Radioactivity

Radioactivity is a spontaneous process, during which an unstable nucleus of a certain element transforms into more stable nucleus of a different element. During the process, loosened particles are emitted from the nucleus in the form of radiation.

Whether the nucleus is stable or unstable is decided by so called valley of stability, which is defined by ratio of protons and neutrons in nucleus. For nuclides with lower mass number, the stable ratio is around 1 : 1. With higher mass number the ratio changes towards 2 : 3 (P : N). If the ratio of a nuclide deviates from the valley of stability ratio, the said nuclide is considered unstable.

Nucleus
The atomic nucleus is the small, dense region consisting of protons and neutrons (together they are called nucleons) at the center of the atom. The diameter of a nucleus is approx. 10-15 m and it represents vast majority of mass of the atom.

Proton is a nuclear particle with a positive charge equal to so called elementary charge (1,6 * 10-19 C). Number of protons in the nucleus is represented as atomic number (Z).

Neutron is a nuclear particle without any charge. As opposed to the proton, the neutron is unstable. As itself it decays into the proton and into other particles. The amount of neutrons in the nucleus is represented with neutron number (N).

An element is defined by amount of its protons and neutrons. From the structure of the nucleus we can assume its stability and mass. Probability of the decay depends on the ratio of N and Z.

Nuclides
A nuclide is an atomic species defined by the specific constitution of its nucleus, i.e., by its number of protons Z and its number of neutrons N.

Carbon, as an example, is found as a mix of two isotopes, 12C and 13C. In tables, the relative atomic mass of carbon is 12,011 which coresponds to this natural mix of isotopes instead of 12,000 which would be relative atomic mass of pure 12C.

Isotopes
Isotopes are variants of a particular chemical element which differ in neutron number. All isotopes of a given element have the same number of protons in each atom. They differ in physical but not chemical characteristics. The main differences are in mass and stability which is direct consequence of different neutron number. As an example, isotope of carbon 12C, which is the most common (98,9%), is stable. Isotope 13C, whose nucleus contains one more neutron, is still stable, but not as common (1,1%). Isotope 14C with two more neutrons in its nucleus is already unstable and that's why we call it radionuclide. Radionuclide is an isotope which decays while emitting radiation.

Isotopes almost do not differ in their chemical attributes. One minor difference might be in speed of their reactions. Heavier isotopes are usually slower in their reactions.

Isobars
Isobars are atoms of different chemical elements which have the same number of nucleons but different atomic number. An example of isobars would be 40S, 40Cl, 40Ar, 40K, and 40Ca. The nuclei of these nuclides contain 40 nucleons but they contain different numbers of protons and neutrons. $$ N + Z = const. $$

Isotones
Isotones are atoms of different chemical elements which have the same number of neutrons but different atomic and mass number. As an example we can use 12B and 13C. Borium has 5 protons, carbon has 6 protons but both have 7 neutrons. The biggest groups of isotones are with 50 neutrons (86Kr,88Sr,89Y,90Zr,92Mo) and with 82 neutrons (138Ba,139La,140Ce,141Pr,142Nd,144Sm). No stable isotones have 19, 21, 35, 39, 45, 61, 71, 89, 115, 123 or 127 neutrons.

α-particles
α-particle constits of two protons and two neutrons which are bond together identically to the nucleus of helium. It has non-zero mass and because it contains two protons it has positive charge +2e. This radiation is emitted during the α-decay of an isotope of a heavy element. Emitted energy is equal to the loss of mass of the system.

Nuclide which is result of the α-decay has lower atomic number by two and mass number lower by four (it's shifted by two places to the left in the PTOE). The emitted particle has much lower mass than the original nucleus which means that the kinetic energy of the the new nucleus is negligible. The newly created nucleus returns back from the excited state by emitting quantums of γ-radiation. γ-radiation usually accompanies α particles during α-decay.

α-particles are highly ionizing form of particle radiation and they have very low penetration depth, which means they can be absorbed by just a few centimetres of air or by a sheet of paper. Superficial effect on human skin is negligible as all of the α-particles absorbed by cells squamous epithelium. Exposing inner epithelia to α-particles can lead to cancerous process as they can damage genetic material. α-particles can be used for medical purposes. In smaller doses they activate defense systems of the cells.

Among elements which undergo α-decay belongs for example uranium, radium or radon.

