
\section{Results of X-ray scans}
The mechanical accuracy  muon chambers is checked in the X-ray tomograph at CERN. All types, but about 10\% of all chambers, are certified by this device. 

Here we discribe the tomograph briefly, a detailed description can be found elsewhere~\cite{woudstra}.
The tomograph consists of a unit with two X-ray beams that runs over the chamber in the Z direction.
The shadows of the beams is measured with scintillator counters. From the pattern of these shadows,  the wire positions can be extracted with an accuracy better than 10\mum. 

Figure~\ref{tomobol0} presents a 'wire-map' of the chamber near one of the outer cross-plates.
\begin{figure}[b]
\begin{center}
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
\put(-1,0){\mbox{\epsfysize=10cm\epsffile{tomobol.eps}}}
\end{picture}
\end{center}
\label{tomobol0}
\caption{\sl  Wire-map of BOL-0.}
\end{figure}

 In Fig.~\ref{guitomo}c~and~d we plot the residuals from straight line
 fits throught the data of the X-ray tomograph scans for BOL0.
 In this case the data from individual layers in each multilayer
 at the HV and RO side  are combined such that they correspond to 
 the same side of the assembly table at which they were glued. 
 In this case the error bars represent the rms of the tomagraph data
 at each tube position.
 The systematic deviations in the X-ray data are clearly correlated with
 and of the same size as the planarity laser data. This shows
 that tube positionning and its QA are  well under control.

 In Fig.~\ref{ztomras} we compare the z deviations  described above with those
 obtained from the analysis of the X-ray tomograph data for chambers
 BOL0,BOL2 and BOL3. In order to compare different multilayers,
 the Rasnik deviations are shifted such that the average deviation
 is zero for the HV- and RO-side respectively. This corresponds 
 to the combined fit applied to the tomograph data 
 for a scan through  each end of the chamber with the zero position taken at
 the middle between the two multilayers.
  The two sets of data ar nicely correlated with
 an rms of the difference of 6 \mum.

 In Fig.~\ref{tomrasy} we compare the Y-pitch deviations  described above with those
 obtained from the analysis of the X-ray tomograph data for chambers
 BOL0,BOL2 and BOL3. The two sets of data ar nicely correlated with
 an rms of the difference of 7 \mum.


Table~\ref{tomorms} gives the RMS for....
\begin{table}[thb]
\begin{center}
\begin{tabular}{||l|c|c|c||} \hline
            &   BOL0    & BOL2& BOL3 \\         
Rms Z (\mum)&   15      &12&    13 \\ \hline
Rms Y (\mum)&   13      &17&    14 \\ \hline
\end{tabular}
\caption{\sl The RMS results.}
\label{tomorms}
\end{center}
\end{table}



The site parameters are listed in Table~\ref{sitepar}.
\begin{table}[thb]
\begin{center}
\begin{tabular}{||l|c|c||} \hline
      &  Nominal & NIKHEF  \\ 
Z-pitch & 30.035 & 30.0353 \\
Y-pitch &26.011 & 26.027 \\
dY-Multilayer & 399.032 & 398.984 \\ \hline
\end{tabular}
\caption{\sl The side parameters extracted from a the tomograph scans of BOL0.}
\label{sitepar}
\end{center}
\end{table}

