218
C. W. Fabjan and D. Fournier
quantitative studies of the energy resolution of high purity Ge crystals, operated at
low temperature (77 K) for γ spectroscopy have been made. A rather comprehensive
discussion is given in [21]. After subtraction of the electronics noise, the width of the
higher energy lines (above 0.5 MeV) is narrower than calculated assuming statistical
independence of the created electron-hole pairs (~2.9 eV are needed to create such a
pair). The reason for this was first understood by Fano [22]. Fundamentally it is due
to the fact that the pairs created are not statistically independent, but are correlated
by the constraint that the total energy loss must precisely be equal to the energy
of the incident photon (in the limit of a device in which all energy losses lead to a
detected signal, in a proportional way, the line width vanishes).
Calling σ the rms of the energy ε used to create an electron-hole pair, the
actual resolution should be σ /(ε
√
Np), smaller than 1/
√
Np by a factor
√
F,
where F = (σ /ε) 2 is the Fano factor. Monte-Carlo simulations [23] reproduce the
phenomenon and give F ~ 0.1 for semiconductor devices, in reasonable agreement
with measurements [21].
When two energy loss mechanisms compete, e.g. ionization and scintillation,
the total energy constrain remains, but with a binomial sharing between the two
mechanisms. It is thus expected that summing up the two contributions, assumed to
be read out independently, will lead to an improved energy resolution (it should be
remembered however that a certain fraction of the energy lost in the medium goes
to heat.
This was first demonstrated with a liquid argon gridded cell exposed to La ions
with an energy of 1.2 GeV/nucleon traversing the cell [24]. In this set-up both
scintillation photons and electrons from electron-ion pairs were detected (see Sect.
6.3.3 for the collection mechanism). More recently, detailed studies of scintillation
and ionization yields were made in liquid xenon using 662-keV γ-rays from a 137 Cs
source [25]. With decreasing voltage applied to the sensitive liquid Xe volume,
the scintillation signal increases while the ionization one decreases, as expected
from recombination of electrons-ions giving rise to additional photons. The spectra
obtained with scintillation alone, ionization alone, and their sum are shown in Fig.
6.16, together with the correlation between the two signals.
The ratio between scintillation and ionization depends also on the nature and
energy of the particle making the deposit. Low energy nuclear recoils are highly
ionizing, giving rise to more recombination and thus an increased light over charge
ratio.
As discussed in Sect. 6.3.1, noble liquid detectors (using either argon or xenon)
have been developed in the last decade which allowed pushing the limits of dark
matter searches. They rely heavily on the existence of two correlated signals
(ionization and scintillation) for a given energy deposit, exploiting in particular
the ratio between the two to distinguish nuclear recoils from photon or muon
background (see Sect. 6.7.2).
When the energy loss per unit length becomes very high (i.e. for low values
of β and/or high values of the electric charge Ze for ions) saturation effects are
observed in liquid ionization detectors, and also in scintillators. Empirically, the
C. W. Fabjan and D. Fournier
quantitative studies of the energy resolution of high purity Ge crystals, operated at
low temperature (77 K) for γ spectroscopy have been made. A rather comprehensive
discussion is given in [21]. After subtraction of the electronics noise, the width of the
higher energy lines (above 0.5 MeV) is narrower than calculated assuming statistical
independence of the created electron-hole pairs (~2.9 eV are needed to create such a
pair). The reason for this was first understood by Fano [22]. Fundamentally it is due
to the fact that the pairs created are not statistically independent, but are correlated
by the constraint that the total energy loss must precisely be equal to the energy
of the incident photon (in the limit of a device in which all energy losses lead to a
detected signal, in a proportional way, the line width vanishes).
Calling σ the rms of the energy ε used to create an electron-hole pair, the
actual resolution should be σ /(ε
√
Np), smaller than 1/
√
Np by a factor
√
F,
where F = (σ /ε) 2 is the Fano factor. Monte-Carlo simulations [23] reproduce the
phenomenon and give F ~ 0.1 for semiconductor devices, in reasonable agreement
with measurements [21].
When two energy loss mechanisms compete, e.g. ionization and scintillation,
the total energy constrain remains, but with a binomial sharing between the two
mechanisms. It is thus expected that summing up the two contributions, assumed to
be read out independently, will lead to an improved energy resolution (it should be
remembered however that a certain fraction of the energy lost in the medium goes
to heat.
This was first demonstrated with a liquid argon gridded cell exposed to La ions
with an energy of 1.2 GeV/nucleon traversing the cell [24]. In this set-up both
scintillation photons and electrons from electron-ion pairs were detected (see Sect.
6.3.3 for the collection mechanism). More recently, detailed studies of scintillation
and ionization yields were made in liquid xenon using 662-keV γ-rays from a 137 Cs
source [25]. With decreasing voltage applied to the sensitive liquid Xe volume,
the scintillation signal increases while the ionization one decreases, as expected
from recombination of electrons-ions giving rise to additional photons. The spectra
obtained with scintillation alone, ionization alone, and their sum are shown in Fig.
6.16, together with the correlation between the two signals.
The ratio between scintillation and ionization depends also on the nature and
energy of the particle making the deposit. Low energy nuclear recoils are highly
ionizing, giving rise to more recombination and thus an increased light over charge
ratio.
As discussed in Sect. 6.3.1, noble liquid detectors (using either argon or xenon)
have been developed in the last decade which allowed pushing the limits of dark
matter searches. They rely heavily on the existence of two correlated signals
(ionization and scintillation) for a given energy deposit, exploiting in particular
the ratio between the two to distinguish nuclear recoils from photon or muon
background (see Sect. 6.7.2).
When the energy loss per unit length becomes very high (i.e. for low values
of β and/or high values of the electric charge Ze for ions) saturation effects are
observed in liquid ionization detectors, and also in scintillators. Empirically, the