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

Here we describe 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(2,0){\mbox{\epsfysize=12cm\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 average of the residuals 
 from straight line fits 
 throught the data of the X-ray tomograph scans for BOL0, BOL2 and BOL3.
 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. 
 The error bars represent the rms of the tomograph data
 at each tube position.

 Comparing  these distributions with those obtained from the
 End-Plug Laser checks before the tubes were glued 
 ( Fig.~\ref{guitomo}a~and~b ) 
 one can see that
 the small systematic deviations in the X-ray data are clearly correlated with
 and of the same size as the laser data. This shows
 that tube positionning and its QA are  well under control.

 In Fig.~\ref{ztomras} we compare the layer Z-pitch  deviations
 obtained from the analysis of the X-ray tomograph data 
 with those derived from the RASAS monitoring at the time
 the layers were glued. 
 In order to compare different multilayers,
 the RASAS 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.

 Similarly In Fig.~\ref{tomrasy} we compare the Y-pitch deviations 
 derived from the RASAS data at the end of each layer glueing period
 with those obtained from the analysis of the tomograph data.
 Again the two sets of data are well 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}

