X-Rays and Gamma Rays - hdbk110997

DOE-HDBK-1109-97

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

Instructor=s Guide

Lesson Plan

Instructor=s Notes

ii. X-Rays and Gamma Rays.

Objective 2

X-rays and gamma rays both ionize atoms. The

energy required for ionization varies with material

(e.g., 34 eV in air, 25 eV in tissue) but is generally

in the range of several eV. A 100 keV X-ray can

potentially create thousands of ions.

As discussed in Module 101, the distinction

between X-rays and gamma rays is their origin, or

method of production. Gamma rays originate from

within the nucleus; X-rays originate from atomic

electrons and from free electrons decelerating in the

vicinity of atoms (i.e., bremsstrahlung).

In addition, gamma photons often have more

energy than X-ray photons. For example,

diagnostic X-rays are about 40 keV, whereas

gammas from cobalt-60 are over 1 MeV. However,

there are many exceptions. For example, gammas

from technicium-99m are 140 keV, and the energy

of X-rays from a high-energy radiographic machine

may be as high as 10 MeV.

X-RAY PRODUCTION

Radiation-producing devices produce X-rays by

accelerating electrons through an electrical voltage

potential and stopping them in a target. Many devices

that use a high voltage and a source of electrons

produce X-rays as an unwanted byproduct of device

operation. These are called incidental X-rays.