[Halld-tagger] Photon flux calibration

Eugene Chudakov gen at jlab.org
Wed Jul 11 09:00:24 EDT 2012 

I do not know if this has been already discussed.  At a high beam
intensity with a 0.001 converter the PS coincidence rate would be
about 100kHz. We can read them out with a prescaling factor, say 10
and analyze them later. If the TAC calibration is not needed one can
use a thin converter. Reading out only some FPGA scalers might be
possible, but many details will be lost.

Eugene

On Wed, 11 Jul 2012, Michael Dugger wrote:

>
> Hi,
>
> I have a quick question.
>
> For production runs are we going to be storing the PS information in terms of 
> individual tagger energy bins?
>
> -Michael
>
> On Wed, 11 Jul 2012, Eugene Chudakov wrote:
>
>> Hi Mark,
>> 
>> Yes, as far as I understand it, all the measurements discussed are
>> done in coincidence with a signal in the tagger, separately in each
>> small energy bin.
>> 
>> a) The photon flux after the radiator and the rate in the tagger are
>> irrelevant, apart from some second-order effects with the accidentals.
>> 
>> b) Only interactions of the beam photons in coincidence with the
>> tagger are used.
>> 
>> c) For example, in order to measure the eta cross section, we need to
>> find the eta production rate per incident photon:
>> 
>> N(eta*tagger)/(N(PS*tagger)*K(TAC*tagger)/K(PS*tagger))
>> 
>> Here K are the rates in a special run with a low beam intensity.  The
>> collimator acceptance is irrelevant, the tagger efficiency (the
>> probability of the radiated electron to be detected in the tagger for
>> a beam photon in the hall) cancels out separately for the low
>> intensity run and for the main run (they may be different).
>> 
>> This scheme does not seem sensitive to the way the low intensity is
>> obtained.  What matters is the stability of the PS and its sensitivity
>> to the photon beam spot profile. The latter can be simulated and later
>> measured with a harp. The former needs monitoring - are there good
>> methods? One should also optimize the thresholds for the TAC and PS.
>> 
>> The rate K(PS) is limited by the capability of the TAC. Assuming a 1nA
>> on a 1.e-4 radiator and a collimator acceptance of 0.3 one gets about
>> 0.5MHz of photons E>1GeV. Let us assume it is OK. Let us take 50MeV energy
>> bins, the PS converter of 1.e-3 conversion probability, a 20% PS
>> acceptance.  The PS rate per bin would be about 0.2Hz. In 1h one
>> gets 700 events/bin.  The K(TAC)/K(PS) should be a smooth curve, the
>> stat. accuracy seems to be OK.
>> 
>> As I have said, I do not see why not to use a thin radiator at 50nA
>> instead of 1e-4 one at 1nA. Can one use a 10-20um carbon thread?
>> 
>> Eugene
>> 
>> 
>> ------------------------------------------------------
>> Eugene Chudakov
>> http://www.jlab.org/~gen
>> 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, Mark M. Ito wrote:
>> 
>>> 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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>

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