- Katılım
- 17 Eyl 2008
- Konular
- 31,034
- Mesajlar
- 0
- Online süresi
- 5m 10s
- Reaksiyon Skoru
- 208
- Altın Konu
- 0
- TM Yaşı
- 17 Yıl 9 Ay 3 Gün
- Başarım Puanı
- 719
- MmoLira
- 40
- DevLira
- 0
HERAKLES Otomatik Avlı kalıcı sunucu. 19 Haziran'da açılıyor. Atius & Wizard güvencesiyle hemen kayıt ol, ön kayıt ödülleri aktif. HEMEN TIKLA!
Except for H all nuclei have more than 1 p+. Since like charges repel, how can any nucleus be stable? The electrostatic +ve forces are not the only one's present in a nucleus. p+ in fact do repel each other but also at work in the nucleus is a "strong force" which acts to overcome the electrostatic force of repulsion within the nucleus, and it binds nucleons into a package. This "strong force" has some of its own peculiar characteristics. It decreases far more rapidly with distance than an electrostatic force. The strong force exerted by one nucleon on another nucleon falls to zero within the nucleus. The strong force between two adjacent nucleons, therefore, does not contribute anything to the binding of the nucleons on the other side of the nucleus. The electrostatic force of repulsion of p+ in the nucleus does not fall off to zero. p+ in one nuclear region repel p+ in all other regions. Such repulsions are toned down by the intervening no because they help separate the p+.
When nuclei carry large numbers of p+ without enough intermingled neutrons to dilute the electrostatic repulsions, the result is an unstable nucleus. Fission is a possible consequence of this instability only in U-235 and other fissile material. Other mechanisms used by all isotopes, including U-235, include ejection of small nuclear fragments and high-energy electromagnetic radiation in order to achieve stability.
Definitions
radionuclides - isotopes whose nuclei emit particles or energy
radioactivity - the emission itself
radioactive - materials that have the ability to be radionuclides
Types of Radioactivity
Alpha radiation, symbol ''
Alpha radiation is actually a particle of 42He striped of it's electrons which gives it a very strong charge of +2. Alpha particles are very massive in comparison with the other types of radioactive particles below. It may reach up to 1/10 of light speed in a particle accelerator. Within a few centimetres of travelling through the air the alpha particle will collide with the air molecules, lose kinetic energy in the collisions, pick up electrons and become a neutral stable helium atom.
Alpha particles cannot penetrate skin, although enough exposure will cause a severe skin burn.
eg. of decay
22286Rn ------> 42He + 21884Po + radiation
When an alpha particle is ejected the mass number drops by 4, the atomic number drops by 2. The particle also emits gamma radiation which sheds excess energy leaving a more stable isotope.
Beta radiation, symbol ''
Beta radiation is actually a high speed electron, 0-1e. Beta particles arise from the decay of a neutron. A neutron first decays into a p+ and an e- and then the e- is ejected. This e- did not come from the e- configuration cloud. It did not exist before decay occurred. In addition to the e- (), a small massless, electrically neutral "antineutrino" is also ejected. Think of the antineutrino as a holder which held the e- in place on the p+ so that a no is formed.
*
eg. of decay
10no ----> 11p+ + o-1e- + antineutrino + radiation
Each isotope gives off its own unique characteristic beta energy. ie; speed varies from zero to a fixed limit. Since an e- has only 1/7000th the mass of an alpha, the is less likely to collide with the molecules of the substance through which it travels. (ie. air)
The fastest particle travel at 300 cm/sec in dry air. Only the highest energy can penetrate skin. However, an antineutrino can travel 3000 light years in water before striking a p+ in the water molecule. Since most of our radiation detectors depend on collisions between radiation particles and a gas, antineutrinos where only isolated very recently.
examples of Decay
13153I ----> 0-1e- + antineutrino + radiation + 13154Xe
31H ----> 0-1e- + antineutrino + radiation + 32He
When a particle is ejected the atomic number increase by 1 but the mass number remains constant.
Gamma radiation, symbol ''
Gamma radiation is high energy photons of the electromagnetic spectrum and usually accompanies and . The emission of an or can leave the nucleus in an excited state. The emission of a photon strips energy from the nucleus relaxing it into a more stable state. radiation is very penetrating but it can be blocked with lead and concrete. In dry air radiation penetrates 400 metres, or 50 cm of tissue or 30 mm of lead.
The energy units most often used to describe the energy for radiation is the electron volt (eV). One eV is the energy 1 e- receives when accelerated by a charge of 1 V. A simple device used to determine this energy is two plates with a charge. You then measure the amount of electricity needed to hold an e- in place against gravity.
1 eV = 1.602 x 10-19 J
1 KeV = 103 eV
1 MeV = 106 eV
1 GeV = 109 eV
Gamma Therapeutic Radiations
The Co-60 radiation machine used in the treatment of cancers produces radiation in the range of 1.173-1.332 MeV. An standard X-ray machine used in diagnostic medicine has energies of about 100 KeV or less. X-rays are also high-energy electromagnetic radiation but are of less energy than rays.
