Irradiation chambers and on-line controls/ measurements
Vacuum chambers for irradiation and ion beam analysis :
The triple beam chamber receives one beam line coming from each of the accelerators, allowing single, dual or triple beam irradiations. Each one of the beam lines, converging towards the triple beam chamber with an incidence angle of 15 °, is equipped with a raster scanner unit able to spread the beam homogeneously over an area 2 cm in diameter on the sample surface.
Each beam line can be equipped with an energy degrader in order to produce a roughly homogeneous irradiation damage and implantation(s) along the whole ion range. The degraders are constituted by rotating wheels mounted with suitable thin foils, typically 25 to 250 µg.cm-2 carbon foils or aluminum from 0.8 to 5 µm thick. Degraders induce ion loss due to divergence after scattering in the foils and straggling in the thickest foils. These phenomena have to be taken into account for the calculation of the implantation profiles. Self-ion effect due to the superimposition of the damage region with implantation is taken into account by selecting an appropriate observation depth in the sample.
The sample holder mounted on the rear part of the chamber covers the temperature range from liquid nitrogen to 800°C. On the top of the chamber, a pneumatic system holds three groups of seven Faraday cups designed to control the beam intensity.
A second vacuum chamber is linked to Épiméthée. It can be used for single beam irradiation or ion beam analysis. A multi-pin Faraday cups device and a heating/cooling stage sample holder are also available in this chamber. In this chamber, samples are irradiated at normal incidence. This layout allows connecting a user-designed chamber behind the single beam chamber and performing in situ diagnostics such as mechanical test or mass spectrometry analysis.
A third vacuum chamber is implemented on Pandore. It is an ion beam analysis chamber equipped with two X-ray detectors, a high purity germanium detector for gamma-ray detection and two surface barrier detectors (100 and 1500 µm) usable for Rutherford backscattering (RBS), elastic recoil detection (ERDA) and nuclear reaction analysis (NRA) measurements. This chamber, currently under revamping, is used for irradiated and non-irradiated sample characterization.
On-line current and dose measurement :
Current integration provides an accurate measurement of implanted species concentration. During irradiation, current is intermittently measured (typically for 30 seconds, every 10 or 20 minutes) using a mobile multi-pin Faraday cups device. This approach is satisfactory with stable beams which is the case with the recent NEC accelerators. Beam profiler monitoring and dose post-controls performed by IBA confirm the validity of this approach. In the triple beam chamber, the device is composed of three groups of seven Faraday mini-cups for spatial uniformity control of each beam. Currents, numbers of incident particles and accumulated doses are regularly monitored during irradiation and the beam intensities can be adjusted at any time in order to attain the requested dose for each ion in the same irradiation time.
Continuous temperature measurement :
A heating-cooling sample stage has been developed to provide precise temperature control from Liquid Nitrogen temperature to 800°C. Low temperature irradiation/implantations (at -172 °C and from 1°C to 20 °C) are managed by flowing LN in the sample stage. Higher temperature (from 20 °C to 800°C) is obtained by heating the sample stage. One to five thermocouples are used to measure the sample temperature and another thermocouple (type K) is used for temperature regulation via PID control. A 2D infrared thermal imaging camera records the sample surface temperature during irradiation. At higher temperature, a bi-chromatic pyrometer can be implemented. Robust coupling of the samples to the sample holder and of the sample holder to the stage is important to achieve homogenous temperature. In most cases, sample holders are custom-built for each experiment.
Another sample stage made of copper is used to cool samples down to liquid nitrogen temperature under higher beam intensities.
Additionnal chambers :
Users of the facility can adapt appropriate chambers for specific applications. Three such user-designed chambers have been used up to now for in situ tensile tests on a fiber and for on line gas analysis during irradiation experiments.