sPHENIX HCal discussion

[Craig Woody <woody AT bnl.gov>]

Hi All,
  Following yesterday's discussion about mapping the tiles, I thought I would add a little information and point out a few things to watch out for when using a Sr90 source to measure scintillators. The first thing to remember is that the electrons coming from the source are produced from beta decay and have a broad spectrum. While the end point of the spectrum is 2.28 MeV, most of the electrons are much lower energy than this. The most energetic electrons are actually the result of Y90 decay which is part of the decay sequence. I've attached a few slides which give the decay scheme and energy spectra of the electrons. The average energy is less than 1 MeV, and these range out pretty quickly in plastic scintillator. I've also included some plots of the range-energy and dE/dx curves for electrons in plastic. In general, the electrons from the source will range out in a few millimeters. Generally this is ok if you're just interested in depositing energy in the scintillator, but if you're trying to study light collection, especially with possible strong surface effects, most of the energy is going to be deposited near the surface and it may give some misleading results.
  The other thing we heard about was the trigger bias. In general, as we all know, it's always better to have your trigger be independent from the thing you are measuring. However, in this case, there seemed to be an additional problem in that the trigger was very close to the noise. If the light yield is indeed only ~ 10 pixels per MeV, and the average energy is perhaps 0.5 MeV, that means you're trying to trigger on  5 pixels firing, which is very close to the single photoelectron noise (and remember, this spectrum extends to several photoelectrons due to cross talk and afterpulsing). This could make getting an unbiased trigger very tricky at best. A better way to trigger would be to use an independent trigger counter, which has to be very thin (~ 0.5 mm) to allow the electrons to pass through, which is something we've often used in the past with Sr90. Another way would be to couple a light guide on one long edge of the tile (giving up the reflectivity of one edge shouldn't change the light collection significantly) and trigger on the direct light in the scintillator with a PMT. This should give a large signal and could also tell you about any position dependence of where the light is produced without introducing any factors due to the WLS fibers. I also agree with Eric's suggestion that measuring the tiles with cosmic rays should be able to determine whether there is any inherent asymmetry in the tile from one end to the other. At least you could be sure then that light is being produced uniformly throughout the thickness of the scintillator.
  Anyhow, I thought I would offer this after hearing yesterday's discussion. My personal feeling from seeing the data was that there was some asymmetry in the light collection within the tile which was over and above any of the effects we've been talking about. However, it certainly needs further investigation.

Cheers,
Craig

On 6/25/2015 1:16 PM, John Lajoie wrote:

Dear sPHENIX HCAL Folks:

    This coming Monday, June 29th, at 4PM BNL time we will kick off the first bi-weekly HCAL meeting. This meeting will alternate weeks with the EMCal meeting chaired by Anne, and is intended to be the forum for the discussion of all things HCAL.  Any and all interested parties are welcome.

    This week we'll start off with status reports from the tile testing at Colorado and BNL.  If anyone would like to add a contribution, by all means contact me and we will work you into the agenda.

    An Indico agenda with BlueJeans coordinates can be found at:

https://indico.bnl.gov/conferenceDisplay.py?confId=1251

Regards,
John Lajoie

--

John Lajoie

PHENIX Deputy Spokesperson

Professor of Physics

Iowa State University

 

(515) 294-6952

lajoie AT iastate.edu

Contact me: john.lajoie

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