\section{Introduction}
\label{sec:intro}

\acs{ATHENA}, the Advanced Telescope for High Energy Astrophysics, is the second large-class
mission (L2) within the \acs{ESA} Cosmic Vision program. Due to steadily increasing cost, ATHENA was redefined into NewAthena in 2023. While the \acs{ATHENA} scientific community had requested the addition of a \ac{AHEPaM} to the \acs{ATHENA} spacecraft (\acs{S/C}), this has now been removed from \acs{ESA}'s responsibility. If possible, \acs{AHEPaM} should be provided as a member-state-funded contribution to NewAthena. This document gives a top-level summary of the development of \acs{AHEPaM} for the original \acs{ATHENA} mission. 

\acs{ATHENA} (and NewAthena) will observe the hot and energetic Universe in the X-ray spectral region and has been conceived to address two key questions in modern astrophysics:
How does ordinary matter form the large-scale structures that we see today?
How do black holes grow and shape the Universe?
ATHENA will comprise two instruments, the Wide Field Imager (\acs{WFI}) and the X-ray
Integral Field Unit (\acs{X-IFU}).
The \acs{WFI} consists of an active pixel sensor camera with a field of view of 40' x 40' (50' goal), a high count-rate capability and high time resolution.
The \acs{X-IFU} provides spatially resolved high resolution (2.5 eV) spectroscopy within a field of view of 5' diameter (7' goal). The cooled Transition Edge Sensor (TES) technology of this instrument provides the necessary energy resolution, while providing exceptional efficiency compared to dispersive spectrometers flown on the current generation of X-ray observatories.

%The ATHENA scientific community requested the addition of a \acs{ATHENA} High Energy Particle Monitor (\acs{AHEPaM}) to the \acs{ATHENA} spacecraft. 
The need for \acs{AHEPaM} is driven by the calibration requirements [CAL-BKG-R-001] and [CAL-BKG-R-002] in \cite{cal-req-esa}. These are translated into 1\% knowledge on the energy ranges of protons (0.1-2 GeV), helium ions (1-3 GeV), and electrons (0.05-1 GeV).

The main goal of \acs{AHEPaM} was to ensure that the requirements on the knowledge of the “Non-X-ray Background” (\acs{NXB}) are met. The NXB (also known as “internal particle background”) is due to high-energy particles (primarily Galactic Cosmic Rays (\acs{GCR})) that interact with the spacecraft and instruments and create showers of secondary
particles. Many of the latter are detected as soft X-ray events. Cosmic ray particles are modulated by the solar cycle because the heliospheric magnetic field, occasionally interrupted by solar particle events which can results in an additional \acs{NXB}. In particular, the \acs{GCR} flux is at a minimum during solar activity maximum and, conversely, the cosmic ray flux is at a maximum during solar activity minimum.

This executive report summarizes the findings of the work performed at \acs{CAU} in a manner suitable for non-experts in the field and is appropriate for publication. It summarizes the key properties of the \acs{AHEPaM} (in Tab.~\ref{tab:key-properties}), describes the design of \acs{AHEPaM}, compares its expected performance with the original measurement requirements, discusses possible future trade studies, and gives an assessment of the current Technology Readiness Level (\acs{TRL}). 