β-particles
β-particles are emitted by nuclei which undergo β-decay. They can be either positively β+ (positrons) or negatively β- (electrons) charged. β+-particles are emitted during tranformation of proton into neutron inside the nucleus. The new element is shifted one place to the left in the PTOE. β--particles are emitted during transformation of neutron into proton inside the nucleus. The new element is shifted one place to the right in the PTOE. β-particles are emitted at very high velocity and they have higher penetration depth than α-particles. They can penetrate materials with low density or low thiccness. As an example for shading of β-particles we can use a tin foil.

γ radiation
γ radiation is electromagnetic radiation with very short wavelength, great amount of energy and high penetration rate. Unlike α a β radiation, which are corpuscular, the γ radiation permeates into matter with more ease and its perfect shading is almost impossible (the intensity of radiation can be lowered by using layers of materials containing heavy elements, for instance lead).

Radionuclides are unstable nuclides. We distinguish natural radionuclides (found in nature) and synthetic radionuclides (created via nuclear reactions). From this we also distinguish natural and artificial radioactivity.

Natural radioactivity
It is spontaneous transition of unstable nuclei to their stable forms. During this process the nuclei are emitting radiation. Natural radioactivity was discovered by H. Becquerel in 1896 and studied by Marie and Pierre Curie.

Induced radioactivity
Transformation of atoms caused by nucleus reactions. This was discovered by Frederic and Irene Curie in 1934 during the test based on strafing aluminium with alfa particles.

Radioactive balance

 * Condition ,when is during some time converted the same amount of atoms in radioactive isotopes line.

A - there is no balance. Physical transfiguration half-life of mother-element (T1) is shorter then transfiguration half-life of the daughter-element (T2)

B - transient radioactive balance, T1 is longer then T2

C - permanent radioactive balance, transfiguration half-life T1 is much longer then T2 element

Nucleus instability

 * Not every atom nuclei are stable. Most of them transform spontaneously into different nuclei.

Radioactive transmutation half-life

 * During this process, unstable atoms beams their energy (as electromagnetic wave or as a particle).


 * Physical half-life
 * during this period, is half of original nuclei transformed
 * Biological half-life
 * during this time is half of particular element separate out of the body
 * Effective half-life
 * combination of biological and physical half-life. It means, that it is period, during which is radioactivity lowered to half, thanks to physical and biological transition.

Nuclear energy
thumb | 220px | Mezinárodní symbol radioaktivity Nuclear reactions are based on transmutation of the nuclei with higher bounding energy to the nuclei with the lower bounding energy. Difference between these energies is excited and used. Bounding energy is value characterising the stability of the nucleus. Most stable nuclei have nucleic number between 30 to 130. It is energy, released during the creation of the nucleus from two nucleons. Nuclear energy can be divided into two types:
 * Nucleus synthesis (Thermonuclear reaction) – Heavier nuclei are made of the lighter nuclei and huge amount of energy is emitted during this process. The conditions for this reaction are very high temperature and pressure ( for example in the stars ).
 * Nucleus fission (Fission reaction) – Two mid-weighted atoms nuclei are created while few neutrons and energy is released. Recently created neutrons are used to another fission reaction. We have two types of reactions, controlled and uncontrolled. During the controlled reaction, the neutrons are used one by one stepwisely. We use this process in powerstation to transfrom nuclear energy to electric energy. On the other hand, the uncontrolled reaction cause chain reaction and created neutrons release the energy immediately which why it is used in nuclear weapons.

Nuclear powerplants
Nuclear fission reaction is taking place in nuclear reactor. Whole process is compound of various parts, in order to prevent leak of radiation. Released thermal energy starts to boil water and flowing water steam runs a turbine. Heavy water (deuterium oxide) is used here as moderator for slowing down fission of neutrons. For controlling velocity of nuclear reaction are used cadmium control rods (are able to absorb generated neutrons) or boric acid.

Štěpná reakce probíhá v jaderném reaktoru, celý blok sestává z několika okruhů, aby se zamezilo únikům radioaktivity. Uvolněná tepelná energie zahřívá vodní páru, která následně pohání turbínu. Ke zpomalení štěpících se neutronů se používá těžká voda (oxid deuteria) = moderátor. K řízení rychlosti jaderné reakce se používají kadmiové regulační tyče, které rády pohlcují vzniklé neutrony a kyselina boritá.

