Abstract: An apparatus and method for using &agr;-particle energy loss to measure the thickness and stoichiometry of films grown by molecular beam epitaxy and other methods. The apparatus for measuring the thickness of films grown on a substrate in a growth chamber, comprises a protective housing having an aperture opening into the growth chamber, a solid state detector disposed in the protective housing, a shutter for opening and closing the aperture, a shield disposed in the housing between the aperture and the solid state detector for shielding the detector, and a calibration source disposed between the shield and the detector for calibrating the measurements made by the detector. A second calibration source disposed between the shutter and the shield, for measuring deposition on the shield.