[Halld-tagger] Fixed array count rates (fwd)

[Alexander Somov]

Hello Dan, all,

1.5 MHz PMT rate for 9 GeV photons seems to be reasonable.
It should even be a little bit smaller for a 1.1 uA current
and 10-4 X0 radiator and should not exceed ~3 MHz per counter
for high-lumi runs (10^8 photons/sec).

Another thing which we might consider is to provide a continuous
energy coverage in the low-energy region after the microscope,
let's say between 7.5 GeV and 8.3 GeV. In the current design the
counters are sampled in this region. This would not cost much,
we will need  about 14 extra cnts and can 'reuse' some of them
which we are not going to install in the microscope region.

This design would help us to decrease a little bit the wrong
energy assignment due to missing hits (due to cnt sampling).
Some simple estimates: the probability to have two hits, one in
the microscope and another in the region 7.5 - 8.3 GeV is

P ~ 10^-2 x 10^10 x 2 ns = 20%,
where 10^10 is the flux of uncollimated photons in the
energy range between 1.2 MeV - 12 GeV (5x10^7 phot/sec lumi)
and 10^-2 is the logs ratio.

The probability to miss one hit due to cnt sampling is about
10%. For the typical detector energy resolution of 600-800 MeV
(depending on the physics channel), the wrong energy assignment
could be on a percent (few percent) level especially for hits in
the microscope low-energy region.

The cnt rate at 7 GeV region is about 3 MHz, a factor of two higher
than that at 9 GeV. We can operate them during data taking.

I don't know how difficult it would be to technically install these cnt
near the micro box (?).

-

My understanding is that your are going to arrange some PMTs in
two rows, if we position them at the top of the counters. They
will be situated a few cm apart from each other. Is 8 cm the
'nominal' position, i.e., the center line between 2 rows ?


Cheers,
        Alex




---------- Forwarded message ----------
Date: Wed, 19 Sep 2012 14:37:20 -0400
From: Daniel Sober <sober at cua.edu>
To: Richard Jones <richard.t.jones at uconn.edu>
Cc: Franz Klein <kleinf at cua.edu>, Alexander Somov <somov at jlab.org>
Subject: Fixed array count rates

Richard,
Thanks for the spectrum.  It confirms that, as I had been assuming, the
differential rate dN/dk at 9 GeV is about half the maximum value on the
coherent peak, and that it falls as ~1/k above that.  Thus, near 9 GeV,
a 4-mm-wide counter will have about the same rate as a 2-mm column of
fibers at the peak.  If we increase the counter width to 5 mm, the count
rate will be about 25% higher.  Is is reasonable to assume that the
maximum channel count rates which can be handled by the SIPMs and the
fixed-array PMTs are equal (~1.5 MHz?), or is there some additional
difference that needs to be considered before I recalculate the counter
sizes and positions without gaps?
Franz and I have decided that a more reasonable counter displacement is
8 cm (perpendicular) behind the nominal focal plane. Any arguments?
Dan


On 9/19/2012 1:17 PM, Richard Jones wrote:
> Dan,
>
> Just cut and paste the text from the email into your browser. Sorry
> for the confusion, the text itself should be correct.
>
> -Richard J.
>
> On 9/19/2012 11:31 AM, Daniel Sober wrote:
>> Richard,
>> All four links actually point to the same file, "...from9.gif"
>> Please send a file that contains the coherent peak.
>> Thanks,
>> Dan
>>
>>
>> On 9/18/2012 5:23 PM, Richard Jones wrote:
>>> Dan,
>>>
>>> Right, here they are:
>>>
>>>   * http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from9.gif
>>>   * http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from9.eps
>>>     <http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from9.gif>
>>>   * http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from3.gif
>>>     <http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from9.gif>
>>>   * http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from3.eps
>>>     <http://zeus.phys.uconn.edu/halld/cobrems/1e8ontarg_from9.gif>
>>>
>>> -Richard J.
>>>
>>>
>>>
>>> On 9/18/2012 4:14 PM, Daniel Sober wrote:
>>>> Richard,
>>>> Reminder:  You said you would send a typical coherent
>>>> bremsstrahlung spectrum so I could check my rate assumptions.
>>>> Dan
>>>> --
>>>> /Daniel Sober
>>>> Professor
>>>> Physics Department
>>>> The Catholic University of America
>>>> Washington, DC 20064
>>>> Phone: (202) 319-5856, -5315
>>>> E-mail: sober at cua.edu/
>>>
>>
>> --
>> /Daniel Sober
>> Professor
>> Physics Department
>> The Catholic University of America
>> Washington, DC 20064
>> Phone: (202) 319-5856, -5315
>> E-mail: sober at cua.edu/
>

-- 
/Daniel Sober
Professor
Physics Department
The Catholic University of America
Washington, DC 20064
Phone: (202) 319-5856, -5315
E-mail: sober at cua.edu/