Radiotherapy
Radiotherapy is a conservative method, which is used for curing malignant tumors. Using various types of ionizing radiation, affected tissue is facing it in order to cause necrosis of the tumor. The goal, except eliminating the affected tissue, is to cause as little damage to the healthy tissue as possible. Selection of the type and intenzity used radiation is based on the character of the tumor. Brachytherapy requires placing the source of radiation on the surface of the tumor or in the tumor. Teletherapy is carried out with source situated outside of the organism. From the physical point of view we distinguish two types of used ionizing radiation - electromagnetic and corpuscular. Electromagnetic includes radiation X (rtg) and γ. Corpuscular radiation includes particles such as protons, neutrons, α-particles and electrons (β-particles). According to the source there is wide range of methods - proton therapy, linear or circular accelerators treatment (X radiation) or cobalt therapy (γ radiation).

Radiosensitivity of cells describes ability to react to radiation. Stem cells are more likely to be sensitive to radiation than mature or highly differentiated cells are.

Radioterapie je konzervativní metoda, která se používá na léčbu zhoubných nádorů v lidském organismu. Dochází k ozařování postižené tkáně, kdy využíváme různé druhy ionizujícího záření. Cílem je zničení nádoru a co nejmenší poškození okolní zdravé tkáně. Dle charakteru nádoru se volí různá intenzita i druh záření. Při teleterapii se provádí ozařování pomocí zdroje, jenž se nachází mimo organismus. Brachyterapie vyžaduje umístění zdroje na povrch nádoru či přímo dovnitř nádoru. Z fyzikálního hlediska používané ionizující záření je dvojího typu - elektromagnetické a korpuskulární. Elektromagnetické zahrnuje záření X (rtg) a záření γ. Korpuskulární záření, také částicové obsahuje protony, neutrony, α-částice i elektrony (β-částice). Dle zdroje existuje celá řada metod - protonová léčba, léčba pomocí lineárního či kruhové urychlovače (X-záření) či hloubková terapie pomocí kobaltu (záření γ).

Radiosensitivita buněk je vlastnost, která charakterizuje schopnost buňky reagovat na ozáření. Minimálně diferencovaná buňka vykazuje větší citlivost na záření než buňky, které jsou zralé, popřípadě vysoce diferencované.

Age analysis of archeological findings
Age determination of archeological findings is done with radiocarbon method of dating.

Carbon is able to create 3 base isotopes - 12C, 13C a 14C. Isotope 14C is radioactive, that means radionuclide with half-life 5730 years. There are all 3 isotopes in a constant ratio in nature. We are able to determine age of dead organism based on number of decays per minute in 1 g of radioactive carbon. When an organism dies its shift of carbon supplies stops and in that moment the ratio of carbon isotopes is same as in nature. But immediately after that redionuclides start to decay and the ratio between isotopes growths.

Uhlík je schopen tvořit tři základní izotopy - 12C, 13C a 14C. (Viz Izotopy) Izotop 14C je radioaktivní, tzv. radionuklid s poločasem rozpadu 5730 let. V přírodě jsou zastoupeny všechny tři izotopy a ve stálém poměru. Z tohoto vyplívá, že při odumření organismu čili při zastavení přísunu uhlíku je poměr izotopů stejný jako v okolní přírodě. Radionuklid se začíná rozpadat a poměr mezi izotopy roste. Dle počtů rozpadů za minutu v 1 g radioaktivního uhlíku je možné určit stáří nálezu.

History
As first discoverer of radioactivity is considered Henry Becquerel in 1896. When he placed fluorescence material between photographic panels and they were untouched, he found out that minerals emitting more than light radiation. Then at the start of 20th century Marie Curie Sklodowska was researching radioactivity and besides everything else she discovered new elements (radium and polonium). She were trying to find out why uranium ore is more radioactive than pure uranium. After four years she discovered polonium (named after her homeland) then she discovered even more radioactive radium. M. R. Sklodowska was the first woman to win a Nobel prize. First nuclear reactor was put into operation in 1942 in USA. First nuclear power station was opened in 1956 in Great Britain.

Henry Becquerel jako první v roce 1896 objevil radioaktivitu díky zjištění, že minerály vyzařují jiné záření než světelné, když položil fluorescenční materiál mezi fotografické desky a fotografické desky byly netknuté. Následně pak Marie Curie Sklodowská na počátku 20. století zkoumala radioaktivitu a mimo jiné objevila nové prvky (radium a polonium). Zkoumala, proč uranová ruda je radioaktivnější, než samotný uran. Po čtyřech letech objevila polonium (pojmenovala ho po své vlasti), poté objevila ještě radioaktivnější radium. M.R. Sklodowská byla první ženou, která získala Nobelovu cenu za fyziku. První jaderný reaktor byl uveden do provozu v USA v roce 1942. První jaderná elektrárna byla otevřena v roce 1956 ve Velké Británii.

Externí odkazy

 * http://www.surao.cz/cze