What type of emission causes Pu-239 to decay to U-235? Show by reaction A. gammaB. betaC. positron D. alpha
The correct answer and explanation is:
The correct answer is D. alpha emission.
Explanation:
Plutonium-239 (94239Pu)(^{239}_{94}Pu) primarily undergoes alpha decay to transform into Uranium-235 (92235U)(^{235}_{92}U).
Alpha decay is a type of radioactive decay where an unstable nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons (essentially a helium-4 nucleus: 24He^{4}_{2}He). When Pu-239 emits an alpha particle, it loses 2 protons and 2 neutrons, reducing its atomic number by 2 and its mass number by 4.
The nuclear reaction can be written as: ^{239}_{94}Pu \rightarrow \, ^{235}_{92}U + \, ^{4}_{2}He
- Before decay: 94239Pu^{239}_{94}Pu (94 protons, 145 neutrons)
- After decay: 92235U^{235}_{92}U (92 protons, 143 neutrons) + alpha particle 24He^{4}_{2}He
Why not other types of emission?
- Gamma emission (A): Gamma rays are high-energy photons emitted from an excited nucleus returning to a lower energy state. Gamma emission does not change the atomic number or mass number, so it cannot transform Pu-239 into U-235.
- Beta emission (B): Beta decay involves the conversion of a neutron into a proton (beta-minus) or a proton into a neutron (beta-plus, or positron emission). This changes the atomic number by one but not the mass number. Beta decay would convert Pu-239 into an element with atomic number 93 (Neptunium), not uranium.
- Positron emission (C): Positron emission is a type of beta-plus decay that decreases the atomic number by one. This is not observed in Pu-239 decay.
Summary
Pu-239 transforms into U-235 by losing an alpha particle (24He^{4}_{2}He), reducing its proton count by 2 and neutron count by 2. This is the classic alpha decay pathway for this isotope and is fundamental in nuclear physics and nuclear reactors where Pu-239 acts as a fissile material.