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Measurements and Results

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 $\sim$100 MeV corresponds to pions with a stopping power $\sim$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 $\approx$20 KHz. The cross-sectional area of the beam was $\sim$2$\times$2 cm$^2$, and proton contamination in the beam was $\sim$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.

Figure 6: Top panel: RF spectrum from one of the Pol.Hi.Tech. multi-clad runs, with the pion beam striking 100 cm from PMT1. Middle panel: the TOF spectrum between counters PF and PD. These are separated by 413 cm. Bottom panel: RF versus TOF.
\includegraphics [height=110.0mm,bbllx=130pt,bblly=150pt,bburx=485pt,bbury=700pt,
angle=0,clip=]{particle_id1.ps} \includegraphics [height=110.0mm,bbllx=100pt,bblly=350pt,bburx=515pt,bbury=700pt,
angle=0,clip=]{particle_id2.ps}

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.

Figure 7: Top panels: Fiber Left and Right TDC spectra, with TDC cuts applied to exclude the TDC timeouts. Bottom panels: Fiber Left and Right ADC spectra. The solid line shows the spectrum with only the TDC cuts applied, and the dashed line has ADC cuts as well, to eliminate the pedestals.
\includegraphics [height=110.0mm,bbllx=130pt,bblly=150pt,bburx=485pt,bbury=700pt,
angle=0,clip=]{tdcadc.ps}

The total length of the fiber bundles was $\approx$300 cm. However, due to spatial limitations in the experimental area, measurements were taken with beam intercepting the fiber bundle at distances from $\approx$40 cm to $\approx$260 cm. Several measurements were carried out.

  1. Group-1: Scans of all fiber bundles were performed, in 10 cm increments, measured from the beam left end of the bundle. These measurements were used to extract the attenuation length and timing resolution of the different fibers.
  2. Group-2: Data were taken at two different configurations of the Pol.Hi.Tech. multi-clad fiber bundle with respect to the beam line, shown in Figure 8. This was done to ensure that the relative orientation of the fiber stack with respect to the beam did not affect the results of the group-1 measurements. The latter were taken in the vertical orientation.

    Figure 8: Two different configurations of the Pol.Hi.Tech. fiber bundle 5/4 stack. Left, the one used for the attenuation length and timing resolution measurements presented below (group-1). Right, the configuration rotated by 90$^0$ to test the validity of our results (group-2).


    \includegraphics [height=60mm,angle=0,clip=]{stack.eps}

  3. Group-3: The Pol.Hi.Tech. multi-clad fiber bundle was scanned in front of the beam in the vertical direction from 135.5 cm to 137.5 cm from the floor, with the beam striking the midpoint of the bundle. Again, this was aimed at cross-checking the dependence on the fiber bundle orientation. The nominal beam height was measured to be 136.5 cm.
  4. Group-4: Again, the Pol.Hi.Tech. multi-clad fiber bundle was used to perform a forward/reverse measurement, where the same point along the fibers was measured with the fiber facing forward or backward. This was done to ensure that differences in the stray magnetic field from the Cyclotron did not affect one fiber PMT differently than the other.
  5. Group-5: The two Kuraray fibers were placed on top of each other in the vertical position, to further investigate the dependence of the results on fiber thickness traversed.



Subsections
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Next: ADC Information Up: TRIUMF Tests Previous: Setup
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2001-10-29