Re: Fiber test stand at JLAB

[Beni Zihlmann <zihlmann@jlab.org>]

Hi George,
concerning figure 2, the relative hight between the two bumps also 
depend on the width of the two individual peaks. However a mu of
2.4 is in the area where the two bumps are about the same height. At a 
mu of 2.2 the second bump is smaller and at 2.8 it is clearly higher.
see also NIM A339 (1994)468-476.  So qualitatively it makes all sense.

concerning the gain of the XP2020 I agree it is rather low. I use a base 
VD124K/T optimized for timing (Type C) where I would expect a gain
of 4*10^7 at 2500V and I us it at 2200V. However as you know that quotes 
only an average gain and the variation in gain for different PMTs
of the very same type can be rather large.

cheers,
Beni

> Hi Beni:
>
> Kathryn has completed the measurements of the number of photoelectrons 
> for the three Kuraray fibers and two Bicron and we're now double 
> checking to make sure the numbers stand the test of different 
> methodology.  As they stand now, they're in agreement with the numbers 
> in your report and Kathryn will post them when we're done.
>
> I have some questions on your report for clarifications:
>
> When I look at figure 2 (the LED ADC spectrum) the mu value gives you 
> ~2.5 photoelectrons.  However, with a mean of ~2.5 pe's I would expect 
> the height of the second peak to be a bit higher than that of the 1st. 
> and it's not.
>
> The gain of the XP2020 at -2200V that comes out at 2 x 10^6 is low for 
> the 12-stage PMT.  I looked back at measurements we had done a number 
> of years ago with a number of XP2020's and we were getting more like 
> 10^7 at 2000 V with voltage divider Type A.   Did you use such a 
> highly "progressive" dynode chain to increase the peak pulse current?  
> I would have thought that for low light measurements a Type A dynode 
> chain will give you excellent gain at low bias and you wouldn't need 
> an external amplifier that can never do much good to peak width.
>
> In any case, as I stated above, Kathryn gets consistent numbers but I 
> am looking to attach some measure of "error" in the extraction to 
> increase confidence because dealing with small numbers of 
> photoelectrons to begin with, agreement can come down to decimal numbers.
>
> Cheers,
>
> George
>
> On 26-Aug-08, at 7:42 AM, Beni Zihlmann wrote:
>
>> Dear Colleagues,
>> I put description of the fiber test stand here at JLAB on the wiki 
>> together with
>> some first results. Have a look at:
>> http://www.jlab.org/Hall-D/software/wiki/index.php/Fiber_Test_Stand_at_JLAB 
>>
>>
>> cheers,
>> Beni
>>
>


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Benedikt Zihlmann
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