Zelda L. Kravitz
X-rays are produced in a cathode ray tube (Figure 1). These tubes produce a stream of electrons moving in a straight line from the cathode (-) to the anode (+). The electric potential between the two terminals is expressed in kilovolts (kV) and the higher it is, the greater the energy imparted to the electron stream. The number of electrons, or current, is measured in milliamperes (mA). The higher the amperage, the greater the flow of electrons.
The cathode (negative side of the tube) consists of one or two filaments made of tungsten. An x-ray generator supplies electrical current which flows through the filaments heating them to extremely high temperatures. When they get hot enough, electrons fly off from the tungsten filament and are propelled through the vacuum of the tube towards the anode. The reason that tungsten is used in the wires is that it has an extremely high melting point (3380o) and doesn’t vaporize easily. The electrons move with tremendous speeds once they leave the cathode; although the distance between the cathode and anode is only from one to three cm, they may accelerate to half the speed of light (186,000 miles/second) by the time they hit the anode target.
This target is made of a small square of tungsten embedded in a larger piece of copper. The projectile electrons strike the tungsten target giving off huge amounts of heat, but since the tungsten has such a high melting point, it is not altered and the heat is conducted away by the copper part of the anode.
The atomic basis for this heat production is interesting. The speeding electrons have different energies. Those with the lowest energies may strike outer orbital electrons of the tungsten target just hard enough to make them vibrate in place. This vibration causes corresponding low energy electromagnetic waves in the wave length of infrared to be given off. Infrared is, of course, heat. More than ninety-five percent of the energy given off in an x-ray tube is heat.
The remaining less than five % of the energy emitted when the electrons strike the target is in the form of x-rays. There are two ways in which these x-rays are produced: by Bremsstrahlung radiation and by Characteristic radiation.
Bremsstrahlung is a German word. Brems means “braking or slowing down” and strahlung is “radiation”. Bremsstrahlung, which produces 85% of the x-rays in a tube, is caused by the attraction of the projectile electron with its negative charge to the nucleus of the tungsten atom in the anode with its positive charge. This attraction causes the electron to change its path and, therefore, to slow down (Figure 2). Part of its kinetic energy is converted to a photon of x-ray energy. An electron may be involved in many of these interactions with a tungsten nucleus and each time, some of its kinetic energy is converted to x-rays until it finally stops moving. Tungsten is advantageous as the target material because heavier atoms (it has an atomic number of 74) produce more bremsstrahlung than lighter atoms. The reason why we can’t use an even heavier atom such as gold (Z = 79) instead of tungsten is that gold melts at 1063°C (whereas tungsten melts at 3380°C).
Bremsstrahlung radiation accounts for about 85% of the total x-ray production of a tube, and characteristic radiation makes up the other 15%. Characteristic radiation occurs when a projectile electron actually knocks out an inner orbital electron of a tungsten atom in the anode target. To take its place, an electron from an outer shell drops down into the lower orbit and in so doing gives off a photon of x-ray energy equivalent to the difference between the electron binding energy of the two shells (Figure 3). For instance: if an electron in the K shell of the tungsten atom is knocked out (binding energy of the K shell is 69.5 keV) and an electron from the L shell (binding energy = 12.1 keV) drops down to take its place, then an x-ray photon of 57.4 keV will be emitted. All K shell x-rays of tungsten have an effective energy of from 57 to 69 keV. The same phenomenon is true for the other shells as well. Whenever an electron drops from a higher shell to a lower shell, a definite amount of energy is always given off. Since this kind of energy is based on the energy levels of the shells, it is called Characteristic energy. Characteristic energy, with its definite bands of emitted energy, stands in contrast to Bremsstrahlung radiation, where x-rays are produced with a continuous spectrum of energy.
One interesting note should be added about mammography (breast radiography). It is desirable to have low energy x-rays when this procedure is done. Since atoms with lower atomic numbers have lower shell energies than those with higher atomic numbers, a molybdenum target is used in the anode instead of tungsten. The K shell effective energies of the molybdenum atom are in the range of 17.9 to 19.5 keV.
