Which unit is used to express photon energy on readout of a detector spectrum?

• A. keV
• B. mSv
• C. Gy
• D. CPM

Answer: (A)

Photon energy on a detector’s spectrum is shown in energy units, because the axis represents how much energy each detected photon carries. In X-ray applications, that energy is routinely expressed in electronvolts, with kilo-electronvolts (keV) being the practical unit for the energies involved. The detector’s energy calibration maps channels or bins to keV, so peaks and features line up with the photon energies you’re measuring.

The other options don’t express photon energy: millisieverts and grays quantify dose and energy deposited, not the energy per photon. CPM is a count rate, telling you how many photons are detected per minute, but not their individual energies. Therefore keV is the appropriate unit to express photon energy on a detector spectrum.