[Halld-cal] Assignment of "noisy" SiPMs

[Elton Smith]

Dear collaborators,

At the Cal Working group meeting on Tuesday, Orlando showed spectra of 
some SiPMs that had higher noise rate than typical. Yi checked that the 
sensor we were looking at corresponded to a high dark rate by Hamamatsu 
also, verifying the effect. These "noisy" units still fall within our 
specification, having noise rates of perhaps 20-30% higher than average. 
Note that once the sensors are irradiated, the dark rate due to 
irradiation will dominate for all sensors. USM is continuing to re-check 
the behavior of sensors that have been set aside based on initial 
measurements. There was some discussion about placing some of these 
units in regions of the detector where they would have the least impact.

There is now some urgency in making a decision regarding how to handle 
these "noisy" units. The assignment of sensors to a particular board is 
in process and we need to decide what to do with these units now so that 
the jumper maps for the electronic boards can be finalized.

I suggest the following:
1) We need a list of all noisy SiPM identified by USM, and the criteria 
for designating "noisy" SiPMs. This list will not be complete because 
not all sensors have been tested. We can compare this list with noisy 
SiPMs from Hamamatsu. If the USM list is verified, we can use the 
Hamamatsu data to complete our list.

2) I propose to assign the "noisy" SiPMs in the following way:
a) Place all noisy SiPMs in the outer layers (sum of 4)
b) Place all noisy SiPMs on the downstream boards
c) Do not place two noisy SiPMs in the same sum.

My reasoning is as follows: a) the outer layer does not have TDCs, so is 
not used as heavily as the other layers. Most of the energy is also 
deposited in the inner layers, especially for the hits at low angles, 
which constitute the majority of events. b) Use the downstream because 
they will register the highest light output and the noise rate will be 
relatively less important c) minimize the impact of noisy SiPMs by not 
having more than one contribute to a single readout channel.

Thus, with this e-mail I am first requesting that USM provide the most 
complete list of noisy channels they can at this time. Cross reference 
to the Hamamatsu measurements can be done by USM or JLab. Second, if 
anyone has any comments/suggestions/feedback to my proposal for 
assignment of the SiPMs, please pass them on to the mailing list.

Thanks, Elton.




-- 
Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave STE 16
Newport News, VA 23606
(757) 269-7625
(757) 269-6331 fax