Measurement of Charge Cloud Size in X-Ray SOI Pixel Sensors

Abstract

We report on a measurement of the size of charge clouds produced by X-ray photons in X-ray SOI (Silicon-On-Insulator) pixel sensor named XRPIX. We carry out a beam scanning experiment of XRPIX using a monochromatic X-ray beam at 5.0 keV collimated to ∼ 10 µm with a 4-µmφ pinhole, and obtain the spatial distribution of single-pixel events at a subpixel scale. The standard deviation of charge clouds of 5.0 keV X-ray is estimated to be σcloud = 4.30 ± 0.07 µm. Compared to the detector response simulation, the estimated charge cloud size is well explained by a combination of photoelectron range, thermal diffusion, and Coulomb repulsion. Moreover, by analyzing the fraction of multi-pixel events in various energies, we find that the energy dependence of the charge cloud size is also consistent with the simulation.

Development of the detector simulation framework for the Wideband Hybrid X-ray Imager onboard FORCE

“…sensors have been reported [16,6,7]. Also, the disruption and/or weakening of the electric field near the pixel edges, which enhances the charge cloud diffusion, has been discussed [5].…”

“…The physical properties considered as the free parameters included the electric field structure and the Coulomb repulsion factor on the charge cloud size. The Coulomb repulsion factor has been estimated analytically assuming a simple condition (spherically symmetric Gaussian distribution of a charge cloud) [7], but this condition may not be applied to actual sensors. Thus we treated this factor as a free parameter.…”

“…3 (a), the cross-sectional view of the electric potential is shown with colors, and the electric field directions are drawn with solid black lines. First we assumed the sensor parameters as follows: the electric field structure which was obtained in the preceding TCAD simulation and the Coulomb repulsion factor of 1.45 at 5.9 keV and 1.55 at 30.9 keV, which were derived in the analytical calculations by Hagino et al (2019) [7]. Then the comparison showed that the simulation results had significantly higher line centroids in the charge-sharing event spectra ( 55 Fe case), and the lower number fraction of the chargesharing events ( 133 Ba case) than the measurements had (see the dashed black lines and the red lines in Fig.…”

“…Finally, we modified the Coulomb repulsion factors to be 1.00 at 5.9 keV and 1.32 at 30.9 keV in order to fit the measured event type distribution. Considering the thicker sensitive layer and the lower back-bias voltage than those in Hagino et al (2019) [7], the drift time (t d ) of the carriers should be longer in our measurements. Since the Coulomb repulsion factor is estimated to be roughly proportional to t −1/6 d [7], the factor is expected to be smaller in our measurements as we obtained here, although it should be noted that this factor is introduced just for a phenomenological treatment.…”

“…Matsumura et al (2015) [5] and Negishi et al (2019) [6] studied the spatial distribution of the charge collection efficiency (CCE) and an electric field structure in the sensors in order to evaluate the XRPIX sensor performance. Hagino et al (2019) [7] modeled the charge cloud size and verified it by comparing it to measured number fractions of chargesharing events. However, there have been no response modeling studies of the XRPIX sensors for hard Xray photon interactions including charge-sharing event spectra, which are necessary for the detector designing and evaluating the scientific capabilities of FORCE.…”