[Halld-tagger] Some results on electron acceptance

[Alexander Somov]

Hello Dan,

There are some descriptions of the quadrupole along with some basic 
resolution studies in the same doc GlueX-doc-1368, p 11. The distance
between the radiator and the center of the magnet is very short - 83.9 cm 
( it was changed from 81 cm in the writeup to 83.9 cm). The magnet is 
positioned very close to the goniometer in order to maximize the effect 
of the field, I surmise - there is not much room for the alternative target 
ladder; the quad has to be moved. But I would not spend much time now 
reoptimizing.

It's important to decide on the number of counters we need/can afford
at the end-point energy. Perhaps we can come up with the technical
design so that we can increase the number of cnts in the future.


Cheers,
        Alex



On Thu, 28 Jun 2012, 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>/
>>>> 
>>>> 
>>> 
>>> -- 
>>> /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/
>
>
>