\begin{table}[h]
    \centering
    \begin{tabular}{|lccl|}\hline
                        &  \multicolumn{2}{c}{\bf Value} &               \\\hline
       {\bf Property}  &  {\bf Required} & {\bf Achieved}& {\bf Remarks} \\\hline
       Mass  &  15~kg  & $\leq$ 15~kg  &   \\\hline
       Power  &  15~W  &  $\leq$ 15W & during continuous load  \\\hline
       Envelope  & 300x250x200mm   & 249x259x235mm  & detailed in \cite{ahepam-micd}   \\\hline
       Data rate  &   1~kbps  & $\ge$ 1~kbps  & 1~kbps can be achieved, more is preferred$^*$.  \\\hline
       Energy ranges: & & & \\\hline
       Protons   &  0.1-2~GeV  & 0.1-$\ge$2~GeV   &  \\\hline
       Electrons   &  0.05-1~GeV  & 0.05-$\ge$1~GeV  &   \\\hline
       Helium   &  1-3~GeV  &  0.01-$\ge$2~GeV/nucleon &   \\\hline
       %\multicolumn{4}{|l|}{{\bf Expected time resolution:}}\\\hline
       \multicolumn{4}{|l|}{{\bf Expected uncertainties for given channel numbers and time resolutions:}}\\\hline
       \multicolumn{4}{|l|}{{\em High species-resolution mode:}}  \\\hline 
       Protons & 5~ch, 10ks, 1\%  & 5~ch, 10ks, 2.7\%  &  detailed in sec. 1.4 \cite{ahepam-djf}  \\\hline 
       Electrons & 2~ch, 3ks, 5\%    & 2~ch, 50ks, 5.5\% &  detailed in sec. 1.4 \cite{ahepam-djf}  \\\hline 
       %Helium &   &  &   \\\hline 
       \multicolumn{4}{|l|}{{\em High statistics mode:}}    \\\hline 
       Protons & 5~ch, 10ks, 1\%    & 5~ch, 10ks, 1.2\% & detailed in sec. 1.4 \cite{ahepam-djf}  \\\hline 
       Electrons & 2~ch, 3ks, 5\%    & 2~ch, 50ks, 3.5\% &  detailed in sec. 1.4 \cite{ahepam-djf}  \\\hline 
       %Helium &   &     &\\\hline 
       &   &    & \\\hline 
    \end{tabular}
    \caption{Key properties of \acs{AHEPaM}. $^*$ An increase to a few kbps would provide highly valuable data for solar \& heliospheric science, see sec.~\ref{sec:trades}. For details see \cite{ahepam-micd}, \cite{ahepam-djf} and \cite{ahepam-req}. Percent values given reflect both the statistical and systematic uncertainties (the detailed calculation of the uncertainties is given in chapter 1.5 in \cite{ahepam-djf}).}
    \label{tab:key-properties}
\end{table}


\section{AHEPaM Design}

\begin{table}[]
    \centering
    \begin{tabular}{|c|c|c|c|c|} \hline
      Requirement   & Protons & Electrons  & He ions   &  Motivation \\\hline
      Energy range   & 0.1 - 2.0 GeV & 0.05 - 1.0 GeV  & 1 - 3 GeV   & Fluxes  \\
      Abs. Precision & \acs{N/A} & \acs{N/A}  & \acs{N/A}   & Abs. calibration  \\
      Rel. Precision &  0.5\% & 1\%  & \acs{N/A}   & Spectral shape  \\
      Rel. to prot. prec.   & \acs{N/A} & 2.5\%  &  5\%  & Normalization  \\
      Stat. acc. $>$3ks  & 1\% in 2 bands & 5\% in 2 bands  &  10\% in 1 band  & Temporal variations  \\
      Stat. acc. $>$10ks  & 1\% in 5 bands & 5\% in 5 bands  &  \acs{N/A}  & Spectro-temp. variations  \\\hline
    \end{tabular}
    \caption{Original measurement requirements.}
    \label{tab:orig-meas-req}
\end{table}

The design of \acs{AHEPaM} was driven by the original measurement requirements which are summarized in Tab.~\ref{tab:orig-meas-req}. The statistical accuracy of 1\% in 2 energy bands within 3 ks for protons (and 5\% for electrons) in the ~1 GeV energy range determined the size, and thus mass, and envelope of \acs{AHEPaM}. Figure~\ref{fig:GCR-spec} shows typical \acs{GCR} spectra of protons (in red) and electrons (blue) as well as two straight-forward fits of a force-field solution to the data. The peak flux of protons (at about 0.5 GeV) can be seen as a few thousand particles per (m$^2$ s sr GeV). The flux of electrons is approximately 2\% of that, similar to that of helium ions (green). The large energy range to be covered by \acs{AHEPaM} requires a substantial amount of matter to slow down particles. 

\begin{figure}
    \centering
    \includegraphics[width=0.6\linewidth]{cau-ath-er_i1-0/GCR-spectra.pdf}   
    \caption{Measurements of galactic cosmic ray protons (red), electrons (green) and corresponding model fits (green, helium).}
    \label{fig:GCR-spec}
\end{figure}

\acs{AHEPaM} utilizes a combination of \ac{SSD}, \ac{BGO} scintillator and Cherenkov detectors. The combination of these different measurement techniques allows for a separation of high energy-electrons and protons. As described below, protons are easily separated from $\alpha$-particles. Fig.~\ref{fig:basic-arrangement} shows the combination of those different sensors and their mounting (blue) on top of the housing of the different electronics boards (green). The entire instrument will be covered (purple) for thermal reasons and to reduce electromagnetic interference (\acs{EMI}) from other sources. 

