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

Chapter 3
The Double Chooz
experiment
tel-00821629, version 1 - 11 May 2013
A nuclear reactor core is an abundant source of ¯⌫ e emitted by the -decays
occurring to the products of the nuclear fission. Since the energy released
during the -decays is smaller than ⇠ 10 MeV, no µ nor ⌧ are produced,
providing a pure flux of ¯⌫ e . Therefore, only disappearance experiments can
be performed using the nuclear reactors as source of neutrinos.
The probability for an ¯⌫ e to remain in an ¯⌫ e state, at a given distance L
from its production point is obtained from Eq. 2.18:
✓ m
P (¯⌫ e ! ¯⌫ e )=1 sin 2 (2✓ 13 )sin 2 2
1.27 13 (eV 2 ◆
)L(km)
(3.1)
E(GeV )
where m 2 13 ' m2 23 . The disappearance probability is shown in Fig. 3.1,
for two di↵erent values of m 2 23 . Given the reactor ¯⌫ e mean energy of
hE¯⌫e i'3 MeV and the mass splitting m 2 23 ' 2 ⇥ 10 3 eV 2 ,thefirstminimum
of the oscillation probability is found at a distance L ' 1100 m from
the ¯⌫ e source.
The ¯⌫ e are detected through inverse -decay reaction (IBD) in liquid scintillator:
¯⌫ e + p ! e + + n (3.2)
The IBD reaction has a kinematical energy threshold of 1.8 MeV and provide
a clear signal signature. The e + quickly looses its energy before it
annihilates with an electron of the scintillator, producing two of 511 keV.
The neutron could be captured on hydrogen, producing of 2.2 MeV. The
signal then consists in a coincidence between a prompt event, produced by
the positron with an energy related to the ¯⌫ e energy, and a delayed event,
produced by the neutron capture.
The signal signature is improved by doping the scintillator with gadolinium
( 155 Gd and 157 Gd) since it provides a better energy signature, with the emission
of ⇠ 8MeV rays, and speed up the neutron capture process thanks
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