Particle Physics Booklet - Particle Data Group - Lawrence Berkeley ...

254 28. Detectors at accelerators
In all cases there is a premium on small λI/ρ and X0/ρ (both with
units of length). These quantities are shown for Z>20 for the chemical
elements in Fig. 28.21.
These considerations are for sampling calorimeters consisting of metallic
absorber sandwiched or (threaded) with an active material which generates
signal. The active medium may be a scintillator, an ionizing noble liquid,
a gas chamber, a semiconductor, or a Cherenkov radiator.
There are also homogeneous calorimeters, in which the entire volume
is sensitive, i.e., contributes signal. Homogeneous calorimeters (so far
usually electromagnetic) may be built with inorganic heavy (high density,
high 〈Z〉) scintillating crystals, or non-scintillating Cherenkov radiators
such as lead glass and lead fluoride. Scintillation light and/or ionization
in noble liquids can be detected. Nuclear interaction lengths in inorganic
crystals range from 17.8 cm (LuAlO3) to 42.2 cm (NaI).
28.9.1. Electromagnetic calorimeters :
Revised October 2009 by R.-Y. Zhu (California Inst. of Technology).
The development of electromagnetic showers is discussed in the section
on “Passage of Particles Through Matter” (Sec. 27 of this Review).
The energy resolution σE/E of a calorimeter can be parametrized as
a/ √ E ⊕ b ⊕ c/E, where⊕represents addition in quadrature and E is in
GeV. The stochastic term a represents statistics-related fluctuations such
as intrinsic shower fluctuations, photoelectron statistics, dead material at
the front of the calorimeter, and sampling fluctuations. For a fixed number
of radiation lengths, the stochastic term a for a sampling calorimeter is
expected to be proportional to � t/f, wheretis plate thickness and f
is sampling fraction [123,124]. While a is at a few percent level for a
homogeneous calorimeter, it is typically 10% for sampling calorimeters.
The main contributions to the systematic, or constant, term b are
detector non-uniformity and calibration uncertainty. In the case of the
hadronic cascades discussed below, non-compensation also contributes to
the constant term. One additional contribution to the constant term for
calorimeters built for modern high-energy physics experiments, operated
in a high-beam intensity environment, is radiation damage of the active
medium. This can be minimized by developing radiation-hard active
media [47] and by frequent in situ calibration and monitoring [46,124].
28.9.2. Hadronic calorimeters : [1–5,124]
Written April 2008 by D. E. Groom (LBNL).
Most large hadron calorimeters are sampling calorimeters which are
parts of complicated 4π detectors at colliding beam facilities. Typically,
the basic structure is plates of absorber (Fe, Pb, Cu, or occasionally U or
W) alternating with plastic scintillators (plates, tiles, bars), liquid argon
(LAr), or gaseous detectors. The ionization is measured directly, as in LAr
calorimeters, or via scintillation light observed by photodetectors (usually
PMT’s). Waveshifting fibers are often used to solve difficult problems of
geometry and light collection uniformity. Silicon sensors are being studied
for ILC detectors; in this case e-h pairs are collected.
In an inelastic hadronic collision a significant fraction fem of the
energy is removed from further hadronic interaction by the production
of secondary π0 ’s and η’s, whose decay photons generate high-energy
electromagnetic (EM) cascades. Charged secondaries (π ± , p, ...)deposit
energy via ionization and excitation, but also interact with nuclei,
producing spallation protons and neutrons, evaporation neutrons, and
recoiling nuclei in highly excited states. The charged collision products
produce detectable ionization, as do the showering γ-rays from the prompt
de-excitation of highly excited nuclei. The recoiling nuclei generate little