We routinely characterize the phantom circle. It is made of plexi glass and attached are 2 unlabeled slabs. It is usually a 3-4 hour process. After many trials, we were able to lower our standard deviation for our measurements. We commonly use an ion chamber which has to warm up before use. We do a series of measurements: one in the middle and six around the center. Overall, it was good practice to learn how to use an ion chamber and get consistent results.