sPHENIX HCal discussion

[Ron Belmont <ron.belmont AT colorado.edu>]

Hi All,

Greetings from FTBF.  Many thanks Mike L, Sean, and Edward, who all sent me private replies with very useful information.  The short version is: the SiPMs in the March shipment were S12572-015P, whereas previous shipments have been -025P.  The -015P have significantly lower gain (~2.3E5, compared to ~5.15E5 for the -025P), which explains both the reduction in light yield in our cosmics test presented about 2 weeks ago and the significantly increased difficulty in determining the single PE peak spacing.  On Friday we did some tests operating the -015P SiPMs with the manufacturer's specified operating voltage---this reduced the noise a bit, but we were already quite close to the correct voltage (which is around 68.3 for the -015P, we had been running at around 67.9).  We were still not able to find the single PE peaks.

A group of us at FNAL will be phoning into the HCal meeting on Tuesday, so I'll present a few slides on this along with additional results from our outer HCal tile testing.

Regards,

Ron

On Thu, Apr 14, 2016 at 7:06 PM, Ron Belmont <ron.belmont AT colorado.edu> wrote:

Hi All,

As discussed in the last HCal meeting, the light yields in the HCal tiles we received in March were lower than the light yields in previously tested tiles, the cause being theretofore unknown.  We have done some significant testing of the SiPMs that were sent to us, and that is the main part of the issue.  Attached as a pdf is a set of slides detailing these things.

Here's the short version:

When comparing light yields from the SiPMs received in March to those received this past October, the light yield is lower by at least a factor of 4.  This is for a fixed LED bias voltage, on a fixed position on a single outer HCal tile.  The LED bias, position, and tile all being held constant, the SiPM is the only thing changing.  The table shows that the five March SiPMs are consistent with each other, the three October SiPMs are consistent with each other, but the two groups are very different.  Furthermore the performance is sufficiently poor that it is not possible for us to extract single photoelectron peaks.

Since Brookhaven did not send any kind of documentation with these SiPMs in the March shipment, we don't know if we're operating them at the manufacturers recommend op voltage, and we don't even know if they're the same model as the previous shipments.  It would great if someone at Brookhaven could tell us what kind of SiPMs these are, if known, and what their recommend op voltages are, if known.  We think this will shed significant light on the matter, one way or another.

Regards,

Ron

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Ron Belmont
Postdoctoral Research Associate
University of Colorado, Boulder
ron.belmont AT colorado.edu
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Ron Belmont
Postdoctoral Research Associate
University of Colorado, Boulder
ron.belmont AT colorado.edu
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