The desired location of the pots is as close to the beam
as possible while maintaining acceptable halo rates in the pots and
backgrounds at DØ. Shortly after the start of each store, once
scraping is completed and the beams are stable, it will be possible
to insert the pots.
The halo rates will be monitored by
the trigger scintillator as the pots are slowly moved in
(full range of motion should be about one minute). The
point of closest approach will be unambiguous, signalled by a rather sharp
increase in the rates. The pots can then be slightly
withdrawn so that the rates will be acceptable for data taking.
It will be possible to calculate how many 
this position
corresponds to using Beam Position Monitors (BPM's). The exact
details of this procedure will be worked out in conjunction with
the Beams Division. Similar procedures will be used for
location of the collimators in Run II on a store-by-store basis.
Opposite side pots should be moved in as symmetrically as possible.
A sample of elastic events can then be used off-line to determine
the position of the beam, using the rates of elastic events in each
pair of pots. An alternate procedure involves plotting the rates
versus position as the pots are inserted,
and then fitting a gaussian to the central part of the
distribution to determine the beam location.
We estimate a beam position uncertainty of about 100 
m,
which is comparable to the position resolution of the detector
and has been
included in the momentum resolution
calculation discussed in Sec. 3.3.4.
These procedures to determine the beam position can be applied (with minor modifications) even in a crossing angle case where the pots are not symmetrically located, but the error will be a little larger. The trigger scintillators can be used to monitor beam stability as well. These procedures can easily be automated based on experience developed during the commissioning of the upgraded DØ detector.