A computer-controlled X-ray powder diffraction system with an automatic sample changer

A 35-position Philips sample changer has been added to a PDP-8/I computer-controlled X-ray powder diffraction system which has operated in a single-sample mode for the past four years [Medrud, Harter, Stephenson & Kane, (1972), Program and Abstracts Amer. Cryst. Assoc. Annual Meeting, Albuquerque, NM, p. 58]. The system operation, instrumentation and data processing are discussed. The computer controls the diffractometer and the sample changer as well as the data acquisition and on-line processing. Subroutines provide for sample identification, correction of 20 values from an external standard, and shutdown of the system in the event of hardware malfunction. The diffraction data can be recorded on either a teletype or magnetic tape as well as on an analog strip-chart recorder. For ease of interpretation, each sample run is automatically started on a major grid division of the strip chart with an interrupt generated by a recorder-mounted photoelectric device. All essential elements of conventional manual diffractometer operation were included in the system plan. Digital intensity profiles taken at fast scan rates (2 to 4 deg min^-1) are more precise than the corresponding analog profiles. As a result, the precision of peak location can be increased up to a factor of five.