[Halld-tagger] first results on fiber light yields

[Fernando J. Barbosa]

Hi Richard,

>From the pictures, I get for each set the following peak amplitudes:
1) 64mV, 278 channels=4.3 ch/mV
2) 46mV, 248 ch=5.4 ch/mV
3) 76 mV, 295 ch=3.9 ch/mV

Clearly not linear where a 40% change on the scope corresponds to 16% on the ADC. This is because we are looking at peak values and the pulse rise time is less than 4ns (ADC sampling). If you are triggering the ADC from the laser pulser, try adding cables in 1ns increments to see how the ADC amplitude changes. The pulse risetime is just too fast for the ADC to sample properly (must be > 4ns). Because the pulse duration is much longer than the pulse risetime, charge should be OK as it is to be used in the experiment. The scope is the better tool for your tests of the fibers unless you put a low pass filter to increase the pulse risetime.

Best regards,
Fernando




----- Original Message -----
From: Alexander Somov <somov at jlab.org>
To: Richard Jones <richard.t.jones at uconn.edu>
Cc: Hall D beam working group <halld-tagger at jlab.org>
Sent: Wed, 12 Feb 2014 22:02:13 -0500 (EST)
Subject: Re: [Halld-tagger] first results on fiber light yields


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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