High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions
HYMNS at CERN
"The cosmos is within us. We are made of star-stuff. We are a way for the universe to know itself." (Carl Sagan)
The HYMNS project was funded by the European Research Council via an ERC Consolidator Grant (2016-2022) . In HYMNS we developed a radically new detection concept, called i-TED , in order to recreate and measure in the laboratory nuclear reactions induced by neutrons that take place inside Red Giant stars, like Betelgeuse . These are nucleosynthesis reactions taking place during core He burning and shell C-burning in Massive Stars (MSs) and also during H burning in low-mass Asymptotic-Giant-Branch (AGB) stars. Understanding such reactions is a fascinating topic of research and a unique tool to learn about the origin of the elements heavier than iron in our Universe, and to disentangle first-hand information on the physical conditions and mechanisms that drive stellar evolution and the chemical enrichment our Milky Way galaxy.
HYMNS allowed us to significantly push further the detection limits for the neutron-capture stellar rates of astrophysical interest. This statement is nicely illustrated in the figure below (adapted from this review article ). This figure illustrates how advances in facility upgrades and detector developments over the last 25 years, have been of pivotal importance for tackling increasingly challenging neutron-capture experiments. The horizontal axis of this figure shows the seven s-process (radioactive) branching nuclides, that we could successfully measure since 2001 at CERN n_TOF. The vertical axis shows a figure of merit indicating the difficulty level of each experiment (each branching isotope). Note the logarithmic vertical scale.
The red-rectancle in the figure above indicates the contribution of the present ERC-Grant to push higher the previous limits in detection sensitivity, thus enabling the very first measurements of neutron-capture cross sections on the two radioactive
nuclei Nb-94 and Se-79. These are two highly debated s-process branching points, which have a remarkable impact in our understanding of red-giant (AGB- and Massive-) stellar structure and evolution (see the discussion in this open-access article).
In this astrophysical context, neutron-induced reactions are the main subject of interest. Because of the very weak strength of the (radiative capture) reaction channel, when compared to other competing channels like elastic scattering, an aspect of pivotal importance for i-TED is the gamma-ray detection efficiency. i-TED exploits advanced gamma-ray Compton imaging techniques in order to attain a superior level of signal-to-background ratio or detection sensitivity. i-TED shows an excellent detection efficiency thanks to the unique combination of largest monlithic scintillation crystals available nowadays together with innovative Machine-Learning analysis techniques. We have implemented PETsys readout electronics, originally developed for TOF-PET medical imaging, for developing this high-resolution Compton imager.
The full i-TED array consisting of four high-efficiency Compton modules was successfully set-up at the CERN n_TOF neutron time-of-flight facility, as shown in the picture below.
The picture below shows a prototype of one i-TED module being characterized at the HYMNS laboratory of IFIC (CSIC-University of Valencia). Each module consists of five large LaCl3(Ce) scintillation crystals optically coupled to large-area silicon photomultipliers (SiPMs).
The final scientific objective of HYMNS is to apply the developed i-TED array for the measurement of stellar neutron capture rates on radioactive isotopes of astrophysical interest, such as Selenium-79 (Se-79) . The latter will allow us to explore and constrain the thermal conditions during core He-burning and shell C-burning in Massive Stars, before they explode as supernovae. The measurement of Neobium-94 (Nb-94) will allow us to disentangle the contribution from thermally-pulsing AGB-stars and other different environments to the chemical compositon of our Galaxy.
The experiments were carried out at the CERN n_TOF facility during 2022. The schematic figure below offers an idea about the complex accelerator-complex existing at CERN. The astrophysics experiments of the present project were carried out at the two neutron beam-lines of 20 m (EAR2) and 185 m (EAR1).
The picture below shows one of the set-ups with i-TED prototypes used for its commissioning at CERN n_TOF EAR1, back in 2021. The aim of these first preliminary measurements was to perform in-beam calibration and proof-of-concept measurements. The capture-sample was attached to the horizontal neutron beam-pipe. The latter is made from carbon fibre, in order to reduce experimental backgrounds due to neutron scattering. The two similar detectors in diagonal angle from the left are two state-of-the-art C6D6 liquid scintillation detectors, also with C-fibre housing, used as reference. All our i-TED modules are pointing to the capture-sample, two of them are set-up at 90deg, and the other two at 125deg. Notice the high-precission micro-metric positioning stage at the base of each i-TED, which enables remote control and focusing (dynamic electronic collimation technique). The black vertical boxes host the readout ASIC electronics and ancillary cooling based on Peltier modules.
