\section{Introduction}
\label{sec:introduction}
The high precision of the next-gen astrophysical x-ray observations which will be preformed by \ac{ATHENA} require a detailed understanding and monitoring of the background signals for the missions main instruments. For previous mission, it has been shown that the majority of the background is caused by particle radiation, mostly protons, electrons and helium particles of the \ac{GCR} \cite{gastaldello-etal-2022}. This correlation is clearly visible in figure \ref{fig:introduction}, presenting the unfocussed background of XMM-Newton as function of the proton flux at 1~GeV observed by SOHO/EPHIN. Any correction of this particle background relies of course on instruments measuring these particle and thus, the uncertainties of those instrument drive the uncertainty of the x-ray telescopes background correction.\newline
In this project we have proposed, designed and modelled a particle instrument - \ac{AHEPaM} - for the \ac{ATHENA} mission that would provide precise measurements of particle in the energy regime which dominates the background of the proposed \ac{ATHENA} x-ray instruments (see figure \ref{fig:introduction}, right). \newline
\ac{AHEPaM} is designed with special emphasis on separating electrons and protons in the high-energy range (electrons above tens of MeV, protons in the GeV range). Furthermore the instrument is scoped such that a sufficiently large geometry factor is achieved resulting in high statistical accuracy for the measured particle fluxes. Both, the particle separation and the expected statistical uncertainties, have been extensively modeled with respect to the expected \ac{GCR} radiation environment and expected uncertainties for the different particle fluxes are presented for different temporal resolutions.\newline
\ac{AHEPaM} has been also validated to match engineering requirements using mechanical stress simulations as well as a thermal model. \newline
This report is structured as follows, the basic instrument design including the general setup of the entire instrument and its detectors in particular as well as mechanical, thermal and electronics design features are presented in section \ref{sec:basic-design}, section \ref{sec:performance-analysis} provides detailed calculations of the measurement capabilities of \ac{AHEPaM} with respect to the expected \ac{GCR} radiation field. Section  \ref{sec:trade-off-report} discusses technological assessment with special emphasis on whether or not a Cherenkov detector should be used in the \ac{AHEPaM} instrument. A brief summary is given in section \ref{sec:summary}.
\begin{figure}[!b]
  \centering
    \includegraphics[width=6cm]{cau-ath-fr_i1-0/figs_introduction/gastaldello.PNG}
  \includegraphics[width=8cm]{cau-ath-fr_i1-0/figs_introduction/lotti.png}
  \caption{left: correlation between the XMM-Newton background and flux of high-energy protons \cite{gastaldello-etal-2022}; right:The cumulative energy distribution of primary electrons, protons and Helium that contribute to the unrejected background, either directly or through the production of secondary particles \cite{lotti-etal-2021}. Modified to show the energy ranges of the proposed AHEPaM as respectively colored boxes and arrows for the corresponding integral channel.}
  \label{fig:introduction}
\end{figure}
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