Charge summing mode (CSM)

1. Improved spatial resolution

The reduction of the charge sharing phenomenon leads to an improved spatial resolution.

Here, we present a simple example in which a slit with a 50 μm opening was measured under identical experimental conditions, first using SPM and then CSM. The X-ray source was polychromatic, operating at 60kV and 30 mA.

The images recorded are shown on the top of the figure. Ten lines perpendicular to the slit were analyzed in each case, as indicated by the blue and red lines, to determine the spatial resolution obtained in each case. The higher counts observed with SPM compared to CSM are attributed to double-counting caused by charge sharing when using the SPM.

By plotting the normalized intensity per pixel across the ten lines for each configuration (bottom panel in the figure on top), it becomes evident that CSM provides higher spatial resolution compared to SPM. In this context, resolution refers to how sharply the slit is observed in the measurement. The higher the resolution, the narrower the gradient region over which counts originating from the slit are distributed.

The measured resolution improved from 88.6 μm with SPM to 82.4 μm with CSM, representing an improvement of approximately 6 μm solely from the use of CSM. Note that the measured spatial resolution in this experiment was limited by the large beam focus size employed (0.2 mm × 12 mm). In case of high-resolution X-ray setups, such as synchrotron sources or X-ray tube sources with micro/nano focus, the CSM mode can significantly enhance the quality of the experimental data.

2. Separating a signal from background

When background radiation is present in a measurement, CSM can significantly improve the separation of the signal of interest from unwanted contributions. To demonstrate the advantage of CSM over SPM in such scenarios, we conducted the following experiment:

Using an X-ray source, we positioned two different fluorescent foils in front of the detector to simulate signal and background components. The foils and their respective fluorescence energies were:

Mo (Molybdenum) – 17 keV (acting as background)
Ag (Silver) – 22 keV (acting as the signal)
The energy separation between background and signal is only 5 keV. This narrow

gap requires a careful threshold selection to isolate the Ag signal while minimizing Mo contribution, without excessive signal loss.

The experimental setup is as illustrated:

First, only shutter a was opened, and the detector collected data corresponding to the background. In the second step, only shutter b was opened, to acquire the pure signal. A foil with a pinhole was placed between shutter b and the detector to enhance the visual distinction between the signal and the background. Finally, both shutters a and b were opened simultaneously, and measurements were acquired using both SPM and CSM modes. A comparison of the results is presented below.

In the first step, only the SPM was activated, and different energy thresholds were tested to determine which setting provides the highest overall counts with minimal noise and which threshold achieves better signal separation while maintaining a high count rate. The following figure presents the best-performing threshold values:

A region of interest (ROI) of 250 pixels in length and 16 pixels in width, centered on the pinhole opening, was selected for further analysis (b). For each line of the detector, the counts from the central 16 pixels were averaged. The resulting profiles were then plotted for the different acquisition scenarios.

(d) presents the results obtained from this analysis. The mean intensity is shown as a function of pixel line position for the SPM under three conditions: (a) background only, (b) background plus signal with a low threshold, and (c) background plus signal with a high threshold.

It should be noted that the high threshold energy lies between the Mo and Ag fluorescence energies. As a result, the Mo signal is excluded from the measurement, leading to a lower total count. Nevertheless, although the total number of counts decreases with the high threshold, the signal-to-background ratio (S/B) is improved compared to the low-threshold case, allowing for the separation of Ag and Mo signals.

A similar analysis was performed using CSM. In this case, two thresholds were applied at 7 keV and 20 keV. Under this condition, the counting of a photon follows the condition: photons with energies below 7 keV were completely rejected, while pixels registering energies above 7 keV were counted only if the energies of their four neighboring pixels also exceeded 20 keV.

The pure Ag signal was also measured using CSM (red stars in panel d). A comparison of the background, Ag signal, and mixed signals is presented here:

In (d) from the following Figure, a direct comparison is shown between the pure signal (red stars), the signal obtained with SPM (green), and the signal obtained with CSM (purple) when both signals are mixed. The results clearly demonstrate the superior signal-to- background ratio provided by CSM.

This example demonstrates the improvement of energy resolution when using CSM. This is particularly important when dual-threshold measurements are performed, as CSM allows for a more precise separation of high-energy photons.
At high energies, using SPM in dual-threshold mode can lead to an overcounting of photons when the subtraction of signals is performed. Thus, a high-energy photon that has its energy spread among several pixels can be counted as several low-energy photons.

3. Count correction

At high energies (see below for more details about each sensor recommendation), using the CSM correct the miscounting of photons caused by charge sharing among pixels.

1. Silicon (Si):

Charge summing mode is not available for silicon sensors. The main advantage of silicon is its ability to detect low-energy X-rays in the range of 6–25 keV. In this lower
part of the spectrum, the use of a low threshold is essential. However, when CSM is employed, the noise from all the summed pixels is also combined, requiring the use of
a higher threshold. As a result, low-energy X-rays would no longer be detectable.

Furthermore, the thinness of Si sensors (typically 300 µm or 500 µm), the high crystal quality (which reduces charge diffusion), and the application of a high bias
voltage help minimize charge-sharing losses. Therefore, the absence of CSM does not significantly impact detection performance in this context.

2. High Z materials:

X-Spectrum detectors can be equipped with GaAs (500 µm), CdTe (1000 µm), or CdZnTe (2000 µm) sensor materials. The increased thickness of these high-Z materials makes charge carriers more susceptible to diffusion, which enhances charge sharing effects. Additionally, high-Z sensors are typically employed for detecting high-
energy photons, a condition that make charge sharing even more pronounced. To address this, detectors with these materials are equipped with CSM, to help mitigate
the impact of charge sharing on signal accuracy.

The CSM is available for 12- and 24-bit mode for high-Z sensor detectors. To apply CSM when acquiring data:
Before acquiring an image.

• Turn the charge summing mode selection on.
• Set the lower threshold. The lower threshold is related to the local maximumand can be modified to account 1⁄4 of the beam energy to avoid overcounting. Here you may consider possible noise/ fluorescence of your sample and set the minimum threshold above this value. The standard values used to suppress noise is 7 keV.
• Set the upper threshold: The Upper threshold is related to the sum of the neighboring pixels. To limit charge sharing losses, we recommend operating with threshold about 5 keV below the expected event energy. If a tighter energy cutoff is required, the threshold can be increased, but this will result in an overall loss of counts. Standard recommended values are above 15 keV.
• Here is an example at X-Spectrum GUI:

Important Note: When using CSM, start with a high energy for the upper threshold (e.g., 20 keV) and gradually decrease it to the desired level (e.g., 15 keV). This helps to reduce electronic noise present in the measurement.