Test in Naples on Scintillator Strips:

Time Resolution

In order to test the scintillator strips used in the 3 planes of the ToF system of Pamela, a telescope was built with 4 layers of streamer tubes ( Y coordinate ) each one equipped with pick-up strips ( X coordinate ).

The streamer tubes used are these of the MACRO exp. at Gran Sasso, so their section (3x3 cm2) is not optimized for our goal. In the X direction, pick-up strips of 0.5 cm pitch or less, obtained by OR-ing several wires of 64 conductors flat cable [1], provide a better space resolution, then the scintillators to be tested are placed along this axis, on the third plane of the telescope.

The trigger is provided by a couple of  30x 30cm2 paddles, on the top and at the bottom of the telescope.

Cosmic ray tracks are reconstructed in the Y-Z and  X-Z  views and the position in the test scintillator is extrapolated with a space resolution of about 1 cm (Fig.1 )

The X  position of the track in the scintillator (in the plots called 'xscin')  is compared  to the  half difference in time recorded by the 2 PMT's  = (T1 - T2)/2 as shown in ( Fig.2 ) . Taking into account light guide lengths, the light propagation speed in the scintillator is estimated to be about 19 cm/ns, in agreement with the value of n=1.58 provided by Bicron.

The time resolution obtained (without time walk corrections) is about 130 ps for S11,S12 as is possible to see in Fig.3   and is in the range 140-150 ps for S31 and S32. This is probably due to the presence of a couple of cookies (6 mm of silicon) behind the PMT's of S3 scintillators instead of only one (3 mm)  in the case of S1 scintillators.

Fig.4 shows the different response in charge of the same PMT with 3 mm silicon cookie or 6 mm. The mean value of the charge distribution with 6 mm silicon  is typically 15-20 % less than  in the other case. The silicon cookies are the solution adopted  after the vibration tests, in order to avoid a  too hard contact between light guides and PMT's.

The S3 and S1 scintillator layers are submitted  to vibration tests in the Galileo labs in Florence. Fig. 7 e Fig. 8 show for S31 that time resolution before and after vibrations is not modified. Results of vibration tests also suggested to give a different shape to light guide edges. Time resolution is not affected by the introduction of  this different coupling (Fig.9).

Each scintillator is characterized in charge and in time with a couple of PMT Fig.5. Charge distributions obtained on PMT 984 and 955 are well fitted with a Landau curve ( Fig.6 )

Notes: