Decaimento radioativo

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O decaimento radioativo (ou radioatividade) é a propriedade de alguns átomos que os faz expelir espontaneamente energia como partículas ou raios. Átomos radioativos emitem radiação ionizante quando decaem, o que significa que eles têm energia suficiente para romper ligações químicas em moléculas ou remover elétrons ligados de átomos, criando assim moléculas ou átomos carregados (íons).[1]

Propriedades

Átomos radioativos são formas instáveis de átomos que também são conhecidos como radionuclídeos. Um átomo é instável (radioativo) se as forças entre as partículas que compõem o núcleo estão desequilibradas--se o núcleo tem um excesso de energia interna. A instabilidade do núcleo de um radionuclídeo pode resultar de um excesso de nêutrons ou prótons. Um núcleo instável irá continuamente vibrar e se contorcer e, mais cedo ou mais tarde, tentar alcançar a estabilidade por alguma combinação de meios:

  • Ejetar nêutrons e prótons
  • Conversão de um para o outro com a ejeção de uma partícula beta ou pósitron
  • Libertar a energia adicional por emissão de fotóns (isto é raios gama).[2]

Produto

Ficheiro:Uranium decayseries.JPG
Cadeia de decaimento de elementos radioativos em chumbo.

Decaimento radioativo ocorre quando o núcleo instável emite radiação (desintegra). O radionuclídeo é assim transformado em diferentes nuclidos (muitas vezes chamado de nuclide filho). Continuará a decair até que as forças no núcleo estejam equilibradas. Por exemplo, quando um radionuclídeo decai, ele se tornará um isótopo diferente do mesmo elemento se o número de nêutrons mudar e um elemento diferente se o número de prótons mudar.

Frequentemente, quando um radionuclídeo decai, o produto de decaimento (o novo nuclídeo) também é radioativo. Isto é verdade para a maioria dos materiais radioativos que ocorrem naturalmente. Para se tornarem estáveis, esses materiais devem passar por várias etapas, tornando-se uma série de diferentes nuclídeos e liberando energia como partículas ou raios em cada etapa. As séries de transformações que um dado radionuclídeo sofrerá, bem como o tipo de radiação que ele emite, são características do radionuclídeo. Isso é chamado de "cadeia de decaimento".[2]

O radionuclídeo sofrerá decaimento se houver um grupo de partículas com uma massa total menor que possa ser alcançada por decomposição ou por fissão nuclear (o núcleo se divide em núcleos menores). Todos os elementos com um número atômico superior a 83 (o número atômico do bismuto) são radioativos. Além disso, vários elementos com números atômicos menores possuem isótopos radioativos que ocorrem naturalmente. Físicos nucleares também criaram dois elementos sintéticos com números atômicos inferiores a 83 para preencher duas lacunas na tabela periódica; ambos são radioativos.

Rate

Every radioactive element or isotope decays at its own rate. The most common published statistic on the rate of decay of any radionuclide is the half-life. This is the hypothetical amount of time that must pass for half of the element or isotope to decay to its next daughter nuclide. Under normal circumstances, an isotope's half-life does not change, nor has any nuclear physicist ever produced a change in any isotope's half-life. However, the RATE Group has developed clear and convincing evidence that the half-lives of all then-naturally-occurring radioactive elements was accelerated greatly at the time of the Global Flood--and furthermore, this change might have triggered that event. (See: Accelerated decay).

Decay types

Radio nuclides of different types can be involved in several different reactions that produce radiant energy. The three main types of ionizing radiation are alpha, beta, and gamma.

