Electron Capture

Electron capture is a type of radioactive decay that happens inside an atom’s nucleus when one of its inner (K or L shell) electrons is pulled in and combined with a proton. When this happens, the proton turns into a neutron. A new atom is formed, and a tiny particle called a neutrino is released. Electron capture usually happens in atoms that have too many protons and cannot release energy by other means.

American physicist Luis Alvarez discovered electron capture in 1937.

Equation

The general equation for electron capture can be written as:

\[ _{Z}^{A}\text{X} + e^{-} \rightarrow \text{ } _{Z-1}^{A}\text{Y} + \nu_e \]

Where:

– $ _{Z}^{A}\text{X} $ represents the parent nucleus

– e^– is the captured electron

– $ _{Z-1}^{A}\text{Y} $ is the daughter nucleus

– ν_e is the neutrino emitted during the process

– Z is the atomic number or the number of protons in the nucleus

– A is the atomic mass or the total number of protons and neutrons in the nucleus

During an electron capture, the parent nucleus loses a proton, and its atomic number (Z) is reduced by one. Then, it transforms into a new element. However, the overall mass (A) of the atom stays the same. A neutrino is released during the process.

Examples

1. Beryllium-7 captures an electron, converting one proton into a neutron, and transforms into Lithium-7, with the emission of a neutrino.

\[ _{4}^{7}\text{Be} + e^{-} \rightarrow _{3}^{7}\text{Li} + \nu_e \]

2. Krypton-81 captures an electron, causing a proton to convert into a neutron, which results in Bromine-81 and the emission of a neutrino.

\[ _{36}^{81}\text{Kr} + e^{-} \rightarrow _{35}^{81}\text{Br} + \nu_e \]

Comparing Electron Capture with Beta Minus Decay and Positron Emission

Electron capture, beta minus decay, and positron emission are all nuclear processes that change the composition of an atom’s nucleus, but they differ in how they occur and their outcomes. Here is a table comparing and contracting the three processes.

Process	Mechanism	Change in Atomic Number	Reaction Conditions
Electron Capture	Inner electron is absorbed Proton → Neutron Neutrino emitted	Decreases by 1	Occurs in proton-rich nuclei, especially in heavier elements where positron emission is not energetically feasible.
Beta Minus Decay	Neutron → Proton Beta particles (electron) and antineutrino are emitted	Increases by 1	Occurs in neutron-rich nuclei
Positron Emission	Proton → Neutron Positrons and neutrinos are emitted	Decreases by 1	Occurs in proton-rich nuclei and requires sufficient energy to emit the positron.

Applications

Medical Imaging: Used in positron emission tomography (PET) scans, where electron capture isotopes are employed for detecting metabolic activity in tissues.
Environmental Dating: Krypton-81 electron capture is used in groundwater and ice core dating, helping scientists estimate the age of ancient water reserves.
Astrophysics: Occurs during the supernova explosion of massive stars, which helps in the transformation of protons into neutrons. This process is important for the formation of neutron stars, which are remnants of stars that have undergone such an explosive collapse.

FAQs

1. What is the difference between electron capture and beta decay?

Ans. The nucleus absorbs an electron from the atom’s electron cloud in electron capture. The nucleus emits a beta particle (either an electron or a positron) from itself during beta decay.