X-rays characteristics

X-Rays
X-rays are a form of electromagnetic radiation. They belong to the short-wavelength, high-frequency end of the electromagnetic spectrum, between the gamma and the ultraviolet radiation. They have wavelengths in the range of 10^-8m to 10^-10m. Their frequency range is 3x10^16Hz to 3x10^19Hz. X-rays are produced by the movement of electrons in atoms. When a photon collides with another atom, the atom may absorb the photon’s energy causing an electron to jump to a higher energy level. This can only happen if the energy level of the photon matches the energy difference between the two electron levels. The electron then falls back to its original energy level, releasing the extra energy in the form of a light photon.

The soft tissue in our body is composed of atoms that do not absorb x-ray photons very well, because their energy levels do not match the energy of the photons. However, the bone tissue absorbs these same photons quite well, due to the calcium atoms which have higher energy levels between its atoms that match the photons energy.

There are two types of X-rays, according to their photon energy:

1. Soft x-rays

These x-rays are defined by having photon energies below 10keV. They have less energy than the hard x-rays, therefor they have higher wavelength. Soft x-rays are used in radiography to take pictures of bones and internal organs. Because they have a relatively low frequency, they do not cause much damage to tissues, unless they are repeated too often.

2. Hard x-rays

Hard x-rays have photon energies above 10 keV. They have smaller wavelength than the soft x-rays. These x-rays are used in radiotherapy, a treatment for cancer. Due to their high frequency, they destroy molecules within specific cells, thus destroying tissue. Another use for these x-rays is airport security. Hard x-rays are used in security scanners to examine baggage.

Production of X-rays

The X-rays were discovered in 8th November in 1895 when Wilhelm Conrad Roentgen was working with a cathode ray tube in his laboratory. X-rays for medical diagnostic procedures are produced in a X-ray tube.

X-ray tube

The tube itself is evacuated, and contains two electrodes:

Cathode: the heated filament acts as the cathode (negative) from which electrons are emitted

Anode: the anode (positive) is made of a heavy metal, usually tungsten.

An external power supply produces a voltage of up to 200kV between the two electrodes. This accelerates the electrons across the gap between the cathode and the anode. The kinetic energy of an electron arriving at the anode is around 200kV. When the electrons strike the anode at high speed, they lose some of their kinetic energy in the form of X-ray photons which emerge in all directions.

Only a small fraction of kinetic energy of electrons are converted into x-rays. Most of the incident energy is transfered to the anode, which becomes hot. Some x-ray tubes have water circulating through the anode to remove this excess of heat. The x-rays that emerge from the x-ray tube have a range of energies, represented in a X-ray spectrum. This spectrum have two components: the Brehmsstrahlung radiation and the characteristic x-rays. These arise from different ways is related to the way which an individual electron loses it's energy when crashes into the anode.

When the electron striking into anode loses its energy and interacts with the electric fields of the anode nucleus this may result in a single x-ray photon or several photons. These all contribute to Brehmsstrahlung radiation.

An electron may cause a rearrangement of the electrons in the anode atom in which an electron drops from a high energy level to a lower energy level. As it does so, it emits a photon with a defined frequency. This contributes to the characteristic x-rays that are characteristic of the anode (if the anode is made of copper instead of tungsten the characteristic x-rays will be different).