UBC Theses and Dissertations
A low gain fine mesh photomultiplier tube for pure CsI Fujimoto, Derek Jun
The increased luminosity of the upgraded SuperKEKB accelerator in turn mandates an upgrade to the Belle detector. One proposed upgrade is to exchange the existing thallium doped cesium iodide scintillation crystals (CsI(Tl)) in the endcap calorimeter with pure cesium iodide (CsI). One advantage of pure CsI is its shorter decay time constant. This would reduce the amount of time taken to process each event, which in turn reduces the chance of simultaneously measuring the energy of different two particles (pileup). Hamamatsu Photonics has produced the R11283 photomultiplier tube with a nominal average gain of 255 ± 11, ideal for measuring the light produced by scintillation in pure CsI while in a magnetic field. A prototype array of 16 photomultiplier tubes was built and tested at TRIUMF. This work documents the characterization of the photomultiplier tube as well as University of Montreal’s pre-amplification and shaper electronics. The primary results can be split into four distinct measurements: the electronic noise, the short term stability, the excess noise factor, and the lifetime. The electronic noise was initially measured with cosmic rays and was found to be (77 ± 2) keV using a Belle II pure CsI crystal. The short term stability was measured with a set of calibration sources, and the variation over a week was (0.28 ± 0.03)% after temperature corrections. The excess noise factor was found to be (1.9 ± 0.1 ± 0.4) using a pulsed UV laser. This result was accompanied by an additional electronic noise measurement of 1730 ± 33 electrons at the anode. The lifetime was found using a UV LED array and a ²⁰⁷Bi source, with the gain × quantum efficiency reduced to (93 ± 3)% after about 48 days of aging in real time. This was equivalent to 70 years of standard Belle II operation with 7 C having passed through the anode. There were several sets of aging behaviours observed, with some evidence that the anode charge is not the sole factor in aging.
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