[Halld-tagger] Some results on electron acceptance

[Alexander Somov]

Dear Dan,

Thanks, interesting.

What beam profile did you use for the gap acceptance
estimation (a pencil beam) ?

1. For the standard GlueX running with a 3.4 mm radiator
the acceptance for collimated photons is going to be
slightly larger. For a typical bremsstrahlung angle of about
2 mrad for 300 MeV electrons, the average radial displacement
of photons at the collimator is
(2e-3 * 0.3 / 11.7)*76 m ~ 3.9 mm,
i.e., if the electron is scattered to a large angle the photon
doesn't make it through the collimator. The bremsstrahlung angle
dominates the multiple scat. and the beam emittance. Ok, we need
to take the beam profile and the effect of the quadrupole into
account. The bg is a concern.

For a 5 mm radiator, the situation is slightly worse.
We have checked the tagging efficiencies using a Geant simulation,
with a  'realistic' beam parameters and ray tracing through  the
quad and  the dipole magnets, GlueX-doc-1368, Fig 15.

The quadrupole field has to be optimized for runs with a 5 mm
collimator for the best end-point efficiency. Currently we
optimized it for the best vertical resolution in the microscope.


2. It may be a good idea to move the goniometer a little bit closer
to the dipole, though it might be too late. As the quad is positioned
close to the goniometer, we will need to move the quadrupole as well.
I recall that there was a discussion about this long time ago; may be
I've missed something important regarding the correct positions.

We can check with accelerator people how critical the current
goniometer position is; they have several monitors in front
of the goniometer; the quad stands may have already been installed.


3. Pushing to .980 would be Ok. I would also install a few more
counters at the end point region (the counters are wide in this area)?

Ideally it would be nice to double the number of counters from the
end-point to the microscope, we will need like 80 more (seems to be too
expensive, though). The fixed-array energy resolution is dominated by the
counter size for Ee ~> 0.5 GeV. Any thoughts about this ?


Cheers,
        Alex





On Thu, 28 Jun 2012, Daniel Sober wrote:

> Dear Alex and Richard,
> Sorry for the error.  I should get a full set of magnet dimensions so that I 
> can do things correctly the first time.
> Attached is the calculation for the 3 cm gap.  If there is any dedicated 
> amorphous-radiator running with interest in the endpoint region, one could 
> improve things by putting the radiator closer:  a distance of 1.5 m (instead 
> of 3.19 m) would increase the gap acceptance from .858 to .948 at k/E0 = 
> 0.980 and from .895 to 0.962 at k/E0=0.975 (11.7 GeV) without substantially 
> changing the energy resolution all the way down to the microscope.
> My primary goal in resuscitating these codes is to work out the counter 
> placement at the high energies.  Should I consider pushing to .980 (11.76 
> GeV)?
> Dan
>
> On 6/27/2012 9:22 PM, Richard Jones wrote:
>> Dan,
>> 
>> Some things to keep in mind:
>>
>>  1. remember the quadrupole is vertically focusing, and can be tuned
>>     to improve things near the endpoint when the physics requires
>>     endpoint energies
>>  2. the gap is 3cm
>>  3. there is significant scraping at 11.7 GeV under GlueX running
>>     conditions
>>  4. we chose 11.7 GeV because things get impossible above that, even
>>     with the quad
>> 
>> -Richard J.
>>
>>  1. On 6/27/2012 4:49 PM, Daniel Sober wrote:
>> 
>>> I have put the current tagger magnet into my old codes and come up with at 
>>> least one interesting result that needs investigating:  Using a realistic 
>>> bremsstrahlung calculation integrated over photon angles (Maximon and 
>>> Lepretre, 1985) for an amorphous gold radiator,
>>> the fraction of the bremsstrahlung electron cone clearing the 2 cm magnet 
>>> gap gets bad rather quickly as k/E0 > 0.95, with only 81% transmitted at 
>>> k/E0 = 0.98.  See the attached files, one for the full range and the other 
>>> in fine steps near the endpoint. Some of the numbers in the header 
>>> (especially "FULL-ENERGY ANGLE") may not make sense to you, but they 
>>> generate what we need.  The second column (Gap frac.) gives the fraction 
>>> of electrons clearing the 2 cm gap, neglecting the Rogowski chamfer which 
>>> will make things a little better -- I will need a detailed drawing of the 
>>> pole shape to account for this effect.  The subsequent columns give the 
>>> fraction passing through a given detector full width.  (The "negative" 
>>> fractions just flag the cases where the magnet gap is the limiting 
>>> aperture.)
>>> 
>>> I am not set up to calculate coherent bremsstrahlung or the effect of 
>>> photon collimation, but with some work I could plug in the appropriate 
>>> electron angular distributions if I had them.
>>> 
>>> 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 <mailto: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/
>
>
>