Assessment of the Imaging Performance of the CITIUS High-Resolution Detector for Heavy Charged Particles and Neutrons

Abstract

We report on the assessment of the imaging performance of CITIUS -- a high-speed X-ray detector developed for the large-scale synchrotron radiation facility SPring-8-II -- for heavy charged particles and neutrons. To characterize the detector response, an irradiation experiment was performed using alpha particles from an $^{241}$Am source at four back-bias voltages of 400V, 300 V, 200 V, and 170 V, thereby controlling the amount of charge diffusion. A Geant4 model of the experiment was constructed, and four model parameters were determined by template fitting to the measured signal cluster shape distributions. The best-fit values are: an intrinsic energy spread of 5% for the source, a gold fraction of 0.4 for the Au-Pd coating, a lateral charge diffusion spread of 26.5 $μ$m over a drift distance of 650 $μ$m at 400V back-bias, and a per-pixel readout noise of 10000 $e^{-}$ in the medium-gain channel. Using the obtained sensor model, simulations were performed for 4 MeV alpha particles and cold neutrons to evaluate the expected spatial resolution. In both cases, simulated CITIUS, when operated in a gain-selecting mode between high and medium gains, yields a substantial improvement: at a pixel size of 70 $μ$m for example, the resolution improves from 9.1 $μ$m to 1.2 $μ$m for alpha particles, and from 26 $μ$m to 1.9 $μ$m for cold neutrons. These results suggest that two key features of CITIUS -- its gain-selecting architecture and the substantial charge sharing enabled by the long carrier drift distance -- extend its imaging capabilities beyond X-rays to heavy charged particles and neutrons.

Figures (6)

• Figure 1: Experimental setup. The $^{241}$Am source has an $8 \times 8\mm^2$ active area and is placed 9.4 from the CITIUS sensor surface. Alpha particles lose energy in the Au–-Pd coating of the source, the air gap, and the dead layer on the sensor surface before reaching the active region.
• Figure 2: (a) Measured distribution of $m_{2,y}$ versus $m_0$, with the projected distributions shown as histograms along the top and left axes. Cross markers indicate the best-fit template distribution. Dashed arrows indicate the fitting range, defined as the bins within the half-maximum of the distribution peak. (b) $\chi^2$ distributions for the source parameters (top) and the sensor parameters (bottom). The source parameter $\chi^2$ is obtained by summing over all four back-bias conditions. The sensor parameter $\chi^2$ is shown for $V_b = 400V$ as a representative example. For clarity, the vertical axis is plotted as $100 - \chi^2/\mathrm{ndf}$. The best-fit values are $\sigma_s = 5\%$, $r = 0.4$, and, for $V_b = 400V$, $\sigma_d = 26.5\um$ and $\sigma_n = 10000e^-$.
• Figure 3: Back-bias dependence of the charge diffusion spread $\sigma_d$. Sufficient charge sharing is observed across all conditions, suggesting that CITIUS is well suited also for alpha particle and neutron imaging. The full depletion voltage was $V_b = 200V$.
• Figure 4: Spatial resolution as a function of the incident angle for alpha particles. Open Squares, triangles, and circles represent the single-gain configuration with $\sigma_n = 10ke^-$, the single-gain configuration with $\sigma_n = 1ke^-$, and the multi-gain configuration with $\sigma_n = 10ke^-$ and $40e^-$, respectively. For each, solid and dashed lines represent the RMS measure and the Gaussian fit measure, respectively. Dash-dotted lines indicate the results for isotropic incidence. At large incident angles, a discrepancy between the two measures arises due to the ring-shaped PSF.
• Figure 5: Spatial resolution as a function of the pixel size for alpha particles. Open Squares, triangles, and circles represent the single-gain configuration with $\sigma_n = 10ke^-$, the single-gain configuration with $\sigma_n = 1ke^-$, and the multi-gain configuration with $\sigma_n = 10ke^-$ and $40e^-$, respectively. For each, solid and dashed lines represent the RMS measure and the Gaussian fit measure, respectively. Small filled circles represent the effect of systematic pixel-to-pixel non-uniformity, which may arise from gain or charge transport variations, as an example, with a random gain gradient $\delta / \mathrm{pixel}$ in one direction, uniformly distributed in the range $0.5\% < \delta < 1.5\%$. The pixel window size used for each pixel size is indicated along the top of the figure.