Implications of Power and X-Ray Production

DOE-HDBK-1109-97

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

Student's Guide

X-rays can scatter off a target to the surrounding area, off a wall and into an adjacent

room, and over and around shielding. A common mistake is to install thick shielding

walls around an X-ray source but ignore the roof; X-rays can scatter off air molecules over

shielding walls to create a radiation field known as skyshine. The emanation of X-rays

through and around penetrations in shielding walls is called radiation streaming.

ii. Implications of Power and X-Ray Production.

When high-speed electrons strike the anode target, most of their energy is converted to

heat in the target, but a portion is radiated away as X-rays. As stated previously, the

electrical power of an electrical circuit is given by:

P=VxI

P is the power in watts or joules/second, V is the potential difference in volts, and I is the

current in amps.

The power developed in the anode of an X-ray tube can be calculated using this

relationship. Consider a 150 kilovolt (kVp) machine, with a current of 50 milliamps

(mA).

P = [150,000 (V)] [0.050 (I)] = 7500 W.

This is about the same heat load as would be found in the heating element of an electric

stove. This power is delivered over a very short period of time, typically less than 1

second. More powerful X-ray machines use higher voltages and currents and may develop

power as high as 50,000 W or more. Cooling the anode is a problem that must be

addressed in the design of X-ray machines. Tungsten is used because of its high melting

temperature, and copper is used because of its excellent thermal conductivity. These

elements may be used together, with a tungsten anode being embedded in a large piece of

copper.