*
X-rays - are made deliberately in a X-ray Cathode tube. They are made by directing a high energy e- beam, just like the electron gun at the back of your TV, at a metal target. This e- beam knocks e- out of the metal's atomic orbitals.
Diagram of an X-Ray Tube and Target Atom.
In the metal target holes are created in the orbitals. If the e- knocked out comes from a lower energy level then a higher energy e- drops down to fill in the hole. This cascade of e- transitions from a higher to a lower level cause the release of electromagnetic energy in the X-ray range of emissions. X-rays come from e- level transition in the orbitals and radiation comes from transitions between nuclear energy levels.
Penetrating Ability
*
Radiation Disintegration Series
Often a radionuclide decays not to a stable isotope but to another radioactive isotope. The process will continue through a series until a stable isotope forms. There are four well charted disintegration series, the Thorium, Neptunium, Uranium and Actinium Series. Your teacher will provide copies of these series for inclusion in your databooks.
Other Radiations
The first three radiations occur naturally. Since the advent of nuclear fission reactors and particle accelerators some very unnatural radioisotopes have been created, identified and studied. Because these unnatural radioisotopes are very unstable they exhibit several types of special emission.
Positron particle, symbol 0+1e+
Think of this emission as a positive electron. It has the same mass as an electron but with a positive charge. It is formed when a p+ decays into a neutron. The positive charge holder is another massless particle called the neutrino.
eg. of positron emission
11p+ ----> 10n0 + 0+1e+ + neutrino
5427Co ---> 5426Fe + 0+1e+ + neutrino
116C ----> 115B + 0+1e+ + neutrino
When the positron moves out through the outer orbital e- there is of course an attraction. When the 0+1e- (positron) and the 0-1e- (electron) collide they annihilate each other. Their masses change into 2 photons of radiation of what is called "annihilation radiation photons" each with an energy of 511 KeV. The positron is called "antimatter" because it destroys a particle of ordinary matter. To be called antimatter a particle must have a counterpart of ordinary matter and they must collide to complete annihilation. The neutrino, being uncharged and massless, plays no part in this annihilation.
Neutron Emission
When the number of neutrons in an isotope are to high the isotope can eject neutrons in order to stabilize the nucleus. This reaction simply lowers the mass of the isotope without changing it to a new element.
eg. 8736Kr ----> 8636Kr + no
Electron Capture or K-Capture
This event is very rare among natural isotopes but is quite common in the synthetic isotopes.
*
eg. 5023V + 0-1e- -----> 5022Ti + X-rays
*
Below is a diagram showing how a K shell electron gets captured and pulled into the nucleus. A high energy level electron falls to fill the hole left. As the e- falls from the higher to a lower orbital it must release energy, most of which is dispersed as X-rays.
An orbital K shell e- is drawn into the nucleus where it neutralizes a p+, therefore the atomic number drops by 1. A hole is left in the K shell and the atom emits X-rays as the outer orbital e- falls in to fill the hole. Normally the rate of decay is independent of the oxidation state, pressure, temperature, or combination with other elements. This appears to be true for and decay. When the decay is by e- capture however, very small differences have been noted.
eg. Be-7 decays faster as a metal than it does as an oxide.
The e- density next to the beryllium nucleus is higher than the density on the BeO, therefore it takes longer to pull in stray K shell electrons.
When nuclei carry large numbers of p+ without enough intermingled neutrons to dilute the electrostatic repulsions, the result is an unstable nucleus. Fission is a possible consequence of this instability only in U-235 and other fissile material. Other mechanisms used by all isotopes, including U-235, include ejection of small nuclear fragments and high-energy electromagnetic radiation in order to achieve stability.
Definitions
radionuclides - isotopes whose nuclei emit particles or energy
radioactivity - the emission itself
radioactive - materials that have the ability to be radionuclides
Types of Radioactivity
Alpha radiation, symbol ''
Alpha radiation is actually a particle of 42He striped of it's electrons which gives it a very strong charge of +2. Alpha particles are very massive in comparison with the other types of radioactive particles below. It may reach up to 1/10 of light speed in a particle accelerator. Within a few centimetres of travelling through the air the alpha particle will collide with the air molecules, lose kinetic energy in the collisions, pick up electrons and become a neutral stable helium atom.
Alpha particles cannot penetrate skin, although enough exposure will cause a severe skin burn.
eg. of decay
22286Rn ------> 42He + 21884Po + radiation
When an alpha particle is ejected the mass number drops by 4, the atomic number drops by 2. The particle also emits gamma radiation which sheds excess energy leaving a more stable isotope.
Beta radiation, symbol ''
Beta radiation is actually a high speed electron, 0-1e. Beta particles arise from the decay of a neutron. A neutron first decays into a p+ and an e- and then the e- is ejected. This e- did not come from the e- configuration cloud. It did not exist before decay occurred. In addition to the e- (), a small massless, electrically neutral "antineutrino" is also ejected. Think of the antineutrino as a holder which held the e- in place on the p+ so that a no is formed.
