96
H. J. Hilke and W. Riegler
Space resolution in drift chambers is influenced by the clustering in several ways.
The arrival time of the first n electrons, where n times gas amplification is the
threshold for the electronics, depends both on the spatial distribution of the clusters
and the cluster size. For large clusters, δ-electrons, ionization may extend far off the
trajectory.
4.2.1.3 Total Number of Ion Pairs
The detector response is related to the cluster statistics but also to the total ionization
n T , e.g., in energy measurements. A quantity W has been introduced to denote the
average energy lost by the ionizing particle for the creation of one ion pair:
W = E i /n E ,
(4.6)
where E i is the initial kinetic energy and n E the average total number of ion pairs
after full dissipation of E j .
Measurements of W by total absorption of low energy particles show that it is
practically independent of energy above a few keV for electrons and above a few
MeV for α-particles. For that reason the differential value w, defined by
w = x < dE/dx > / < n T >
(4.7)
may be used alternatively, as is usually done in Particle Physics, to relate the average
total number of ion pairs n T , created in the track segment of length x, to the average
energy lost by the ionizing particle. For relativistic particles, dE/dx can not be
obtained directly from the difference of initial and final energy (about 270 keV/m
for γ = 4 in Ar), as it is below the measurement resolution. Therefore, w has to be
extrapolated from measurements of lower energy particles. For the rare gases one
finds w/I = 1.7 − 1.8 and for common molecular gases w/I = 2.1 − 2.5, where
I is the ionization potential, indicating the significant fraction of dE/dx spent on
excitation. Values for photons and electrons are the same, also for α particles in rare
gases; in some organic vapours they may be up to 15% higher for α-particles. At
low energy, close to the ionization potential, W increases.
In gas mixtures, where an excitation level of component A is higher than I
of component B, excited molecules of A often produce a substantial increase in
ionization, as has e.g. been observed even with minute impurities in He and Ne:
adding 0.13% of Ar to He changed W from 41.3 to 29.7 eV per ion pair. This energy
transfer is called Jesse effect or Penning effect, if metastable states are involved. It
is also possible that more than one electron is ejected from a single atom, e.g., by
Auger effect following inner shell ionization.
The distribution of n T in small gas segments is very broad, see an example
in Fig. 4.2 [24]. To describe the measurement result, it is thus appropriate to
use the most probable value instead of the mean, since the mean of a small
number of measurements will depend strongly on some events from the long tail
H. J. Hilke and W. Riegler
Space resolution in drift chambers is influenced by the clustering in several ways.
The arrival time of the first n electrons, where n times gas amplification is the
threshold for the electronics, depends both on the spatial distribution of the clusters
and the cluster size. For large clusters, δ-electrons, ionization may extend far off the
trajectory.
4.2.1.3 Total Number of Ion Pairs
The detector response is related to the cluster statistics but also to the total ionization
n T , e.g., in energy measurements. A quantity W has been introduced to denote the
average energy lost by the ionizing particle for the creation of one ion pair:
W = E i /n E ,
(4.6)
where E i is the initial kinetic energy and n E the average total number of ion pairs
after full dissipation of E j .
Measurements of W by total absorption of low energy particles show that it is
practically independent of energy above a few keV for electrons and above a few
MeV for α-particles. For that reason the differential value w, defined by
w = x < dE/dx > / < n T >
(4.7)
may be used alternatively, as is usually done in Particle Physics, to relate the average
total number of ion pairs n T , created in the track segment of length x, to the average
energy lost by the ionizing particle. For relativistic particles, dE/dx can not be
obtained directly from the difference of initial and final energy (about 270 keV/m
for γ = 4 in Ar), as it is below the measurement resolution. Therefore, w has to be
extrapolated from measurements of lower energy particles. For the rare gases one
finds w/I = 1.7 − 1.8 and for common molecular gases w/I = 2.1 − 2.5, where
I is the ionization potential, indicating the significant fraction of dE/dx spent on
excitation. Values for photons and electrons are the same, also for α particles in rare
gases; in some organic vapours they may be up to 15% higher for α-particles. At
low energy, close to the ionization potential, W increases.
In gas mixtures, where an excitation level of component A is higher than I
of component B, excited molecules of A often produce a substantial increase in
ionization, as has e.g. been observed even with minute impurities in He and Ne:
adding 0.13% of Ar to He changed W from 41.3 to 29.7 eV per ion pair. This energy
transfer is called Jesse effect or Penning effect, if metastable states are involved. It
is also possible that more than one electron is ejected from a single atom, e.g., by
Auger effect following inner shell ionization.
The distribution of n T in small gas segments is very broad, see an example
in Fig. 4.2 [24]. To describe the measurement result, it is thus appropriate to
use the most probable value instead of the mean, since the mean of a small
number of measurements will depend strongly on some events from the long tail