6.3.1 NEUTRON TDC SPECTRUM
Since the neutron counters were not part of the trigger only a fraction of
about 10-3 of the recorded events have a neutron counter signal. The
neutron TDC spectrum for such events is shown in Fig. 6.5. The events above TDC
channel 115 are neutrons from reaction 6.1, which have a velocity of
0.894 cm/ns. The prompt peak in the TDC channels 20-40 are photons from
shower backsplash in the CsI-array and in the tag counters, which was
identified as following: About 20 cm of lead placed in the flight path of the
neutrons between target and neutron counter enhanced the prompt peak over the
neutron peak. Since the probability of penetrating 20 cm of lead is very low
for photons, they must come from a source other than the degrader or the
target. Photons below 20 MeV can be detected by the neutron counter with a
probability of ca 10-20% . The time difference between the prompt peak and the
neutron peak agrees with the TOF difference for neutrons and photons at a
distance of 0.8 m from the target. The remaining events in Fig. 6.5 have a flat
time distribution and are accidental. In order to determine the energy of the
photon detected in the CsI-box, events which fulfill
119<= tn <=127, (6.9)
where tn denotes the TDC channel of the neutron, were used
for the analysis.
Figure
6.5:
TDC spectrum for all the events with a neutron signal. One TDC channel
corresponds to 0.89 ns. The peak around channel 25 arises from shower
backsplash in the CsI-crystals and in the tag counters. The events above
channel 115 are neutrons from
p -p-> p 0n: Events below channel 127
are dominated by unscattered neutrons; above channel 127 scattered neutrons
dominate the rate.