TXRF of Light Elements

THe ED analyis of light elements is a challenging task as there are inherent problems for the analysis of low Z elements:

Since the energy of the emitted fluorescence radiation is generally lower than 2 keV, we have to consider self absorption in the sample. This can be avoided by preparing the sample as a thin film. Additionally, attenuation on the way from the sample to the sensitive volume of the detector crystal occurs, thus, all measurements have to be performed in vacuum.

The intensity I of the fluorescence Kα radiation is influenced by the following factors, which may result in low fluorescence intensities:

fig50.jpg

I0 is the intensity of the primary radiation, G a geometric factor, τK the photoelectric mass absorption coefficient of the K-Shell, ωK the fluorescence yield, ε the detector efficiency, A the absorption coefficient, and pα the transition probability, ci the concentration of the element I in the sample. The following parameters can be controlled better:

  • τK:          special x-ray source is needed to excite low Z elements efficiently
  • ωK:         low for low Z materials, but natural restriction
  • ε:           special detector with thin Si-deadlayer and contact layer; ultrathin entrance windows required
  • A:          self absorption in the sample, can be neglected for thin film approximation

The measurement set-up consisted of a chamber suitable for 6”-Wafers (rotatable sample carrier), a 1.9 kW Cr anode X-ray tube (long fine focus) as the source. A multilayer monochromator W/C, an adjustable slit-system modifies the primary spectrum to a monoenergetic beam. A Si(Li) detector (Oxford Premiumgrade B35; 30 mm²) with an ultra thin window (Moxtec AP3.1 300 nm on a Si grid support) (4) equipped with an electron trap (Æ 6mm) to suppress noise originating from Auger-, Photo- and Compton electrons collects the photons. The electron trap consists of 2 high field permanent magnets 6x4x1 mounted inside a cylindrical collimator made of Fe with a center hole of 6 mm diameter and attached to the detector snout.

Detection limits (LLD) using the Cr anode tube at operating conditions of 30kV and 30mA for 100 s live time, which were extrapolated for 1000s in all cases. The conversion from pg to atoms/cm² assumes a relevant inspected area of 0.5 cm².

Element

LLD[pg]

1012Atoms/cm2

Na

180

9,4

Mg

150

7,4

Al

140

6,2

Table 1: detection limits