  1. Alpha decay- Two protons and two neutrons emitted from nucleus
  2. Beta decay- A neutron emits an electron and an antineutrino and becomes a proton
  3. Gamma decay- Excited nucleus releases a high-energy photon
  4. Positron emission- A proton emits a positron and a neutrino and becomes a neutrino
  5. Internal conversion- Excited nucleus transfers energy to an orbiting electron and ejects it
  6. Proton emission- A proton is ejected from nucleus
  7. Neutron emission- A neutron is ejected from nucleus
  8. Electron capture- A proton combines with an orbiting electron, emits a neutrino and becomes a neutron
  9. Spontaneous fission- Nucleus disintegrates into two or more random smaller nuclei and other particles
  10. Cluster decay- Nucleus emits a certain type of smaller nucleus that are larger than an alpha particle
  11. Double-beta decay- two neutrons emit two electrons and two antineutrons become two protons
Property Alpha radiation Beta radiation Gamma radiation
Composition Alpha particle (helium nucleus) Beta particle (electron) High-energy electromagnetic radiation
Symbol Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): ^{{4}}_{{2}}{\mathrm {He}}

or Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): \alpha

Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): ^{{0}}_{{-1}}e

or Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): \beta

Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): \gamma

Charge 2+ 1- 0
Mass 4 1/1837 0
Penetrating power Low Moderate Very high

[3]

Alpha

Alpha, beta, and gamma radiation have differing abilities to penetrate substances. Alpha particles have low power and can be shielded by a sheet of paper or by human skin. Some beta particles can be stopped by human skin, but some need a thicker shield (like wood) to stop them. Gamma rays are the most penetrating of the three types of radiation, requiring a shield at least as thick as a concrete wall.[2]

Alpha radiation are helium nuclei that have been emitted from a radioactive source. The Alpha particle includes two protons and two neutrons and has a 2+ charge. An alpha particle can be written as 42He or as α in nuclear equations. The atomic number of the daughter atom is reduced by 2 and its mass number is lower by 4 when an atom loses an alpha particle.[4]

For example, examine the following chemical equation. Superscripts represents the mass numbers and subscripts represents the atomic numbers.

Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): ^{{238}}_{{92}}{\mathrm {U}}\to _{{90}}^{{234}}{\mathrm {Th}}+_{{2}}^{{4}}{\mathrm {He}}

(α emission)

The sum of the atomic masses of Thorium and alpha particle is equal to that of Uranium. As are the sums of the atomic numbers.[5]

Beta

There are 3 types of Beta decay: electron emission, electron capture, and positron emission. [5] During electron emission, a neutron changes into a proton with the loss of an electron. For example, 31H becomes 32He with the loss of 0-1e.

A beta particle can be written as 0-1e or β in nuclear equations. The superscript 0 shows that electron has very small mass compared to proton. Since its subscript is -1, the electron has negative charge.[6]

Falhou ao verificar gramática (Falha na conversão para PNG;

verifique se o latex, dvips, gs e convert foram correctamente instalados): ^{{14}}_{{6}}{\mathrm {C}}\to _{{7}}^{{14}}{\mathrm {N}}+_{{-1}}^{{0}}e

(β emission)

Since Carbon-14 emits a beta particle, the nitrogen-14 atom has the same atomic mass number (both of their superscripts are same), but its atomic number is increased by 1. It means that it contains one more proton and one fewer neutron.

Gamma

A gamma ray is a high-energy photon emitted by a radioisotope. Sometimes, nuclei emit gamma rays with alpha or beta particles during radioactive decay as you can see in the following equation.

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verifique se o latex, dvips, gs e convert foram correctamente instalados): ^{{230}}_{{90}}{\mathrm {Th}}\to _{{88}}^{{226}}{\mathrm {Ra}}+_{{2}}^{{4}}{\mathrm {He}}+\gamma


Since gamma rays do not have any mass, it does not affect the atomic number or mass number of an atom. [7]

History of discovery

Radioactivity was first discovered by accident in 1896 by a French scientist, Henri Becquerel. He was experimenting with fluorescent and phosphorescent materials to help understand the properties of x-rays and their ability to expose photographic film, which had been discovered in 1895 by Wilhelm Roentgen. Upon seeing x-ray exposed film, he immediately thought of putting some phosphorescent rocks on photographic paper to see if it would darken the film in the same way.[8]

He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper.[9] As Becquerel had anticipated, the phosphorescent salts had produced an image on the film. He theorized that the uranium absorbed the sun’s energy and then emitted it as x-rays. His theories were proven false when it became overcast in Paris putting off further experiments for a couple of days. He placed the photographic plates and the uranium salt in a drawer and for some unknown reason, decided to develop the photographic plates anyway.[10] He was surprised to find a strong and clear image exposed onto the film, proving that the uranium emitted radiation without an external source of energy such as the sun. During this fortuitous sequence of events Becquerel had discovered radioactivity.[9]

