carried out recently (September, 2018) and the data is being analyzed.
An objective of HYMNS is to apply the new apparatus i-TED, once fully developed, to the measurement of key nuclei of astrophysical interest. One such nucleus is 79Se, which is one of the HYMNS objectives. A major challenge here was to obtain a suitable sample of 79Se, which is radioactive and has a half-life of 3x105 y. Thus, a collaboration was established with the Isotope and Target Chemistry Laboratory of the Paul Scherrer Institute (PSI) in Villigen, Switzerland and with the Institut Laue-Langevin (ILL) in Grenoble, France. After several optimization tests at the PSI laboratory a suitable primary 78Se sample was produced, which is presently being irradiated at the large-flux reactor of ILL-Grenoble in order to produce (via neutron-activation) a small content of 79Se for the posterior measurement at CERN n_TOF using i-TED. At the time of writing this report a total of 42 full-power days equivalent have been used, which correspond to about 3 mg of 79Se.

Final results

A significant improvement with respect to state of the art techniques has been accomplished in terms of high energy resolution position-sensitive radiation detectors. The i-TED position-sensitive radiation detectors, based on monolithic crystals, represent the largest of its kind with SiPM readout. Their spatial resolution, as reported in [arXiv:1811.05469], is the best result reported so far for this kind of detectors. Although HYMNS has a clear focus on nuclear astrophysics, the developed apparatus and techniques could well find applications in other fields, such as molecular imaging in nuclear medicine, or homeland security.
Finally, the most significant step-forward in terms of radiation detection techniques is due to the first application of a novel system such as i-TED, with high energy and position resolution, to the field of neutron capture experiments using simultaneously the neutron time-of-flight method and the Compton imaging technique. It is in this respect that i-TED, with an energy resolution of ~5% and its directional imaging-sensitivity, is expected to provide a breakthrough in the field of neutron capture measurements, allowing for a superior background discrimination and improvement in signal-to-background ratio.
As reported in the previous section, we have carried out validation and proof-of-principle measurements with an i-TED demonstrator at CERN n_TOF. The data are being analyzed and the results will be published in the near future. The full 4-pi i-TED system will be developed at the HYMNS lab of the HI in 2019 and 2020. Finally, it is planned to carry out the 79Se neutron capture measurement over the stellar energy range of interest using i-TED during the last year of the project. The astrophysical interpretation of the results obtained will be carried out in close collaboration with theoretical astrophysicists.