The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-ray coded-aperture mask instrument with a primary field-of-view of \(4.6^\circ \times 4.6^\circ \) (FWHM). The instrument collimators become increasingly transparent at energies above \(\sim \)100 keV, making CZTI sensitive to radiation from the entire sky. While this has enabled CZTI to detect a large number of off-axis transient sources, calculating the source flux or spectrum requires knowledge of the direction and energy dependent attenuation of the radiation incident upon the detector. Here, we present a GEANT4-based mass model of CZTI and AstroSat that can be used to simulate the satellite response to the incident radiation, and to calculate an effective “response file” for converting the source counts into fluxes and spectra. We provide details of the geometry and interaction physics, and validate the model by comparing the simulations of imaging and flux studies with observations. Spectroscopic validation of the mass model is discussed in a companion paper, Chattopadhyay et al. (J. Astrophys. Astr., vol. 42 (2021) https://doi.org/10.1007/s12036-021-09718-2).

AstroSat 上的镉锌碲化物成像仪 (CZTI) 是一种硬 X 射线编码孔径掩模仪器，主视场为\(4.6^\circ \times 4.6^\circ \) (FWHM)。仪器准直器在能量高于\(\sim \)100 keV，使 CZTI 对来自整个天空的辐射敏感。虽然这使 CZTI 能够检测大量离轴瞬态源，但计算源通量或光谱需要了解入射到检测器上的辐射的方向和能量相关衰减。在这里，我们提出了一个基于 GEANT4 的 CZTI 和 AstroSat 质量模型，可用于模拟卫星对入射辐射的响应，并计算用于将源计数转换为通量和光谱的有效“响应文件”。我们提供几何和相互作用物理学的详细信息，并通过将成像和通量研究的模拟与观察进行比较来验证模型。Chattopadhyay等人在一篇配套论文中讨论了质量模型的光谱验证. （J. Astrophys. Astr.，第 42 卷（2021 年）https://doi.org/10.1007/s12036-021-09718-2）。