pair spectrometer+PrimEx (fwd)

[Jim Stewart <jstewart@jlab.org>]

---------- Forwarded message ----------
Date: Wed, 14 May 2008 11:27:08 -0400 (EDT)
From: Franz Klein <fklein@jlab.org>
To: Jim Stewart <jstewart@jlab.org>
Cc: Richard Jones <jonesrt@uconnvm.uconn.edu>,
     Elke-Caroline Aschenauer <elke@jlab.org>, Dan Sober <sober@cua.edu>
Subject: pair spectrometer+PrimEx


Jim,

some thoughts about pair spectrometer and PrimEx requirements:

1) PrimEx wants to run high rate tagged beam at ~90-96+% of E0.
Shall we make the tagger vacuum window a bit larger to cover an additional
1-2% of low-energy electrons?
Shall we have finer segmentation than the already doubled number of
counters above 9GeV photon energy (i.e. 120 instead of 80 counters for
Eg>9 GeV or finer segmentation above 10.x GeV)?

2) Pair spectrometer
(on the wiki are some basic specs of the PrimEx PS, Hradchya has 2 design
options, one based on PrimEx magnet, the other was presented on Friday):

What is the purpose of the pair spectrometer:
a) monitoring of absolute collimated photon flux
b) monitoring of collimated photon spectrum
c) absolute energy measurement
d) polarimetry?

ad a)
Accuracy of photon flux measurement requested by PrimEx group is ~1%.
In order to do so, we need to the transmission function (using total
absorption counter) and ensure that the conditions don't change when
moving from ~0.2-0.5nA to 1uA; in both conditions the PS response has to
be the consistent, which requires hodoscopes with good segmentation (say
16 channels) on both sides ('symmetric') running with good linearity
between a few Hz and several 10^5Hz.
Changing converters inbetween might be an option but then we've to know
the converter thickness very well.

ad b)
That can be done with an asymmetric configuration (when using offline
'coincidence' with the tagger) but we might have to see the spectrum
online (epics scalers) since our usual monitoring (plotting the diamond
tagger spectrum normalized to a tagger spectrum from amorphous carbon -
and fitting the coherent edge in that spectrum) works only well when the
coherent peak is pronounced.
To see the spectrum online we need high segmentation on both sides (say 64
scint. fibers).

ad c)
The absolute energy calibration of the tagger will be a special setup
using scint. fibers - if the Bdl is large enough - or mustrips triggered
by coincidence of 2nd row of PS counters and tagger counters.

ad d)
If pair polarimetry with PS is an option, we need additional vertical
segmentation (say 4 thin scints. or scint.fibers).

Sorry that I cannot come up with a more useful conceptional design at this
moment: it's not difficult to estimate but depends on Bdl and size of the
vacuum chamber (=distance to 1st row of hodoscope conters).

Greetings

Franz