RE: Primex beam requirement

["Gan, Liping" <ganl@uncw.edu>]

Dear Sascha,

Thanks for your comments. Here are my answers:



1. Photon flux stability: 1 % 
  Why do you need such a precision ? You tag each beam photon
  in the energy range of interest, so that you know exactly what
  the beam flux is, i.e. you know the luminosity (regardless
  whether the beam flux is stable or not).

Answer:
For a D=5mm collimator, only abut 30% tagged photons will be able to pass through 
collimator and  hit on target. On the other hand, you may have an electron detected by 
tagger without producing a photon 
on the target due Moller scattering. Therefore,  it is not true that "You tag each beam photon
  in the energy range of interest, so that you know exactly what the beam flux is". Only way to
 know exact photon flux is TAC run, however, you can not run TAC during production run. In addition, 
you can not run TAC with high beam intensity. Therefore, maintain the flux stability is important.

 

2. Photon intensity on target: 6.4 10^6 photons/sec/GeV
   Is it in the 'signal' region 10.5 - 11.7 GeV ?

Answer: Yes.

3. (Beam) energy resolution: 10-3
  Where did this number come from ?
  Is the eta production Xsection such a steap function of energy ?
  (we are talking about inclusive X section  measurements, aren't we)


Answer: 
No. We are not  "talking about inclusive X section  measurements". We are talking 
about differential cross section measurement on eta coherent production. The Primakoff amplitude will be extracted from this differential cross section. Therefore, the angular shape of Primakoff is very important for such extraction. The angle of the peak for the Primakoff is proportional to the 4th power of the beam energy. That is why we need knowledge of beam energy 
at 10e-3 level.

Best regards

Liping



-----Original Message-----
From: Alexander Somov [mailto:somov@jlab.org]
Sent: Fri 12/5/2008 10:19 AM
To: Gan, Liping
Cc: Richard Jones; Hall D beam working group
Subject: RE: Primex beam requirement
 
Dear Liping,

I have a couple of questions relatet to your table:

1. Photon flux stability: 1 %
  Why do you need such a precision ? You tag each beam photon
  in the energy range of interest, so that you know exactly what
  the beam flux is, i.e. you know the luminosity (regardless
  whether the beam flux is stable or not).

   ( If you use a reference channel for the Xsection measurement,
   like Compton scattering, than, of course, the flux stability
   is important)

2. Photon intensity on target: 6.4 10^6 phot/sec/GeV
   Is it in the 'signal' region 10.5 - 11.7 GeV ?


3. (Beam) energy resolution: 10-3
  Where did this number come from ?
  Is the eta production Xsection such a steap function of energy ?
  (we are talking about inclusive X section  measurements, aren't we)

Thanks,
       Sascha




On Thu, 4 Dec 2008, Gan, Liping wrote:

> Dear Richard,
>
> According our discussion at the meeting, here is updated beam requirement for PrimEx experiment.
>
> Best regards,
>
> Liping
>
>
>