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Chapter 513 Radioisotopes

But why are some isotopes radioactive?

This has to start with the history of the discovery of isotopes. Since the discovery of radioactivity in the late 19th century, by the beginning of the 20th century, humans on the blue planet had discovered more than 30 types of radioactive elements. It has also been proven that although some radioactive elements have significantly different radioactivity, their chemical properties are exactly the same.

Then in 1910, the British chemist F. Soddy proposed a hypothesis that chemical elements exist in different varieties with different relative atomic mass and radioactivity but the same other physical and chemical properties. These varieties should be in the same position in the periodic table and are called isotopes.

Soon, the relative atomic mass of lead was found to be 206.08 and that of another 208 from different radioactive elements.

In 1897, American physicist W. Thomson discovered the electron. Then in 1912 he improved the instrument for measuring electrons and used the effect of magnetic field to make a magnetic separator, which was the predecessor of the mass spectrometer.

When he used neon gas for measurement, no matter how the neon was purified, he got two parabolas on the screen, one representing the mass of 20 neon and the other representing the mass of 22 neon.

This was the first stable isotope discovered, that is, a non-radioactive isotope. When FW Aston built the first mass spectrometer, he further proved that neon did have two isotopes with different atomic masses, and discovered more than 200 isotopes from more than 70 other elements. So far, 109 elements have been discovered, and only 20 elements have no stable isotopes, but all elements have radioactive isotopes.

Most natural elements are mixtures of several isotopes, with about 300 stable isotopes and more than 1,500 radioactive isotopes.

The discovery of isotopes has deepened the understanding of the atomic structure of the Blue Planet. This not only gave the concept of elements a new meaning, but also brought about a major change in the standard of relative atomic mass, proving once again that it is the number of protons and nuclear charge, rather than the atomic mass number, that determines the chemical properties of elements.

Radioactive isotopes are unstable and will change. The nuclei of radioactive isotopes are very unstable and will continuously and spontaneously emit radiation until they become another stable isotope. This is called nuclear decay.

When radioactive isotopes undergo nuclear decay, they can emit alpha rays, beta rays, gamma rays, and electron capture, etc. However, radioactive isotopes do not necessarily emit these rays at the same time when they undergo nuclear decay.

The speed of nuclear decay is not affected by external conditions such as temperature, pressure, electromagnetic field, etc., nor is it affected by the state of the element. It is only related to the nuclide itself. The speed of radioactive isotope decay is usually expressed in terms of "half-life".

The half-life is the time it takes for a certain number of radioactive isotope atoms to decrease to half of their initial value. For example, the half-life of phosphorus 32 is 14.3 days, which means that if there were originally one million phosphorus 32 atoms, after 14.3 days, only 500,000 would be left.

The longer the half-life, the slower the decay, and the shorter the half-life, the faster the decay. Half-life is a characteristic constant of radioactive isotopes. Different radioactive isotopes have different half-lives, and the types and quantities of radiation emitted during decay are also different.

A radioisotope is an atom with an unstable nucleus. Each atom has many isotopes. Each group of isotopes has the same atomic number but different atomic weights. If the atom is radioactive, it is called a physical radionuclide or radioisotope. Radioisotopes undergo radioactive decay, emitting gamma rays and subatomic particles. Chemists and biologists use radioisotope technology to affect our food, water, and health. However, they are aware of the dangers and have developed safety guidelines for their use. Some radioisotopes occur naturally, while others are artificially created.

The decrease in the number of radioactive isotope atoms follows an exponential law. As time goes by, the number of radioactive atoms decreases in geometric progression, which can be expressed as: N=N0e-λt Here, N is the number of radioactive atoms remaining after decaying for t time, N0 is the initial number of radioactive atoms, and λ is the decay constant, which is a constant related to the properties of this radioactive isotope, λ=y(t)=e-0.693t/τ, where τ refers to the half-life.

The SI system is used for calculating radioactive intensity. In the SI, the unit of radioactive intensity is expressed as Becquerel, abbreviated as Becquerel, which is a nuclear decay that occurs in 1 second, and the symbol is Bq. 1Bq=1dps=2.703×10-11Ci. This unit reduces the conversion steps in practical applications and is convenient for use.

When the radiation emitted by radioactive isotopes hits various substances, various effects will occur, including the effect of radiation on substances and the effect of substances on radiation. For example, radiation can make photographic film and nuclear emulsion sensitive to light; make some substances produce fluorescence; can penetrate substances of a certain thickness, and in the process of penetrating substances, can be partially absorbed by the substances, or scattered; and can also ionize the molecules of some substances.

In addition, when radiation irradiates humans, animals and plants, it will cause physiological changes in the organisms. The interaction between radiation and matter is a process of energy transfer and energy loss for nuclear radiation, and a physical reaction and absorption process of external energy for the irradiated matter.

Due to their different properties, various rays have different characteristics in their interactions with matter. These characteristics are often related to the density and atomic number of the matter. When alpha rays pass through matter, they transfer their radiation energy to the matter mainly through ionization and excitation. Their range is very short, about one centimeter in the air and only twenty-three micrometers in lead metal.

When electrons pass near the nucleus, they are accelerated by the Coulomb field and radiate electromagnetic waves, which is called Bremsstrahlung. Bremsstrahlung is a continuous electromagnetic radiation, and its occurrence probability is proportional to the energy of the beta ray and the atomic number of the substance. Therefore, low-density materials are used in protection to reduce Bremsstrahlung.

β rays can be absorbed by aluminum layers that are not too thick. γ rays have the strongest penetrating power and the longest range. The range of 1MeV r rays in the air is about meters. R rays acting on matter can produce photoelectric effect, Compton effect and electron pair effect. It will not be completely absorbed by matter, but will gradually weaken as the thickness of the matter increases.

Although radioactive isotopes are harmful to the human body, they are also very useful.

For example, radiography technology can display the internal conditions of an object on a photograph. The application of this is naturally the use of X-rays in hospitals to check for fractures, etc., which is referred to as X-rays.

In addition to filming, there are also applications in measurement technology, such as the determination of the age of paleontology, monitoring and controlling the thickness of materials in the production process, etc. The most famous of these is the famous carbon-14 dating method.

In the treatment of cardiovascular diseases, radioactive isotopes are used as tracers, the most famous of which is cardiovascular contrast agent.

Then there is the use of energy from radioactive isotopes as energy for spacecraft, artificial hearts, etc., which are collectively called nuclear batteries.

(End of this chapter)