Do only electrons emit a photon when accelerated in EM field? Do protons, for example, radiate?
Synchrotron radiation is a form of photon emission by relativistic charged particles being accelerated (deflected) in a transverse magnetic field. One of the unresolved problems in the proposed SSC (Superconducting Super Collider) project near Dallas, Texas was soft (ultra violet) synchroton radiation from highly relativistic 15 TeV protons desorbing residual gas molecules from the vacuum chamber walls (at 2 kelvin), and causing an unacceptable rise in the residual gas pressure.
Here is a paragraph from the CERN LHC Design Book on proton synchrotron radiation]
12.2 BEAM VACUUM REQUIREMENTS
The LHC presents several original requirements with respect to classical vacuum systems. It has to provide adequate beam lifetime in a cryogenic system, where heat input to the 1.9 K helium circuit must be minimised and where significant quantities of gas can be condensed on the vacuum chamber. The following four main heat sources have been identified and quantified at nominal intensity and energy:
• Synchrotron light radiated by the high energy circulating proton beams (0.2 W m-1 per beam, with a
critical energy of about 44 eV);
• Energy loss by nuclear scattering (30 mW m-1 per beam);
• Image currents (0.2 W m-1 per beam);
• Energy dissipated during the development of electrons clouds, which will form when the surfaces seen by the beams have a secondary electron yield which is too high.
Reducing the heat input to the cryogenic system introduces constraints on the design (e.g. the necessity of a beam screen), on the materials (e.g. the introduction of a copper layer) and on the gas density to be achieved in the LHC vacuum system. In addition, other more classical constraints are set by the lifetime, the stability of the beams, which in turn sets the acceptable longitudinal and transverse impedance [3, 4] and locally by the background conditions in the interaction regions. The vacuum lifetime is dominated by the nuclear scattering of protons on the residual gas. The cross sections for such an interaction at 7 TeV vary with the gas species [5, 6] and are given in Tab. 12.1, together with the gas density and pressure (at 5 K) compatible with the requested 100 hour lifetime. This number ensures that the contribution of beam-gas collisions to the decay of the beam intensity is small as compared to other loss mechanisms; it also reduces the energy lost by scattered protons in the cryomagnets to below the nominal value of 30 mW m-1 per beam.
The previous post discusses non-nuclear proton bremsstrahlung in magnetic fields (synchrotron radiation), where there are no strong interactions. The geometric "cross section" of a proton is about 40 millibarns (40 x 10-27 cm2)*. For electrons producing bremsstrahlung on striking a proton, the total "cross section" is about 3 millibarns (3 x 10-27 cm2) per nucleus for a gamma=2. For a proton hitting a proton and producing bremsstrahlung at gamma = 2, the total cross section is about 18372 times smaller, or about a nanobarn. Very roughly, at gamma=2, the cross section for a proton collision producing one or more pions is about 50 millibarns. So observing proton-nuclear bremsstrahlung in the background of strong interaction debris is extremely difficult. But proton-proton bremsstrahlung has been observered at 100-MeV or lower (below pion production threshold) is reported in the literature (mostly pay per view).
* 1 Barn = 1 x 10-24 cm2. If you had a hypothetical mole of individual hydrogen atoms in a 1-cm x 1 cm x 1 cm box, the percentage of area covered by hydrogen atoms with a 40 geometric millibarn cross section is about 2.4%.
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