Effect of Voltage and Current on Photon Energy and Power

DOE-HDBK-1109-97

Radiological Safety Training for Radiation-Producing (X-Ray) Devices

Student's Guide

ii. Characteristic X-Rays.

When electrons change from one atomic orbit to another, characteristic X-rays are

produced. The individual photon energies are characteristic of the type of atom and can

be used to identify very small quantities of a particular element. For this reason, they are

important in analytical X-ray applications at research laboratories.

EFFECT OF VOLTAGE AND CURRENT ON PHOTON ENERGY AND POWER

It is important to distinguish between the energy of individual photons in an X-ray beam and

the total energy of all the photons in the beam. It is also important to distinguish between

average power and peak power in a pulsed X-ray device.

Typically, the individual photon energy is given in electron volts (eV), whereas the power of a

beam is given in watts (W). An individual 100 keV photon has more energy than an

individual 10 keV photon. However, an X-ray beam consists of a spectrum (a distribution) of

photon energies and the rate at which energy is delivered by a beam is determined by the

number of photons of each energy. If there are many more low energy photons, it is possible

for the low energy component to deliver more energy.

The photon energy distribution may be varied by changing the voltage. The number of

photons emitted may be varied by changing the current.

Voltage.

The power supplies for many X-ray devices do not produce a constant potential (D.C.)

high voltage but instead energize the X-ray tube with a time varying or pulsating high

voltage. In addition, since the bremsstrahlung X-rays produced are a spectrum of energies

up to a maximum equal to the electron accelerating maximum voltage, the accelerating

voltage of the X-ray device is often described in terms of the peak kilovoltage or kVp.