The material sources, or effusions cells, are independently heated until the desired material flux is achieved. Changes in the temperature of a cell as small as 0.5ºC can lead to flux changes on the order of one percent. To control thicknesses to about 1%, highly stable control loops with Tungsten-Rhenium thermocouples and proportion, integral, and derivative (PID) controllers are used as well as a chilled panel separating the cells to prevent thermal crosstalk. Computer controlled shutters are positioned in front of each of the effusion cells to be able to shutter the flux reaching the sample within a fraction of a second. Since shutters in front of the effusion cells reflect some of the power back into the cell, opening the shutters is known to cause flux transients on the order of a few percent with a time constant of several minutes. These transients are difficult to measure and depend on cell and system-specific factors such as the shutter type and location, heater element design, and thermocouple placement. The sources are located approximately 10 cm from the sample in the growth position. In production systems, which are designed for larger wafers and greater uniformity, this distance is larger.