Radioactivity

To perform the test, a photographic film was used, which blackened under the influence of these rays. The experiment has shown that solar radiation is not necessary for the test material to blacken the film. However, this contradicted the initial hypothesis, so further research had to be conducted. This time, the task was to confirm or deny X-rays as the factor causing the change in the film. The fact that X-rays do not carry any charge, so they are not bent by the magnetic field, was used. Becquerel placed a material containing uranium and the photographic film in a vacuum chamber located in a magnetic field. As a result of the experiment, it was discovered that the rays emitted by the tested sample bend in the magnetic field, which meant it was not the same as X-rays. In the course of continuing his research, the scientist showed that there are three types of radiation that materials can emit – neutral, positively charged and negatively charged. Based on these assumptions, subsequent researchers put forward their theses and conducted further experiments. Maria Skłodowska Curie and Pierre Curie discovered next radioactive elements: polonium and radium. Ernest Rutherford, a Nobel Prize winner in chemistry, also devoted a part of his career to radioactivity and named the types of radiation discovered by Becquerel with the Greek letters of the alphabet: alpha, beta and gamma.

Where does radioactivity come from?

Research has shown that radioactivity is a feature of some elements. This indicates its close relationship with their atomic level, and more specifically – with the atomic nucleus. Each of the three types of radiation is a quantum of energy that an unstable nucleus can emit. This means that the radiation is actually the result of their decay. The occurrence of such unstable nuclei is in practice caused by their ratio of protons to neutrons. Due to the different types of instability, there are also different types of radiation. The key to understanding radioactivity are isotopes and their differences at the atomic level. For example, the most common carbon isotope ¹²C is not radioactive, while ¹⁴C is radioactive. Their atomic number, and therefore the number of protons, is the same. The difference also cannot be due to the number of electrons, because the atom would then be a positively or negatively charged ion. The only possibility, in the case of isotopes, are differences in the number of neutrons in the nucleus. ¹⁴C has two more neutrons than ¹²C, so it is also heavier. The existence of isotopes for all elements means that the number of radioactive nuclei is also large. Such unstable and radioactive isotopes are called radioisotopes, but their prevalence is not as high as those visible in the periodic table of elements.

Why can the nucleus be unstable?

The protons and neutrons in the nucleus are subject to large nuclear forces that hold them together, overcoming the electrostatic repulsion between the protons. Unlike protons, neutrons have a positive effect on strengthening the nuclear force. It was shown that the ratio of the number of neutrons to protons should be about 1.5:1. For lighter atoms below 20u, the stable ratio is 1:1. Otherwise, the nuclei tend to disintegrate. All isotopes of elements with an atomic weight above 208 are unstable.

Picture 1 Graph of the area of stability depending on the number of nucleons in the nucleus. Source: http://ch302.cm.utexas.edu/nuclear/radioactivity/selector.php?name=band-stability

The graph presented above shows the dependence of the stability of the nucleus on the number of individual nucleons. The black line corresponds to a 1:1 ratio of neutrons to protons. Stable isotopes are marked with black squares and unstable isotopes with corresponding colours according to the legend. The stability of radioactive isotopes is greater the closer they are to a stable ratio. According to the graph, we can distinguish three types of decays:

1. Alpha decay, especially in massive nuclei, emitting two protons and two neutrons,
2. Beta minus decay, when the nucleus has too few protons, resulting in the emission of electrons,
3. Beta plus decay, if the atomic nucleus has an excess of protons, with the emission of positrons.

These types of decays involve a change in the number of protons in the nucleus, and therefore also a change of the chemical element from one to another.

Alpha (α) radiation

Experimentally, in 1909 E. Rutherford and T. Royds showed that alpha particles are identical to helium ions. After passing the alpha radiation through the thin walls of a vacuum chamber, the image of the spectral lines on the optical spectrometer obtained in the gas chamber clearly confirmed this. For example, the ²⁴⁰Pu nucleus undergoes alpha radiation, according to the reaction:

Alpha radiation is characterized by positively charged particles with a range of several centimetres in air and very low permeability. A piece of paper will stop them.

Beta (β) radiation

In the case of beta-minus radiation, electrons coming from the nucleus are emitted. Since nuclei are not characterized by the presence of electrons, they are formed only during decay, and in addition to them, a second particle is also emitted – the electron anti-neutrino. Due to the increase in the number of protons in the molecule, the beta decaying element transforms into another element with a higher atomic number. An example of a decay course is consistent with this mechanism:

This radiation is characterized by negatively charged particles with a range of several dozen centimetres in the air and greater permeability compared to alpha radiation. It is stopped by an aluminium sheet with a thickness of about 3 to 4 mm.

Beta-plus decay occurs similarly, but it emits positron particles and electron neutrinos. The number of protons in the nucleus decreases and the element that undergoes it is transformed into another one with a lower atomic number, as in the example:

Gamma (γ) radiation

It is the only type of radiation that does not change an element into another, because it does not emit any particles, only gamma radiation itself. It is one of the types of electromagnetic waves, similarly to infra-red or ultraviolet light, but the gamma radiation wave is the shortest.   The decay process causes the transition of the excited nucleus of the atom to a state of lower energy, resulting in the emission of photons with the energy equivalent of the nucleus of the atom in individual states. The diagram of the course of gamma decay can be written as:

The characteristic of gamma radiation is its identification with an electromagnetic wave of high frequency and light-like nature. The range in air is theoretically unlimited, while permeability is the greatest of all types of radioactivity. It is stopped only by a thick wall or a fifteen centimetre layer of lead.