[Halld-tagger] Photon flux calibration

[Michael Dugger]

Hi,

I agree with Richard.

The TAC runs will be used, in part, to measure the fraction of photons 
surviving the collimator. The degree of photon polarization is influenced 
by the beam collimation because the coherent spectrum has a better 
probability of surviving the collimator than the incoherent spectrum. This 
means that the tagging efficiency will be dependent upon the degree of 
polarization.

-Michael

On Tue, 10 Jul 2012, Richard Jones wrote:

> Eugene,
>
> One needs to measure the ratio of the pair spectrometer rate to the TAC 
> counter *for a particular set of beam photon populations.*  The populations 
> are defined by those beam photons that are in coincidence with each of the 
> tagger detector channels.  None of this is meaningful without the tagger in 
> coincidence.  As soon as you change the radiator, the population being 
> selected by the tagger coincidences changes.
>
> -Richard Jones
>
>
>
>
> On 7/10/2012 10:52 AM, Eugene Chudakov wrote:
>> Hi,
>> 
>> The yesterday's discussion on the photon flux calibration did not
>> convince me that one desperately needs a 1nA current.
>> 
>> One needs to measure the ratio of the pair spectrometer rate to the
>> total absorption counter rate (for a given energy bin in the tagger).
>> This ratio should not be very sensitive to the type of the
>> radiator. Both detectors see the same photon beam. So, instead of
>> using a 1nA beam current run one may use a thin radiator or a scanning
>> wire with a 50nA run. I suppose it is easy to simulate the acceptance
>> of the pair spectrometer to find out what would be the dependence on
>> reasonable shifts in the beam spot profile (say, a 20% variation of
>> the radiator thickness across the beam).  One should also keep in mind
>> that a low current beam might have a different profile with respect to
>> the full current beam, so this kind of uncertainty always exists.
>> 
>> Eugene
>> 
>> 
>> On Mon, 9 Jul 2012, Richard Jones wrote:
>> 
>>> Hello,
>>> 
>>> Please remember our biweekly working group meeting this morning at 
>>> 11:30EST.
>>> The draft agenda is posted in the usual place.  Please install links in 
>>> the
>>> agenda page to any materials that you will be presenting.
>>> 
>>> -Richard J.
>>> 
>>> 
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>
>
>