Marie Curie, who was one of Becquerel's students and her husband Pierre, continued to study radiation while working in Becquerel's lab. While testing an ore of uranium (pitchblende), for its ability to turn air into a conductor of electricity, she discovered that a much more active element than uranium must exist within the ore. She named this new element polonium, and coined the term radioactivity to describe the process.[11] Henri Becquerel, Marie and Pierre Curie jointly received the Nobel Prize in physics in 1903 for their discovery of radioactivity and their other contributions in this area.[10]

Accelerated nuclear decay

Main article: Accelerated decay

In 1970 through 1971, Robert Gentry provided two lines of evidence that, he suspected, supported the idea of an unknown form of radiation: giant radioactive halos and unique lead isotope ratios.

“Previously unreported lead isotope ratios, that is, values for the lead-206/lead-207 ratio ranging from about 20 to 60, primarily radiogenic in origin but unsupported by uranium decay, have been determined in the inclusions of certain polonium halos by means of ion microprobe techniques. Evidence for radiogenic lead-208 unsupported by thorium decay may also be inferred from the existence of a composite polonium halo type with rings from the radioactive precursors of lead-208. Several new dwarf halo sizes, seem to indicate the existence of unknown, very low-energy alpha-emitters. Furthermore, the three-ring "X halo" also provides evidence for an unknown series of genetically related alpha-emitters with energies in the range from 3 to 7 million electron volts.”- Robert Gentry [12]
“A new group of giant radioactive halos has been found with radii in excess of anything previously discovered. Since alternate explanations for these giant halos are inconclusive at present, the possibility is considered that they originate with unknown alpha radioactivity, either from isomers of known elements or from superheavy elements.”-Robert Gentry [13]

Since then, the RATE Group developed definitive evidence that what Gentry had discovered was not evidence of a new type of radiation, but of radioactive acceleration.

Dr. Walt Brown followed up on this and made the most radical proposal to date: that all radioactive elements and isotopes formed on the earth itself,[14] during the severe earthquakes attendant upon the global flood.

Uses

Ficheiro:Geiger counter in use.jpg
Geiger counter used to measure radioactive emissions from a sample.

Describe the uses of radioactive decay here.

Nuclear Powerplants

Age Dating

Radioactive Tracers

Cancer Treatment

Sterilizing

Smoke Detectors

Genetic Studies

Some references:

References

  1. Radiation and Radioactivity by the U.S. Environmental Protection Agency
  2. 2,0 2,1 2,2 Why Are Some Atoms Radioactive? by the U.S. Environmental Protection Agency.
  3. Antony Wilbraham, et. al. Chemistry. Upper Saddle River, New Jersey: Prentice Hall, 2008. p801
  4. Alpha decay by Wikipedia
  5. 5,0 5,1 Radioactive Decay The Bodner Group. Bodner Research Web.
  6. Beta Decay Physics 2000. by the Department of Physics. University of Colorado at Boulder.
  7. Gamma ray by Wikipedia
  8. Discovery of Radioactivity by Dr. V B Kamble. Vigyan Prasar Science Portal
  9. 9,0 9,1 The Discovery of Radioactivity by the Lawrence Berkeley National Laboratory
  10. 10,0 10,1 The Discovery of Radioactivity by Duke University Department of Chemistry.
  11. The Discovery Of Radioactivity: The Dawn of the Nuclear Age by Fran Slowiczek, Ed.D and Pamela M. Peters, Ph.D., Access Excellence.
  12. Gentry, R.V. 1971. Radiohalos: Some Unique Pb Isotope Ratios and Unknown Alpha Radioactivity. Science 173, 727.
  13. Gentry, R.V. 1970. Giant Radioactive Halos: Indicators of Unknown Alpha-Radioactivity? Science 169, 670.
  14. The Origin of Earth's Radioactivity by Dr. Walt Brown, Center for Scientific Creation, Online Edition

External links

Radioactive.jpg

Creationist

Secular

Ver também