Types of Radioactivity: Alpha Beta and Gamma Decay

The process of spontaneous disintegration of some unstable atomic nuclei is known as radioactivity. In other words, the phenomenon of spontaneous emission of radiations by heavy elements is called radioactivity. The elements which show this phenomenon is called radioactive elements. Radioactivity is a continuous and irreversible nuclear phenomenon.

• Radioactivity was discovered by Henry Becquerel in uranium salt in the year 1896.
• After the discovery of radioactivity in Uranium, Pierre Curie and Madame Curie discovered a new radioactive element called radium.
• Some examples of radioactive substances are Uranium, Radium, Thorium, Neptunium, etc.
• The radioactivity of a sample cannot be controlled by any physical ( pressure, temperature, electric or magnetic field) or chemical changes.
• All the elements with atomic number (Z) > 82 and naturally radioactive.
• The conversion of lighter elements into radioactive elements by the bombardment of fast-moving particles is called artificial or induced radioactivity.
• Radioactivity is a nuclear event and not an atomic one. Hence the electronic configuration of the atom doesn’t have any relation with radioactivity.

When the radiation enters into an external electric field, it splits into three parts: alpha rays, beta rays, and gamma rays.

Alpha (α) Decay

The spontaneous emission of an alpha particle from a radioactive nucleus is called Alpha decay. α decay occurs when the nucleus emits α particles. 

This process involves the spontaneous emission of nucleons since α particles (₂He⁴) contain two protons and two neutrons therefore the emission of α particles causes the nucleus to get transmuted into a daughter nucleus having atomic number (Z) two less and atomic mass (A) four less.

Let us consider some examples of α decay: 

• ₈₈Ra²²⁶  ⇢  ₈₆Rn²²² + ₂He⁴
• ₉₂U²³⁸  ⇢  ₉₀Th²³⁴ + ₂He⁴
• ₉₄Pu²⁴²  ⇢  ₉₂U²³⁸ + ₂He⁴

Properties of alpha decay:

• Alpha particles have charge +2e and mass 4u.
• Alpha particles have a kinetic energy of 5MeV.
• Nearly 90% of the 2500 known nuclides are radioactive. They are not stable but decay into other nuclides.
• When unstable nuclides decay into different nuclides, they usually emit alpha or beta particles.
• Alpha emission occurs principally with nuclei that are too large to be stable.
• Alpha decay is mainly governed by strong nuclear force and electromagnetic force.

Beta (β) Decay

The spontaneous process of emission of beta particles from a radioactive nucleus is called Beta-decay.

The nucleus achieves greater stability in beta decay. In beta decay, either a neutron is converted into a proton or a proton is converted into a neutron. 

The general reaction for beta decay is given as:

_ZX^A  ⇢  _Z+1Y^A+ _-1e⁰

Beta-decay is mainly of three types: Beta-minus (β^–), Beta-plus (β⁺), and electron capture.

Beta-minus (β^–)

In beta-minus, the neutron inside the nucleus is converted into a proton and an electron like a particle.

Those nuclei having more neutrons (N)  than protons (Z)  become unstable and tend to be beta-minus decay (β^–). A β^– particle is like an electron. The emission of β^– involves the transformation of a neutron into a proton, an electron, and a third particle called Antineutrino.

β^– decay usually occurs with nuclides for which the neutron-to-proton ratio (N/Z ratio)  is too large for stability. In β^– decay, N decreases by 1, Z  increases by 1 and A remains the same. β^– decay can occur whenever the neutral atomic mass of the original atom is larger than that of the final atom.

For Example:                

₁₅P³² ⇢  ₁₆S³²+ _-1e⁰

Beta-plus (β⁺)

In a  β⁺ decay, a proton is converted into a neutron and a positron (_-1e⁰ ) is emitted if a nucleus has more protons than neutrons.

Nuclides for which N/Z is too small for stability can emit a positron, the electron’s antiparticle, which is identical to the electron but with a positive charge. The basic process is called beta-plus (β⁺) decay.

p ⇢  n + β⁺ + v (v=neutrino)

β⁺ decay can occur whenever the neutral atomic mass of the original atom is at least two electron masses larger than that of the final atom. There are a few nuclides for which β⁺ emission is not energetically possible but in which an orbital electron can combine with a proton in the nucleus to form a neutron and a neutrino. The neutron remains in the nucleus and the neutrino is emitted. 

For Example:                     

₁₁Ne²² ⇢ ₁₀Ne²² + _-1e⁰

Electron Capture

Electron capture, nucleus absorbs one of the inner electrons revolving around it and hence a nuclear proton becomes a neutron and a neutrino (v) is emitted.

The process is represented as:  

₁H¹ + _-1e⁰  ⇢   ₀n¹ + v (v=neutrino)

Electron capture is compatible with a positron emission as both processes lead to the same nuclear transformation. However, electron capture occurs more frequently than positron emission in heavy elements. This is because the orbits of electrons in heavy elements have small radii and hence orbital electrons are very close to the nucleus.

