Nuclear reaction codes development for the particles and nuclei production in meteoroids and planetary atmospheres

Hirtz, Jason (2019). Nuclear reaction codes development for the particles and nuclei production in meteoroids and planetary atmospheres (Unpublished). (Dissertation, Faculty of Science, University of Bern, Physics Institute)

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Through history, humans always tried answering the two existential questions: “Where do we come from?” and “Where shall we go?”. While it belongs to philosophers, to governments, to social sciences, and to each of us to answer the question “Where shall we go?”, we should be able to have at least a partial answer to the question “Where do we come from?”. Only knowing the past enables one to make useful predictions for the future.
The question “Where do we come from?” is not only a philosophical one. It is foremost a question with an answer based on facts and observations, which can be studied scientifically. Thereby a major task of historians, archaeologists, palaeontologists, geologists, and (astro)physicists is to study facts and observations and to understand how the world evolved. While the scientists study the history at different time scales; the sum of their researches allows us to have a better understanding of what happened since the beginning of the universe. With a good knowledge of our past, we can understand the present and predict the future more precisely. The different studies are very often based on the traces left by the various events that happened in the past. These traces are often analysed using methods coming either from physics (14C dating) or from chemistry (chemical composition analysis). Based on the experimental data, scientists propose scenarios to explain the observations. Finally, a good theory also studies the likelihood of the hypothesis considering all available information.
The group in Berne at the time of writing this thesis comprised of four members: a geologist, a chemist, a physicist, and an astrophysicist. The objective of our group is mainly focused on the study of meteorite histories. “Where do they come from?”, “How long was their journey?”, and “How long have they been on Earth?” are common questions we try to answer. This quest is aided and guided by the measurements of isotopes in meteorites. They are produced by cosmic rays and known as cosmogenic nuclides. The group studies also cosmogenic nuclide production high in the atmosphere as well as at the surface of moons and planets. The analysis of cosmogenic nuclide abundances in the studied objects gives crucial information about their irradiation histories. However, we can go one step further and try to understand the history of the cosmic rays and, by extension, to better understand the history of the solar system, galaxy, and the universe. The actual works of the different group members are very different but complementary in the objective to answer the question “Where do meteorites, cosmic rays, etc. come from?”. And, as we know: “We” are all stardust, “We” are the result of the interaction between different types of stars, dust, and cosmic rays since the beginning of the universe. In this sense, we can also say that the objective of our group is to help answering the fundamental question “Where do we come from?”.
Nowadays, we understand the origin and the physics of the cosmic rays relatively well, even if fun- damental questions are still not answered. Cosmic rays are free elementary particles and atomic nuclei travelling though the universe. They are mainly produced by dying stars, mostly supernova explosions. Then, they propagate in the vacuum of space with a velocity often close to the speed of light. Finally, the particles end their lives in hard collisions with massive objects like planets and meteoroids (the name of
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CHAPTER I. INTRODUCTION
meteorites before they fall on Earth or another planet). In addition, there are also interactions within the galactic cosmic rays (GCR) itself, i.e., GCR particles interacting with other GCR particles. Taking into account their quantities and their effects, the cosmic ray spectrum relevant for our studies is dominated by hadrons (mainly protons and alpha particles) with energies in the range of few GeV . When such particles collide with a nucleus in a meteoroid, a planet, or an atmosphere, it results in a phenomenon known as nuclear spallation.
Nuclear spallation (hereafter simply called spallation) is a complex phenomenon, which happens when a light particle collides with a heavier nucleus with energies in the energy range of a few GeV . This collision leads to a fragmentation of the involved nucleus, often leading to a smaller nuclide and to the emission of hadrons and small clusters. A good example of such a type of reaction is the case of a proton colliding with a lead nucleus at a relatively high energy. This can result in the formation of a gold nucleus with the ejection of some protons and neutrons. It does make us wonder what would the alchemists of the past centuries say if they knew that lead can naturally turn into gold while being irradiated in space? Could we perhaps find the famous philosopher’s stone in the cosmic rays? Behind this nice but very specific example, it is important to emphasis that the chemical composition of an object in space can be and actually is modified. The nuclei in the objects exposed to cosmic rays can be transformed into other elements through spallation reactions and this is a naturally occurring phenomenon. The study of the isotope