Energetic particles are typically measured with so-called particle telescopes which combine different kinds of detectors to measure the energy that a particle deposits in the detector. A clever combination allows to determine the particle energy and determine what kind of particle (electron, proton, $\alpha$-particle) it was. Particles in the energy range to be covered by \acs{AHEPaM} typically loose only a fraction of their energy in a detector, this can be approximated\footnote{The energy deposited in a detector can be modeled much more accurately with the sophisticated \acs{GEANT4} simulation package which was developed at \acs{CERN} \cite{agostinelli-etal-2003}. This software package was used extensively in the development of \acs{AHEPaM}.} by the Bethe-Bloch equation, the relevant parts for this discussion are given in eq.~\ref{eq:bethe-bloch},
\begin{equation}
\frac{{\rm d}E}{{\rm d}x} \sim \frac{Z^2 n_e}{E},
    \label{eq:bethe-bloch}
\end{equation}
where $E$ is the particle kinetic energy, $Z$ its nuclear charge,  $n_e$ is the electron density in the detector material, and d$x$ the detector thickness. For example, a 500 MeV proton looses less than 100 keV in a typical silicon solid-state detector (\acs{SSD}). This means that the total energy of a particle in the required energy range can not be measured within a reasonably-sized detector. That the deposited energy is proportional to $Z^2$ assures that protons and helium nuclei can easily be distinguished. The difficulty lies in separating electrons from protons. If a particle is faster than the speed of light in the detector, it produces Cherenkov radiation. Because electrons in the required energy range basically travel at the speed of light in vacuum, \acs{AHEPaM} also uses this measurement technique to discriminate electrons from protons because the latter are much slower (at the same kinetic energy) and therefore do not produce Cherenkov radiation. If the particle has enough energy, it can also produce a shower of secondary particles, an effect that is also used in \acs{AHEPaM}. Thus the driving requirements for \acs{AHEPaM} were the large energy range, the high counting statistics, and the discrimination between electrons and protons. These were met by using the combination of multiple measurement techniques described in the following and that can be seen in Fig.~\ref{fig:AHEPaM-concept}.

\begin{figure}
\begin{subfigure}[]{0.45\linewidth}
    \centering
    \includegraphics[height=5.5cm,]{cau-ath-ddc-0006_i1-0/media/ahepam-fm_top_telescope.png}
    \caption{\acs{AHEPaM} detector concept. The front silicon solid-state detector (\acs{SSD}s) is shown in silver-grey, the others are shown in red, Cherenkov detectors in yellow, \acs{BGO} detectors in dark red, and photo-multiplier tubes (\acs{PMT}s) in green. The small white areas on the \acs{BGO} detectors are the photo-diodes.}
    \label{fig:AHEPaM-concept}
\end{subfigure}
\begin{subfigure}[]{0.45\linewidth}
  \centering
  %\includegraphics[width=7cm]{../cau-ath-djf-0007_i1-0/media/figs_sim/sim_model_invert.png}
  \includegraphics[height=5cm]{cau-ath-djf-0007_i1-0/media/figs_sim/shower_el_500MeV.png}
  \caption{Visualisation of a simulation run with an electron at 500~MeV entering from the left.}
  \label{fig:geometry_sketch}
\end{subfigure}
\caption{\acs{AHEPaM} combines three different measurement techniques.}
\label{fig:AHEPaM-measurement-concept}
\end{figure}


