Intro Spectroscopy

PIXL is part of the Mars 2020 payload and is a micro-focus X-ray fluorescence (XRF) spectrometer for fine-scale measurements of elemental distribution in rocks and soils. PIXL has an arm-mounted sensor head, body-mounted electronics, and a rover deck-mounted calibration / reference target.

In XRF spectroscopy, X-rays hit a target surface, causing the target to fluoresce. The characteristic energies and number of emitted fluorescent X-rays are determined by the target’s elemental composition. Semiconductor detector electronics count the emitted X-rays and sort the counts by energy into histograms to produce a spectrum. This process occurs through the following steps. Upon striking the detector, an X-ray photon produces a number of electrons proportional to the photon energy. Readout electronics convert this charge into a voltage pulse and measure the height of the pulse. A digital pulse processor counts the number of pulses of a specific pulse height. The result is a histogram of X-ray counts vs. X-ray pulse height. This histogram is calibrated using a material with a few known peaks to convert the histogram to a spectrum of X-ray counts vs. X-ray energy. Detector temperature and the temperature-dependent resistance of the cable between the sensor head and the body-mounted electronics may affect this calibration. To facilitate energy calibration and determining its dependence on temperature (and any other variables) the calibration process can be mostly automated.

The X-ray spectrum yields information about the elemental composition of the spot measured by the primary X-ray beam. Its information is encoded in the energy and intensity of the X-rays. To extract this information the peaks corresponding to the emission lines of each element must be identified and their net intensity extracted. This part of the process is usually called spectrum processing. Once the net intensities for a peak associated with each element are found, they are converted to element abundances by accounting for the atomic effects of each element and the effect of overall composition on the intensity that escapes from the target being analyzed. A predicted intensity is calculated using a physics-based model for X-ray interactions referred to as the fundamental parameters model. The target composition input to this model is adjusted until the predicted intensity matches the intensity from the X-ray spectrum.