XRD instrument overview

 

We have a Phillips PW-1840 X-ray diffraction machine. This is a computer controlled instrument with both a chart recorder and computer data acquisition. It has a goniometer with an automatic divergence slit and a scan angle of ~0 to 120° 2θ. It is currently set up with a cobalt tube using an iron filter. Samples can be introduced as smears on short glass slides (about 5 mm shorter than a typical petrographic microscope slide) or in a shallow well in aluminum holders.

Power supply

The most important thing to remember during XRD operation is to not destroy the instrument. An important relationship in this regard is the formula:

 

A*V = W.

 

 

The power supply is composed of a high voltage transformer, a rectifier, and control circuitry. It supplies well-regulated high-voltage D.C. potential and current to the X-ray tube. The voltage knob on the power supply goes up to 50 kV (kilovolts; 50,000 volts), which is its maximum rating. The current gauge goes up to 100 mA (milliamperes), but in fact the actual power supply limit is 60 mA. Because the X-ray tube is rated to 1800 W, the electron beam anode in the tube will melt long before the power supply limit is reached. Therefore, the power control knobs must NEVER be set to exceed 1800 W. Normally they are set to 45 kV and 35 mA, which is ~1600 W.

 

To give you some idea of why high power might damage the tube, think of the line focus on the cobalt anode. It is 0.4 mm by 10 mm, an area of 4 mm2. Running at 1600 W, the power density at the line focus is 400 W/mm2, which is 40 kW/cm2, which is 400 MW/m2. This is roughly five times the radient power density on the surface of the sun. Cooling water on the back of the anode carries away the excess heat, but only to a point.

Goniometer

The goniometer is the device that holds the sample and the X-ray detector. A line between the middle of the sample and the X-ray tube target is at a 6° angle to the surface of the target (X-ray "takeoff angle"). During a scan, the sample rotates about an axis parallel to the line focus on the target at a rate measured in °θ/minute. The detector rotates about the same axis at a rate of 2°θ/minute. As a result, the angle of incidence of X-rays on the plane of the sample and the angle of diffraction of X-rays to the detector are always equal. As a technical note, surface characteristics, such as roughness, become very important at low 2θ angles, making 1 or 2°2θ a practical routine limit.

 

Our goniometer has an automatic divergence slit that widens as the 2θ angle increases. The slit widens in such a way that the area of the sample illuminated by X-rays remains constant at all 2θ angles. This allows scans to be done at angles below 1°2θ; very difficult on instruments with fixed divergence slits.

 

The detector on our instrument is a solid state silicon device, instead of a more traditional Geiger tube, proportional counter, or scintillation counter. The latter detectors measure discrete X-ray absorption events. In our instrument a small voltage is put across the silicon detector. Electron-hole pairs that are produced in the silicon by X-rays produce an excess leakage current through the device that is proportional to deposited X-ray energy and approximately proportional to the count rate of other detector types. It is this leakage current that is the measured signal.

Chart recorder

The chart recorder can make a permanent record of the X-ray diffraction scan, though it is rarely used. Data are also be read into the computer, the usual way data are acquired and displayed. The chart recorder is under computer control during normal operation. Moving the switch from REMOTE to LOCAL and pushing down the left toggle switch on the left side of the chart recorder advances the paper. If you set CHART to zero and press the START button, then the chart recorder acts as a rate meter for the X-ray detector.