[Halld-tagger] Photon flux calibration

Wed Jul 11 03:45:15 EDT 2012

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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