The history of the study of radioactivity began on March 1, 1896, when the famous French scientist Henri Becquerel accidentally discovered a strangeness in the emission of uranium salts. It turned out that photographic plates located in the same box with the sample are illuminated. This was due to the strange, highly penetrating radiation that uranium possessed. This property was found in the heaviest elements that complete the periodic table. He was given the name "radioactivity".
We introduce the characteristics of radioactivity
This process is the spontaneous transformation of an atom of an isotope of an element into another isotope with the simultaneous release of elementary particles (electrons, nuclei of helium atoms). The transformation of atoms turned out to be spontaneous, not requiring the absorption of energy from the outside. The main quantity characterizing the process of energy release during radioactive decay is called activity.
The activity of a radioactive sample is the probable number of decays of a given sample per unit time. In
SI (System International) its unit of measurement is called becquerel (Bq). In 1 becquerel, the activity of such a sample is taken, in which on average 1 decomposition per second occurs.
A = λN, where λ is the decay constant, N is the number of active atoms in the sample.
Allocate α, β, γ-decays. The corresponding equations are called displacement rules:
title | What's happening | Reaction equation |
α –decay | the transformation of the atomic nucleus X into a nucleus Y with the release of the nucleus of a helium atom | Z A X → Z-2 Y A-4 + 2 He 4 |
β - decay | the transformation of the atomic nucleus X into a nucleus Y with the release of an electron | Z A X → Z + 1 Y A + -1 e A |
γ - decay | not accompanied by a change in the nucleus, energy is released in the form of an electromagnetic wave | Z X A → Z X A + γ |
Time interval in radioactivity
It is impossible to establish the moment of particle breakdown for a given specific atom. For him, it is more like an “accident” than a pattern. The energy release characterizing this process is defined as the activity of the sample.
It is noticed that it changes over time. Although the individual elements exhibit surprising constancy of the degree of radiation, there are substances whose activity decreases several times in a fairly short period of time. Amazing variety! Is it possible to find a pattern in these processes?
It has been established that there exists a time during which exactly half of the atoms of a given sample undergo decay. This time interval is called the half-life. What is the point of introducing this concept?
What is the half-life?
It seems that in a time equal to the period, exactly half of all active atoms in a given sample decay. But does this mean that in two half-lives, all active atoms will completely decay? Not at all. After a certain moment, half of the radioactive elements remain in the sample, after the same period of time, another half of the remaining atoms decays, and so on. In this case, the radiation remains for a long time, significantly exceeding the half-life. This means that active atoms are retained in the sample regardless of radiation
Half-life is a quantity that depends solely on the properties of a given substance. The value of the value is determined for many known radioactive isotopes.
Table: “Half-life of the decay of individual isotopes”
| Title | Designation | Type of decay | Half life |
Radium | 88 Ra 219 | alpha | 0.001 seconds |
Magnesium | 12 Mg 27 | beta | 10 minutes |
Radon | 86 Rn 222 | alpha | 3.8 days |
Cobalt | 27 Co 60 | beta gamma | 5.3 years |
Radium | 88 Ra 226 | alpha, gamma | 1620 years |
Uranus | 92 U 238 | alpha, gamma | 4.5 billion years |
The determination of the half-life is performed experimentally. In the course of laboratory studies, the measurement of activity is carried out repeatedly. Since laboratory samples are of minimal size (the safety of the researcher is above all), the experiment is carried out with a different time interval, repeating many times. It is based on the regularity of changes in the activity of substances.
In order to determine the half-life, the activity of a given sample is measured at certain time intervals. Given the fact that this parameter is associated with the number of decaying atoms, using the law of radioactive decay, determine the half-life.
Definition Example for Isotope
Let the number of active elements of the studied isotope at a given moment of time be equal to N, the time interval during which observation t 2 - t 1 is carried out, where the moments of the beginning and end of observation are quite close. Suppose that n is the number of atoms decaying in a given time interval, then n = KN (t 2 - t 1 ).
In this expression, K = 0.693 / T½ is the proportionality coefficient, called the decay constant. T½ is the half-life of the isotope.
We take the time interval per unit. In this case, K = n / N indicates the fraction of the isotope nuclei present that decay per unit time.
Knowing the value of the decay constant, one can also determine the half-life of the decay: T½ = 0.693 / K.
It follows that, per unit time, not a certain number of active atoms decays, but a certain fraction of them.
The Law of Radioactive Decay (SRR)
The half-life is the basis for RRP. The pattern was deduced by Frederico Soddy and Ernest Rutherford based on the results of experimental studies in 1903. Surprisingly, multiple measurements made with instruments that are far from perfect, in the conditions of the beginning of the twentieth century, led to an accurate and reasonable result. He became the basis of the theory of radioactivity. We derive a mathematical record of the law of radioactive decay.
- Let N 0 be the number of active atoms at a given time. After the time interval t has passed, N elements will remain uncompleted.
- By the time moment equal to the half-life, exactly half of the active elements will remain: N = N 0/2 .
- After another half-life, the following remains in the sample: N = N 0/4 = N 0/2 2 active atoms.
- After a lapse of time equal to another half-life, the sample will retain only: N = N 0/8 = N 0/2 3 .
