Positron annihilation versus electron cloud

1. Positron annihilation versus electron cloud Angel H. Angelov Institute for Nuclear Research and Nuclear Energy Bulgarian Academy of Sciences, Sofia “Audiatur et altera pars” NEC'2007, Varna, Bulgaria

2. Electron cloud formation In the accelerator vacuum chamber (beam-pipe) a number of primary electrons is created by a residual gas ionization (PRGE), by a photoelectron emission from the walls (PPE) or by other processes. The electric field generated by the charge of the bunched beam Nb accelerates these primary electrons and they hit the wall of the chamber. Some secondary electrons (SE) are produced. Their number depends on the secondary electron yield of the chamber wall surface (SEY). The secondary electrons are multiplied due to the same reason and this continues until the process saturation. The electron cloud parameters are also relevant to the chamber-beam geometry and to the parameters of the beam. NEC'2007, Varna, Bulgaria

3. Parameters of the EC electrons 1 • The important parameter for estimating the rate of the processes with electrons is their energy distribution (LHC example). • “I” points to the direction of the curve distortion when the secondary electron yield goes higher. • “II” arrow shows the elevation of the distribution curve when the number of the beam bunches increases. NEC'2007, Varna, Bulgaria

4. Parameters of the EC electrons 2 • The secondary electrons populate the very low energy zone. • 1 – primary electron energy • 2 – secondary electrons • Two different situations for the energy distribution of electrons: • 1 – beam pause (“seed” electrons) • 2 – beam on (electron acceleration) • Memory effect! NEC'2007, Varna, Bulgaria

5. Processes with positrons in the accelerator vacuum 1 • The annihilation is a process responsible for the 0.511 MeV gamma radiation from astrophysical objects. The density of the particles in these objects varies from that of the solid matter for the interstellar dust, via the plasma densities for the solar flares to the fairly rare gas for the interstellar media. The temperature of these objects also varies from 80 K for cold media to 10E+5 K for hot ones. • The accelerator vacuum has about 10E+15 m-3 equivalent hydrogen density and electron density from 10E+5 m-3 to 10E+13 m-3. These values coincide with the parameters of the astrophysical object known as H II region. • We shall consider only the processes with thermalized positrons because only 10% of positrons with MeV energies annihilate before their thermalization to eV energies via interactions.There is not a noticeable difference in interactions of positrons with hydrogen atoms and heavier ones and their ions either. NEC'2007, Varna, Bulgaria

6. Processes with positrons in the accelerator vacuum 2 • The thermally averaged annihilation rates per unit target density of the four main processes: • Curve 1 - direct annihilation rate on free electrons without Coulomb interaction. • Curve 2 - With Coulomb interaction • Curve 3 - radiative recombination with free electrons. • Curves 4 and 5 - interactions rates with hydrogen – for a direct annihilation on atom electrons and for a charge exchange with neutral H, respectively. NEC'2007, Varna, Bulgaria

7. Processes with positrons in the accelerator vacuum 3 NEC'2007, Varna, Bulgaria

8. Production of positrons1 • The isotope positrons sources can be of two origins, i.e. preliminary elaborated and embedded in the wall isotopes or created in the wall isotopes by the irradiation of the beam losses. • The examples for the first type are widely described. • The information about the possibility to produce positron emitting isotopes by beam losses can be found for example in the published notes on the LHC project. The average loss rate around the LHC ring is 1.65x10E+11 m-1 y-1 (due to beam-gas interactions only). For the 250 h beam and the general position in the ring this will produce two basic radiation components: - Highly Energetic Hadron Fluence on the inner surface of the pipe with value estimated about 10E+4 to 10E+5 cm-2 s-1. - 1 MeV Neutrons Equivalent Fluence with value 10E+5 to 10E+6 cm-2 s-1. • The LHC beam screen, irradiated by this radiation has on the inner surface 75 m-6 copper sheet. Some of the copper isotopes produced by experimental irradiation of LHC materials are listed in Table 1. NEC'2007, Varna, Bulgaria

9. Table 1 NEC'2007, Varna, Bulgaria

10. Production of positrons2 The positron production, concurrent to the electron production always exists in the vacuum chamber of LHC but the conditions to overcome EC are not favorable. This can be controlled by an especially designed metal alloy or compound for the inner surface of the pipe. Information about some useful elements is shown in table 2: NEC'2007, Varna, Bulgaria

11. Conclusion The positrons in the vacuum accelerator chamber are a possible tool to control the number of the low energy electrons. Experimental evidences for their influence on the EC formation can be obtained by positioning of a positron source near the electron cloud detector and then a comparison of the memory effect times should be made. NEC'2007, Varna, Bulgaria