\section{BI system layout and installation studies}


\begin{figure}[th]
\centering
	\includegraphics[angle=90,width=\textwidth]{BIS78_3D.pdf}
	\caption{Mechanical drawings of a BIS~7/8 sMDT chamber with integrated triple thin-gap RPC. The RPC chamber is supported independently on the same rails 
	in the BIS layer as the sMDT chamber. The same support scheme will be used also for the BIS~1-6 chambers.}
	\label{fig:BIS78_3D_sMDT_RPC} 
\end{figure}
%
\begin{figure}[th]
\centering
 	\includegraphics[angle=90,width=\textwidth]{BIS78_RPC.pdf}
	\caption{Mechanical drawing of a BIS triple thin-gap RPC chamber with only 50 mm thickness fitting into the BIS layer together with the BIS sMDTs.}
	\label{fig:BIS78_RPC} 
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{BIS78_3D_RPC_support}
	\caption{Mechanical drawings of a BIS~7/8 RPC chamber with rail supports.}
	\label{fig:BIS78_3D_RPC_support} 
\end{figure}
%


The design and construction of new BIS78 sMDT chambers with 2 x 4 tube layers and integrated thin-gap RPCs with three gas gaps for the replacement of the current BIS~7 and BIS~8 MDT chambers 
in the 2019-20 LHC shutdown is a pilot project for the complete replacement of the BIS MDT chambers by integrated sMDT and RPC chambers for HL-LHC. 
The drawings of a BIS~7/8 sMDT detector with integrated triple thin-gap RPC chamber of only 50~mm thickness are shown in  
\ref{fig:BIS78_3D_sMDT_RPC}. The preliminary BIS RPC design is shown in Figure~\ref{fig:  
The design of the remaining new BIS~1-6 sMDT and RPC chambers will be very similar to the BIS~7/8 design.

The common challenge for the mechanical design is the small available space 
and the control of gravitational deformations of the thin chambers with minimal space for support structure. 
The RPC chambers are mounted on their own support frame and will be installed on the existing rails in the BIS layer independently from the sMDTs (see Figure~\ref{fig:BIS78_3D_RPC_support}).
Otherwise the weight of the RPCs would distort too much the BIS sMDT chambers with their thin support structure. 
In order to provide space for the RPC rail supports in between the sMDT chambers, a gap of 20~mm is required between adjacent sMDT chambers which is achieved by removing one tube per layer of each BIS~1-6 chamber
compared to the maximum possible. This does not affect width in z of the Faraday cages on the Readoud and high-voltage ends of the chambers which contain the unchanged number of 
readout and HV distribution boards since the RPC supports have to pass between the sMDT chambers only in two locations along the tubes (see Figure~\ref{fig:BIS_RPC}).  
Also, there needs to be no extra gap between adjacent RPCs. 
The resulting tube layout of a sector of the new BIS sMDT layer for LHC phase 2, including the BIS~7/8 chambers, is shown in Figures~\ref{fig:BIS_layout}.
The chamber readout service modules (CSMs) will be mounted on the toroid coils as close as possible to each chamber such that the signal cable length between mezzanine 
cards and CSMs will not be longer than 4~m.







\label{sec:Barrel_layout}
The RPC BI system aims to almost fully cover the $|\eta|<1.05$ region, with very limited unavoidable acceptance holes. Figures \ref{Rpc:Layout2} and \ref{Rpc:Layout1}shows the proposed layout scheme which is divided as usual in 16 sectors following the same naming scheme of the MDTs. The space availability and possible installation procedures for the RPC on the BI chambers have been studied in detail as it is one of the critical factors. For the large sectors between the barrel toroid coils,  the RPC can be installed on top of the existing MDT chambers.
In the small sectors, corresponding to the coils, it is proposed to extend to all BIS stations the same layout of the new BIS78 chambers, i.e. replacing the present MDT BIS chamber with an muon station integrating a small tubes MDT (sMDT) chamber and the new low-profile RPC triplets, in order to respect the present envelopes. The sMDT chambers are discussed in Section~\ref{sec:sMDT chambers}.
\begin{figure}[htbp]
\begin{center}
\includegraphics[height=0.35\textheight]{BI_layout2}

\end{center}
\caption{\label{Rpc:Layout2}The schematic BI layout in the R-$\Phi$ and R-Z cross sections. The new chamber positions are highlighted by the red boxes.}
\end{figure}
\begin{figure}[htbp]
\begin{center}

\includegraphics[height=0.35\textheight]{BI_layout1}
\end{center}
\caption{\label{Rpc:Layout1}A more detailed description of the proposed layout for the odd and the even sectors}
\end{figure}

A systematic study has been carried out by the ATLAS Technical Coordination~\cite{tatiana1} \cite{tatiana2} to evaluate the available space and the installation procedures. A summary of this study is reported in Chapter~\ref{sec:Installation}. This study produced  preliminary  drawings of the envelopes of the RPC chambers and of their support structures, taking also into account the limitations related to the need to leave space for the projective alignment lines that connect the different MDTs layers. An example for a particular sector is shown in figure~\ref{Tatiana:layout}. This permitted to start the study of different installation scenarios and to setup a Montecarlo simulation to optimize the chamber layout.
\begin{figure}[htbp]
\begin{center}
\includegraphics[height=0.7\textheight, angle=-90]{RPC_BIL_sec_01_C}
\end{center}
\caption{\label{Tatiana:layout}The layout drawing for the BIL of sector 1 side C}
\end{figure}
In Figure \ref{Rpc:layout_sim} it is shown the result of an acceptance study using simulated MDT truth hits and applying a realistic BI RPC geometry including cutouts for alignment, services, and inefficient regions near chamber edges. The chamber layout  used here is the first iteration, not yet optimized for maximizing the coverage, as it is visible in the example. This notwithstanding the total acceptance of the BI chamber is 91\%, to be compared to 95\% of the MDT chambers, which combined with the present RPC system provides already a 95\% of trigger acceptance. This type of study will be used for determining the production layout so to obtain the best possible result.
\begin{figure}[htbp]
\begin{center}
\includegraphics[height=0.4\textheight]{BI_layout_sim}
\end{center}
\caption{\label{Rpc:layout_sim}Montecarlo simulation for the BIL of sector 1 side C acceptance}
\end{figure}

The preliminary layout can be summarized as follows:
\begin{itemize}
 \item 236 RPC triplet Units of BIS and BIL type, according to the layout under study;
\item in the the 96 chambers of small sectors, an integrated muon station composed of the new RPC triplet and the corresponding sMDT, will replace the present BIS MDTs, extending to the rest of the BIS the solution implemented for the 16 BIS78 stations in Phase-I.
  \item The total covered surface is about 470 m$^{2}$, for a total detector area of 1400 m$^{2}$, one fifth of the present RPC system.
  \item the total number of FE channels will be 135000 in a conservative scenario, scaling the strip pitch with the distance from the IP, as requested by the physics performance.
  \item The total number of trigger boxes (PAD) is 400, and these PADs will be very similar to the ones being designed for the scheduled replacement of the trigger electronics in the present RPC system.
\end{itemize}