Development of novel detectors and techniques
Technically, HYMNS involved the development of a new concept of detection system called i-TED (Total Energy Detector with gamma-ray imaging capability). i-TED was designed to provide a superior level of sensitivity and selectivity for the measurement of neutron-capture reactions of astrophysical interest. At the HYMNS-laboratory of IFIC we carried out the design, assembly, calibration and validation of all the i-TED modules. The pictures below show the evolution of this new system, from the technical design up to the full implementation stage:
Design of detector encapsulation has been accomplished with the contribution of the mechanics department at IFIC. For convenience, prototype development is carried out via 3D printing using as basis material PLA (polylactic acid), a commonly used printing element. However, detailed Geant4 Monte Carlo simulations have shown that, for neutron capture experiments, aluminum represents a better choice in terms of neutron sensitivity.
The production-run detectors were encapsulated in thin aluminum housing in order to further reduce the sensitivity to neutron-induced gamma-ray backgrounds. The pictures below show another view of one i-TED Compton module. The apertures are designed to access the SiPM photosensor rear connectors, while preventing light coming into the sensitive detector volume.
Interface printed-circuit boards (PCBs) specifically designed by our team at IFIC, help to match the SiPM electrical pulses to the PETsys ASIC-based frontend readout electronics (not visible in the pictures). The rear detector is accomodated onto a linear stage, which can be remotely controlled with micro-metric precision. This allows us to implement a dynamic-collimation technique, which enables a high-level of versatility in terms of detection efficiency and image resolution.
At HYMNS we strove to continuously upgrade and integrate both hardware and software in order to cope with the highest requirements in terms of counting rates, resolution and overall detector performance.
Machine-Learning and Artificial Intelligence are one of our main working tools for pushing further the performance of our developments, and their applicability to different experimental conditions and environments. The figures below show one example of artificial intelligence (Autoencoders) applied to the spatial detector response to recover missing pixels due to dead-time effects or other possible experimental issues:
We integrate and develop the up-to-date Machine Learning and Artificial intelligence techniques to tackle different problems as it is displayed in the following figure:
We have included the use of boosted decision trees and artificial neural networks for supervised task as background discrimination, position reconstruction in LaCl3 crystals, correction of position reconstruction distortions and detection of suitable detected events for Compton imaging. In addition to that, we have investigated also specific neural network architectures such as Autoencoders, GANs for unsupervised task as anomaly detection.
HYMNS Knowledge & Technology Transfer to Society
HYMNS has delivered two spin-off projects aimed at exploring the commercial and social innovation potential of the techniques and concepts achieved herein. The following two ERC-funded projects focus on health applications.
The ERC-Proof-of-Concept project AMA (Advanced imaging system for Medical Applications) aims at transferring the gamma-ray imaging techniques and instruments of HYMNS to fields such as radioguided surgery and emerging neutron-capture cancer therapy treatments (BNCT).
The ERC-Proof-of-Concept project GNVISION (Gamma-Neutron Vision aimed at improved cancer treatments in Hadron Therapy) aims at simultaneously imaging both gamma-rays and thermal neutrons, which are ineluctably produced during the proton-therapy tumor treatments.
GNVision cosists of an upgraded (and patented ) i-TED imager, supplemented with some innovative features, that allow one to simultaneously detect both high-energy gamma-rays and also thermal neutrons. In this video we explain why and how these developments can help society towards a cleaner environment and also contribute in the field of hadron-therapy cancer treatments.
Recently, we have successfully tested the applicability of our system for ion-range monitoring in different European hadron therapy centers. The picture below shows some of the first proof-of-concept tests carried out with two i-TED detectors at the National Accelerators Center (CNA) in Sevilla. Some publications related to this work and contributions to conferences are already available (see public documents section).
After a very successful implementation at the low beam-energy of the CNA cyclotron, we carried out a more realistic experiment at the CPO Hadron Therapy Center of Orsay, France. A picture of the set-up with a large water phantome can be seen in the picture below.
The results obtained are very encouraging, we are presently working on the data analysis and dissemination of the latter experiments.
HYMNS Outreach
Visit us! We show our laboratory regularly to high-school and undergraduate students and explain the research and developments carried out in the framework of this project.
