[Halld-tagger] Photon flux calibration

[Gan, Liping]

Hi, Eugene,

In order to measure the ratio of the pair spectrometer rate to the
total absorption counter rate (for a given energy bin in the tagger), one need enough pairproduction rate for the TAC run. According to Sascha's calculation, the PS rate per energy bin is a few Hz level for a 10^-3 R.L. convertor (if I remember correctly) during production run. Therefore, for the TAC run, the expected rate will be about 2-3 orders of magnitude less. It would be difficult to use PS for calibration during the TAC run. It can provide relative monitoring only over the longer time period. For PrimEx runs, our PS convertor was the  physics target which is ~1.5-2 orders of magnigude thicker than what we planned in Hall D. 

Liping




________________________________________
From: halld-tagger-bounces at jlab.org [halld-tagger-bounces at jlab.org] On Behalf Of Eugene Chudakov [gen at jlab.org]
Sent: Tuesday, July 10, 2012 7:55 PM
To: Richard Jones
Cc: halld-tagger at jlab.org
Subject: Re: [Halld-tagger] Photon flux calibration

Richard,

Basically, the PS is doing the same thing as the TAC (they see the
same beam), but the TAC's efficiency is about 100% while the PS-es
efficiency is unknown to a 1% accuracy, but can be calibrated, say
with the 1nA beam.

The acceptance of the collimator and the efficiency of the tagger may
be different for different radiators (I suppose it will not be a big
factor, perhaps a few percents). However, I do not see why the ratio
of the tagged rates in the TAC and in the PS (for a given tagger
energy bin) should depend on the radiator, apart from small
geometrical effects associated with the beam spot in the PS. I do not
assume that using a thinner radiator would distort the tagger energy
measurement with respect to the normal radiator. The energy dependence
of the correction factor must be a smooth function anyway. We just
need to calibrate the PS with the TAC using some radiator, and the
calibration should work for another radiator. I may be missing
something - please explain.

Eugene

------------------------------------------------------
Eugene Chudakov
phone (757) 269 6959  fax (757) 269 6331
Thomas Jefferson National Accelerator Facility
12000 Jefferson Ave,
Newport News, VA 23606 USA

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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