%\section{Soudan Option}
\subsection{Introduction}

An alternate to WIPP~\cite{APP-DET:wipp} for the location of the long-baseline neutrino detector is the Soudan Mine in northern Minnesota.

The Soudan Site is located in the Soudan Mine State Park, Minnesota; it has 
been the location of an underground neutrino facility since  1984 and is 
currently being prepared for the installation of the MINOS~\cite{APP-DET:minos} experiment,
which is scheduled to start taking data  in 2004. 

 The Soudan site has 
both advantages and disadvantages compared with WIPP. The advantages include closer distance to BNL, higher event rate, existing infrastructure, and an operating
underground neutrino detector. The closer distance means that the storage ring
at the Neutrino Factory can have a smaller inclination angle 
($7.7^{\circ}$ Soudan \textit{vs.} $13.1^{\circ}$ WIPP) which 
translates into easier and less expensive civil engineering. The 1700 km location also means  a higher neutrino rate per kton at 
Soudan than at WIPP. A detector at Soudan could be 1/3 the size of one
at WIPP and still have the same number of $\nu$-interactions/year (see Table~\ref{DET:rates}.)
\begin{table}[!htb]
\begin{center}
\caption{Event rates at possible detectors at Soudan and WIPP.}
\label{DET:rates}
\begin{tabular}{|lccccc|}
\hline
$E_{\mu}$ Muon Ring & Baseline & ${E_{{\nu}_\mu}}$ & ${E_{{\nu}_e}}$ & 
$N({\nu_{\mu}}$ CC) & $N({\nu_{e}}$ CC) \\
(GeV) &(km) & & & (per kton-year) & (per kton-year) \\
 \hline
$10_{\textrm{BNL-SOU}}$ & 1700 & 7.5 & 6.5 & 260 & 120 \\ 
$20_{\textrm{BNL-SOU}}$ & 1700 & 15 & 13 & 2150 & 960 \\ 
$20_{\textrm{BNL-WIPP}}$ & 2900 & 15 & 13 & 740 & 330 \\ \hline
\end{tabular}
\end{center}
\end{table}

 The site has infrastructure well matched
to the requirements of a physics experiment, and offers the potential to use
an existing detector (MINOS) upgraded for the Neutrino Factory program.
The closer distance to BNL is also a disadvantage of the Soudan site.
At 1700 km, both the matter oscillation and the CP violating term, $\delta$,
are harder to observe (See Fig.~\ref{fg:cp}). However, the measurement of
$\theta_{13}$ and the improvement in both $\Delta{m_{23}}^{2}$ and 
$\theta_{23}$ should not be significantly affected by the shorter distance.


\subsection{Site}
As noted the Soudan Site has two excavated halls available for physics 
experiments: the MINOS hall and
a neighboring hall in which Soudan II  was located. 
The halls are not optimally oriented with respect to BNL. MINOS
is rotated $39^{\circ}$ with respect to a $\nu$-beam incident from Brookhaven.
The second hall is also pointed away from BNL at a significant angle, but 
may be wide enough to allow for the installation of a detector rotated in an
appropriate direction  for the Neutrino Factory.
Services such as elevator access,
electricity, water and cranes, though available in both halls, would presumably
need to be upgraded if a larger experiment were installed. 
\begin{figure}
\begin{center}
%\mbox{\epsfxsize 3.5in
%\epsfbox{cavcolorsm.ps}}
\includegraphics[width=10cm]{../template/report/ps-and-eps/cavcolorsm.ps}
\caption[The Soudan Site ]{The Soudan Site. The two underground experimental halls are located 
at 714 m below the surface. The MINOS hall is on the left and the Soudan II 
hall is on the right.}
\label{fg:cavcolorsm}
\end{center}
\end{figure}
\subsection{Detector}
The MINOS detector could be upgraded to increase its mass for a future
experiment at the Neutrino Factory. MINOS is a 5 kton detector composed
of layers of 4-cm steel absorber plates interleaved with layers of
scintillator slats (Fig.~\ref{fg:minos_3}). There is a field coil 
that runs through the center
of each plate down the length of the detector. The coil magnetizes
the plates, producing a toroidal field with a field of 1.5~T.
The MINOS detector operates similarly to the Steel/Scintillator/PDT
detector described in Chapter~\ref{detector-chapter}. The magnetic
field enables the identification of the sign of the leading muon resulting
from $\nu$ CC interactions. The momentum and energy of the muon can be
determined by a combination of bending in the $\vec{B}$-field and
range. Background events are rejected through momentum and isolation
cuts, range-out and timing. The MINOS detector is approximately 40\%
steel by volume. The thin steel plates give the experiment the ability
to identify muons down to 1~GeV/$c$ and to possibly measure $\nu_{e}$
events. An upgrade to MINOS would add a front section to the 
detector that was 80\% steel by volume, including 20-cm thick 
magnetized-steel absorber plates interleaved with standard MINOS 
scintillator slats. This could allow the detector to approach  a total
mass of 15~kton. Such an upgrade would totally fill the available
space in the MINOS hall, and would not resolve the issue of the
detector angle relative to the incident beam angle. An alternate solution would
be to build a 15 kton detector in the second hall available at Soudan. The
second hall is wider than the MINOS hall, and could accommodate a
detector installed at an angle rotated toward BNL. With this solution,
the angle between the detector's major axis and the neutrino beam 
would be smaller, though not $0^{\circ}$. The choice between these
alternate solutions requires additional study.
%\vskip{10cm}
\clearpage
\begin{figure}[!bth]
\begin{center}
%\mbox{\epsfxsize 3.5in
%\epsfbox{minos_3.ps}}
\includegraphics[width=100mm,angle=-90]{../template/report/ps-and-eps/minos.ps}
\caption[Two views of the MINOS detector ]{Two views of the Minos detector showing both its general features
and its configuration in the experiment hall. }
\label{fg:minos_3}
\end{center}
\end{figure}