To measure the low fluxes of \acs{GCR} particles \acs{AHEPaM} had to have a large collecting area, and a large field of view (\acs{FOV}), the product is equivalent to the "collecting power" or geometric factor. This is determined by the area of the front and rear detectors of the particle telescope, and by its length. The \acs{AHEPaM} developed in this contract maximizes the geometry factor by its compact design and by allowing to measure particles from the front and back, thus doubling the geometry factor. To achieve this, it is designed to be symmetric about its middle plane, as can be seen in Fig.\ref{fig:AHEPaM-concept} which shows a \acs{CAD} view of the arrangements of the various detectors in \acs{AHEPaM}. A particle entering \acs{AHEPaM} from the lower left will first trigger the front \acs{SSD} which is shown in silver-grey. If it is an electron, it will produce Cherenkov radiation in the Cherenkov detector (shown in yellow), whereas slower protons or helium nuclei will not. The threshold velocity, $v_{th} = c/n$, for producing Cherenkov radiation is determined by the refractive index of the material, $n$. The particle then hits the next \acs{SSD} (shown in red), traverses the high-density \acs{BGO} scintillator, the central \acs{SSD}, and exits \acs{AHEPaM} on a "symmetric" path through the following \acs{BGO}, \acs{SSD}, Cherenkov, and final \acs{SSD} on the upper right. This design is extremely compact and thus maximizes the geometric factor of \acs{AHEPaM} and allows determination of the energy losses in multiple detectors. The process of energy loss is stochastic, the distribution of deposited energy is described by the Landau distribution which is so skewed towards larger energy depositions that only its most probable values is defined, but not its mean. Because this could mimic an energy deposition of a heavier particle, the \acs{SSD}s are arranged in back-to-back pairs and the minimum of the energy deposition is used for the data processing in \acs{AHEPaM}. Thus the five detectors seen in Fig.~\ref{fig:AHEPaM-concept} are actually pairs of detectors. The Cherenkov detectors only produce typically 200 photons per electron, they are read out with \acs{PMT}s which provide sufficient amplification of this very weak signal. The energy deposited in the high-density ($\rho =$ 7.13 g/cm$^3$) \acs{BGO} is converted into abundant scintillation light by that material which is read out with extremely compact photodiodes. That signal is proportional to the energy that the particle looses in the \acs{BGO}. The central \acs{SSD} performs another precise measurement of the particle's energy loss before it continues into the symmetric part of \acs{AHEPaM}. Figure~\ref{fig:geometry_sketch} shows such an example for a 500 MeV electron. The energy resolution of an \acs{SSD} is inversely proportional to its area, therefore \acs{AHEPaM}'s \acs{SSD}s are divided into many segments which are amplified and read out separately.  This segmentation also allows \acs{AHEPaM} to detect particle showers that high-energy particles can produce when they interact with matter, especially the high-density \acs{BGO}. They also allow to correct for variations in the path lengths of individual particle tracks by reconstructing the approximate track geometry from the detector segments that were hit.
% measurement technique explained with plot
\begin{figure}
  \centering
  \includegraphics[width=16cm]{../cau-ath-djf-0007_i1-0/media/figs_sim/pene_A-E_cher.png}
  \caption{Expected count rates as function of the energy losses in BGO2 and BGO1 for penetrating particles utilizing a Cherenkov detector which removes input from ions below 2~GeV/nuc. The rows correspond to 1) proton, 2) helium and 3) electron simulations. The columns are based on 1) the proton, 2) the electron and 3) the helium trigger. See report \cite{ahepam-djf} for a more detailed description of the simulations and model setup.}
  \label{fig:pene_supertrigger_cherenkov}
\end{figure}
The electro-magnetic showering in the \acs{BGO}, which is more likely for electrons than for ions, allows for separating those species. This effect is shown in Fig. \ref{fig:pene_supertrigger_cherenkov} which shows simulations with expected fluxes (see Fig. \ref{fig:GCR-spec}) of protons (first row), helium particles (second row) and electrons (third row). A pre-selection of particle types has been performed for this plot utilizing different thresholds in the \acp{SSD}, with columns one, two and three presenting the proton, electron and helium pre-selection, respectively. Given the higher flux of protons compared to electrons this pre-selection is obviously not sufficient since protons populate the electron pre-selection (first row, second column). However, using the showering in the \acp{BGO} those particles can be separated in this plot which presents the energy deposition in the second \ac{BGO} (y-axis) vs.~the first \ac{BGO} (x-axis). The blue box is the proposed electron selection box and it has been shown that the proton contamination in there is only 4\% using the Cherenkov and 28\% without the Cherenkov as an additional selection criteria. As discussed in detail in \cite{ahepam-djf}, this contamination can be corrected for by statistical means.

The proposed sensor design of the \acs{AHEPaM} is sketched in Fig. \ref{fig:telescope-specs}. The design of the \acp{SSD} is driven by the need of i) defining appropriate opening angles of the telescope and tracking the particles trajectories and ii) removing particles traversing the instrument under oblique angles and thus missing one or more of the detectors. Segmentation of the detectors allows for tracking the particles. Requiring a set combination of these segments in the different detectors also defines the opening angle of the instrument. The \acp{SSD} at location SDB, SDC and SDD have an additional large outer segment which serves as an anti-coincidence, i.e., whenever a particles triggers those segments it is discarded. Another detail of the \ac{SSD} setup is that instead of a single \ac{SSD} a stack of two detectors is used (cf. colored positions sketched in Fig. \ref{fig:telescope-specs}). Selecting the minimum of the energy-losses in either of the two reduces the natural distribution of energy-losses caused by the statistical nature of the involved physical processes, i.e., results in narrower distributions and hence better identification of the particle species and primary energy.


