Introduction Ion-induced desorption is a severe luminosity limitation of low charge state heavy-ion accelerators. Studying the phenomena for almost 10 years using different target materials and various ion beams and energies, provides a rather clear picture of the physical processes involved in ion-induced desorption at room temperature [1]. Because of limited experimental data [2], the situation is less obvious for gas desorption of cryogenic surfaces. Due to the importance of cryogenic surfaces in future heavy ion accelerators, we investigated the desorption behaviour in combination with the surface coverage of cryogenic surfaces at different temperatures. For the irradiation experiment we used two disk-shaped targets, bare copper and gold-coated (7μm) copper with a nickel diffusion barrier (2μm). The targets were mounted on a commercial vacuum cold head including radiation shielding and temperature diagnostics. A resistive heater allowed the control of the target temperature by means of a software power control (PID). The cold head including the target was mounted at the LINAC-3 beamline at CERN [1,2]. Before cooling, the whole setup was UHV-baked to achieve low pressures. This procedure should help to keep the surface gas coverage of the target as low as possible. From single shot measurements the pumping speed of the setup was measured by the decay of the pressure bump from the beam pulse. This information is necessary, since the surface of the target and shielding contribute to the pumping speed. During ion irradiation of the targets, desorption yields were measured by the pressure-rise method with a Bayard-Aplert vacuum gauge and a residual gas analyzer (RGA), both calibrated. The desorption yields were derived by the ideal gas law as follows:

In particle therapy for cancer the dosimetry of the charged-particle radiation is mostly performed by measuring the ionization produced in air-filled ionization chambers. The conversion of the reading of an ionization chamber into absorbed dose requires W-values, which are defined as the average energy needed to produce an ion pair by a particle in a gas. Because of the increasing importance of ion therapy and the lack of experimental W-values for charged particles in air, new measurements of W-values are being carried out at PTB and GSI. Existing measurements for protons in air indicate an uncertainty of 4 % [1,2] and the main goal of the present studies is to achieve an accuracy of about 1 %. A new experimental set-up has been developed at PTB for the measurement of W-values in argon, nitrogen and air for protons and helium ions at energies from 0.5 to 3.5 MeV/u at PTB and for carbon ions between 3.6 and 7.0 MeV/u at GSI.

The ion-induced desorption experiment, installed in the CERN Heavy-Ion Accelerator LINAC 3, has been used to study the dynamic outgassing of cryogenic surfaces. Two different targets, bare and gold-coated copper, were bombarded under perpendicular impact with 4: 2 MeV/u Pb(54+) ions. Partial pressure rises of H(2), CH(4), CO, and CO(2) and effective desorption yields were measured at 300, 77, and 6.3 K using single shot and continuous ion bombardment techniques. We find that the heavy-ion-induced desorption yield is temperature dependent and investigate the influence of CO gas cryosorbed at 6.3 K. The gain in desorption yield reduction at cryogenic temperature vanishes after several monolayers of CO are cryosorbed on both targets. In this paper we describe the new cryogenic target assembly, the temperature-dependent pressure rise, desorption yield, and gas adsorption measurements.

For the fixed-target heavy-ion experiment CBM at FAIR, a low-mass silicon tracking detector system is being developed that can track at high rates the hundreds of charged particles that will be created when an intense beam of heavy nuclei interacts with the target [1]. The detector is based on silicon microstrip detector technology. Fast self-triggering front-end electronics will be located at the periphery of the tracking system. The distance between the detectors and the electronics will be bridged with thin long aluminium cables structured into microstrip lines and insulated with polyimide. In the CBM environment, the exposure to ionizing radiation is estimated to about 100-200 kGy in several years of operation. For reliable operation, radiation-induced changes of the bulk and/or surface conductivity of the dielectric material have to be excluded.

