[Halld-tagger] Photon flux calibration

[Michael Dugger]

Eugene,

What you propose should work as long as we determine the tagger rate 
of "good" hits using out-of-time tagger events during production running. 
Once the tagger rate of good hits has been determined, we can find the 
tagger efficiency using the calibrated PS. Doing things in this manner 
basically means that the job of the PS is to get a tagging efficiency, and 
the job of the TAC is to calibrate the PS (using special runs without 
concern for the tagging efficiency).

In hall-B we use the out-of-time tagger events to determine the photon 
flux in a similar manner. The only real difference is that we use the TAC 
runs to get the tagging efficiency. Having the tagging efficiency measured 
during the production running is probably better than determining the 
tagging efficiency with special running conditions.

For the hall-B data, the absolute photon normalization was problematic for 
linearly polarized photon beams. My feeling is that problems we had with 
the absolute normalization for linearly polarized photons was due to the 
tagging efficiency being determined using special running conditions. 
Luckily for us, we were only interested in a relative normalization, and 
the problems we saw for the absolute normalization was not an issue we 
cared much about.

Take care,
Michael


On Tue, 10 Jul 2012, Eugene Chudakov wrote:

> Hi Michael,
>
> Let us assume that both the TAC and the PS are neutral to the photon
> polarization. You can calibrate the PS with respect to the TAC with an
> amorphous radiator and use this calibration for the diamond
> radiator. The collimator acceptance at 9GeV may be higher with the diamond
> than with the amorphous radiator, and the PS will see it.  The PS just
> detects the full photon flux. As I said before, there is a caveat: if
> the diamond radiator provides a much smaller beam spot on the PS
> converter and the PS acceptance is very sensitive to the location of
> the origin of the pair, there might be some distortions. A dependence
> of the PS acceptance on the polarization may also distort the
> measurement. Both effects can be evaluated (of course, the
> polarization dependence is not relevant for the Primakoff-eta).
>
> Eugene
>
>
> On Tue, 10 Jul 2012, Michael Dugger wrote:
>
>> 
>> 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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>>> 
>>> 
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>