RE: Primex beam requirement

[Alexander Somov <somov@jlab.org>]

Dear Liping,

Thanks a lot for the prompt reply and details.

Regarding the flux stability:
you have a separate requirement on the beam position
stability of +-200 microns, which is related to the flux
stability. If you impose an additional requirement on
the beam emittance stability it would provide you with the
constant tagging efficiency, i.e., you could (perhaps) rely
on the tagging hodoscopes; anyway, the pair spectrometer will
be essential for the measurements.

Cheers,
        Sascha



On Fri, 5 Dec 2008, Gan, Liping wrote:

>
> 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
>>
>>
>>
>
>
>
>