Final answer:
The boxlike device under the tube housing that varies the size of the radiation field is known as a collimator, which is crucial in medical imaging and radiation therapy to focus and shape the radiation beam.
Step-by-step explanation:
The boxlike device attached under the tube housing in radiographic equipment that allows the operator to vary the size of the radiation field is called a collimator. This device is particularly important in medical imaging and radiation therapy, such as with a cobalt-60 machine used for cancer treatment. The collimator directs the radiation to target specific areas, like a tumor, and minimizes exposure to surrounding tissues. By adjusting the collimator, the radiation beam can be shaped and sized appropriately to the area that needs to be imaged or treated. The lead collimator is also an integral part of the Anger or gamma camera, focusing gamma rays to produce an image. In this context, the collimator helps to direct the gamma rays through holes and towards the scintillator tubes, where the gamma rays create light flashes that are converted into electrical signals, thereby creating an image.