*
eg. of decay
10no ----> 11p+ + o-1e- + antineutrino + radiation
Each isotope gives off its own unique characteristic beta energy. ie; speed varies from zero to a fixed limit. Since an e- has only 1/7000th the mass of an alpha, the is less likely to collide with the molecules of the substance through which it travels. (ie. air)
The fastest particle travel at 300 cm/sec in dry air. Only the highest energy can penetrate skin. However, an antineutrino can travel 3000 light years in water before striking a p+ in the water molecule. Since most of our radiation detectors depend on collisions between radiation particles and a gas, antineutrinos where only isolated very recently.
examples of Decay
13153I ----> 0-1e- + antineutrino + radiation + 13154Xe
31H ----> 0-1e- + antineutrino + radiation + 32He
When a particle is ejected the atomic number increase by 1 but the mass number remains constant.
Gamma radiation, symbol ''
Gamma radiation is high energy photons of the electromagnetic spectrum and usually accompanies and . The emission of an or can leave the nucleus in an excited state. The emission of a photon strips energy from the nucleus relaxing it into a more stable state. radiation is very penetrating but it can be blocked with lead and concrete. In dry air radiation penetrates 400 metres, or 50 cm of tissue or 30 mm of lead.
The energy units most often used to describe the energy for radiation is the electron volt (eV). One eV is the energy 1 e- receives when accelerated by a charge of 1 V. A simple device used to determine this energy is two plates with a charge. You then measure the amount of electricity needed to hold an e- in place against gravity.
1 eV = 1.602 x 10-19 J
1 KeV = 103 eV
1 MeV = 106 eV
1 GeV = 109 eV
Gamma Therapeutic Radiations
The Co-60 radiation machine used in the treatment of cancers produces radiation in the range of 1.173-1.332 MeV. An standard X-ray machine used in diagnostic medicine has energies of about 100 KeV or less. X-rays are also high-energy electromagnetic radiation but are of less energy than rays.
*
X-rays - are made deliberately in a X-ray Cathode tube. They are made by directing a high energy e- beam, just like the electron gun at the back of your TV, at a metal target. This e- beam knocks e- out of the metal's atomic orbitals.
Diagram of an X-Ray Tube and Target Atom.
In the metal target holes are created in the orbitals. If the e- knocked out comes from a lower energy level then a higher energy e- drops down to fill in the hole. This cascade of e- transitions from a higher to a lower level cause the release of electromagnetic energy in the X-ray range of emissions. X-rays come from e- level transition in the orbitals and radiation comes from transitions between nuclear energy levels.
Penetrating Ability
*
Radiation Disintegration Series
Often a radionuclide decays not to a stable isotope but to another radioactive isotope. The process will continue through a series until a stable isotope forms. There are four well charted disintegration series, the Thorium, Neptunium, Uranium and Actinium Series. Your teacher will provide copies of these series for inclusion in your databooks.
Other Radiations
The first three radiations occur naturally. Since the advent of nuclear fission reactors and particle accelerators some very unnatural radioisotopes have been created, identified and studied. Because these unnatural radioisotopes are very unstable they exhibit several types of special emission.
Positron particle, symbol 0+1e+
Think of this emission as a positive electron. It has the same mass as an electron but with a positive charge. It is formed when a p+ decays into a neutron. The positive charge holder is another massless particle called the neutrino.
eg. of positron emission
11p+ ----> 10n0 + 0+1e+ + neutrino
5427Co ---> 5426Fe + 0+1e+ + neutrino
116C ----> 115B + 0+1e+ + neutrino
When the positron moves out through the outer orbital e- there is of course an attraction. When the 0+1e- (positron) and the 0-1e- (electron) collide they annihilate each other. Their masses change into 2 photons of radiation of what is called "annihilation radiation photons" each with an energy of 511 KeV. The positron is called "antimatter" because it destroys a particle of ordinary matter. To be called antimatter a particle must have a counterpart of ordinary matter and they must collide to complete annihilation. The neutrino, being uncharged and massless, plays no part in this annihilation.
Neutron Emission
When the number of neutrons in an isotope are to high the isotope can eject neutrons in order to stabilize the nucleus. This reaction simply lowers the mass of the isotope without changing it to a new element.
eg. 8736Kr ----> 8636Kr + no
Electron Capture or K-Capture
This event is very rare among natural isotopes but is quite common in the synthetic isotopes.
*
eg. 5023V + 0-1e- -----> 5022Ti + X-rays
*
Below is a diagram showing how a K shell electron gets captured and pulled into the nucleus. A high energy level electron falls to fill the hole left. As the e- falls from the higher to a lower orbital it must release energy, most of which is dispersed as X-rays.
An orbital K shell e- is drawn into the nucleus where it neutralizes a p+, therefore the atomic number drops by 1. A hole is left in the K shell and the atom emits X-rays as the outer orbital e- falls in to fill the hole. Normally the rate of decay is independent of the oxidation state, pressure, temperature, or combination with other elements. This appears to be true for and decay. When the decay is by e- capture however, very small differences have been noted.
eg. Be-7 decays faster as a metal than it does as an oxide.
The e- density next to the beryllium nucleus is higher than the density on the BeO, therefore it takes longer to pull in stray K shell electrons.