To monitor the stability of the LAC PMTs we have chosen radioactive light pulsers, YAP: Ce+241Am provided by SCIONIX (Holland). The geometrical dimensions of this crystals are 3.0 mm by 0.15 mm (outer diameter 4.0 mm by 1.0 mm thick) (Fig. 1).
They are hermetically sealed and
chemically stable, and yield a rate of 20± 4 counts/s. From Table 2 we see that the YAP
crystal has good emission intensity (40% to respect to NaI), it is not hygroscopic and has
a short decaytime (27 ns).Before installing them in the experimental apparatus, we
measured, at the Frascati National Laboratories, the resolution of the energy spectrum
given by one of these light pulsers mounted directly on the center of a 2" EMI 9954A
PMT window. Among 512 light pulsers, the s/peak ratio is distributed between 2% and 4%
with the peak around 3% (see Fig. 2).
The basic idea of our gain monitoring system is very simple. The 5.49 MeV alpha particle from 241Am causes scintillation in the YAP crystal and provides a continuous signal on the PMT (each phototube has is own light pulser) which is recorded through the same electronic chain used during regular data taking.
The light pulsers are coupled in air to the photo-multipliers. The 32x32 mm2 central region of the 2" bialkali-cathode sensitive area of the PMT is covered by the group of 8 light guides [11] so we installed the pulsers out of this region choosing the best peripheral position in terms of quantum efficiency (Fig 3).
We did not glue the pulser directly onto the
photo-cathode in order to have the possibility to increase and decrease the amount of
light on the sensitive surface. This is achieved simply changing the distance between
pulser crystal and the cathode. As already said above, the photo-multipliers are
positioned on the top of each LAC module in a light-tight aluminum box. In order to move
the pulsers from outside the box they have been mounted at the end of a steel cable that
slides inside a steel guide. So, screwing a cursor outside of the aluminum box moves the
steel cable allowing the distance between pulser and photo-multiplier to vary. This, in
turn, changes the light intensity on the photo-cathode according to the pulser position.