Radiation and Radioactivity (I)

Often, when we talk about the danger of nuclear energy we talk about the danger of radiation. Also, when we talk about the danger, not yet demonstrated by the way, of mobile telephones we talk about radiation too. Is it correct to use the word radiation in both cases? Is it always the same radiation?

Historically, radiation is a combination of oscillating electric and magnetic fields propagating in a medium. In other words, it is a moving wave. Opposite to sound waves which need a physical medium to propagate, electromagnetic waves, radiation, can also propagate in the vacuum such as the outer space.

As a side note, due to the vacuum is empty, if we ignore quantum fluctuations that continuously create particle and antiparticle pairs of course, and thus there is nothing that helps the propagation of sound waves, the big explosions heard when the starship Enterprise shoots a photon torpedo against a Klingon’s bird of prey should not be possible to be heard, but Star Trek is Star Trek and this fact can be forgiven.

These electromagnetic waves are characterised by an energy that depends on its wavelength (distance between two points with the same phase, such as crests) or its frequency (number of waves per second).

A wave

Wavelength and frequency are inversely related, so if the wavelength is high or there distance between two crests is large its frequency is low or there are few waves per second and vice versa. It we take the frequency as the reference magnitude, we have that the higher the frequency the greater the energy.

c=λν and E=hν

Mathematical relations between wavelength λ and frequency ν, and between the energy E and the frequency ν. c is the speed of light which is constant with an approximate value of 300.000 km/s and h is the Plank constant with a value of  6,63 x10-34 joules per second and that, as it names indicates, is constant.

Up to now, this does not say much about radiation, thus to go deeper we have to know that every body with a temperature T above absolute zero, which is equivalent to -273.15 Celsius (pure water freezes at 0 Celsius), emits radiation. If we take into account that the third principle of thermodynamics tells us that it is impossible to reach the absolute zero in a finite number of steps, we come to the conclusion that everything emits radiation! And if everything emits radiation, we have a problem, a big one, if every radiation is of the same kind! Because if it is of the same kind as the mobile telephone radiation and of the same kind of the one that escapes from nuclear power plants, since everything emits radiation we should have disappeared from the earth a long time ago.

Keep calm because it is not as bad. As I mentioned earlier, radiation is a wave that propagates with a frequency and thus it has an energy. Depending on the frequency we have a type of radiation or another and this is known as the electromagnetic spectrum.

Now is when things become more interesting. Electromagnetic radiation is classified as non-ionizing radiation and ionizing radiation. Non ionizing radiation is the one that doesn’t carry enough energy (its frequency is not high enough) so as to liberate one or more electrons from the atoms or molecules it collides with, while the ionizing radiation has enough energy to liberate such electrons. I guess we can now deduce what radiation is the bad on, cannot we?

The limit between what it is known as ionizing and non-ionizing radiation is not, however, very clear. If we have in mind that the ionization energy of a hydrogen atom is 13.6 eV (1 EV or electronvolt is the energy that an electron requires so overcome an electric potential barrier of 1 volt), the radiation that makes the hydrogen to lose an electron will be ionizing. Notwithstanding, matter, including ourselves, is made of different chemical elements which make that there is a wide range of ionization energies. In any case if we consider the hydrogen ionization energy as the lowest limit, by looking at the electromagnetic spectrum we have that for radiation with energies in the ultraviolet range and above towards gamma rays, it will be considered as ionizing radiation. Anyway, there is not an agreement on this limit yet, because as I said everything depends on the material the radiation is colliding with considering the ultraviolet frequency range as a lower limit.

We have seen that the electromagnetic radiation is everywhere and cannot avoid it; in fact, we are a large radiation source. Because we are visible to others, it means that we are emitting radiation, in this case in the visible frequency range. Additionally, we usually have a body temperature above 36 Celsius and thus we are also emitting radiation in the infrared frequency range. Therefore, is radiation dangerous? The answer is, in general terms, no. What we need to do is to use the correct terms when we speak and say that what it is dangerous is the ionizing radiation, especially at high energies.



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