246
C. W. Fabjan and D. Fournier
6.3.3 From Ionization to Electrical Signal in Dense Materials
One major avenue for calorimetry instrumentation is the measurement of the
ionization charge produced in dense, active materials. In the presence of an applied
electric field the charges move, inducing a current in readout electrodes proportional
to the liberated charge and hence to the energy deposited by the showering particle.
Electric charges are much easier to transport and to collect compared to light, which
is the basic, decisive advantage of this concept.
This technique was introduced in the early 1970s [87] using liquefied argon
as the active material. It has matured into one of the most widely used methods
for calorimetry instrumentation, in particular, of sampling calorimeters. Noble
liquid ionization calorimeters offer a number of attractive advantages, especially
for instruments in the difficult environment of colliders. They are characterized by
intrinsic stability and excellent uniformity of response (the only amplification is
in the electronics chain which is fairly easy to calibrate), relative ease of a high
segmentation and reasonable cost.
Other materials than argon are suitable for this method of detection, in particular
the heavier noble liquids (Kr, Xe). In liquid helium and liquid neon, electrons are
trapped in nano-scale cavities, and drift with characteristic speeds about a thousand
times slower than electrons in other noble liquids. Solid neon was found to be usable
at low rate [88]. Some saturated molecules like Tetramethylpentane (TMP), which
is a liquid at room temperature, have also been tried. High purity at the ppb-level,
required to avoid electron trapping, has limited their use compared to noble liquids,
which however require cryogenic operation. The properties of noble liquids for
ionization calorimetry are given in Table 6.3. Besides the value of dE/dx and X 0
specific to the material, important parameters are the mean energy needed to create
an electron-ion pair, the electron drift speed as a function of the electric field, and
the dielectric constant, which affects the capacitance of a readout cell. Since the
ions have a much smaller drift velocity compared to electrons, a track crossing a
gap (and depositing charge uniformly) will give rise to a triangular current (see Fig.
6.35) given by Eq. (6.30) where +Q 0 and –Q 0 are the liberated charges, d the gap,
and v the drift velocity of electrons. The resulting current is
I (t) = Qv/d
(6.30)
with Q = Q 0 (1 − vt/d). This formula is easily derived by remembering that a
point charge q at a distance x from one of the parallel planar electrodes defining the
gap of width d, induces a charge –q(d − x)/d on this electrode, and –xq/d on the
other one. 2
2 In case of test cells with a grid at an intermediate potential in between the two electrodes, all
charges of the grid-cathode region contribute with the same weight to the anode signal.
C. W. Fabjan and D. Fournier
6.3.3 From Ionization to Electrical Signal in Dense Materials
One major avenue for calorimetry instrumentation is the measurement of the
ionization charge produced in dense, active materials. In the presence of an applied
electric field the charges move, inducing a current in readout electrodes proportional
to the liberated charge and hence to the energy deposited by the showering particle.
Electric charges are much easier to transport and to collect compared to light, which
is the basic, decisive advantage of this concept.
This technique was introduced in the early 1970s [87] using liquefied argon
as the active material. It has matured into one of the most widely used methods
for calorimetry instrumentation, in particular, of sampling calorimeters. Noble
liquid ionization calorimeters offer a number of attractive advantages, especially
for instruments in the difficult environment of colliders. They are characterized by
intrinsic stability and excellent uniformity of response (the only amplification is
in the electronics chain which is fairly easy to calibrate), relative ease of a high
segmentation and reasonable cost.
Other materials than argon are suitable for this method of detection, in particular
the heavier noble liquids (Kr, Xe). In liquid helium and liquid neon, electrons are
trapped in nano-scale cavities, and drift with characteristic speeds about a thousand
times slower than electrons in other noble liquids. Solid neon was found to be usable
at low rate [88]. Some saturated molecules like Tetramethylpentane (TMP), which
is a liquid at room temperature, have also been tried. High purity at the ppb-level,
required to avoid electron trapping, has limited their use compared to noble liquids,
which however require cryogenic operation. The properties of noble liquids for
ionization calorimetry are given in Table 6.3. Besides the value of dE/dx and X 0
specific to the material, important parameters are the mean energy needed to create
an electron-ion pair, the electron drift speed as a function of the electric field, and
the dielectric constant, which affects the capacitance of a readout cell. Since the
ions have a much smaller drift velocity compared to electrons, a track crossing a
gap (and depositing charge uniformly) will give rise to a triangular current (see Fig.
6.35) given by Eq. (6.30) where +Q 0 and –Q 0 are the liberated charges, d the gap,
and v the drift velocity of electrons. The resulting current is
I (t) = Qv/d
(6.30)
with Q = Q 0 (1 − vt/d). This formula is easily derived by remembering that a
point charge q at a distance x from one of the parallel planar electrodes defining the
gap of width d, induces a charge –q(d − x)/d on this electrode, and –xq/d on the
other one. 2
2 In case of test cells with a grid at an intermediate potential in between the two electrodes, all
charges of the grid-cathode region contribute with the same weight to the anode signal.