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

5.4 Fast Neutrons analysis 123
estimation of how many FN are in coincidence with an energy deposition in
the IV, i.e. the IVT e ciency ✏ IV T . The tagging e ciency is measured between
33 % and 23 %, depending on the multiplicity condition. The prompt
energy spectrum for the di↵erent IVT multiplicity condition is shown in
Fig. 5.11.
The minimal condition of 2 PMT hit is assumed as IVT condition in the following
analysis. The systematic uncertainties introduced by such condition
are discussed in Sec. 5.4.5.
5.4.2 IVT background
tel-00821629, version 1 - 11 May 2013
The correlation between ID and IV energy shown in Fig. 5.10 presents an
excess of events . 5 MeV in the ID and . 1 MeV in the IV. Such excess,
also visible in the prompt energy spectrum of Fig. 5.11, is understood in
term of backgrounds contaminating the IVT sample. Given the topology of
the IVT events, which require a three-fold coincidence between IV energy
deposition, ID prompt event and the following ID delayed, two categories of
background, with similar topologies, have been identified:
-events: generated by low energy from natural radioactivity. The
undergoes Compton scattering in the IV and in the ID where, in accidental
coincidence with a thermal neutron, it mimics the FN signature.
As the accidental background discussed in Sec. 4.3.1, the energy
distribution is expected to contaminate the IVT sample up to ⇠3 MeV
in the ID. Due to the rather poor light collection e ciency and energy
resolution of the IV, such background deposit energies . 1MeVinthe
IV. The expected event topology is schematised in box 2 of Fig. 5.12.
¯⌫ e + IVT from /n: generated by an accidental coincidence between an
¯⌫ e interacting in the ID and an IV from natural radioactivity or thermal
neutron capture on H. Since the ID energy deposition is produced
by a true ¯⌫ e events, the ID energy distribution shown its spectrum,
with a peak around ⇠3 MeV. As observed for the -background, the
energy distribution in the IV is limited . 1 MeV. The expected event
topology is schematised in box 3 of Fig. 5.12.
As summarised in Tab. 5.3, tagged FN events are expected to show two
kind of correlations, between prompt and delayed event in the ID but also
between the ID and the IV energy deposition. The time and the spatial
correlations between prompt and delayed events in the ID are the results
of a single particle generating both prompt and delayed events, in a similar
way as ¯⌫ e events. The correlation between IV and ID take place since both
signals are generated by the same parent muon. The time correlation between
ID and IV signals is expected in a short time window of few tens ns,
dominated by the particles time of flight between IV and ID.