The Phillips PW-1840 X-ray diffractometer has both a chart recorder (rarely used) and computer data acquisition. It has a goniometer with an automatic divergence slit, and a scan angle of ~0 to 120° 2Θ. It currently has a copper tube with a nickel Kβ filter. Samples can be smears on short glass slides (about 5 mm shorter than a typical petrographic microscope slide) or powder pressed into a shallow well in aluminum holders.
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.
|Characteristic||Power supply||Copper tube|
|Ampere (A), a unit of electrical current||60 mA maximum||55 mA maximum|
|Volt (V) is a unit of electrical potential||50 kV maximum||60 kV maximum|
|Watt (W) is a unit of electrical power: W=A*V||Setting the power supply current and voltage to maximum values at the same time yields 3000 watts. This will destroy the X-ray tube and power supply.||Normally run the tube at 45 kV and 40 mA, which is 1800 watts.|
Always run the instrument at 45 kV and 40 mA, never higher!
The power supply is composed of a high voltage transformer, a rectifier, and control circuitry. It supplies a well-regulated high-voltage DC potential and current to the X-ray tube. The voltage knob on the power supply goes up to 50 kV (kilovolts, 50,000 volts), its maximum rating, but never exceed 45 kV. The current gauge goes up to 100 mA (milliamperes), the power supply limit is 60 mA, but never exceed 40 mA. Normally run the instrument at 45 kV and 40 mA, which is 1800 W.
To give you some idea why higher power might damage the tube, think of the line focus on the copper anode. It is 0.4 mm by 12 mm, an area of 4.8 mm2. Running at 1800 W, the power density on the line focus is 375 W/mm2, or 375 MW/m2, or roughly six times the radiant power density at the sun’s surface. Cooling water on the back of the anode carries away the excess heat, but only to a point.
The goniometer is the device that holds and moves the sample and the X-ray detector. The tube doesn’t move. A line between the middle of the sample and the X-ray tube anode is at a 6° angle to the anode surface (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 X°Θ/minute. The detector rotates about the same axis at a rate of 2X°Θ/minute. As a result, the angle of incidence of X-rays to the sample plane, and the angle of diffraction of X-rays to the detector, are always equal. As a technical note, sample characteristics, such as surface roughness, become very important at low 2Θ angles, making 1 or 2°2Θ a practical routine limit for most samples.
Our goniometer has an automatic divergence slit that widens as the 2Θ angle increases. This allows the sample area illuminated by X-rays to remain constant at all 2Θ angles. This allows scans to be done at low 2Θ angles without having to change slits.
The detector is a solid state silicon device, instead of a more traditional Geiger 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 produced in the silicon by absorbed X-rays produce an excess leakage current that is proportional to the deposited X-ray energy and number of X-rays absorbed per unit time. This leakage current is the measured X-ray signal. Thus, the actual X-ray intensity signal is in microamperes, but the signal is reported as counts per second after multiplying microamperes by a conversion factor.
The chart recorder can make a permanent record of the X-ray diffraction scan, though it is rarely used. Data are usually recorded by the computer. If, on the controller box, you set CHART to zero and press the START button, the recorder acts as a rate meter for the X-ray detector. That is useful for aligning the X-ray optics.