composition of the irradiated objects allows us to get information about their exposure history. Moreover, some of the often studied cosmogenic nuclides are radioactive. Therefore, an equilibrium between production and decay will be reached after some irradiation time, which depends on the half-life of the studied nuclide. This equilibrium not only depends on the irradiation history but is also disturbed if changes happen in the irradiation set-up (e.g., if the meteorite fall on Earth where it is shielded from cosmic rays). Therefore, studying isotope ratios makes it also possible to obtain information about the history of the objects after being exposed to cosmic rays. As an example, for a meteorite, the measurement of some of the isotope ratios involving radioactive cosmogenic nuclides allows us to determine when the meteorite fell on Earth and it also provides information about the journey of the meteorite and finally about the cosmic rays themself.
A reliable interpretation of the measured nuclide abundances and isotope ratios requires a deep un- derstanding of the irradiation process. Notably, the fluxes of primary and secondary cosmic ray particles must be known with high precision as a function of particle types, energy, and depth within the irradiated object as well as a function of the size and chemical composition of the irradiated object. In addition, there is a need to know the interaction cross sections between the different elements present in the target and the cosmic ray particles. The complexity of all involved processes makes it impossible to under- stand cosmogenic nuclides synthesis only based on experimental data. The best solution available for this problem is to carry out sophisticated physical simulations, which allows to study all involved processes at minimal cost in terms of time and money. However, such simulations must be validated by experiments for various representative cases to ensure the reliability of the prediction for unknown and/or not well-studied regions.
This is in this context that my thesis took place. My work was to develop a program simulating the cosmic ray irradiation of meteoroids and (exo)planetary atmospheres. This program is named Cosmic- Transmutation. The objective of this program is to predict the fluxes of light particles (protons, neutrons, and alpha particles) together with the radioisotope production in meteorites and atmospheres. Another point of interest of this program is to study the links between the irradiation spectrum and the effects on the exposed object. This would help to better understand and interpret the experimental data. For example, detection thresholds could be established, which can guide future experiments.
The first two years of my PhD were spent at the CEA Saclay (France), in the DPhN (Département de Physique Nucléaire). My work there was to improve and validate the IntraNuclear Cascade model of Liège (INCL), which describes and quantifies the output of spallation reactions. The code INCL is included in the transport model Geant4. The objectives were threefold. The first objective was an improvement of the INCL model i.e., an extension to high energies, which resulted in a better prediction of spallation
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reactions. The second objective was to develop a deep understanding of the Geant4 and INCL models and of the associated physics, which are crucial parts of the final CosmicTransmutation model. The last objective was to further develop my programming skills, especially in C++, which were highly useful during the development of the CosmicTransmutation model.
After the improvement of the INCL code and its implementation into the public version of Geant4, the last two years of my thesis took place at the University of Bern, in Switzerland and were devoted to the development of the CosmicTransmutation model and to the analysis of first results.
The first phase of this thesis allowed me to answer two important questions coming with the devel- opment of any program: “How much can we trust this program?” and “What are the important input parameters that should be taken into account to make correct predictions?”. In the second phase, I learned what type of predictions are needed to understand the history of meteorites. This thesis develops and explains the work done during the PhD with the development and the validation of the different models I worked on. Additionally, I will discuss how the different types of data can be interpreted.
The thesis is organised as follows. First, chapter II describes the basic concepts of this thesis, which are cosmogenic nuclides, cosmic rays, and spallation. In chapter III, I presents the IntraNuclear Cascade model INCL that has been developed and used all along this PhD. Chapter IV is devoted to the implementation of strange particles into INCL realised at the CEA Saclay. It is followed by chapter V, which describes the variance reduction scheme introduced to boost the study of strange particle production in INCL by avoiding statistical problems. Next, chapter VI corresponds to the final part of the work carried out at CEA Saclay; it presents the results obtained with the newly developed version of INCL. Similarly, chapter VII summarises the development of the CosmicTransmutation model realised at the University of Bern. Thereafter, chapter VIII presents first results obtained with the CosmicTransmutation model. Finally, chapter IX draws a conclusion of the work carried out during this PhD and I will discuss some possibilities for future work.

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