eliminate the necessity to move sample and instrument components during data collection and consequently the inventive apparatus does not need to provide data scan motion or to change tilt angles during data collection therefore the gonistat used by laboratory systems and many existing portable systems to provide the data scan and to set tilt angles can be eliminated reducing the weight and size of the apparatus one embodiment of the inventive handheld diffractometer is particularly useful in on site stress measurements fig3 shows a typical pattern 300 of diffracted x rays from a polycrystalline powder sample 304 the diffraction pattern 300 from such a sample 304 forms a series of diffraction cones 306 314 if a large number of crystals are oriented randomly in the space covered by the incident x ray beam 302 each diffraction cone such as cone 306 corresponds to paths followed by diffracted x rays that are diffracted from the same family of crystalline planes in all the participating crystals polycrystalline materials can be single phase or multi phase solids stress measurement with two dimensional x ray detectors is based on a fundamental relationship between the stress tensor and diffraction cone distortion as shown in fig4 this figure shows two diffraction cones 404 and 406 that are produced when an x ray beam 402 strikes a sample 400 stresses in sample 400 distort the diffraction cone shape so that the bragg angle 2 u03b8 becomes a function of the angle u03b3 that is 2 u03b8 2 u03b8 u03b3 where the particular function is uniquely determined.