history of the study of radioactivity began on March 1, 1896, when the famous French scientist Henri Becquerel accidentally discovered a strange radiation in uranium salts.It turned out that a photographic plate, placed in a box with a sample marred.This brought the country having a high penetrating power of radiation, which had uranium.This property is found at the very heaviest elements, completing the periodic table.He was given the name "radioactivity".
introduce the characteristics of radioactivity
This process - spontaneous transformation of the isotope in a different isotope of the element with the simultaneous release of elementary particles (electrons, nuclei of helium atoms).The conversion of atoms appeared spontaneous, requiring no external energy absorption.The main quantity characterizing the process of energy release during radioactive decay is called activity.
activity of a radioactive sample is the probability of the number of decays of the sample per unit time.In the SI (International System) unit of its called becquerel (Bq).The activity of 1 Bq adopted such a sample, wherein the average disintegration occurs in one second.
A = λN, where λ- decay constant, N - the number of active atoms in the sample.
There are α, β, γ-decay.The corresponding equations are called offset rules:
| name | What happens | reaction equation |
| α decay | transformation of the atomic nucleus Xthe kernel Y with a release of the atomic nucleus of helium | ZAH → Z-2YA-4 + 2He4 |
| β - decay | transformation of the atomic nucleus X kernel Y with a release of an electron | ZAH → Z + + 1YA -1eA |
| γ - decay | not accompanied by changes in the nucleus, the energy released in the form of an electromagnetic wave | ZHA ZXA → + γ |
Timeinterval of radioactivity
the collapse of the particles can not be set for this particular atom.For him, it is rather an "accident" than the pattern.The release of energy that characterizes the process, defined as the activity of the sample.
noticed that it changes over time.Although some elements show a surprising degree of constancy of the radiation, there are substances whose activity is reduced several times in a relatively short period of time.The amazing variety!Is it possible to find a pattern in these processes?
established that there is a time during which exactly half of the atoms of the specimen undergoing decay.This time interval is called "half-life".What is the meaning of the introduction of this concept?
What is the half-life?
It seems that for the time equal to the period, exactly half of all active atoms of the sample breaks.But does this mean that during the two half-lives of all active atoms completely disintegrate?Not at all.After a certain point in the sample is half of the radioactive elements by the same amount of time remaining atoms decomposes even half, and so on.The radiation persists for a long time, significantly greater than the half-life.So active atoms are retained in the sample, regardless of the radiation
half-life - a value that depends only on the properties of the substance.The value defined for many well-known radioactive isotopes.
Table: "The half-life decay of certain isotopes»
| name | designation | View decay | half-life |
| Radium |
88Ra219 | alpha | 0,001 seconds |
| Magnesium | 12Mg27 | beta | 10 minutes |
| Radon | 86Rn222 |
alpha | 3,8 days |
| Cobalt | 27Co60 | beta, gamma | 5,3 years |
| Radium | 88Ra226 | alpha, gamma | 1620 years |
| Uranus | 92U238 | alpha, gamma | 4,5 Ga |
Determining the half-lifeExperimental.In laboratory studies measuring the activity is carried out repeatedly.Since laboratory samples of minimum size (security researcher above all), the experiment is carried out with different intervals, repeated many times.It is based on a pattern of change of activity of substances.
In order to determine the half-life is measured activity of the sample at specific time intervals.Given that the parameter related to the quantity of disintegrated atoms, using the law of radioactive decay half-life is determined.
example for determining isotope
Let the number of active elements under study isotope at a given time is equal to N, the time interval during which the monitored t2- t1, where the beginning and end of the observation are quite close.Assume that n - number of atoms disintegrated in a given time interval, then n = KN (t2- t1).
This expression K = 0,693 / T½ - the coefficient of proportionality is called the decay constant.T½ - the half-life of the isotope.
We take the time interval for the unit.Thus K = n / N indicates the fraction of isotope nuclei present disintegrating per unit time.
Knowing the value of the decay constant, we can determine the half-life and decay: T½ = 0,693 / K.
This implies that per unit of time breaks are not a certain amount of active atoms and determined their share.
Law of radioactive decay (spp)
The half-life is the basis spp.The pattern derived Frederick Soddy and Ernest Rutherford on the basis of the results of experimental studies in 1903.Surprisingly, the multiple measurements made with instruments that are far from perfect in terms of the early twentieth century, led to an accurate and valid results.It became the basis of the theory of radioactivity.We derive mathematical notation of the law of radioactive decay.
- Let N0 - number of active atoms in a given time.After the interval of time t will nondecomposed N elements.
- By the time equal to the half-life remain exactly half of the active elements: N = N0 / 2.
- After another period of half-life in the sample are: N = N0 / N0 = 4/22 active atoms.
- the passage of time equal to another half-life, save only the sample: N = N0 / 8 = N0 / 23.