\begin{figure}[h]
	\begin{subfigure}[]{0.45\linewidth}
		\includegraphics[width=0.9\linewidth]{cau-ath-ddc-0006_i1-0/media/cau-ath-icd-0009_i2-0_telescope.pdf}
		\caption[FM telescope]{\centering{Top-level \acs{CAD}-drawing of the \acs{FM} \acs{AHEPaM} particle telescope. }}
		\label{fig:techdraw-fm-telescope}
	\end{subfigure}
	\hfill
	\begin{subfigure}[]{0.45\linewidth}
		\includegraphics[width=0.9\linewidth]{cau-ath-ddc-0006_i1-0/media/cau-ath-icd-0009_i2-0_detectors.pdf}
		\caption[FM detectors]{\centering{ \acs{CAD}-drawings of the \acs{FM} detectors}. The \acs{FM} combines two pairs of small detector and three pairs of one small and one large detector.}
		\label{fig:techdraw-fm-detectors}
	\end{subfigure}
	\caption[FM telescope]{Geometry of the FM telescopes. The size of the large solid-state detectors is the driver of \acs{AHEPaM}'s \acs{TRL}.}
	\label{fig:telescope-specs}
\end{figure}

Key properties such as mass, power, volume, etc.\,of the AHEPaM developed under this contract are given in Tab.~\ref{tab:key-properties}. One can easily see that \acs{AHEPaM} is indeed very compact and is close to fulfilling the original measurement requirements. It is also clear that it is probably not possible to satisfy all the measurement requirements within the resource allocations foreseen for \acs{AHEPaM}. This is one of the lessons learned from the work performed under this contract. The measurement capabilities of \acs{AHEPaM} are summarized in Tab.~\ref{tab:AHEPaM-data-products} and discussed in more detail in Sec.~\ref{sec:performance}. 

The mechanical, thermal, and electrical interfaces of \acs{AHEPaM} with the \acs{ATHENA} spacecraft were designed to be as straightforward as possible. Due to its allocated mass, \acs{AHEPaM}'s mechanical interface consists of 8 separately attached titanium feet. Titanium has been choosen for its high mechanical strength, while providing comparatively low heat conductivity, which helps to thermally decouple the unit from the (usually warmer) mounting panel. This is an important factor in order to reach the detector head's design temperature of 0$^\circ$C. The thermal design furthermore foresees a dedicated external radiator for the \acs{AHEPaM} electronics box, as well as potentially necessary separate detector radiators. The latter consist of, e.g., \acs{OSR} tiles, and are directly attached to the outer instrument cover surfaces shown in purple in Fig.~\ref{fig:ahepam-with-cover}.

Structural and thermal modeling was performed to ensure that \acs{AHEPaM} would survive environmental testing as well as the launch and space environment. Results from initial thermal modeling show that it is possible to approach the above mentioned design temperature in the hot case by using an external radiator with a footprint of $\sim$0.03m$^2$. Consequently during cold phases, where the instrument is not illuminated by the sun, operational heat in the range of $\sim$5W needs to be considered. (Refer to \cite{ahepam-djf} for details.)

While the first structural \acs{FEA} results show design-compliance with the main dynamic requirements (e.g. 1st natural frequency above 140Hz), the structural response regarding mechanical vibration needs to be improved. There are a few locations of the instrument, e.g. instrument feet and BGO-bracket, where the allowed maximum material stress is exceeded during random vibration loads \cite{ahepam-djf}. While increasing the sheet thicknesses at these locations would mitigate this issue, it might be beneficial to review the structural design again to identify potential improvements.  Note that the initial \acs{SMM} was set up using a relatively coarse mesh and, e.g., assumed a homogeneous plate thickness instead of using the dedicated, and detailed stiffening ribs. Taking these aspects into account, next steps regarding the development of a \acs{FM}-design should include a revised \acs{SMM} which includes the latest design changes\footnote{After ending the \acs{AHEPaM}-contract with ESA, several design changes have been implemented which are not part of this report.} and the relevant structural design aspects. Furthermore a vibration test could be conducted with the available \acs{DM}-parts, which would help to understand the response of the unit and help improving its design. Due to resource constraints the instrument cover was not part of the \acs{DM}. While a concept has been presented, adequate engineering resources should be budgeted to finalize its design.

\acs{AHEPaM} requires standard 28VDC  and communicates with \acs{ATHENA} via UART interface over LVDS.

