Experimental Optimization

Reducing Dead-Time

Dead time refers to the period after each photon detection during which the detector cannot process another event. High dead time leads to signal loss and non-linear responses. The dead time should be monitored using Prospect before running a scan and/or using PyMca while running it. It should remain below 10% for accurate data.

If the measured dead time is quite high, check the signal activity on Prospect and look for signs of abnormal background or electronic noise. Then, it is possible to: - Use aluminum filters to cut low-energy photons that may saturate the detector. - Adjust slit widths to reduce the beam size and thus the photon flux. - Electronic noise near zero energy can occur. To remove electronic noise, apply an “electronic” filter in Prospect settings that excludes counts near zero energy.

Reducing Signal-to-Noise Ratio

Improving the signal-to-noise ratio is important to resolve fine spectral features and ensure reproducibility. Key strategies include: - Optimize counting statistics by increasing acquisition time or averaging multiple scans. - Minimize electronic noise by proper grounding and shielding of cables and electronics. - Use collimators and slits to limit scattered radiation reaching the detector. - Employ aluminum filters to remove low-energy background photons.

Use of Auxiliary Components

Collimators

Collimators are used to reduce the angular divergence of the X-ray beam, thus improving the energy resolution of the measurement. By restricting the beam's angular spread, collimators help limit the contribution of off-axis photons, which may lead to broadening or shifting of spectral features.

They are especially useful when: - Measuring lines requiring high resolution. - Reducing scattering from surrounding components or air. - Working in configurations where spatial filtering improves signal clarity.

However, collimators also reduce the total photon flux. Their use is therefore a trade-off between resolution and intensity.

Placement: Collimators should be placed after the X-ray tube window.

Slits

Slits control the width and height of the X-ray beam, which directly affects both the intensity and the resolution of the signal. Slits are typically used to: - Limit the beam size to match the sample dimensions. - Reduce background signal by spatially filtering unwanted scattering. - Control the solid angle accepted by the detector, thus tuning spectral resolution.

Narrower slits improve resolution but reduce signal intensity. Start with wider slits for alignment and coarse scans, then gradually narrow them for fine measurements.

Placement: Slits can be placed before or after the sample, right before the detector.

Aluminum Filters

Aluminum filters can be used to attenuate low-energy X-rays that contribute to background noise or saturate the detector. These filters exploit the energy-dependent absorption properties of aluminum, which strongly absorbs photons below ~1.5 keV, while having a limited effect on higher-energy signals.

Applications include: - Reducing X-ray background. - Preventing detector pile-up or dead time due to intense low-energy peaks.

Filter thickness should be chosen based on the energy range of interest. Multiple filters can be stacked for greater attenuation.

Placement: Aluminum filters can be placed anywhere along the beam path.