5 Interactions of Beams with Surroundings
191
5.1.3.2 Shielding of Electromagnetic Showers
As mentioned earlier, electromagnetic showers in the multi-GeV range develop
through successive bremsstrahlung and pair production. The particles involved
are photons, electrons and positrons that in dense materials have typical radiation
lengths in the centimeter range. As a result, high-energy electromagnetic showers
are halted by a typical concrete shielding wall having thickness of the order of 1 m or
by a tungsten layer of a few centimeters. It has to be noted that secondary neutrons
are also generated through photo-production, thus associated shielding issues have
to be correctly considered.
5.1.3.3 Shielding of Neutrons
The elastic scattering cross section of neutrons on nuclei is large at all energies and
has a role in attenuating them. The scattered neutron will lose energy on every elastic
collision, especially if the target nucleus is light and so carries away a larger fraction
of the energy as it recoils. Therefore, the presence of hydrogen is most effective in
reducing the neutron kinetic energy. Special care must be taken with respect to:
• neutrons streaming through ventilation channels or other penetrations, where
intermediate and low-energy neutrons readily scatter from the wall. Propagation
down the channel is thus possible via a series of ‘reflections’ even if the channel
is not in the original direction of the neutron;
• the thermalization of neutrons, in the presence of certain elements with high
capture cross-sections (e.g., Boron or Cobalt) that effectively clean the neutron
field but can induce relevant secondary effects (e.g., electronic damage or
activation issues).
Monte Carlo calculations aimed at shielding design optimization may require
dedicated biasing techniques to overcome the associated statistical convergence
challenges.
5.2 Lifetimes, Intensity and Luminosity
Particles which circulate on stable orbits in an accelerator within the geometrical
and energy acceptance can get lost as a result of collisions with other particles. This
leads to a decrease of the number of particles N circulating with time t.
The beam intensity lifetime τ is the inverse of the total loss rate dN/dt,
normalized to the actual intensity N (at the time t)
1
τ
= −
1
N
dN
dt
.
(5.1)
191
5.1.3.2 Shielding of Electromagnetic Showers
As mentioned earlier, electromagnetic showers in the multi-GeV range develop
through successive bremsstrahlung and pair production. The particles involved
are photons, electrons and positrons that in dense materials have typical radiation
lengths in the centimeter range. As a result, high-energy electromagnetic showers
are halted by a typical concrete shielding wall having thickness of the order of 1 m or
by a tungsten layer of a few centimeters. It has to be noted that secondary neutrons
are also generated through photo-production, thus associated shielding issues have
to be correctly considered.
5.1.3.3 Shielding of Neutrons
The elastic scattering cross section of neutrons on nuclei is large at all energies and
has a role in attenuating them. The scattered neutron will lose energy on every elastic
collision, especially if the target nucleus is light and so carries away a larger fraction
of the energy as it recoils. Therefore, the presence of hydrogen is most effective in
reducing the neutron kinetic energy. Special care must be taken with respect to:
• neutrons streaming through ventilation channels or other penetrations, where
intermediate and low-energy neutrons readily scatter from the wall. Propagation
down the channel is thus possible via a series of ‘reflections’ even if the channel
is not in the original direction of the neutron;
• the thermalization of neutrons, in the presence of certain elements with high
capture cross-sections (e.g., Boron or Cobalt) that effectively clean the neutron
field but can induce relevant secondary effects (e.g., electronic damage or
activation issues).
Monte Carlo calculations aimed at shielding design optimization may require
dedicated biasing techniques to overcome the associated statistical convergence
challenges.
5.2 Lifetimes, Intensity and Luminosity
Particles which circulate on stable orbits in an accelerator within the geometrical
and energy acceptance can get lost as a result of collisions with other particles. This
leads to a decrease of the number of particles N circulating with time t.
The beam intensity lifetime τ is the inverse of the total loss rate dN/dt,
normalized to the actual intensity N (at the time t)
1
τ
= −
1
N
dN
dt
.
(5.1)