Re: talk today on the pair spectrometer (fwd)

[Alexander Somov <somov@jlab.org>]

Hello Hrachya,

> 1. You mentioned the hit of two adjacent FSF counters by the same track.
> Is there a quantitative data on that(percentage). May be that was a
> motivation for counters tilting, that was technologically not
> convenient.

Hit multiplicities are on page 25 of the talk. Counters are placed
close to each other in the plane perpendicular to the bamline, therefore
the probability for multiple hits is relatively large (given large
deflection angle tan_theta~0.3). For positrons I estimated the fraction
of N_hit > 1 / N_total ~ 31 %. Here I required both cells to have dE > 0,
but I didn't apply any thresholds. The ratio will be smaller if you
require dE > dE_thresh. The counter length (along z) is 1 cm; we might
also think of making them shorter.

Why do you think that it would be inconvenient to tilt the counters
(we can assemble plastic counters on a plate and attach the plate to the
vacuum window) ?

> 2.The coincidence rate in PS as I've seen is  18.5khz at converter
> thickness of 0.01Xo. But what was taken as a integral photon rate I've
> probably missed...  Could you please remind me?
> I've evaluated an integral coincidence rate in the energy range 6-12 Gev
> at  photon flux 10^8/GeV, an average efficiency 0.1% and radiator thicknes
> of 0.001Xo as 0.6khz.

The rate is estimated for L = 10-2 X0 and lumi 10^8 ph/sec.

I generated events setting up a photon beam energy threshold to
E_photon > 1.2 GeV. The photon rate for this threshold is 5.6 10^9 ph/sec
(it corresponds to the photon rate of 11 10^9 for a beam energy threshold
of 0.12 GeV).

Then, in the energy range of 6-12 GeV, the rate would roughly  be
ln(12/1.2)/ln(12/6) = 0.3 time smaller (0.31 to be exact if you take it
from the energy spectrum). Therefore the integral photon rate
in the range 6-12 GeV is 1.87 10^9. A coincidence rate should be
1.87 10^9 * 0.01 * 10-3 = 18.7 kHz
(what exactly is 0.1% ? Did I multiply it correctly here ?)

Please let me know if you see anything wrong here (I will check it again).


> 3. You didn't mention the method used for random coincidence ID.
I required a coincidence of at least one counter for positrons
with at least one counter for electrons. At the moment I put only
WSF and FSF counters into GEANT (WSB and FSB are not used).

> 4. The use of shielding installed in vacuum chamber is a good idea, Did
> you also see  a need to install a shielding , separating hodoscopes from
> the beam pipe, which  acts as a radiator for small angle deflected
> showering electrons and positrons.

I obviously didn't have time to investigate this option. At the moment
I use two slits for the hodoscope counters (for positrons and electrons,
respectively) at the back plane of a vacuum chamber. Each slit it 62 cm
wide (in X directions) and 3.4 cm height. Slits start at a radial
distance of 19 cm from the beamline and ends at 81 cm. The slit is closed
by 250 micron Kapton window. The size/position is chosen to allow
hight-energy leptons (up to 12 GeV) to go through it. Leptons are
subsequently dumped into a lead band and iron blocks downstream the beam.
I have not yet simulated back splashes from the lead wall behind the
hodoscopes  (will do). Though I think that background from back-splashes
should be tiny.


Cheers,
       Sascha



On Mon, 22 Sep 2008, Hrachya.Hakobyan wrote:

> Hello Sasha,
>
> I've read your talk in the past GlueX meetings and have a few questions on
> the PS subject.
> 1. You mentioned the hit of two adjecent FSF counters by the same track.
> Is there a quantitative data on that(percentage). May be that was a
> motivation for counters tilting, that was technologically not
> convenient.
>   2.The coincidence rate in PS as I've seen is  18.5khz at converter
> thickness of 0.01Xo. But what was taken as a integral photon rate I've
> probably missed...  Could you please remind me?
> I've evaluated an integral coincidence rate in the energy range 6-12 Gev
> at  photon flux 10^8/GeV, an average efficiency 0.1% and radiator thicknes
> of 0.001Xo as 0.6khz.
>
> 3. You didn't mention the method used for random coincidence ID.
>
> 4. The use of shielding installed in vacuum chamber is a good idea, Did
> you also see  a need to install a shielding , separating hodoscopes from
> the beam pipe, which  acts as a radiator for small angle deflected
> showering electrons and positrons.
>
>
> Thanks in advance, Hrachya
>