This box controls the diffractometer and chart recorder during a scan. Normally it is under computer control and you can skip this section. If you wish to do manual scans, or have to set the goniometer at particular positions for X-ray optics alignment, this section is for you.
The twelve numeric keys in the middle have obvious data entry functions. Entering a number such as 1.5 enters just that number, except when entering the chart scale. For the chart scale, the decimal point means "ten to the power of", so in this case 2.5 means 2x105.
Prior to a scan the eight keys on the left change which scan settings the numeric keys in the middle are referring to. During a scan they show either scan settings or the goniometer 2θ angle.
The six keys on the right control the goniometer.
This is a labeled cartoon of the photo of the control box above. See the table below for details.
| Button | Purpose | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| FROM | Start angle for the start of the scan. This can range from 0°2θ to 120°2θ in multiples of 0.01°2θ. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| TO | Stop angle for the end of the scan. This can range from 0°2θ to 120°2θ in multiples of 0.01°2θ. This must be larger than the FROM angle. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| SPEED | Goniometer scan rate: 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, and 0.1°2θ/second. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| CHART | Chart recorder paper speed: 5, 10, 20, and 50 mm/°2θ. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| RANGE | The
full-scale range for the chart paper in nominal counts/second. The chart paper
will record from 0 to the full scale counts/second. Larger peaks will go off
scale. 20,000 counts/second is a good starting point. Range settings are:
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| T.C. | Time constant. This is the time in seconds over which the X-ray detector count rate is averaged. The longer the time constant, the smoother the trace. If the time constant is too short the trace is unnecessarily wiggly because of statistically fluctuations in the count rate. If the time constant is too long the diffraction peaks are broadened, become shorter, and are displaced to higher 2θ angles. In general the time constant should be no longer than 1/50 of the time it takes to scan 1°2θ, which is the amount of time needed to scan 0.02°2θ (1/50 of 1°2θ). For example if the scan rate is 0.01°2θ/second, then 1° is covered in 100 seconds, so 2 seconds would be the longest reasonable time constant. If the scan rate is 0.05°2θ/second, then 1° is covered in 20 seconds, so the time constant should be 0.2 or 0.5 seconds. Time constant values are: 0.2, 0.5, 1, 2, 5, 10, and 20 seconds. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| SLIT | This value is analogous to the "detector slit" on other instruments. Our detector does not really have a slit. Instead, the active area on the solid-state detector is varied. The narrower the slit, the higher the 2θ resolution but the lower the count rate. Slit width values are 0.05, 0.1, 0.2, and 0.3 mm. For clays use 0.3, for most other uses use 0.2 mm. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ANGLE | Press this button to display the current goniometer angle. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| SAMPLE | If we had an automatic sample changer this button would tell the changer which sample to load. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| UP and DOWN | Pressing these buttons will cause the goniometer to go up or down in increments of 0.01°2θ. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| SLEW | This key lets you tell the goniometer to go to a specific °2θ position. To do this, press SLEW; then enter the °2θ position using the numeric keys; then press START. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| START | Starts the goniometer to do a scan using the scan parameters that have been set. The chart recorder advances to the next line and starts automatically. If SLEW has been pressed, then the goniometer simply moves to the entered position. For goniometer adjustments, if the CHART value is zero, START turns the chart recorder on so it acts as a real-time X-ray intensity meter (turn the speed control knob on the recorder to its slowest setting or you will soon run out of paper). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| STOP | Stops the goniometer and chart recorder from doing whatever they were doing. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Schenectady, NY 12308 U.S.A. |