X-rays are produced by a device which beams electrons with an energy between 103 and 106 eV at a metal plate. The electrons interact with the metal plate and are stopped by it. Much of the energy of the incoming electrons is released in the form of X-rays, which are highenergy photons of electromagnetic radiation. An example of such a device is shown below. Electrons are accelerated from the cathode towards the anode by an electric field.
There are two mechanisms by which the X-rays are produced within the metal. The first mechanism is called bremsstrahlung, which is German for "breaking radiation." X-rays are emitted by the electrons as they are brought to rest by interactions with the positive nuclei of the anode.
The second mechanism occurs when an incoming electron knocks an inner electron out of one of the metal atoms of the anode. This electron is replaced by an electron from a higher energy level of the atom, and a photon making up the energy difference is emitted.
X-rays are absorbed by a material when they pass through it. The amount of X-rays absorbed increases with the density of the material. In addition, lower energy X-rays are more likely to be absorbed than higher
energy X-rays. (Note: 1 eV = 1.6 x 1019J; Planck's constant h = 4.1 x 10-15 eV-s; speed of light c = 3 x
108 m/s.)
Which of the following graphs best represents the relationship between the amount of X-rays absorbed per unit length of material and the energy of the X-rays, for lead, bone, and air?
