The beam tests at TRIUMF were conducted in the M11 area in the meson
hall, using pions with 100 MeV incident kinetic energy (195 MeV/c
momentum). The M11 beam energy of 100 MeV corresponds to pions
with a stopping power
20% above that of minimum ionizing
particles. Scans of the magnetic field for the two dipole (bender)
magnets of M11 and for two of the quadrupoles were carried out, in
order to focus of the beam on the trigger/finger counter. The jaws
and slits were set accordingly to yield a coincidence rate (see below)
of
20 KHz. The cross-sectional area of the beam was
2
2 cm
, and proton contamination in the beam was
2%. Pions, muons and electrons were present in the RF
distribution (see Figure 6). The extraction of
the attenuation length for the fibers tested was not dependent on the
particle species, and therefore the entire RF spectrum was used.
As in the cosmic ray case, the same PMT's were used to test all fiber bundles in all configurations. The event trigger was chosen to be a coicidence between the RF signal coming from the cyclotron, the paddle counter (PR), and the finger counter (TR), as shown in Figure 5. Constant fraction discriminators (TENELEC TC455) were used to produce the logic timing signals, thus minimizing time/amplitude correlation. ADC and TDC spectra were recorded for all counters in Figure 5 as was the RF timing spectrum.
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Software cuts placed on the data to ensure good TDC values for all four PMT's, as well as ADC values above pedestal value for the fibers. Specifically, generous cuts were placed around the TDC peaks of FL, FR, PR and TR, and ADC cuts were placed on FL and FR to include the spectra above channel 25, thereby excluding only the pedestals. Representative TDC and ADC plots are shown in Figure 7. The TDC plots have cuts imposed which exclude the timeout peak at channel 4096. The ADCs have the TDC cuts, and are shown with and without pedestal cuts. The ADC lower limit was set at channel 25. Changing this to channel 50 did affected the geometric mean of the fiber PMT's but not the Left/Right ratio from which the attenuation length is extracted.
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The total length of the fiber bundles was 300 cm. However,
due to spatial limitations in the experimental area, measurements were
taken with beam intercepting the fiber bundle at distances from
40 cm to
260 cm. Several measurements were carried
out.
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