Final answer:
An iodine escape peak in gamma-ray spectroscopy occurs when a gamma photon ejects a K-shell electron, resulting in a characteristic X-ray escaping the detector.
Step-by-step explanation:
An iodine escape peak occurs in gamma-ray spectroscopy when an incident gamma photon from a radioactive source interacts with the detector material and ejects a K-shell electron.
As the shell vacancy is filled by an electron from a higher energy level, a characteristic X-ray is emitted, which subsequently escapes the detector without being absorbed. This results in a measured energy peak at a value that corresponds to the initial gamma-ray energy minus the energy of the escaped characteristic X-ray.
For a Tc-99m source, which emits gamma rays with energy around 140 keV, the iodine escape peak would occur at an energy level approximately 28 to 30 keV lower than the primary peak due to the approximate energy of iodine K-shell X-rays. This would result in an iodine escape peak at approximately 110-112 keV.