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Alpha Decay

The elements are composed of the same type of atoms. Atoms are comprised of three fundamental particles, electrons, protons and neutrons. Out of these sub-atomic particles, protons and neutrons are placed in the nucleus whereas electrons are placed around the nucleus in certain energy levels that are known as shells. All elements are placed in the long form of the periodic table according to their atomic number. As we move from left to right in the period of the periodic table, the atomic number increases by one unit in each group.

The stability of any element depends on the ratio of $\frac{neutrons}{protons}$ or $\frac{n}{p}$ ratio. As this ration increases, the stability of atom decreases. That is the reason; heavy elements with high atomic number are not stable and decompose with release of energy in the form of radiations. Such elements are known as radioactive elements. For example uranium is a radioactive element which decomposes to its isomers and other light elements like plutonium.

The radiations which radiate from the radioactive elements can be three types; alpha, beta and gamma rays. Out of these radiations, alpha radiations are positively charged particles whereas beta particles are negatively charged particles. Gamma rays have no charge and consider as neutral radiations. Let’s discuss the alpha decay of radioactive elements.

Related Calculators
calculate radioactive decay half life decay calculator
 

Alpha Decay Definition

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Atoms with more number of neutrons and protons are highly unstable and get stabilized by emission alpha particles. The emission of alpha particles from parent nuclei to form new daughter nuclei is called as alpha decay.
Hence alpha decay is one process which can use to convert an unstable atom to a more stable one. Each alpha particle has 2 protons and 2 neutrons, hence because of emission of alpha particle the atomic number of daughter nuclei is two less compare to parent nuclei and atomic mass decreases by four units.

Alpha Decay

Alpha Particle Decay

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The constituent particle present in alpha rays is called as alpha particle. Each alpha particle carry two units of positive charge and four times atomic mass compare to hydrogen atom i.e. its equivalent to helium nuclei. Hence alpha particles represents as 2He4 with atomic number 2 and mass number 4.
Alpha Particle Decay   
Alpha particles move with a very high speed of around 16x 104 km/sec and ionized the gas through which they pass. They cause luminescence on striking a zinc sulphide plate. Their penetration power is least out of all radioactive particles. It is 100 times less than penetration power of beta particles.

Alpha Beta Gamma Decay

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The process of disintegration of an unstable atomic by the emission of some ionizing particles like alpha, beta particles or gamma rays, it termed as radioactive decay or nuclear decay.

Alpha Beta Gamma Decay
A nuclei which emits energy in the form of radiation is known parent radionuclide, which further transform to an atom with a nucleus in a different nucleus containing different numbers of nucleons. These new elements formed during the radioactive decay are known as daughter nuclei.
The radioactive decay of radioactive nuclei takes place by the emission a certain particle like alpha particles (2He4) , beta particles (-1e0 ) and gamma rays or by proton decay and electron capture.

Some Radioactive Decays

Any radioactive decay follows the law of mass and energy of conservation. Hence the total number of nucleons (neutrons + protons) must be the same on both side of nuclear reaction.

Beta Decay


During beat decay, the parent nuclei emitted beta rays which made up of a beta particle. Beta particle is a fast moving electron or positron which depends on the type on beta decay involved.
Beta Decay

Beta rays or cathode rays first observed by William crook and J.J. Thomson during the gas discharge at high electric voltage. The constituent particles present in beta rays are beta particle or negatively charged electrons. The velocity of beta particles is around velocity of 16x 104 to 24x 104 km/sec which is more than alpha particles. The penetration power of beta rays is much more than alpha particles.

Beta decay can be two types.
  1. Negative beta decay
  2. Positive beta decay or Positron or anti-electron decay
When in a nucleus the number of protons and neutrons is high, it shows beta decay. During beta decay one of the protons or neutrons is converted into the other. In negative beta decay a neutron converts into a proton, an electron, with an anti neutrino particle.

0n1$\rightarrow$ 1p1 + -1e1 + Ï…

For example,
  • Beta decay of tritium (Hydrogen-3) to forms isotope of helium-3;
1H3$\rightarrow$ 2He3 + -1e0+ Ï…
  • Beta decay of thorium-234 to forms Protactinium.
90Th234$\rightarrow$ 91Pa234 + -1e0+ Ï…
  • Emission of beta particles from carbon-6 form nitrogen-5.
6C12 $\rightarrow$ 7N14 +-1e0 +Ï…

While in positron decay a proton decays into a neutron, a positron with a neutrino particle.

1p1$\rightarrow$ 0n1 + 1e1 +neutrino

In positron emission there is no change in mass number of daughter nuclei due to positron emission while atomic number decreases by one. Some examples of positron decay are as follows.

Positron emission from barylium-7 gives lithium-7 and anti neutrino particle.

4Be7$\rightarrow$ 3Li7 + 1e0 + neutrino

Positron emission from fluorine-18 gives oxygen-18 and anti neutrino particle.

9F18$\rightarrow$ 8O18 + 1e0 + neutrino

Positron emission from carbon-11 gives boron-11 and anti neutrino particle.

6C11 $\rightarrow$ 5B11+ 1e0 + neutrino


Nuclear Decay Equations

In some nuclear reaction nucleus captures an electron and converts a proton into a neutron. For example; electron capture with beryllium-7 forms Lithium-7.

Gamma Decay

  • Gamma radiations are high energy electromagnetic spectrum radiate in the form of photons which have no mass and no charge.
  • In nuclear decay gamma rays originate due to the change in structure and energy levels of the atomic nucleus because of alpha or beta emission.
  • Hence gamma decay is secondary effects of atomic disintegration.
  • When nuclei emitted any particle like alpha or beta particles, they become excited and move at higher energy level which makes the nucleus unstable.
  • For getting the stability atom released this extra amount of energy in the form of radiation known as gamma rays.
  • Since gamma rays are neutral and mass less; there is no change in atomic number and mass number in daughter nuclei compare to parent nuclei.
  • For example when Cobalt-60 emitted one beta particle and convert in Nickel-60 with the emission of gamma rays.

Alpha Decay Examples

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Some common examples of alpha decay are as follows.
  • The alpha emission with Plutonium-240 forms Uranium-236.
94Pu240 $\rightarrow$ 92U236 +2He4
 
Alpha Decay Particles
  • Alpha decay of seaborgium-263 to rutherfordium-259
106Sg263 $\rightarrow$ 104Rf259 + 2He4
  • Alpha decay of radioactive isotope of astatine that is astatine-211 forms bismuth-207.
83At211$\rightarrow$ 81Bi207 + 2He4

Alpha Decay Equation

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The general alpha decay equation for a parent nuclei X with atomic number “a” and mass number “b” , which further converts on daughter nuclei Y will be,

aXb $\rightarrow$ a-2 Yb-4 +2He4

Some common examples of alpha decay are as follows.
  • 95 Am241 $\rightarrow$ 93Np237 +2He4
  • 92U238 $\rightarrow$ 90Th234 + 2He4
  • 90Th234 $\rightarrow$ 88Ra230 + 2He4
  • 94Pu239 $\rightarrow$ 92U235 + 2He4
  • 92U238 $\rightarrow$ 90Th234 + 2He4
  • 94Pu239 $\rightarrow$ 92U235 + 2He4
  • 95Am241 $\rightarrow$ 93Np237 + 2He4
  • 84Po218 $\rightarrow$ 82Pb214 + 2He4
  • 92U234 $\rightarrow$ 90Th230+ 2He4
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