[Halld-tagger] first results on fiber light yields

[Richard Jones]

Alex,

| Actually fadc spectra for 1) and 3) agree reasonably well.
Yes, that is the problem.  They agree on the FADC, but not on the scope.

| Why 2) is about 20% wider (was it a different readout channel)?
Because it runs through 100ft of cable, whereas 1 and 3 run through 50ft of
cable.

| (there could be some sampling effects due to the different pulse
| shape around the peak. I would also compare amps without the
| splitter).
I cannot remove the splitter because without it we have no FADC trigger.
 We need to split one of the signals to discriminate it.   I agree there
are details at the 10% level that relate to where the peak falls in the
FADC clock.  But the fall time is slow enough that this cannot be more than
exp(-2ns / 16ns) ~ 12%.  The FADC / scope discrepancy is much greater than
that.  See Fernando's email for the numbers.

-Richard J.



On Wed, Feb 12, 2014 at 10:02 PM, Alexander Somov <somov at jlab.org> wrote:

>
> Hello Richard,
>
> Actually fadc spectra for 1) and 3) agree reasonably well
>
> Why 2) is about 20% wider (was it a different readout channel)?
>
> (there could be some sampling effects due to the different pulse
> shape around the peak. I would also compare amps without the
> splitter).
>
> Cheers,
>       Alex
>
>
>
>
>
>
> On Wed, 12 Feb 2014, Richard Jones wrote:
>
>  Dear colleagues,
>>
>> We are working to obtain light yields for the first article fiber bundle
>> that was produced for the tagger microscope some time back.  We have run
>> into a problem trying to interpret what we see coming from the FADC250
>> module that is installed in the data acquisition crate we are using for
>> fiber QA tests.  Please see the plots on the wiki page linked below that
>> illustrate the problem we are seeing.
>>
>> https://halldweb1.jlab.org/wiki/index.php/Tagger_
>> Microscope#Light_yield_tests
>>
>> When we look at the signals on the scope, we see one set of amplitude
>> ratios that clearly reveal the differences in light yield resulting from
>> different lengths of the fibers that are covered in reflective paint.  The
>> results from the FADC are much less distinguished, and disagree in the
>> ratios of pulse heights.  Comments or suggestions as to what we are doing
>> wrong would be appreciated.
>>
>> -Richard Jones
>>
>>
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