The problem and possible solution
Calibration is necessary for PSD (Position Sensitive Detectors) based instruments to relate channel number to two-theta angle. This is especially so with INEL PSDs where there can be discontinuities not modellable via normal polynomial fitting. (instead use a spline curve fit where the curve has to go through each calibration point).
Hoever a common problem is that most standards do not have many peaks at low angle.
Thus Silver Behenate (c-axis= 58.373 to 58.380 Anstrom) for low to medium angle calibration; combined with an appropriate medium to high angle standard (such as Cubic Y2O3 (IA3 a=b=c=~10.6 Angstrom) - annealed at 1200°C for a few days) may offer a solution covering the entire angular range.
The following is based around the following publications and data:
Obtaining Silver Behenate
Quoting from Blanton, et al (2000), "Eastman Kodak Company provided silver behenate to The Gem Dugout (Smith. 1999). from where it may be purchased for scientific study. One-half of the purchase price is donated to the International Centre for Diffraction Data for the ICDD Crystallographic Scholarship fund.]"
Smith, D.K. (1999). The Gen Dugout. 1652 Princeton Drive, State College, Pennsylvania, 16803, USA. E-mail: email@example.com
Other Available NIST/NBS XRD Standards
Data and Tutorial Run Throughs