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
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}.
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
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.
