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The positron (e+) or antielectron is the antiparticle of the electron (e−). The positron has the same mass as an electron and opposite to the electron an electric charge of +1 e, where e is the elementary charge. A collision of a low-energetic positron and electron follows to annihilation of both colliding particles and as a result of it two or more photons are generated.
The positrons are emitted during radioactive decay of some radioactive isotopes as 11C, 40K and etc.. The short-lived isotopes generating positrons are used for medical imaging — positron emission tomography. The positron annihilation is used in materials research to detect variations in density, defects and displacements — positron annihilation spectroscopy.
The positron emission by isotopes is not able to generate enough positrons that are required for the experiments in high-energy physics (particle physics), where collisions of the high energetic electrons and positrons is used to create diverse subatomic particles to search, for instance, for new kinds of particles and where a high intensity source of positrons is needed. In such experiments, the positrons are generated by pair production from a sufficiently energetic photons (gamma rays or gammas) interacting with atoms in a material.
There are different generation methods of the high energy photons, that have been used in the past or are used in positron sources currently (SLC at SLAC, HERA and DORIS at DESY, e+-e− collider at KEK), or can be used for the future linear colliders.
The multi-MeV photons are generated via bremsstrahlung radiation of GeV electrons interacting with atomic nuclei, when the electron beam passes through a solid target. In the thick targets, these photons also interact with target atoms and generate e+-e−-pairs and more photons. The secondary photons having high enough energy also interact in target and create more particles. Thus, the avalanche of particles (e+, e−, γ and other) is developed. The positron source based on this method is oldest and well tested, therefore, it is usually named as the conventional source [1 Tsunehiko Omori et al., "A conventional positron source for International Linear Collider", Nucl. Instrum. Meth. A672 (2012) 52 ].
The low energy particles, that lost their energy in collisions and did not reached the end (exit side) of the target, are stopped and absorbed. The rest of energy of such particles is deposited in the target material as a heat. If the current of primary electron beam is high and/or the beam spot size on target small, the heat density induced by beam in the target can be too high and the solid target can be broken or melted. An other target issue is the material degradation under an irradiation. The beam and secondary particles interacting with target nuclei can kick them of their normal positions in the crystal lattice. Such permanently missing or extra nucleus in the lattice is a crystal defect and to characterized the level of material damage the units dpa (displacements per atom) is used.
The charged particles deflected by a transverse magnetic field emit the photons (synchrotron radiation). The hundreds-GeV electrons passing through an alternating periodic magnetic field of undulator (or wiggler) can generated needed for positron production multi-MeV photons. The electron beam at the end of undulator can be bended by a dipole magnet, thus, only the straight directed photons strike the target that has to be much thinner in comparison to the target of the conventional e+ source. Therefore, usually the energy deposited in the target of the undulator-based source and induced target damage significantly lower, if, of course, the photon spot size on target is not too small. This can be essential for the development of high current positron source for the future International Linear Collider (ILC) [2 Technical Design Report, 5 Volumes, 12 June 2013 ].
The bended trajectory of charged particle can be achieved not only by applying dipole magnetic field, but also with a precisely aligned mono-crystal: if the angle of the incident electron beam is very small in respect to the atomic string (or plane) in the crystal, the electrons will be attracted by the positive charge of nuclei in the string and, as a result, the beam can be caught and oscillate around the string generating a channeling radiation. The positrons are produced by that photons in the second thin amorphous target. Thus, the target of such source consist from the two parts — mono crystal and amorphous. Therefore, the source was named as the hybrid target source. Currently, this type of e+ source is included in a conceptual design of the Compact Linear Collider (CLIC) at CERN [3 CLIC Conceptual Design Report, 3 Volumes, 12 October 2012 ].
In the Compton-based e+ source, the scattering of laser photon by electron is used to transfer a part of electron energy to a photon (Compton backscattering). Several groups in France and Japan are involved in the Compton-based positron source R&D for future e+-e− colliders [4 Shuhei Miyoshi et al., "Photon generation by laser-Compton scattering at the KEK-ATF", Nucl. Instrum. Meth. A623 (2010) 576 , 5T. Akagi et al., "Production of gamma rays by pulsed laser beam Compton scattering off GeV-electrons using a non-planar four-mirror optical cavity", JINST 7 (2012) P01021, 6I. Chaikovska et al., "Polarized positron source with a Compton multiple interaction point line", Proceedings of IPAC2012, TUPPR012].
The list of publications summarizes my contribution together with contribution of other members of our group at DESY, University Hamburg et al. to the development of the ILC positron source.
[1] Tsunehiko Omori et al., "A conventional positron source for International Linear Collider", Nuclear Instruments and Methods in Physics Research Section A, Vol. 672, P. 52 (2012)
[2] Technical Design Report, 5 Volumes, 12 June 2013
[3] CLIC Conceptual Design Report, 3 Volumes, 12 October 2012
[4] Shuhei Miyoshi et al., "Photon generation by laser-Compton scattering at the KEK-ATF", Nucl. Instrum. Meth. A, Vol. 623, P. 576 (2010), arXiv:1002.3462
[5] T. Akagi et al., "Production of gamma rays by pulsed laser beam Compton scattering off GeV-electrons using a non-planar four-mirror optical cavity", JINST 7 (2012) P01021, arXiv:1111.5834
[6] I. Chaikovska et al., "Polarized positron source with a Compton multiple interaction point line", Proceedings of IPAC2012, New Orleans, Louisiana, USA, TUPPR012, arXiv:1405.2689v1