- By the time when n half-lives pass, N = N 0/2 n active particles will remain in the sample. In this expression, n = t / T½: ratio of study time to half-life.
- ZRR has a slightly different mathematical expression, more convenient in solving problems: N = N 0 2 - t / T½ .
The pattern makes it possible to determine, in addition to the half-life, the number of atoms of the active isotope that have not decayed at a given time. Knowing the number of atoms of the sample at the beginning of the observation, after some time, you can determine the lifetime of this drug.
The formula for the law of radioactive decay helps to determine the half-life only if certain parameters are present: the number of active isotopes in the sample, which is difficult to find out.
Law Consequences
It is possible to write down the formula of ZRR using the concepts of activity and atomic mass of a preparation.
Activity is proportional to the number of radioactive atoms: A = A 0 • 2 -t / T. In this formula, A 0 is the activity of the sample at the initial time, A is the activity after t seconds, T is the half-life.
The mass of the substance can be used in the laws: m = m 0 • 2 -t / T
During any equal periods of time, the absolutely identical fraction of the radioactive atoms available in this preparation decays.
The limits of applicability of the law
The law in every sense is statistical, determining the processes taking place in the microworld. It is clear that the half-life of radioactive elements is a statistical quantity. The probabilistic nature of events in atomic nuclei suggests that an arbitrary nucleus can fall apart at any moment. It is impossible to predict the event, you can only determine its probability at a given time. As a result, the half-life does not make sense:
- for a single atom;
- for a sample of minimum weight.
Atom lifetime
The existence of an atom in its original state can last a second, or maybe millions of years. Talking about the lifetime of this particle is also not necessary. By introducing a value equal to the average value of the lifetime of atoms, we can talk about the existence of atoms of a radioactive isotope, the consequences of radioactive decay. The half-life of an atomic nucleus depends on the properties of a given atom and does not depend on other quantities.
Is it possible to solve the problem: how to find the half-life, knowing the average lifetime?
To determine the half-life, the formula of the relationship between the average lifetime of an atom and the decay constant helps no less.
τ = T 1/2 / ln2 = T 1/2 / 0.693 = 1 / λ.
In this record, τ is the average lifetime, λ is the decay constant.
Use of half-life
The use of ZRR to determine the age of individual samples has become widespread in studies of the late twentieth century. The accuracy of determining the age of fossil artifacts has increased so much that it can give an idea of the life time for millennia BC.
Radiocarbon analysis of fossil organic samples is based on a change in the activity of carbon-14 (the radioactive carbon isotope) present in all organisms. It enters a living organism in the process of metabolism and is contained in it in a certain concentration. After death, the metabolism with the environment ceases. The concentration of radioactive carbon decreases due to natural decay, activity decreases proportionally.
If there is such a value as the half-life, the formula of the law of radioactive decay helps to determine the time from the moment the body stops functioning.
Radioactive Transformation Chains
Radioactivity studies were conducted in laboratory conditions. The amazing ability of radioactive elements to remain active for hours, days, and even years could not but surprise physicists in the early twentieth century. Studies, for example, of thorium, were accompanied by an unexpected result: in a closed ampoule, its activity was significant. At the slightest breath she fell. The conclusion was simple: the conversion of thorium is accompanied by the release of radon (gas). All elements in the process of radioactivity turn into a completely different substance, which is distinguished by both physical and chemical properties. This substance, in turn, is also unstable. At present, three series of similar transformations are known.
Knowledge of such transformations is extremely important in determining the time of inaccessibility of zones infected during atomic and nuclear research or catastrophes. The half-life of plutonium - depending on its isotope - lies in the range from 86 years (Pu 238) to 80 Ma (Pu 244). The concentration of each isotope gives an idea of the period of disinfection of the territory.
The most expensive metal
It is known that in our time there are metals much more expensive than gold, silver and platinum. Plutonium also belongs to them. Interestingly, plutonium created in the process of evolution does not occur in nature. Most of the elements obtained in the laboratory. The use of plutonium-239 in nuclear reactors has enabled it to become extremely popular these days. Obtaining a sufficient amount of this isotope for use in reactors makes it practically priceless.
Plutonium-239 is obtained in vivo as a result of the chain of transformations of uranium-239 into neptunium-239 (half-life is 56 hours). A similar chain allows the accumulation of plutonium in nuclear reactors. The rate of appearance of the required amount exceeds the natural billions of times.
Energy Applications
We can talk a lot about the shortcomings of nuclear energy and the “oddities” of humanity, which almost any discovery uses to destroy their own kind. The discovery of plutonium-239, which is able to take part in a nuclear chain reaction, allowed it to be used as a source of peaceful energy. Uranium-235, which is an analogue of plutonium, is extremely rare on Earth; it is much more difficult to isolate it from uranium ore than to obtain plutonium.
Earth Age
Radioisotope analysis of isotopes of radioactive elements gives a more accurate idea of the lifetime of a particular sample.
Using the chain of transformations of "uranium - thorium" contained in the earth's crust makes it possible to determine the age of our planet. The percentage of these elements on average throughout the earth's crust is the basis of this method. According to recent data, the age of the Earth is 4.6 billion years.