Hi Doug,
your remarks about the kinematical boundaries are very correct.
However, they are valid for the quasielastic breakup only. If it is
desired to provide small-q acceptance also off the quasielastic peak (to
study FSI, MEC, IC ...) either towards threshold or towards the dip
region, this can be done variing either the the missing momentum, i.e. the
neutron time-of flight as well as the neutron angle (at fixed electron
angle). For the latter, it could thus be desirable to have acceptance even
at more backward angles than for the quasielastic ones.
Anyway, the same argument also holds for the forward neutron angles.
However, the point is that the cross section (count rate) off the
quasielastic peak drops dramatically, also at low-Q2, and it might thus be
questionable if such a measurement can be done within a short period (with
LADS at in this position) as compared to having the acceptance there
available once and forever by arranging the detectors according to
version 2.
One advantage of version 2(a) is, that one gains 10 degrees at backward
angle and has to pay that with less than one degree loss at forward angle.
Best regards,
Michael
> 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
> >
> >
> >
> > --
> >
> > +-------------------------------------+--------------------------+
> >| Office: | Home: |
> >| -------------------------------------|--------------------------|
> >| Dr. Michael Kohl | Michael Kohl |
> >| Laboratory for Nuclear Science | 5 Ibbetson Street |
> >| MIT-Bates Linear Accelerator Center | Somerville, MA 02143 |
> >| Middleton, MA 01949 | U.S.A. |
> >| U.S.A. | |
> >| - - - - - - - - - - - - | - - - - - - - - -|
> >| Email: kohlm@mit.edu | K.Michael.Kohl@gmx.de |
> >| Work: +1-617-253-9207 | Home: +1-617-629-3147 |
> >| Fax: +1-617-253-9599 | Mobile: +1-978-580-4190 |
> >| http://blast.lns.mit.edu | |
> > +-------------------------------------+--------------------------+
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
>
>
> 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
>
--+-------------------------------------+--------------------------+ | Office: | Home: | |-------------------------------------|--------------------------| | Dr. Michael Kohl | Michael Kohl | | Laboratory for Nuclear Science | 5 Ibbetson Street | | MIT-Bates Linear Accelerator Center | Somerville, MA 02143 | | Middleton, MA 01949 | U.S.A. | | U.S.A. | | | - - - - - - - - - - - - | - - - - - - - - -| | Email: kohlm@mit.edu | K.Michael.Kohl@gmx.de | | Work: +1-617-253-9207 | Home: +1-617-629-3147 | | Fax: +1-617-253-9599 | Mobile: +1-978-580-4190 | | http://blast.lns.mit.edu | | +-------------------------------------+--------------------------+
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