R. Jenkins

Physics of X-Rays

The X-ray region is normally considered to be that part of the electromagnetic spectrum lying between 0.1–100 A, being bounded by the y-ray region to the short wavelength side and the vacuum ultra-violet region to the long wavelength side. The actual boundary between the X-ray and vacuum ultra-violet region is not clearly defined and for many years the 50–500 Â mid-region has not been exploited by practical spectroscopists to any great degree. Over the last few years however this wavelength range has been examined both from the short wavelength end by the X-ray spectros-copist and from the long wavelength end by workers in the fields of plasma-and astrophysics. It is now common practice to refer to this particular region as the soft X-ray and vacuum ultra-violet region.

Instrumental factors in the detection of low concentrations by X-ray fluorescence spectrometry

A survey is given of detection limits currently obtainable by X-ray fluorescence spectrometry. The significance and limitation of certain instrumental variables are evaluated in the light of recent developments in this field with examples taken from different parts of the wavelength range. Mention is made of possible future trends in instrumentation and their likely effect on detection limits discussed.

Choice of detectors

A choice has to be made between the scintillation counter and the flow proportional counter for the measurement of a certain wavelength.

Quantitative x-ray diffractometry

Very often the quantity of a compound has to be determined in a complex mineral, the composition and absorption coefficients of which are unknown. To correct for these unknown facts, the attenuation of a diffraction line arising from the bottom of the sample holder may be used. The following intensities were collected in measuring the percentage quartz in a mineral with a conventional diffractometer, utilizing a sample holder with a nickel bottom.

Quantitative X-ray diffractometry use of internal standard

The weight fraction of a quartz (SiO2) has to be determined in a natural specimen. To correct for the unknown absorption in this specimen, 200 mg KCl are added to 1000 mg of the sample, both very finely ground, and well mixed. Determine the weight fraction of α quartz, using the following data:

Choice between ratio and absolute counting methods

The choice has to be made between the ratio and absolute method of counting in a counting programme which already involves collection of counts on a standard sample (i.e. whether the absolute or ratio method is chosen the standard is counted anyway, so no time is to be saved by not counting the standard).

Relative line intensities obtained on alkali halides

Table 1 lists the relative line intensities obtained on samples of sodium, potassium and rubidium chlorides, using identical instrumental conditions.

Effect of matric absorption on sensitivity

An ore specimen consisting predominantly of calcium and barium carbonates contained traces of nickel. A specimen containing no barium sulphate with a mass absorption coefficient for Ni Kα of 124 gave the following data 240 c/s per % for nickel background 120 c/s (a) Estimate the lower limit of detection for nickel in an analysis time of 200 s. (b) What would be the effect on this detection limit of adding 20% of barium sulphate to the specimen, assuming that the c/s per % value is inversely proportional to the matrix absorption? Assume also that the background is reduced by 8% by the addition of the barium sulphate. The atomic weights of Ba = 137.4, S = 32.1 and O = 16.

Spurious peaks in X-ray diffractograms

Adiffractogram of quartz exhibited an additional spurious peak at approximately 6° (2θ).The effect was observed both in specimens containing only low atomic number elements and specimens containing high atomic number elements. The patterns were always taken using Cu radiation, proportional counter (Xe-filled) and pulse height selection. The pulse height selector was set at the 45% acceptance level since the resolution of the counter (peak width at half height) was 18% for Cu Kα radiation.

Measurement of, and correction for, absorption in trace analysis of whole rock

A whole rock specimen was being analyzed for trace concentrations of strontium. The method of Norrish and Chappell(10) was used to measure and correct for absorption and the following data were obtained.