Etude et impact du bruit de fond corrélé pour la mesure de l'angle ...

5.4 Fast Neutrons analysis 125
Correlation Fast Neutron -events ¯⌫ e +IV /n
Box 1 Box 2 Box 3
ID only time & space 3 7 3
ID - IV time & space 3 3 7
Table 5.3: Space and time correlation for IVT fast neutron and its backgrounds
tel-00821629, version 1 - 11 May 2013
On the other hand, -events and ¯⌫ e + IV /n events do not present
both kind of correlations. The -events shown correlation between the ID
prompt-like event and the IV event, because both are generated by the same
particle, but it lacks correlation between the prompt-like and the delayedlike
event in the ID. The ¯⌫ e +IV /n show correlation between prompt and
delayed events, since they are generated by a true ¯⌫ e , but it lacks the correlation
between the ID event and the IV event, since due to an accidental
coincidence.
Such di↵erent correlations are exploited in the following two sections to
quantify and reject the background contaminating the IVT sample.
background
In Fig. 5.13 the distance between prompt and delayed events for ¯⌫ e candidates
is shown together with the expected distribution from ¯⌫ e MC and the
contribution from accidental background. The ¯⌫ e candidates are correlated
up to ⇠ 1.5 m, while above such distance the distribution is dominated by
the accidental background. The FNs are expected to have prompt/delayed
spatial correlation similar to the ¯⌫ e events, while background are expected
to have the same distribution as the accidental background.
A comparison between the IVT sample and the contamination from
background is shown in Fig 5.14. The sample is obtained by performing
o↵-time window coincidence between [1010, 1100] µs. A total of 19 events
are found, corresponding to ⇠ 12 % of the IVT sample. The background
contaminates, as expected, the low energy part of the energy spectrum, up
to ⇠ 2 MeV on the ID and up to ⇠ 1 MeV on the IV. The FN sample shows
spatial correlation up to ⇠ 1mwhile background extends beyond 1 m.
The vertex distribution from background shows a distribution close to
the outer most region of the ID, as expected from originating from outside
the ID.
A cut on the distance between prompt and delayed candidate is applied to reduce
background contamination to the IV Tagged sample. The cut value
is set at 1.5 m, where the accidental component become dominant, as shown
in Fig. 5.13. Of the 19 events selected with the o↵-time search, 16 events