In this project, the creation and evolution of radiationinduced defects in semiconductors was studied by ionoluminescence. This technique allows in-situ studies of radiative recombination of electron-hole pairs produced during the irradiation with energetic ions. Damage formation by high energy ions is complex and includes different processes starting with initial defects, followed by diffusion, annihilation, and clustering of defects until finally stable defects are formed. At present, it is not well understood how the properties of intermediate defects influence the final damage state. Some of these defects have a very short lifetime and cannot be investigated after the irradiation. For example, Si-interstitials in p-type silicon are very mobile even at very low temperatures (4.2 K) [1]. Because they easily interact with impurities, it is very challenging to observe them as isolated defects. By exploiting the luminescent recombination of electron-hole pairs created during the initial damage process, we want to get insight into the electronic properties of transient defects. Ion-beam induced luminescence measurements were performed using a dedicated system attached to the M3 branch of the UNILAC. The first experiments were performed with several wide-band gap semiconductors such as ZnO and GaP, as it is much easier to optimise the equipment using visible light. In the future, we plan to extend our investigations to the near infrared range to investigate also semiconductors with smaller band-gaps (e.g. Si, GaAs). The investigated samples were vapour phase grown n-type ZnO single crystals (Helbig). They were mounted on a closed-cycle He-cryostat and irradiated with 4.8 MeV/amu Au ions at different sample temperatures (T= 300 K, 30 K). The samples were irradiated with a flux of approximately 5 10 ions cms. The induced ionoluminescence disappeared after ~ 15 min corresponding to a fluence of ~5 10 ions/cm. Ionoluminescence spectra measured within the first 15 min. are presented in Fig. 1 [2]. All spectra show a broad peak near the band edge of ZnO which are probably of excitonic origin. Additional photoluminescence (PL) investigations after the irradiation revealed pronounced changes in the spectrum, the most obvious being the complete disappearing of the green band between 450 and 600 nm (Fig. 2), which is related to copper impurities. In the samples grown at our Institute, the disappearance of the green band was accompanied by the creation of a new broad band centered at 700 nm (not shown). The responsible defect of this band is not known. Also in all investigated samples, the hydrogen content seems to be reduced in the implanted layer, as the H-related PL-peak is clearly reduced in intensity (not shown). We found that the ionoluminescence spectra are similar but not equal to the PL spectra of the samples. To find the energy levels of the transient defects we have to identify the origin of the peaks in the excitonic region to separate the transient defects from the stable defects.

The planned International Facility for Antiproton and Ion Research (FAIR) will consist of a superconducting double-ring synchrotron offering ion beams of intensity increased by a factor of 100-1000 compared to the existing GSI accelerators. Materials close to the beam tube will be exposed to secondary radiation of neutrons, protons, and heavier particles, possibly limiting reliable function and lifetime of device components. The present study investigates the radiation hardness of insulating components with focus on polyimide used as electrical insulation of the quench heaters of superconducting magnets. In this special application, polyimide has to provide (i) electrical insulation of the heater as well as (ii) heat transmission to the magnet to guarantee thermal exchange to the superconducting coils. In case radiation-induced degradation decreases the thermal conductivity, higher heating powers for a given temperature step would be required. We thus performed measurement of low temperature thermal conductivity of ion-irradiated polyimide.

1GSI, Darmstadt, Germany; 2CERN, Geneva, Switzerland Ion induced desorption is a serious intensity limitation for high current, low charge state heavy ion accelerators like SIS18 and SIS100. Whereas room temperature desorption was intensively investigated in the last years [1, 2 ], there is only one measurement for molecular desorption of a cryogenic surface in the relevant energy regime [3]. Since SIS100 is a cryogenic, super-conducting machine, desorption yield data for cryogenic surfaces in the relevan t temperature regime are required. Therefore two test stands where taken in operation at CERN and GSI in 2009. First GSI results will be presented.

