Sterile neutrino oscillations in MINOS and hadron production in pC collisions Page: 95 of 237
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4.4 The photomultiplier gains and the scintillator light emission
4.4 The photomultiplier gains and the scintillator light
As already explained, the overall detector response is a combination of the photomultiplier
gain, of the scintillation light emission and of the electronics. The effect of the electronics
can be considered negligible. Photomultiplier gains are measured through the light injection
system and a comparison with the overall detector drift can be made. Figure 4.9 shows
the percentage variation from the beam data taking period from May 2005 until June 2009
of the overall detector drift and the percentage variation of the photomultiplier gains, that
have been measured independently. While the overall detector drift is decreasing over time,
the gains are increasing due to ageing. Thus the scintillation light, that is not measured
by any other dedicated system, must be decreasing over time more than the gains are
increasing. As shown in figure 4.9 the photomultipliers gains are increasing by =2.5% a
year at the Near Detector and by =2% a year at the Far Detector.
In a very simple model, the scintillation light emission can be derived by the overall re-
sponse drift divided by the gains, as shown in figure 4.9. The behaviour at the Far Detector
is easier to interpret as the temperature variation is small, allowing simple interpretation
that the ageing is manifesting itself as a linear increase/decrease of the quantities. At the
Near Detector this easy interpretation is not valid as the temperature variation is quite
large and modifies the linear trend. The scintillator light emission seems to be decreasing
by e -3.5% a year at the Near Detector and by e -3% a year at the Far Detector. As the
scintillator light is decreasing more than the gains, the overall detector drift is decreasing.
4.5 Temperature dependences
Figures 4.10 and 4.11 show the detector responses over time and the relative hall temper-
ature measurement over time. It is possible to evaluate the drift response change due to
a temperature variation. As ageing does not affect the response on a scale of a few days,
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Tinti, Gemma Maria & U., /Oxford. Sterile neutrino oscillations in MINOS and hadron production in pC collisions, thesis or dissertation, July 1, 2010; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1015233/m1/95/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.