[BLASTTALK] LADS positions

From: Michael Kohl (kohlm@mit.edu)
Date: Fri Oct 03 2003 - 01:06:21 EDT


Hi,

in recent discussions about the physics program with the LADS detectors it
became obvious that the left-right sector symmetry regarding the
detector is in fact not wanted in case of the neutron counters
and the measurement of G_En. In contrary, it is desired to maximize
the neutron detection efficiency for the low-Q kinematics 0.05<Q2<0.5 on
the right sector, corresponding to neutron angles between 40 and 80
degrees (with the target spin pointing about 35 degrees to the left). In
order to do this, it would be great if it was possible to put two layers
of LADS behind each other (15+20)cm between the BLAST detector and the
already installed Ohio wall, thus increasing the detector thickness from
10cm to 45cm. A schematic view would look like (the LADS bars have
vertical orientation):

    45deg 80deg
    +--------------------------------------------------------+
    | Right-sector Ohio wall |
    +--------------------------------------------------------+
         +--------------------------------------------+
         |X| L20_R || L15_R |
         +--------------------------------------------+
            +------------------------++------------------+
            | L15_L || L20_L |X|
            +------------------------++------------------+
                 +=========================================+
                 | Right-sector TOF |
                 +=========================================+

  
  /
 <----------------------------beam direction------------------------
  \

The leftmost channel of L20_R and the rightmost channel of L20_L are
without phototubes, indicated by "X".

L20_L = left-sector 20cm thick
L20_R = right-sector 20cm thick
L15_L = left-sector 15cm thick
L15_R = right-sector 15cm thick

"Left" and "right" sector of course refers to the old labeling scheme.
 
This proposed setup would offer the advantage of minimized acceptance loss
due to the combined geometry of the LADS/Ohio detectors and the minimal
distance to the target.

However, things are not so easy. Jim Kelsey has checked the solid model
and finds that two layers of LADS would barely fit in the gap between the
TOFs and the Ohio wall, every inch would count. There would be probably an
overlap of the Tof tubes with the inner LADS wall in vertical direction.
The real tolerance needs to be measured during a hall access.
JK expresses his concerns of endangering the Tof tubes during the
installation process, when everything is so tight.
A possible installation procedure would start with the inner LADS layer
(L15_L and L20_L) by moving them as close as possible towards the TOF
detector and then placing the second LADS layer (L20_R and L15_R) into
the gap between the first layer and the Ohio wall. In this way the Tofs
would be protected by the first LADS layer when the second layer comes
down with the crane.
Another question is how often this installation procedure would need to
be repeated in the view of accessing the drift chambers or anything which
needs an open subframe, e.g. to replace wires (how often has this been the
case?)

Alternatives would always result in a smaller acceptance. E.g. the Ohio
wall could be moved away from the target by some amount in order
to open up the space (presently it is in contact with the vertical beam of
the main frame right behind the Ohio wall. Moving it to a bigger distance
means that it would have to be moved behind the vertical beam)

Another possibility is to put only the L20_L+R between TOF and Ohio wall,
and to place the L15_L+R behind the Ohio wall.

I suggest that we should have a meeting early next week, how about Monday
10/6 after the operations meeting? Somebody representing the wire chambers
should also be present.

By the time perhaps Vitaliy has a count rate estimate (assuming
45cm detector thickness between 45 and 80 degrees) available?

Best 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 | | +-------------------------------------+--------------------------+



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