Hi Michael,
Unfortunately I won't be able to be there tomorrow but maybe I can
just make a few comments and perhaps you can pass them on to the others.
For a 850 MeV incident electron 85 degrees for the electron
corresponds to 30 degrees for the neutron. In general we cover only 80
degrees but the curvature of the electrons in the field may allow some data
up to 85 degrees. This is of course the highest Q**2 so lowest count rate
and we benefit from the thicker LADS.
At the other end of the spectrum an electron scattering angle of 15
which is the smallest we can achieve and only then by reversing the field
corresponds to 77 degrees for the neutron. This is low Q*2 so highest rate.
So on the basis of the above I prefer the first option with the
neutron wall only back to 80 degrees and better overlap at the high Q**2.
If there is some need to get neutrons at 90 degrees I suspect this is also
a low Q**2 and high rate so we could always move one of the LADS to cover
the back angle for a few weeks to measure that point specifically.
--On Monday, June 2, 2003 11:35 AM -0400 Michael Kohl <kohlm@MIT.EDU> wrote:
> Dear collaborators,
>
> Jim Kelsey has worked out detailed CAD layouts of the possible final
> positions for the neutron walls (existing NC and refurbished LADS).
>
> Taking into account that four LADS bars out of 28 per sector have to be
> excluded (due to lack of phototubes), there are now two approaches
> optimized for two separate requirements.
>
> The first approach optimizes the forward-angle acceptance, with two LADS
> bars behind each other, resulting in 35cm thickness. The most forward
> angle is thus 27.5 degrees. The two 15+20cm LADS walls overlap between
> 27.5 and 42.7 degrees. The price for this is the reachable most backward
> angle for the NC, which is 80 degrees while providing still a small
> overlap with the LADS walls around 43 degrees.
>
> The second approach covers neutron angles with the NC up to 90 degrees.
> Thereby, the LADS acceptance to forward angles shrinks a little and the
> most forward angle which is covered is 28.3 degrees. The overlap
> with NC is at 46 degrees. The two 15+20cm LADS walls overlap between 28.3
> and 42 degrees. In a slightly modified version, the most forward angle
> would be 29.6 degrees, and the two LADS walls overlap between 29.6 and
> 43.3 degrees.
>
> The second approach is technically less difficult to handle for Jim, as
> the first approach needs an additional support for the NC.
>
> In the present layouts for the LADS positions, the lead glass detectors
> can't be in at the same time.
>
> As there might be different preferences from various experiments, it is
> suggested to have a meeting tomorrow (June 3) at 11am in the conference
> room) in order to discuss this and to come closer towards a decision
> which makes everybody happy.
>
>
> Regards,
>
> Michael
>
>
>
> --
>
> +-------------------------------------+--------------------------+
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>
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Cheers,
Douglas
26-415 M.I.T. Tel: +1 617 258 7199
77 Massachusetts Avenue Fax: +1 617 258 5440
Cambridge, MA 02139, USA E-mail: hasell@mit.edu
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