291— Evaporating Radioactive Wire

We are developing an apparatus to rapidly evaporate extremely small quantities of radioactive materials, to be used as a source to test a new type of radiation detector. Evaporation takes place when the temperature is sufficiently high, which we achieve by delivering a sudden high current through a relatively thin wire. For our purposes, this must be done in a vacuum chamber both to prevent oxidation and to achieve a long mean free path of the evaporated material. This allows the collection of evaporated material without hindrance from the air molecules. Copper wires of different shapes and sizes are used, and high current is supplied through them using a car battery. As the temperature of the wire increases, the resistance of the wire changes. Monitoring current is therefore crucial to determine energy deposited in the wire. A calibrated Hall Sensor is used to measure the current. We are also implementing an optical pyrometer to measure the temperature directly. The temperature must increase rapidly in order to evaporate sufficient material before the wire melts and droplets get scattered violently. Using a high-speed camera to image the explosion helps to evaluate mass of evaporation, and also whether it is isotropic.