[Halld-tagger] Photon flux calibration

[Eugene Chudakov]

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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