Provided is a device comprising a phantom comprising a solid water material, having a square cross-sectional shape, and having a width that varies monotonically along a height of the solid phantom; and an array of radiation detectors disposed within the phantom; wherein the array of radiation detectors is configured to detect radiation within the phantom. Further provided is a linear accelerator having a gantry and comprising the device as described herein installed in a treatment head of the gantry. In certain embodiments, the linear accelerator further comprises software to interface the device with the linear accelerator.