THERMALLY STIMULATED CURRENT SPECTROSCOPY - EXPERIMENTAL-TECHNIQUES FOR THE INVESTIGATION OF SILICON DETECTORS

Radiation damage of silicon detectors can be investigated by means of several techniques. In a high-luminosity environment the thermally stimulated current (TSC) spectroscopy PrOves to be a good experimental method in order to analyze the deep defects induced by irradiation. Two different experimental systems for TSC spectroscopy, developed at the Energy Engineering Department of Florence, are examined in detail. The two systems differ in the cryogenic agent used to cool the silicon detectors: the first one utilizes a helium-gas closed-cycle Gifford-McMahon two-stage cryogenerator; the second one utilizes the helium vapors present inside a liquid helium Dewar. In the two systems two different heating procedures are used in order to perform the thermal scans necessary to carry on the TSC measurements: in the former system an electric heater performs the thermal scan, while in the latter one the heating process is achieved utilizing the temperature variation of the helium vapors inside the Dewar as a function of the height over the surface of the boiling helium. The main advantage of the electric heater is the possibility of obtaining heating rates beta as high as 2.0 K/s, while the helium vapors ensure an excellent temperature matching between the temperature sensor and the sample (with the drawback of lower heating rates). The two methods used to get information by means of TSC spectroscopy, (variation of beta and delayed heating) are briefly described together with an evaluation of the sensitivity of the measurement (no difference has been made between the two systems because the use of a different cryogenic agent does not affect sensitivity). Both systems proved satisfactory, even if the ease of operation of the helium gas makes it preferable in the temperature range examined by our measurements (120-200 K).