The simulation detailed in section \ref{sec:performance-analysis} has been performed individually with and without a Cherenkov detector in order to investigate whether or not the AHEPaM requirements can be full-filled with both setups. The requirement regarding the electron uncertainties has proven to be the most difficult one to achieve due to the contribution of protons to the electron channels. This contamination is significant due to the higher proton flux compared to the electrons expected for the \ac{GCR} (see fig. \ref{fig:adriani-e-p}).\newline
While the methods introduced in \ref{sec:performance-analysis} utilizing thresholds in the different detectors of the instrument have reduced the contamination already significantly even without a Cherenkov detector, this improvement has proven to be insufficient in order to full-fill the given requirements. Introducing the Cherenkov to the setup allowed for a further suppression of the proton contamination and hence significant lower electron uncertainties. Additionally, the Cherenkov allows to separate protons above and below 2~GeV allowing for better energy resolutions up to 2~GeV as well as providing an integral channel for protons above 2~GeV. Hence, from a measurement technique perspective the Cherenkov detector is highly preferred.\newline
From a technical point of view, the Cherenkov detector increases the complexity of the instrument. Especially the high voltage that is necessary in order operate the \ac{PMT} of the Cherenkov detector has to be considered. Furthermore, the Cherenkov detector has to be connected to its \acs{PMT} and the rest of the instrument mechanically and thermally. The technical detailed are further described in section \ref{sec:technology-assessment}. It is important to note that Cherenkov detectors have been already used successfully for space mission in the past (including instruments build in Kiel \cite{ahepam-heritage}). \newline
Based on the analysis above we have decided that a Cherenkov is recommended in order to full-fill the measurement requirements and that the additional technical efforts are both manageable and appropriate for the benefits. However, a de-scoped version of \ac{AHEPaM} without the Cherenkov detectors has been proven to provide the capabilities of separating electrons from protons utilizing the methods described in \cite{ahepam-djf} based on sufficient statistics which could be achieved by integrating over longer time periods. Given the small temporal variations of electrons in the energy range above 50~MeV this de-scoped version is expected to provide electron fluxes within the required systematical and statistical uncertainty range if the requirements on the integration period for electrons would be relaxed.\newline
The cherenkov detectors will be read-out by a \acp{PMT} (see section \ref{subsec:pmtvsother}) and thus a high voltage supply ($>$1kV) is necessary in order to utilize the detector. Therefore, a high voltage supply has to be included in the instrument and HV operation during thermal vacuum conditions have to be ensured. Designing, building and testing such a supply is feasible for us given our experience with such instrumentation. An aerogel cherenkov detector with a \ac{PMT} has been already successfully used by the \ac{KET} instrument build in Kiel aboard the Ulysses spacecraft \cite{ahepam-heritage}.
High voltages and \acp{PMT} have been further used in various instruments from Kiel such as \ac{FaNS} and \ac{MONSTA} on balloon born missions in the scope of the \ac{BEXUS} project.\newline
The same applies for the mechanical and thermal aspects. While the mechanical mounting of the cherenkov is far from trivial, experiences from the mechanical design of solar orbiters \ac{HET} instruments (which include BGO scintillators) can be used. \newline
\subsection{Dimensions of BGOs}
The simulations detailed in section \ref{sec:performance-analysis} have been also performed for various different BGO thicknesses (i.e. 4cm and 6cm in addition to the 2cm presented above) in order to investigate whether or not the separation of electrons and protons can be further improved due to enhanced secondary particle showers caused be electrons in thicker targets. Our analysis, however, has shown that no substantial improvement in particle separation can be achieved by increasing the BGOs thickness. Since the thinner ones are beneficial from a technical point of view (especially the mechanical one), we have settled for a 2~cm thick BGO (similar to the one used by solar orbiters \ac{HET}.
\subsection{PMT vs.\,APDs vs.\,SiPMs}
\label{subsec:pmtvsother}
As the BGOs can easily be read-out by easy to handle photo-diodes and as the simulation analysis showed that the instrument will consist only of Si Detectors, BGOs and the Cherenkov detectors, the decision between PMTs, APDs and SiPMs is only driven by the needs for the read-out of the Cherenkov detectors aerogel. Our analysis has shown that there will only be at best a few hundred photons produced in the aerogel. With that information APDs are out of the consideration as their typical gain would produce charges too small to be easily measured in spacecraft environment. PMTs and SiPMs provide similar gains and therefore are both well suited for the application. The following table shows a comparison of both technologies.
Proofness & Old and proven technology & New modern technology \\\hline
CAU practical experience & CAU practical knowledge & has not been used at CAU yet \\\hline
Sensitive area & Large & very small segments with the need to use arrays of SiPM \\\hline
Sensitivity to noise & One sensitive area with low noise & large number and small distances between single elements leads to cross-talk and a more noisy signal \\\hline
Detector volume & large tube volume & very small single elements, flexibly composable to the needed larger arrays \\\hline
typical operating ambient temperature &\(-30^\circ\mathrm{C}\) to \(+50^\circ\mathrm{C}\)&\(-20^\circ\mathrm{C}\) to \(+60^\circ\mathrm{C}\)\\\hline
Sensitivity to temperature changes & None & high with the need for a control loop \\\hline
\caption{Comparison of the properties of PMTs and SiPMs}
\label{tab:my_label}
\end{table}
The biggest advantage for the SiPM is the comparably low supply-voltage needed. For the primary application as a digital coincidence detector their characteristics, especially their sensitivity to noise, is not expected to be a significant problem. On the other hand first simulations showed, that additional information concerning shower development in the whole instrument might yield a better discrimination of protons and electrons in certain energy ranges. For that application a clearer less noise signal would be crucial and the PMT would be advisable. Additionally a large array of SiPMs would lead to a necessity for a large number of electronics channels. This number is already high due to the complexity of the whole instrument and might be out of the viable scope. Finally SiPM have not been used at CAU before. Also due to the difficult delivery conditions and the additional workload CAU was not able yet to investigate the application of SiPMs.
Therefore the decision was made to use PMTs for this project.
