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5.1.3 Practical Shielding Considerations
In most of the cases around high-energy hadron accelerator operation, the particle
cascades originate from beam interactions due to four basic source terms:
• beam–beam interactions (at and close to the experiments);
• beam–residual gas interactions (all along the accelerator);
• direct losses: beam cleaning at collimator locations or beam dump;
• spurious losses (random locations around the accelerator).
The emerging secondary particle cascade is then again defining a multi particletype and energy spectrum, interacting with the various materials around the
accelerator. Therefore, in order to reduce the radiation levels and the corresponding
fluences, shielding material can be employed to initiate and absorb showers. If
the shielding material is thick, the cascades continue until most of the charged
particles and photons have been absorbed, except for neutrons and secondary
photons.
The following aspects have to be considered in the shielding conceptual design:
5.1.3.1 Radiation Attenuation
The propagation of high-energy hadrons features an exponential decrease due to
nuclear reactions, while their energy loss, in case they are charged, is mostly due to
ionization. The latter accounts for the majority of the energy loss in electromagnetic
showers and for about 2/3 of the energy deposited in hadronic showers. Most of
the other 1/3 of the hadronic energy is carried away by neutrons. Being neutral
these are not affected by ionization losses, are decoupled from the rest of the
shower, are subject to elastic and inelastic collisions and are considerably more penetrating than the charged component. Hadronic showers above 100 MeV progress
through a variety of hadronic and nuclear processes that result in secondaries
that are predominantly pions (π + , π − ,π 0 ), followed in importance by nucleons
(protons, neutrons), strange mesons/baryons and photons. The π 0 component,
appearing whenever hadron energies are above the pion production threshold,
is particularly important, since it decays immediately to two photons producing
electromagnetic showers and shifting the shower energy from the hadronic to
the electromagnetic sector. Hadrons above few tens of MeV undergo nuclear
reactions that increase the neutron multiplicity. The spallation process breaks the
nucleus into a few large fragments. Additional neutrons may also be produced just
during this process, as well as by the subsequent evaporation from these excited
fragments.
Since the fragments have large mass (M) and total charge (Z) and ionize, they
will be stopped quickly in dense shield materials.
M. Brugger et al.
5.1.3 Practical Shielding Considerations
In most of the cases around high-energy hadron accelerator operation, the particle
cascades originate from beam interactions due to four basic source terms:
• beam–beam interactions (at and close to the experiments);
• beam–residual gas interactions (all along the accelerator);
• direct losses: beam cleaning at collimator locations or beam dump;
• spurious losses (random locations around the accelerator).
The emerging secondary particle cascade is then again defining a multi particletype and energy spectrum, interacting with the various materials around the
accelerator. Therefore, in order to reduce the radiation levels and the corresponding
fluences, shielding material can be employed to initiate and absorb showers. If
the shielding material is thick, the cascades continue until most of the charged
particles and photons have been absorbed, except for neutrons and secondary
photons.
The following aspects have to be considered in the shielding conceptual design:
5.1.3.1 Radiation Attenuation
The propagation of high-energy hadrons features an exponential decrease due to
nuclear reactions, while their energy loss, in case they are charged, is mostly due to
ionization. The latter accounts for the majority of the energy loss in electromagnetic
showers and for about 2/3 of the energy deposited in hadronic showers. Most of
the other 1/3 of the hadronic energy is carried away by neutrons. Being neutral
these are not affected by ionization losses, are decoupled from the rest of the
shower, are subject to elastic and inelastic collisions and are considerably more penetrating than the charged component. Hadronic showers above 100 MeV progress
through a variety of hadronic and nuclear processes that result in secondaries
that are predominantly pions (π + , π − ,π 0 ), followed in importance by nucleons
(protons, neutrons), strange mesons/baryons and photons. The π 0 component,
appearing whenever hadron energies are above the pion production threshold,
is particularly important, since it decays immediately to two photons producing
electromagnetic showers and shifting the shower energy from the hadronic to
the electromagnetic sector. Hadrons above few tens of MeV undergo nuclear
reactions that increase the neutron multiplicity. The spallation process breaks the
nucleus into a few large fragments. Additional neutrons may also be produced just
during this process, as well as by the subsequent evaporation from these excited
fragments.
Since the fragments have large mass (M) and total charge (Z) and ionize, they
will be stopped quickly in dense shield materials.