an epitaxial growth reactor for processing wafers of a semiconductor material by exposing it to a reactive gas flow . a wall positioned at a slight distance from the wafer or group of wafers which is exposed to the reactive gas in a double wall , with a very narrow space between the two walls , and this space is filled with a mixture whose composition can be varied and , consequently , the thermal conductivity can be adjusted . the mixture used is a hydrogen / argon mixture , the proportion of each of these gasses in the mixture being adjustable . the interior wall of the double wall is a quartz plate and the exterior wall is made of metal . relevant fig . : 1 .