[Halld-tagger] Photon flux calibration

Tue Jul 10 22:01:39 EDT 2012

Folks,

The following may be known to all, but I thought that the discussion 
might benefit from a restatement of "tradition."

As Richard mentioned, having the tagger in coincidence is key to the 
traditional normalization scheme for tagged photon beams.

The main advantage of the traditional approach is  that it has no 
dependence on the efficiency of a particular tagger counter. Those 
efficiencies are unknowns. One does not even bother to measure them.

The basic assumption is that there are bins in photon energy 
corresponding to the acceptance of particular tagger counters. For each 
of these bins in photon energy, the corresponding tagger counter is 
_required_ for any valid event. The attitude then for each bin is that 
if the corresponding tagger counter does not fire, it is not a photon. 
With that attitude,  the beam rate in that energy bin is _defined_ as 
the rate in the corresponding tagger counter. And nicely, with this 
attitude, the efficiency of the tagger counter does not matter for 
normalization; the beam rate determination reduces to measuring the rate 
in the relevant tagger counters.

The only thing that defeats this scheme are the cases where the tagger 
counter fires, and thus we have a valid traditional photon, but that 
photon does not get to the target. One standard scenario is that the 
photon hits a collimator. The probability that things work out, and the 
photon makes it to the target, is traditionally called the tagging 
efficiency. Note that this is an unfortunate terminology; the tagging 
efficiency has nothing to do with the efficiency of the tagger! Some 
have opted to use the term "tagging ratio" instead to avoid this confusion.

The total absorption counter method purports to measure all photons that 
make it to the target, and thus to measure the tagging ratio directly. 
But it suffers from the requirement that the beam rate needs to be low 
enough not the blow the TAC out of the water, as we all realize. 
Traditionally (again, yikes!) the pair spectrometer is used as a 
relative rate monitor; its rate relative to the TAC is measured in the 
TAC runs, and thus, properly scaled, it can act as a proxy for the TAC 
at standard running rates.

   -- Mark

On 07/10/2012 07:55 PM, Eugene Chudakov wrote:
> 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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