- By the time when the half-life will be held n periods, will remain in the sample N = N0 / 2n active particles.In this expression, n = t / T½: the ratio of time to study the half-life.
- spp has a slightly different mathematical expressions, more convenient in solving problems: N = N02-t / T½.
regularity to determine the half-life in addition to the number of atoms of active isotope nondecomposed at a given time.Knowing the number of atoms of the sample at baseline, after some time it is possible to determine the lifetime of the drug.
determine the half-life of radioactive decay law formula helps only if certain parameters: the number of active isotopes in the sample, it is quite difficult to learn.
consequence of the law
Record formula spp possible, using the concept of activity and the mass of the atoms of the drug.
activity is proportional to the number of radioactive atoms: A = A0 • 2-t / T.In this formula, A0 - activity of the sample at the initial time, A - activity after t seconds, T - the half-life.
mass of a substance can be used in patterns: m = m0 • 2-t / T
during any equal intervals of time breaks completely the same proportion of the radioactive atoms available in this preparation.
limits of applicability of the law
law in all respects is a statistical, defining processes in the microcosm.It is understood that the half-life of radioactive elements - the statistical value.The probabilistic nature of events in atomic nuclei suggests that arbitrary kernel can fall apart at any moment.Predict the event it is impossible, we can only determine its credibility at a time.As a result, the half-life does not make sense:
- for a single atom;
- sample minimum weight.
Lifetime atom
existence of the atom in its original condition may last for a second, and perhaps millions of years.Talking about the lifetime of a given particle is also not necessary.Enter the amount, equal to the average lifetime of the atoms, you can talk about the existence of atoms of a radioactive isotope effects of radioactive decay.The half-life of the nucleus of an atom depends on the nature of the atom and does not depend on other variables.
it possible to solve the problem: how to find a half-life, knowing the average lifetime?
determine the half-life formula relating the average lifetime of the atom and the decay constant help, no less.
τ = T1 / 2 / ln2 = T1 / 2 / 0.693 = 1 / λ.
This entry τ - the average lifetime, λ - the decay constant.
Using half-life
spp Application for determining the age of the individual samples is widespread in studies of the late twentieth century.The accuracy of determining the age of fossils artifacts has increased so much that can give an idea of the life time of the millennium BC.
Radiocarbon analysis of fossil specimens based on the change in the activity of carbon-14 (radiocarbon) present in all organisms.He gets into a living organism in the process of metabolism and found in it a certain concentration.After the death of metabolism with the environment is terminated.The concentration of radioactive carbon falls due to natural decay, the activity is reduced in proportion.
With such values, the half-life, the formula of the law of radioactive decay helps to determine the time of the termination of life of the organism.
Chains radioactive transformation
radioactivity studies were conducted in the laboratory.The amazing ability of radioactive elements remain active for hours, days or even years could not come as a surprise to physicists of the early twentieth century.Studies, for example, thorium accompanied by an unexpected result: in a sealed vial of his activity was significant.At the slightest whiff of it fell.The conclusion was simple: the conversion of thorium accompanied by the release of radon (gas).All elements in the process of radioactivity turn into a completely different matter and different physical and chemical properties.This compound, in turn, is also unstable.It is now known three rows of similar transformations.
Knowledge of these transformations are extremely important in determining the time of inaccessibility areas contaminated in the process of atomic and nuclear research, or disasters.The half life of plutonium - depending on its isotopes - in the range from 86 (238 Pu) to 80 million years (244 Pu).The concentration of each isotope gives an indication of the period decontamination area.
most expensive metal
known that nowadays there are much more expensive metal than gold, silver and platinum.These include the plutonium.Interestingly, in the nature created in the evolution of the plutonium is not found.Most of the elements obtained in the laboratory.Operation of plutonium-239 in nuclear reactors has enabled him to become extremely popular these days.Obtaining sufficient for use in a number of reactors of the isotope makes it practically invaluable.
Plutonium-239 is obtained in vivo as a result of chain reactions of uranium-239, neptunium-239 (half-life - 56 hours).A similar chain allows you to accumulate plutonium in nuclear reactors.The rate of occurrence of the required number exceeds the natural billions of times.
Application in Energy
can talk a lot about the shortcomings of nuclear power and the "strangeness" of humanity, which uses almost any opening for the destruction of their own kind.The discovery of plutonium-239, which is able to participate in a nuclear chain reaction, allowed to use it as a source of peaceful energy.Uranium-235 is an analogue of the plutonium found on Earth extremely rare, select it from the uranium ore is much more difficult than getting the plutonium.
Age Earth
radioisotope analysis of isotopes of radioactive elements gives a more accurate picture of the life time of a sample.
use chain transformations "uranium - thorium" contained in the earth's crust, makes it possible to determine the age of our planet.The percentage of these elements in average throughout the crust underlies this method.According to the latest data, the age of the Earth is 4.6 billion years old.
