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

28. Detectors at accelerators 255
or no detectable signal. The neutrons lose kinetic energy in elastic
collisions over hundreds of ns, gradually thermalize and are captured,
with the production of more γ-rays—usually outside the acceptance gate
of the electronics. Between endothermic spallation losses, nuclear recoils,
and late neutron capture, a significant fraction of the hadronic energy
(20%–35%, depending on the absorber and energy of the incident particle)
is invisible.
For h/e �= 1, whenh and e are the hadronic and electromagnetic
calorimeter responses, respectively, fluctuations in fem significantly
contribute to the resolution, in particular contributing a larger fraction of
the variance at high energies. Since the fem distribution has a tail on the
high side, the calorimeter response is non-Gaussian with a high-energy tail
if h/e < 1. Noncompensation (h/e �= 1) thus seriously degrades resolution
as well as producing a nonlinear response.
It is clearly desirable to compensate the response, i.e., to design
the calorimeter such that h/e = 1. This is possible only in a sampling
calorimeter, where several variables can be chosen or tuned:
1. Decrease the EM sensitivity. The absorber usually has higher 〈Z〉 than
does the sensor, the EM energy deposit rate, relative to minimum
ionization, is greater than this ratio in the sensor.
2. Increase the hadronic sensitivity. The abundant neutrons have a
large n-p scattering cross section, with the production of low-energy
scattered protons in hydrogenous sampling materials such as butanefilled
proportional counters or plastic scintillator. (When scattering off
a nucleus with mass number A, a neutron can at most lose 4/(1 + A) 2
of its kinetic energy.)
Motivated very much by the work of Brau, Gabriel, Brückmann, and
Wigmans [134], several groups built calorimeters which were very nearly
compensating. The degree of compensation was sensitive to the acceptance
gate width, and so could be somewhat tuned.
The average longitudinal distribution rises to a smooth peak, increasing
slowly with energy but about one nuclear interaction length (λI )into
the calorimeter. After several interaction lengths its fall is reasonably
exponential. It has been found that a gamma distribution fairly well
describes the longitudinal development of an EM shower, as discussed in
Sec. 27.5.
The transverse energy deposit is characterized by a central core
dominated by EM cascades, together with a wide “skirt” produced by
wide-angle hadronic interactions [139].
Further discussion and all references may be found in the full Review
of Particle Physics.The numbering of references and equations used here
corresponds to that version.