Assessing the Sensitivity of a Single Crystal CVD Diamond Detector with Synchrotron Microbeam Techniques

In this study, X-ray sensitivity mapping was performed using a synchrotron microbeam at the National Synchrotron Light Source (NSLS, USA) on a low-purity single crystal CVD diamond sample labeled VS-Pt. This CVD-grown sample was chosen for its unique characteristics, including thin nitrogen lines and substrate areas, which can be identified and observed through synchrotron microbeam mapping. The objective of the study was to investigate the effects of varying pulsed bias voltage, beam size, beam step displacement, and device annealing on the homogeneity of the current response in the maps. Additionally, the impact of DC bias voltage, bias polarity, and bias pulse width was explored to determine the optimal conditions for synchrotron measurements. Theoretical relationships between the averaged normalized response and the applied bias voltage under synchrotron irradiation were also examined. The results showed that, compared to the annealed sample, the average normalized response of the detector reached saturation at a shorter pulse width for the bias voltage. Bias polarity did not significantly affect the device sensitivity, which contrasted with the annealed sample that exhibited noticeable variations in the images. Sensitivity saturation in the un-annealed sample occurred at 200 V or less. After annealing, the normalized response at –20 V increased linearly with pulse width, while the response at +20 V showed a slight tendency toward saturation, indicating that bias polarity influenced sensitivity. Following annealing, the sensitivity increased, possibly due to photoconductive gain resulting from a sensitizing effect. Moreover, the sensitivity was influenced by the sign of the DC bias, which differed from the pre-annealing response, where minimal differences were observed between images at positive and negative biases. Points near the nitrogen line exhibited slower current rise and decay times, likely due to shallow trap levels, while the substrate region showed lower photocurrent. Bias polarity affected the current amplitude, potentially due to polarization effects. When comparing the theoretical and experimental results for the VS-Pt sample, the theoretical values consistently fell below the experimental data. The sensitizing effect may contribute to this higher-than-expected response. Finally, as the beam size and/or step size decreased, the device sensitivity increased. This phenomenon is likely associated with the priming effect (pre-irradiation), whereby the sample accumulates a larger pre-irradiation dose as the beam or step size decreases. These insights could guide the development of detectors with improved spatial resolution, fast time response and dose-rate dependence, which are crucial for applications in dosimetry for X-ray radiotherapy and radiobiology applications as well as in beam monitors for accelerators and synchrotron radiation measurements.