[Halld-tagger] Some results on electron acceptance

[Richard Jones]

Dan,

For the specs on the quadrupole, please see the following page on the wiki.

https://halldweb1.jlab.org/wiki/index.php/Standard_Beamline_Quadrupole_Specification

For its placement, you may need to contact Alex.

-Richard J.


On 6/28/2012 6:08 PM, Daniel Sober wrote:
> Alex,
> I have used an electron beam with no size or divergence -- the angles are only from (incoherent) bremsstrahlung.  I wasn't suggesting a re-design.  Presumably the goniometer position and quadrupole have been optimized for coherent bremsstrahlung and the microscope.  I was only pointing out that a simple target ladder closer to the dipole would have advantages in the endpoint region if we ever run with an amorphous radiator.
> I can put the quadrupole into my calculations, but I need values for the position, length and gradient.  Is there a document that lists all of this?
> For coherent bremsstrahlung, my acceptance calculations are clearly invalid, and I will not spend a lot of time thinking about how to incorporate a collimated coherent angular distribution unless someone has specific wishes.
> I will proceed with the counter layout calculations up to about 0.980.
> Dan
>
> On 6/28/2012 5:08 PM, Alexander Somov wrote:
>>
>> 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> <mailto: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/ <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 <mailto:sober at cua.edu>/
>
>


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