[Halld-tagger] Photon flux calibration

Gan, Liping ganl at uncw.edu
Thu Jul 12 02:16:32 EDT 2012 

We would like to normalize the flux for each E-counter in the Tagger.

Liping

________________________________________
From: halld-tagger-bounces at jlab.org [halld-tagger-bounces at jlab.org] On Behalf Of dugger at jlab.org [dugger at jlab.org]
Sent: Wednesday, July 11, 2012 9:49 AM
To: Eugene Chudakov
Cc: halld-tagger at jlab.org
Subject: Re: [Halld-tagger] Photon flux calibration

Do we want to be able to normalize the data in energy bins that are finer
than the PS energy resolution (~40 MeV)?

> 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.
>>>>>>>>
>>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Halld-tagger mailing list
>>>>>>> Halld-tagger at jlab.org
>>>>>>> https://mailman.jlab.org/mailman/listinfo/halld-tagger
>>>>>>
>>>>>>
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>>>>
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>>
>


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