\caption{Energy spectrum of primary \acp{GCR}. Taken from \citet{gcr_spectrum}.}
\label{fig:intro:gcr_spectrum}
\end{wrapfigure}
Primary \acp{GCR} are charged particles which reach the top of Earth's atmosphere from outer space. These primary particles originate from sources outside the solar system like supernovae. They consist of \SI{98}{\%} protons and nuclei as well as \SI{2}{\%} electrons. The positively charged particles are made up of about \SI{87}{\%} protons and \SI{12}{\%} helium. The remainder are heavier particles \citep{longair2011high,grupen2018einstieg}. The energy spectra $N(E)$ with $E$ being the particle energy can be described by power laws with a spectral index of $\gamma\approx3$:
The spectra are modulated by the solar activity and change with the solar cycle. \citep{ParticleDataGroup:2012pjm,longair2011high}. An example of a \acs{GCR} spectrum can be found in figure \ref{fig:intro:gcr_spectrum}. The particles reach energies up to $10^{20}\,\si{\eV}$. To reach such high energies special acceleration processes are needed. A possible explanation is the stochastic acceleration of particles while crossing shock fronts of supernova remnants (Fermi acceleration) which leads to the observed power laws for the energy spectra \citep{kolanoskiSkript}.
The primary particles from \acs{GCR}s interact with nuclei from Earth's atmosphere and cause a cascade of secondary particles and radiation called \textit{Extensive Air Showers}. Figure \ref{fig:intro:air_shower} shows a sketch of an air shower. The secondary particles are produced by many different particle interactions. There are three different components of secondary particles. The \textit{muonic component} consists of muons and neutrinos, the \textit{hardronic component} of pions, kaons, neutrons, protons and other nuclei and the \textit{electromagnetic component} is made up of electrons, positrons and photons \citep{ParticleDataGroup:2012pjm,Zilles2017,kolanoskiSkript}. The secondary particles decay, lose energy and induce the production of new particles while propagating through the atmosphere. All these processes lead to a characteristic particle profile with a maximum at a height of about \SI{20}{km}, the so-called \textit{Regener-Pfotzer maximum}\citep{regener1933new,mcintosh2021regener}. Figure \ref{fig:intro:pfotzer} shows the count rates of \acs{GCR} measurements in different atmospheric heights performed by \citet{mcintosh2021regener}. The Regener-Pfotzer maximum can clearly be seen at a height of \SI{20}{km}. A smaller and lighter version of \ac{CHAOS} without the Cherenkov detector was built and flown on a weather ballon as part of the application to the \ac{BEXUS} program . The measurements performed during this flight can be seen in figure \ref{fig:intro:count_rate_chaos_junior}. Again, the Regener-Pfotzer maximum can be seen at altitudes of about \SI{20}{km}. Often, only the term \textit{Pfotzer maximum} is used.
\caption{High altitude measurements of \acs{GCR}s. The Regener-Pfotzer maximum can be seen at an altitude of approximately \SI{20}{km}. Figure 2 taken from \citet{mcintosh2021regener}.}
\caption{Measurements during the balloon flight of \ac{CHAOS}junior. The Regener-Pfotzer maximum can be seen at an altitude of approximately \SI{20}{km}.}
\label{fig:intro:count_rate_chaos_junior}
\end{minipage}
\end{figure}
\subsection{Cherenkov Effect}
\begin{figure}[htb]
\begin{minipage}[t]{0.6\linewidth}% [b] => Ausrichtung an \caption
\caption{Illustration of a charged particle passing through a medium with $n$ being the refractive index of the medium and $c$ the speed of light. Subfigure \textbf{(a)} shows a particle velocity smaller $c/n$ and \textbf{(b)} greater $c/n$. Figure 1 taken from \citet{2016cerenkov}.}
\caption{Formation of a Cherenkov cone due to the positive interference of Cherenkov radiation.}
\label{fig:intro:cherenkov_cone}
\end{minipage}
\end{figure}
The Cherenkov effect occurs when the velocity of a charged particle in a transparent medium (refractive index $n$) exceeds the phase velocity of light in that medium ($v \geq c/n$). Pavel Cherenkov was the first to observe this effect in 1934 while working with radioactive samples in water. The emission of blue light in the visible spectrum is characteristic for the Cherenkov effect.\\
The Cherenkov radiation arises from the polarization of the atoms of the dielectric medium due to the Coulomb field of the charged particle. The atoms are now electrical dipoles. Figure \ref{fig:intro:cherenkov_dipols} shows this phenomenon. If the speed of the particle does not exceed the speed of light in the medium, the polarisation is symmetric and no dipole field forms at large scales. However, if the particle's speed exceeds the speed of light in the medium, the polarisation becomes asymmetric along the particle's path, creating a temporary dipole field. This oscillating dipole emits the Cherenkov radiation. The emission of light is limited to a cone with an aperture angle of $2\Theta$ (seen figure \ref{fig:intro:cherenkov_cone}), since it is cancelled out in all other directions by interference effects:
Furthermore, the spectrum of the Cherekov radiation is continuous and contains all wavelengths which lead to a real value for $\Theta(\lambda)$. Due to the formation of the Cherenkov cone, the Cherenkov effect is often compared to a sonic boom \citep{frank1991coherent}.\\
The speed of a particle is related to its mass and its kinetic energy. Therefore, particles with different masses need different energies to exceed the speed of light in the medium and cause the Cherenkov effect. For example, an electron needs approximately 1.1\,MeV to create Cherenkov radiation in our detector (refractive index $n=1.05$), whereas heavier ions need much larger energies.
The primary \acp{GCR} reaching the Earth's atmosphere are made up of various particle species. Current particle detectors have limitations differentiating between those particles. \ac{CHAOS} uses a novel combination of detectors which allows to separate between light and heavy particles, such as electrons and ions, based on the energy-dependent light emission of a Cherenkov aerogel scintillator.
\item[Obj. 1.1:] To measure Primary Galactic Cosmic Rays above the Pfotzer maximum which is at around 20 km height and separating between light and heavy particles. (Scientific)
\item[Obj. 1.2:] To test a new mechanical design and electronics which can potentially be used for further projects at the Department for Extraterrestrial Physics, such as the \ac{AHEPaM}. (Technical)
To achieve the outlined objectives, \ac{CHAOS} uses a combination of multiple \acfp{SSD}, a \acf{BGO} scintillation crystal and a Cherenkov aerogel scintillator. The light produced in the \ac{BGO} is measured by photodiodes while a \ac{PMT} measures the light from the Cherenkov scintillator. Particle species can be identified using the measured energy losses of incident particles provided by the \acp{SSD} and \ac{BGO}. The Cherenkov detector further differentiates between light and heavy particles because they trigger the detector at different energies. All detectors are connected to an electronics stack and placed inside a pressure housing which itself is placed inside a thermobox.\\
The \ac{CHAOS} design is based on the design of the \ac{AHEPaM} instrument, currently under development at the Department for Extraterrestrial Physics at \ac{CAU} Kiel.
\item\replaced{mainly responsible for project management, testing and calibration}{responsible for coordination and controlling of the team as well as electronics and mechanics}
\item\replaced{mainly responsible for project management, documentation, integration and data analysis}{responsible for coordination and controlling of the team, documentation as well as science and data analysis}
\item\replaced{transferred to \ac{FH} Kiel in March 2024 and left the project}{will transfer to \acf{FH} Kiel in March 2024, but remains a part of \ac{CHAOS}}
