Computer-Controlled X-Ray and Neutron Diffraction Experiments

Abstract

The use of on-line computers for control and acquisition of data from x-ray and neutron diffractometers has continuously improved and expanded. Systems vary from a small 4K core computer to a time-sharing system with a medium or large computer. The choice of a single time-shared computer or an individual standalone system must be based on one’s own particular environment. As large high-speed electronic computers became available, increasingly complex chemical and magnetic structures have been analyzed and solved; this has created a demand for rapid, reliable, and versatile means of obtaining diffraction data. Since small computers have been developed at reduced cost and with increased storage capacity, they must be considered for use in diffraction experimentation. Therefore, in x-ray and neutron scattering, small computers are needed for data acquisition and large computers are needed for data analysis.

The ARgonne Computer Aided Diffraction Equipment (ARCADE) is an example of an effort to develop more automated methods. The system utilizes an IBM-1130 with 4K core memory and a 500K disc storage. The computer is used to (1) control the experiment, (2) collect the data, (3) make decisions during the experiment, and (4) process the data, which will be used in a larger computer to solve the problem.

The computer may be coupled to various parts of the experiment. Both the sample crystal and the detector are positioned by de stepping motors that are position-controlled by optical encoders to an accuracy of ±0.005°. Pulses from both the detector and clock/monitor systems are recorded by the computer. The computer may also be used to control a rate meter, strip chart, console printer, temperature controller, or rf generator for the flipping of a polarized neutron beam. The data may be recorded on disc, paper tapes, or transmitted to a larger computer.

A generalized software control program is basic to the system. This control program must always remain in core. The experimenter can use the program, in a straightforward conversational mode, by means of a keyboard console or by automatic coupling of a series of programs. The ARCADE system now has a repertoire of approximately 150 programs that involve several hundred subroutines. The programs can be categorized into the following groups: control, orientation, data collection, and data reduction. Although the computer is small, the large disc storage permits programs to be linked together to extend the range of experimental control. Changes and additions are constantly being made to adjust to the experimental needs.

Work performed under the auspices of the U. S. Atomic Energy Commission.