RADIATION DOSE MEASUREMENTS AND A STUDY OF DAMAGE TO ACCELERATOR COMPONENTS IN THE TRISTAN e+e- COLLIDER AT 27.5 GeV

The TRISTAN e+ecollider has been operated at a beam energy of 27.5 GeV, with a characteristic energy of the synchrotron radiation of 187 keV. The radiation in the TRISTAN tunnel was measured using them~oluminescence dosimeters. The dose distribution in the arc sections is influenced by the iron yoke and by gaps in the lead shields. The dose rate on the aluminum chamber surface is about IO6 Gy/Ah, where 1 Ah accumulates in about 10 days of operation. A 1Omm lead shield reduces the dose rate to 104 to 105 Gy/Ah. The dose rate at shielding gaps reaches 105 Gy/Ah. Lead shields of 5 mm thickness reduce the dose rate bv a factor of 20 to 80. Radiation damage to the aluminum vacuum system is mainly from HN03, produced from NOx. Connectors of distributed ion pumps and cables of beam position monitors were damaged by fluorine gas, resulting from the decomposition of Teflon insulators in cables and connectors. Reading and writing functions of 64 kbit RAM’s and EPROM’s were lost at 700 Gy. Operational amplifiers broke at 3~10~ Gy. Nitril-Butadiene rubber hoses for magnet cooling water were hardened by a radiation dose of 2x105 Gy. Magnet coils have survived 106 to IO7 Gy. When the beam energy was raised from 25 to 27.5 GeV, the dose rate doubled in the straight sections, while increasing five-fold at the center of the bending magnets. Therefore tight shielding will be required when the beam energy is raised as planned. The elecuon(e-)-positron colliding ring, TRISTAN (main ring. TMR) was operated first in October 1986 at a beam energy of 25 GeV. Since then the beam energy was gradually increased to 26.5 GeV(May 1987), 27 GeV (June 1987) 27.5 GeV (October 1987) and 28 GeV(January 1988). The characteristic energy of 187 keV for 27.5 GeV operation is the highest in the world. As the beam energy and current were increased, radiation damage of accelerator components became increasingly apparent. Therefore the radiation dose distribution was monitored and several methods were applied to control the damage. This paper describes dose distributions. some examples of radiation damage, and methods to control the damage and corrosion. 2 3 ‘IO’ z.5 2 6.10’ 7 6 ~10 p7y od of DOS Rate WremenE All doses were measured with thermoluminescence dosimeters (TLD: Be0 [UD-170LJ). All doses are quoted as absorded radiation in air. The dosimeters were encapsulated with plastic cases to avoid fading due to visible light, and were evenly spaced on strings spanned across the tunnel, and along various components. TLD’s were also mounted inside the lead shielding plates to study the attenuation of dose. Measurements were carried out at 27.5 GeV for both the electron mode and the electron-positron mode. To avoid saturation, the exposure time was limited to 120 seconds. The beam currents were also restricted, to 0.33 mA for electrons only, and to 0.5 mA for the electron -positron mode. The contribution of high energy electrons to the total dose can be neglected because the beam loss during the exposure is negligibly small, except near the beam stopper. All measurements are normalized to a dose of 1 Ah of circulating beam, either electrons or electrons plus positrons.