Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 28

Bremsstrahlung (from the German meaning “braking radiation”) is the production of X‐rays when a charged particle undergoes a sudden change in velocity. When a high‐speed electron collides with an atomic nucleus electric field, there is an abrupt change in the particle's velocity, and a fraction of the particle's kinetic energy is converted into X‐rays. This fraction is extremely small for low‐energy betas and for low atomic numbered absorbers, but it increases with increasing energy and with increasing atomic number. For this reason, beta shields are made of materials of low atomic number. In practice, beta‐shielding material of atomic number higher than 13 (Al) is seldom used.

Bremsstrahlung production is of importance in two cases, the first being when it is deliberately used to generate useful X‐rays. In this application, electrons are emitted from the cathode in a specially designed high vacuum diode, Figure 3, and are accelerated across a high voltage (∼100 kV in medical diagnostic X‐ray units). When electrons strike the high atomic numbered tungsten anode, approximately 1/1000th of their kinetic energy is converted into electromagnetic X‐ray energy. The intensity of the X‐rays increases as the beam of accelerated electrons increase and as the accelerating voltage increases; the penetrating power of the X‐rays depends only on the high voltage, and increases with increasing high voltage. To protect against unwanted bremsstrahlung, X‐ray tubes are enclosed in lead shields that have shuttered apertures through which the useful beam escapes.

Second, bremsstrahlung X‐rays are an unwanted side effect of shielding betas or in an instrument or other device in which electrons are accelerated across high voltages, such as an electron microscope, a klystron microwave generator, or an electron beam metallurgical furnace. Since these devices are not intended to be used as an X‐ray source, these unwanted X‐rays can pose a serious hazard if the user or the industrial hygienist is unaware of their existence.

FIGURE 3 Stationary target X‐ray tube. X‐rays are formed via bremsstrahlung in the tungsten target. http://www.osha.gov/SLTC/radiationionizing/introtoionizing/ion3.gif.