The design of \acs{AHEPaM} that resulted from this study is shown in Fig.~\ref{fig:basic-arrangement}. The left-hand figure (Fig.~\ref{fig:ahepam-wo-cover}) shows the particle telescope that was shown in detail in Fig.~\ref{fig:AHEPaM-concept} in pale blue mounted to the top of the electronics box (\acs{EBox}), shown in dark green. This chassis provides the structural support for the particle telescope and provides shielded routing of the cables which carry the analog signals from the sensor head to the \acs{EBox}. 



\begin{figure}[ht]
	\begin{subfigure}[]{0.45\linewidth}
		\includegraphics[width=0.9\linewidth]{cau-ath-ddc-0006_i1-0/media/ahepam_without-cover.png}
		\caption{\centering{The instrument cover (not visible) shields the telescope (blue) against light and electronic noise.}}
		\label{fig:ahepam-wo-cover}
	\end{subfigure}
	\hfill
    \begin{subfigure}[]{0.45\linewidth}
		\includegraphics[width=0.9\linewidth]{cau-ath-ddc-0006_i1-0/media/ahepam_with-cover.png}
		\caption{\centering{\acs{AHEPaM} with its instrument cover (purple) attached to the top of the \acs{EBox} (green).}}
		\label{fig:ahepam-with-cover}
	\end{subfigure}
	\caption[Two \acs{CAD} views of \acs{AHEPaM}'s telescope on top of the \acs{EBox}, without and with the instrument cover.]{Two \acs{CAD}-views of AHEPaM's particle telescope on top of the \acs{EBox}, without and with cover.}
	\label{fig:basic-arrangement}
\end{figure}



\section{Expected Performance}
\label{sec:performance}

Detailed simulations were performed with \acs{GEANT4} \cite{agostinelli-etal-2003} to determine the geometry factors and responses of different combinations of detectors in \acs{AHEPaM} which are key to understanding the expected performance of \acs{AHEPaM}. The required discrimination between electrons and protons was achieved by selecting a refractive index $n$=1.05 of the Cherenkov detector which is close to that of vacuum. Thus this detector only triggers on protons with kinetic energies above $\sim 2$ GeV, which lies well beyond the maximum of the \acs{GCR} flux and at the upper limit of the proton measurement requirement (see Tab.~\ref{tab:orig-meas-req}). That means that most protons are correctly separated from electrons by this technique alone. Additional measurements in \acs{AHEPaM} further improve this discrimination. The main challenge for \acs{AHEPaM}, however, is to meet the required statistical accuracy, i.e., to acquire sufficient counting statistics to meet the requirements given in Tab.~\ref{tab:orig-meas-req}. To increase counting statistics only particles from one hemisphere were simulated, exploiting the symmetry of \acs{AHEPaM} (see Fig.~\ref{fig:AHEPaM-concept}). Therefore, only results from one hemisphere (i.e., $2\pi$ sr) are reported here. During solar quiet times, i.e., in the absence of a solar particle event, the \acs{GCR} particle radiation background is essentially isotropic. This, however, is also the situation when the count rates are small, in other words, it is the limiting case for determining \acs{AHEPaM}s measuring capabilities. This also means that the $2\pi$-sr results reported here can effectively be doubled, i.e., their uncertainties divided by the appropriate factor, $\sqrt{2}$. We do not correct for this geometric factor in this report because solar particle events can be very an-isotropic during their onset times, i.e., in the first hours of the event. 

The concept for \acs{AHEPaM} which was developed in this contract continuously provides two classes of data products, high resolution and high statistics. The high-resolution data product is much better at discriminating between protons and electrons than the high-statistics data product. It requires particles to traverse the entire \acs{AHEPaM} particle telescope, i.e., to hit the front-most and rear-most detectors in Fig.~\ref{fig:AHEPaM-concept}. The field of view for this data product is narrow and consequently its geometric factor ("gathering power") is limited, and hence only a  fraction of all particles is measured. The high-statistics data product, on the other hand, provides data at high counting statistics, but at the cost of reduced discrimination between electrons and protons. This is achieved by relaxing the requirement that all detectors of the \acs{AHEPaM} telescope are triggered which results in a larger geometric factor. The measurement capabilities of both data products are summarized in Tab.~\ref{tab:AHEPaM-data-products}. Note that the instrument's hard- and software can be designed such that both, the high species-resolution and high statistic mode are performed in parallel. Comparison with the requirements listed in Tab.~\ref{tab:orig-meas-req} shows that \acs{AHEPaM} is close to meeting the measurement requirements for protons. In fact, accounting for the $2\pi$-sr simulation, the high-statistics data products meet the original requirement. It has been also shown that the helium requirements can be fulfilled as long as the proton requirements are met. However, those for electrons can not be met, primarily because their flux is much lower than the proton flux (see Fig.~\ref{fig:GCR-spec}). The flux of electrons in the energy range below about 50 MeV is dominated by solar and Jovian electrons \cite{vogt-etal-2018, eraker-and-simpson-1981}. Solar energetic electron events at energies above 1 MeV were measured already in the 1970's and 1980's by the MEH experiment on ISEE 3 and showed only a few events exceeding energies of 50 MeV (for details see \cite{moses-etal-1989}). Thus, the flux of electrons above 50 MeV is dominated by the slowly varying galactic contribution. Therefore, we propose to relax the requirement on the time resolution of the electron flux. Possible time-scales for such re-defined requirements should be linked to physical processes such as Forbush decreases (1 day time resolution) or Carrington rotations (28 days).



