Abstract
The emission of electromagnetic radiation by relativistic bare heavy ions penetrating ordinary matter is investigated. Our main aim is to determine the bremsstrahlung which we define as the radiation emitted when the projectile does not break up. It pertains to collisions without nuclear contact ('ultraperipheral collisions'). Requirement of coherent action of the nucleons in order to keep the penetrating projectile intact limits bremsstrahlung to relatively soft photons. The spectrum shows a resonance structure with peak position near 2{gamma} times the position of the giant dipole resonance, that is, near 25{gamma} MeV for a lead ion ({gamma}{identical_to}E/Mc{sup 2} is the Lorentz factor of the projectile of energy E and mass M). The maximum exceeds the bremsstrahlung from a hypothetical structureless, pointlike particle of the same charge and mass as the incoming nucleus, but rapid depletion follows on the high-energy side of the peak. As a result of its relative softness, bremsstrahlung never dominates the energy-loss process for heavy ions. As to the emission of electromagnetic radiation in collisions with nuclear break-up, it appears modest when pertaining to incoherent action of the projectile nucleons in noncontact collisions. In collisions with nuclear contact, though, substantial radiation is emitted. It overshoots the bremsstrahlung. However, more » despite the violence of contact events, the associated photon emission only exceeds the radiation from a hypothetical structureless pointlike nucleus [emitted energy per unit photon-energy interval essentially constant up to ({gamma}-1)Mc{sup 2}] at relatively low photon energies (for lead roughly below 0.2{gamma} GeV, a limit which is about an order of magnitude above the position of the bremsstrahlung peak). Results are presented for bare lead ions penetrating a solid lead target at energies of 158 GeV/n ({gamma}=170) and beyond. « less
Highlights
Emission of bremsstrahlung is the primary cause of slowing down for relativistic electrons penetrating a substance at high energies, that is, at energies beyond, roughly, 800 MeV/(Zt + 1.2), where Zt is the atomic number of the target
Theoretical studies of energy-loss processes for relativistic bare heavy ions have pointed toward a similar dominance of radiative losses at energies far beyond the rest energy of the heavy projectile [1,2]
A rough estimate presented in Ref. [4] shows that as a result of strong moderation due to internal structure, bremsstrahlung never dominates the energy-loss process for a bare nucleus: For bare relativistic heavy ions, atomic excitation and ionization are the most important energy-loss processes up to a relatively high energy beyond which electron-positron pair creation becomes the major energy-loss channel
Summary
Emission of bremsstrahlung is the primary cause of slowing down for relativistic electrons penetrating a substance at high energies, that is, at energies beyond, roughly, 800 MeV/(Zt + 1.2), where Zt is the atomic number of the target. Theoretical studies of energy-loss processes for relativistic bare heavy ions have pointed toward a similar dominance of radiative losses at energies far beyond the rest energy of the heavy projectile [1,2] These studies turn out to overestimate the radiative losses substantially by neglecting the composite nature of the projectile [3,4]. We shall only put the label bremsstrahlung on the radiation in case the projectile remains intact throughout the interaction. This reflects the very meaning of the word: radiation causing moderation of the motion of the projectile (braking). To complete the picture of electromagnetic radiation by a relativistic atomic nucleus penetrating matter, we shall estimate the emission in such events as well as hint at additional photon sources for noncontact collisions
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