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Once properly configured, diffractometer alignment proceeds as follows.(1) Arrange for the X-ray beam to go through the center of rotation. Generally,the center of rotation is found with a pin and a telescope.(2) Arrange for the X-ray beam to be perpendicular to the 2θ axis. This conditionis typically verifyed by comparing X-ray burns made on X-ray sensitive paperwith 2θ near the undeflected beam direction and with 2θ offset by 180°.(3) Set 2θ so that the undeflected X-ray beam direction corresponds to the zero of2θ.(4) Align the χ rotation axis with the laboratory y axis to set the zero of θ.(5) Align the φ rotation axis with the θ rotation axis to set the zero of χ.One way to do (4) and (5) is as follows:(i)(ii)Mount aSi(111) wafer so that the (111) direction is (approximately) along theφ axis.Find the (111) Bragg reflection. Note the values of θ and χ. Call them θ 1andχ 1.(iii) Rotate φ by 180°.(iv) Find the Bragg reflection again. Note thes values of θ and χ. Call them θ 2and χ 2.(v) 1⁄2 (χ 1+ χ 2)corresponds to χ =90° in a correctly aligned spectrometer; 1 ⁄2 (θ 1+ θ 2)corresponds to θ = 1 ⁄2 2θ.The Huber four-circle diffractometer is an example of an instrument that works withfourc. 1 Another common spectrometer configuration has two crossed ± 20° tilt stageson top of full θ and 2θ circles. This configuration is compatible with the four-circlecode if the tilt stage immediately adjacent to the θ circle is χ and the other is φ. Whenthe first tilt stage is zero, χ is at 90°.1 The four-circle Huber has dial readings with all right-handed rotations, so the χ circle should have dialreadings and user readings that are in opposite senses. If the θ circle is offset by 180° then the dial readings andthe user readings of all angles can have the same sense.INTERIM WORK-IN-PROGRESS (8/16/01) NOT FOR GENERAL DISTRIBUTION FOUR-CIRCLE REFERENCE 167