\begin{table}[]
    \centering
    \begin{tabular}{|l|l|l|l|}\hline
%      Data  & Protons  & Electrons   &  Geometry \\
%       Product & Protons  & Electrons   &   Factor\\ \hline
      Data Product & Protons  & Electrons & Geometry Factor\\\hline
      high resolution & 5 bands \@ 10 ks: 2.7\% & 5 bands \@ 100 ks: 6.1\%   & 2.9 cm$^2$ sr (uni-directional \\
      & 2 bands \@ 3 ks: 3.1\%  & 2 bands \@ 50 ks: 5.5\%   &  \\
      high statistics& 5 bands \@ 10 ks: 1.2\%  & 5 bands \@ 50 ks: 5.5\%   & 6.8 cm$^2$ sr (uni-directional) \\
      & 2 bands \@ 3 ks: 1.4\%  & 2 bands \@ 50 ks: 3.5\%   &  \\\hline
    \end{tabular}
    \caption{Measurement capabilities of the current \acs{AHEPaM} design. Percent values given reflect both the statistical and systematic uncertainties (the detailed calculation of the uncertainties is given in chapter 1.5 in \cite{ahepam-djf}). Note that geometry factors are given as uni-directional. Because \acs{AHEPaM} measures in both the "forward" and "backward" directions, the geometry factors are effectively doubled.}
    \label{tab:AHEPaM-data-products}
\end{table}







\section{Trades to be considered and lessons learned}
\label{sec:trades}
\acs{AHEPaM} is also a highly-capable scientific instrument which covers an important gap in the energy coverage of \acs{GCR}s and \acs{SEP}s. It would therefore be highly advisable to also provide its data to the heliophysics community and to increase \acs{AHEPaM}'s telemetry rate from 1 kbps to a few kbps. This would increase the science output of \acs{AHEPaM} significantly. 

During the concept design phase, the development of the baselined measuring concept and thus the telescope together with the front-end electronics had the highest priority. While the detector/ telescope design matured, the detailed mechanical design of this highly complex sensor could not keep up. This led to a situation where the structural design had to include a finalized detector arrangement. Since the simulation results were all based on the fixed telescope geometry, changes based on mechanical considerations (such as the cherenkov detector's mechanical support concept) were very complex to include, mainly due to lack of available space between the detectors. 
In retrospect it would have been better to detail some of the instrument design aspects, e.g. assembly and structural joints of the sub- assemblies, in both fields in parallel, in order to better understand the mutual requirements which they impose on each other. 

The simulations detailed in section 1 of \cite{ahepam-djf} have been performed individually with and without a Cherenkov detector in order to investigate whether or not the \ac{AHEPaM} requirements can be fulfilled 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:GCR-spec}).\newline
While the methods introduced in \cite{ahepam-djf} 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 fulfill the given requirements. Introducing the Cherenkov to the setup allowed for a further suppression of the proton contamination and hence significantly 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 details are further described in section 5 in \cite{ahepam-djf}. It is important to note that Cherenkov detectors have been already used successfully for space mission in the past (including instruments built in Kiel \cite{ahepam-heritage}). \newline
Based on the analysis we have decided that a Cherenkov is recommended in order to fulfill 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 could be relaxed.

%\section{Summary, Conclusions, and Outlook}
\section{Final Assessment and \acs{TRL}}
\label{sec:summary}
In the framework of the \ac{AHEPaM} project it has been shown that the presented design is capable of meeting the stated engineering requirements while fulfilling most of the ambitious measurement requirements with the exception of the electron uncertainty in the required temporal resolution. We propose a relaxation of the above-stated temporal resolution since no variation of the observed electron flux is known to occur on such small time-scales at these high energies. \newline
Furthermore, it has been shown that as a simplified design concept, discarding the Cherenkov detector in the instrument set-up provides the opportunity to meet most of the measurement requirements with only marginally increased uncertainties compared to a "full" \ac{AHEPaM} instrument as described above while significantly decreasing the complexity and engineering challenges.




