[Halld-pid] Time-of-Flight Meeting, Thu. Feb. 14, 10 am, F326

[Beni Zihlmann]

Hi Sasha,
then I think I understand why you get much smaller results for the 
shorter paddels.
The attenuation depends on the light wavelength. A lot of short wave 
length light
that is created in the scintillation process gets  absorbed at rather 
short distances.
So if you have a light detector at short distances and one at large 
distances you
will find a rather short attenuation length. If however the closer light 
detector is
already reasonably far away from the light source then you will measure 
a much
larger attenuation because the light got absorbed already before it 
reaches the
first light detector.
See for example a figure from the fiber tests for the BCAL: 
https://halldweb1.jlab.org/wiki/index.php/File:Fiber13_painted_ratio.gif
In order to define the light attenuation we decided to use only data 
points sufficiently far away from the
light detector. If you would use the data points closer to the light 
detector you would find a much steeper curve and
hence a much smaller attenuation length.

So I guess if you put the light source at the same distance from the 
close PMT as for the long paddles you will
find a similar result as for the long paddles. Of course d gets much 
smaller.

cheers,
Beni


> On Thursday, February 14, 2013, Beni Zihlmann wrote:
>> Hi Sasha,
>>
>> I think you used the wrong distance  "d" to calculate the attenuation
>> length.
>> in order to get your number of 89.7 of the third paddle in hte list I
>> need to
>> use a d=84.66. You however claimed this is a short paddle in which cast I
>> would need to use a d=42.33. but then I get  and attenuation of 44.8. So
>> I think that the paddle with the short attenuation length are still the
>> long
>> BUT narrow paddles.
>>
>> Beni
>>
> Beni,
>
> Sorry, I forgot to mention that ADC data files are not pedestal-corrected.
> Pedestal files were in a different directory. I have now placed them in
> the same dirs as data files. And distance "d" between measurements points
> is the same for all paddles - 84.66cm, or 1/3 of the normal paddle length.
> Of course, the short paddles have only 18.1cm left from the points of LED
> placement to the closest PMT (distance "c") instead of the same 84.66cm
> for long ones.
>
> Sasha
>   
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