For Example:                      

₅₄Xe¹²⁰  + _-1e⁰  ⇢  ₅₃I¹²⁰ + v 

Properties β Decay:

• In a β decay, the neutron inside the nucleus changes into protons, as a result, the charge number remains the same, and the atomic number increases by one.
• In emission of β particle is accompanied by a companion particle having variable energy.
• The companion particle is massless and chargeless and is called Antineutrino.
• The emission of Antineutrino along with β particle conserves the angular momentum during β decay.
• The mass of neutrino and antineutrino is zero. The spin of both is 1/2 in units of h/2π. The charge on both is zero.
• The spin of neutrino is antiparallel to its momentum while that of antineutrino is parallel to its momentum.

Gamma (γ) Decay 

It is the spontaneous process of emission of high energy photons from a radioactive nucleus. The emission of alpha and beta particles leave the daughter nucleus in the excited state which in turn emits one or more Gamma-ray photons in single or successive transitions.

Since the gamma rays are emitted by the daughter nucleus emission of gamma rays for the emission of alpha and beta particles. The energy of gamma-ray is equal to the difference between the energy of the excited state or higher energy state and the ground state of the nucleons.

For Example:                                   

• ₈₂Pb²¹⁰ ⇢ ₈₃Bi^210* + _-1e⁰ + Antineutrino 
• ₈₃Bi^210* ⇢ ₈₃Bi²¹⁰ +  γ-ray

Properties Gamma (γ) Decay:

• When the nucleus is placed in an excited state, either by bombardment with high energy particles or by a radioactive transformation, it can decay to the ground state by emission of one or more photons called gamma rays.
• The order of energy of Gamma Photon is 100 KeV.
• The rest mass of the Gamma Photon is zero.

Terminologies used:

• Antineutrino – It is a very little particle having almost no mass. It is emitted with beta-minus (β^–) particles during radioactive decay.
• Positron – It is the antiparticle of an electron. the properties of a positron are the same as that of an electron except a positron carries a positive charge while an electron carries a negative charge.
• Neutrino – It is a very little neutral particle having almost no mass. It is emitted with β⁺ particles during radioactive decay. It interacts very weakly with materials and hence is not detected easily.

Comparison between Properties of Alpha(α), Beta(β) Particles and Gamma(γ) Rays

Sample Questions

Question 1: Write the symbolic expression for β^– the decay process of ₁₅P³².

Answer: 

The symbolic expression for β^–  decay process of ₁₅P³² is,

₁₅P³² ⇢ ₁₆X³² + _-1e⁰ +  ̅ν

Question 2: Give an example to show that most of the decay energy appears as kinetic energy of α particle.

Answer: 

The mass number of alpha emitters is 210, and in the decay of polonium ₈₄Po²¹⁰, the alpha particle emitted is found to have energy 5.3 MeV.

Therefore from equation, we have

Q = (K.E) α × A/A – 4  

    =  5.3 × 210 / 210 -4

    =  5.4 MeV

This example clearly illustrates that the most of the disintegration energy appears as the kinetic energy of alpha particle.

Question 3: Calculate the disintegration energy ₉₂U²³² (mass = 232.037146 u) decays to ₉₀Th²²⁸ (mass = 228.028731 u) with the emission of an α particle.

Answer: 

Use conservation of energy ₉₂U²³² is the parent, ₉₀Th²²⁸ is the daughter.

Since the mass of the helium is 4.002603u, the total mass in the final state is 228.028731 u + 4.002603 u = 232.031334 u

The mass last when the ₉₂U²³² decays is 232.037146 u – 232.031334 u = 0.005812 u

Since, 1 u = 931.5MeV,the energy Q released is,

Q = (0.005812 u) × (931.5 MeV/u)

    ~5.4 MeV

And this energy appears as kinetic energy of the α particle and the daughter nucleus.

Question 4: Find the maximum energy that a β particle can have in the following decay: 

₈O¹⁹ ⇢ ₉F¹⁹ + –₁e⁰ +  ̅ν

(Given: m(₈O¹⁹) = 19.003576 a.m.u, m(₉F¹⁹) = 18.998403 a.m.u and m(_-1e⁰) = 0.000549 a.m.u.)

Answer: 

The Rest mass of  ̅ν = 0

Q- value of β decay = m(O) – {m(F) + m(e)}

                                = 19.003576 – (18.998403 + 0.000549)

                                = 19.003576 – 18.998952 

                                = 0.004624 a.m.u

Since 1 a.m.u = 931 MeV

                      = 0.004624 × 931 MeV 

                      = 4.3049 MeV

The energy is shared by β^– particle and antineutrino ( ̅ν ). If  ̅ν does not get any share then the maximum K.E of β^– particle is 4.3049 MeV.

Question 5: Given the following atomic masses: ₉₂U²³⁸  = 238.05079u, ₉₀Th²³⁴  = 234.04363u, ₉₁Pa²³⁷ = 237.05121u; ₁H¹ = 1.00783, ₂He⁴ = 4.00260u. Show that  ₉₂U²³⁸  cannot spontaneously emit a proton.

Answer: 

 ₉₂U²³⁸  —> ₉₁Pa²³⁷ + ₁H¹

 Mass Defect,

Δm = ( 238.05079 – 237.05121 – 1.00783) u

=> Energy released Q = -0.00825u

= -0.00825 × 931.5 = -7.68 MeV

Since, Q value is negative, so proton cannot be emitted spontaneously.