When producing K-characteristic rays, there must be a tube potential of at least?

To produce K-characteristic rays, a specific minimum tube potential is required to ensure that the incident electrons possess enough energy to knock inner-shell electrons from the target atoms, allowing for the transition of outer-shell electrons to fill these vacancies. This electron transition results in the emission of characteristic X-rays, specifically the K-series X-rays.

The minimum tube potential needed to achieve these conditions is generally around 70 kV. At this voltage, the electrons have sufficient kinetic energy to displace K-shell electrons in the target material (often tungsten in radiographic applications). As a result, the outer-shell electrons drop to fill the K-shell vacancies, emitting K-characteristic X-rays in the process.

Lower voltages, such as 60 kV, do not provide enough energy for this interaction to occur effectively. Therefore, 70 kV is established as the threshold for generating K-characteristics reliably in radiographic practices, making it the correct answer.