Etude de bruit de fond induit par les muons dans l'expérience ...

tel-00724955, version 1 - 23 Aug 2012
3.1 Bolometers 53
The thermistors of the EDELWEISS standard bolometers are 7 mm 3 Neutron
Transmutation Doped (NTD) Germanium crystals glued on a sputtered gold pad on
the main Germanium crystal, see Figure 3.3b. These NTD sensors are sensitive to
the global temperature variations of the absorber, having no resolution power of the
time evolution of the phonon signal and thus without the possibility to determine
the position of the interaction, in contrast to the sensors described in Section 3.1.3.
The NTD sensors are polarised by individual constant currents I. In this manner
the rise of temperature in the absorber gives rise to a variation ∆R of the thermal
resistance and induces a voltage fluctuation ∆V , as shown in Figure 3.2 right, corresponding
to the heat signal:
∆V = ∆R · I (3.8)
For example, for the above mentioned temperature rise of ∆T ∼ 10 µK, the voltage
change is ∆V ∼ 1 µV.
Though increasing the applied voltage, and thus the electrical field, would in
principle improve charge collection for ionization, a moderate voltage, typically between
±3 V and ±9 V depending on the detector, is essential to limit additional
heating of the crystal. This effect is generally known as the Neganov-Luke-effect
[151]. It is analog to the Joule effect in metals. The charge carriers acquire energy
during their drift in the crystal and release this energy via phonons. The released
energy is proportional to the number of charge and to the applied voltage of polarisation:
ELuke = NIV = ER
V (3.9)
ɛ
The total measured energy Etot is then equal to the sum of ELuke and the recoil
energy ER, reduced by a potential heat quenching factor Q ′ in the case of a nuclear
recoil:
E γ
tot = ER + ER
V = ER 1 +
ɛγ
V
(3.10)
ɛγ
E n tot = Q ′ ER + ER
V = ER Q ′ + QV
(3.11)
Note that in Ge, as already mentioned, Q ′ ≈ 1, see Equation 3.17.
For any incident particle, the normalized heat energy in keV is:
ɛn
EH = Etot
1 + V ɛγ
Hence, the heat energy for an electronic recoil E γ
H and for a nuclear recoil En H is
E γ
H
1 + V
= ER
ɛγ
1 + V ɛγ
E n Q
H = ER
′ + QV ɛγ
1 + V ɛγ
And note that EH = ER still holds for γ-particles.
= ER
ɛγ
(3.12)
(3.13)
(3.14)
3