%{\bf Erwähnt werden im Heritage doc \cite{ahepam-heritage} EPHIN, EPD/HET, KET, RAD und die diversen BEXUS missionen. Ich hab nochmal reingeschaut und wir geben dort doch TRL Zahlen an, allerdigns nur für die einzelnen Methoden (siehe die Tabelle unten): } \newline
%\begin{figure}[ht]
%    \includegraphics[width=\columnwidth]{../cau-ath-spc-0005_i1-0/figures/TRLtable.png}
%\end{figure}
%{\bf Bzgl Ableitung auf deine Tabelle unten: i) dE/dx-C haben wir im Heritage auf 3-4, mit CHAOS finde ich die Erhöhung auf 5 gut. Müssten wir nur erklären, ii) da wir bei dE/dx ein TRL von 7 (u.a. auch bei HET und RAD) haben könnten wir BGO in deiner Tablle auf 7 setzen? Wie und ob sich dieses auch auf die Large SSDs umsetzt bin ich skeptisch. Ist vom Engineering her doch deutlich schwieriger?  }

We have assessed the \acs{TRL} of the critical subsystems of \acs{AHEPaM} in Tab.~\ref{tab:TRL}. While the average TRL is high (see document \cite{ahepam-heritage}), the overall, wrapped-up \acs{TRL} is defined by the lowest \acs{TRL} and thus driven by the large \acs{SSD}s needed for the current design of \acs{AHEPaM}. A dedicated qualification process for these large detectors would improve that assessment but could not be performed in the course of this work on the development of the \acs{AHEPaM} conceptual study. All other critical subsystems have heritage from previous space missions (detailed in \cite{ahepam-heritage}) or from the \acs{CHAOS}/\acs{BEXUS} project which is currently undergoing environmental tests and will be launched in October 2024. This experiment can be considered as a simplified \acs{AHEPaM} which comprises all measurement principles relevant for \acs{AHEPaM} but uses smaller detectors (\acs{SSD}s). It is a high-fidelity demo-model of \acs{AHEPaM} measurement principles and also explains the increased  \acs{TRL} reported here for the Cherenkov detector compared to the \acs{TRL} reported in document \cite{ahepam-heritage} which was written before the tests with \acs{CHAOS}/\acs{BEXUS}. The \acs{CHAOS} \acs{BGO} scintillator is identical to the one foreseen for \acs{AHEPaM}, as is its Cherenkov detector. Thus all critical subsystems of \acs{AHEPaM} have been tested with \acs{CHAOS} except for the large (and expensive) \acs{SSD}s. The \acs{DORN} instrument for Chang'E 6 was developed by \acs{IRAP} in Toulouse but uses our front-end electronics (\acs{FEE}) for the detector read out, as it is foreseen for \acs{AHEPaM}.
\begin{table}[h]
    \begin{tabular}{|lcp{10cm}|}\hline
    {\bf Subsystem} & {\bf \acs{TRL}} & {\bf Rationale for \acs{TRL}} \\\hline
    Electronics & 7 & heritage from \acs{DORN} on Chang'E 6 \\
    Mechanical design & 5 & heritage from multiple missions \\ \hline
    \underline{Measurement Principle:} & & \\
    Cherenkov detector & 5 & Demonstration with \acs{CHAOS}/\acs{BEXUS}\\
    BGO scintillator & 7 & Heritage from \acs{MSL}/\acs{RAD}, Solar Orbiter \acs{HET}, and \acs{CHAOS}/\acs{BEXUS}\\
    Large \acs{SSD}s & 4 & Such \acs{SSD}s are in use at the KArlsruhe TRItium Neutrino (KATRIN) Experiment, but could not be tested in a relevant environment in the \acs{AHEPaM} project.\\ \hline
    Wraped up  \acs{TRL} & 4 & Lowest \acs{TRL} of all subsystems. \\\hline
    \end{tabular}
    \caption{Summary of subsystem-level \acs{TRL}s and overall \acs{TRL}.}
    \label{tab:TRL}
\end{table}

%\section{Next steps}
While the general design and assembly concept of the \acs{DM} worked out as expected, some aspects could be improved and should be revised. During assembly it was, e.g., found that although the Cerenkov detector chassis could be integrated without difficulties besides the BGO sub-assembly, the space between the detectors    is generally too small to allow for an adequate safety distance towards the following structural part. Consequently the spacing between the detectors should be increased, allowing for improved assembly, overall reducing risks during the integration. Moreover the electrical shielding inside the PMT chassis needs to be improved in order to reduce electronics noise. Other improvements such as limiting the number of different screw/bolt types and revising the Cerenkov mounting springs do not directly affect the performance of \acs{AHEPaM}, but would lead to an improved overall instrument design.