[Halld-tagger] Some results on electron acceptance

[Daniel Sober]

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/


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