During long-term operation of the new FAIR facility, parts of the superconducting magnets will be exposed to high radiation levels, cryogenic temperatures, and dynamic mechanical loads (Lorentzian forces during pulsed operation). Depending on the position of the different components, the radiation due to beam losses consists of a cocktail of gammas, neutrons, protons, and heavier particles [1]. Although the number of heavy fragments of the initial projectiles is small compared to neutrons, protons, or light fragments (e.g. α particles), their large energy deposition can induce extensive damage at rather low fluences (dose calculations show that the contribution of heavy ions to the total accumulated dose can reach 80% [2]). In the MeV to GeV energy regime, beam-induced radiation damage strongly depends on the specific sensitivity of the material and scales with fluence and electronic energy loss of the ions. In particular, organic polymers to be used e.g., as cable insulation for the superconducting FAIR magnets, may undergo severe degradation accompanied by outgassing of small volatile radiolysis products [3,4]. This study tackles the dielectric strength of polyimide (Kapton) as electrical insulation and G11-type epoxy/glassfiber composites as structural support material. Kapton foils of thickness 12, 25, and 50 μm were irradiated with 21 and 800 MeV protons (ITEP) and with various heavy ions of MeV-GeV energy (UNILAC, GSI). In addition, three types of 1-mm thick epoxy/glassfiber sheets were exposed to 180-MeV/u Xe ions (SIS, GSI). The irradiation experiments with protons and Xe ions took place in air, while the UNILAC irradiations were performed in vacuum. To test degradation of the insulating properties, breakdown voltage measurements were carried out using a current-limited 20-kV high voltage tester available at CERN. The ramping speed of the DC voltage was 1.3 kV/s. The location of breakdown events was inspected by means of optical microscopy and typically occurred inside the Rogowski-type stainless steal electrodes having a diameter of 10 mm. Any significant geometric influence on the electric field is therefore excluded. The tests took place in air, at room temperature, and at a humidity of 24-30%. No systematic errors due to temperature and/or humidity fluctuations were found. For Kapton, the measurements show an overall decrease of the breakdown voltage with increasing dose (Fig. 1). For light projectiles, such as protons and C ions of rather small electronic energy loss (dE/dx between 0.03 and 0.5 keV/nm), the decrease of the breakdown voltage becomes significant at doses above 1 MGy. In the case of heavy ions (dE/dx .> 16 keV/nm), the breakdown voltage changes at a much lower dose (note the semi-log presentation of Fig. 1). The expected maximum voltage in the superconducting coils of the FAIR magnets is about 3 kV. In the tested dose regime up to ~80 MGy, the degradation due to light ions is insignificant for the operation voltage. The situation is much more crucial for heavy ions, where already a dose of a few kGy results in a severe decrease of the breakdown voltage. At around 0.1 MGy, the values are close to the voltage requirement for the FAIR magnets. These results give a first indication that individual tracks completely passing through the Kapton insulation may represent a serious security risk for the insulation of the FAIR magnet coils.

Glass fibre reinforced plastic (GFRP) is considered as structure support and insulating material for the new superconducting FAIR magnets. GFRP (G11) is a composite material consisting of woven glass fibres (volume fraction ~50 %) and cured polymer resin (e.g. amine-cured epoxy). The material will be exposed to cryogenic temperatures, high radiation levels and dynamic mechanical loads (Lorentzian forces during pulsed operation). Long-term reliability under such extreme conditions is an issue of extreme importance. Although epoxy resin exhibits excellent mechanical and electrical properties, it is probably the weakest component of the GFRP composite because of its rather poor radiation hardness. This report investigates radiation damage in polyepoxy foils exposed to ions from the UNILAC (Xe and U ions of 11.1 MeV/u, range ~145 μm, fluence 10 10 – 4×10 11

In the new superconducting FAIR magnets, polyimide films (Kapton) will be used as insulator. During operation, the material is exposed to high-dose radiation and it has to withstand enormous mechanical forces induced by the magnet field ramping. For better lifetime estimation, this study investigates radiation-induced degradation of the mechanical properties of Kapton by tensile strength measurements. Foils of 25-μm thick Kapton were irradiated with Ti, Sm, and Au ions of 11.1 MeV/u at a fluence between 1x10 10 and 5x10 12 ions/cm 2 . The tensile strength and Young modulus of the samples were measured applying strain on a Zwick tensile tester (model Z020TH) at the Deutsches Kunststoff Institut, Darmstadt. To optimize the irradiation, the samples had a size of 15×30 mm 2 which differs from the DIN norm.