Where Does the Extra Mass in Beta-Plus Decay Come From?

Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of beta-plus decay, specifically addressing the question of where the extra mass comes from when a proton decays into a neutron and emits a positron. Participants explore the implications of mass differences in nuclear processes and the conditions under which beta-plus decay occurs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that in beta-plus decay, a proton decays into a neutron and emits a positron, raising the question of the source of the extra mass since the neutron is heavier than the proton.
  • Another participant explains that this process occurs in nuclei with a high proton-to-neutron ratio, where the mass of the nucleus before decay is greater than after due to binding energy differences.
  • A participant provides specific mass values for the neutron, proton, and positron, calculating a mass difference that seems to indicate an appearance of mass during the decay process.
  • One reply emphasizes that positron emission is a nuclear decay process rather than a simple proton decay, providing an example involving fluorine-18 and oxygen-18 to illustrate energy differences and binding energy considerations.
  • Another participant argues that the isolated case of particle decay does not account for the energy dynamics in nuclei, suggesting that it is energetically favorable for a proton to decay into a neutron and positron in certain nuclear configurations.
  • A later post mentions specific examples of positron decay in light nuclei, providing additional context on proton/neutron ratios and energy release during these processes.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of mass changes during beta-plus decay, with no consensus reached on the source of the extra mass or the implications of the calculations presented.

Contextual Notes

The discussion highlights the complexity of nuclear decay processes and the importance of considering binding energy and nuclear configurations, which may not be fully resolved in the participants' arguments.

repugno
Messages
78
Reaction score
0
Hello all,

In beta-plus decay, a proton decays into a neutron and emmits a positron. If the neutron weighs more than the proton where did the extra mass come from?
 
Physics news on Phys.org
This process occurs in nuclei where the proton to neutron ratio is too high. Under these conditions mass of the nucleus before is greater than the mass of the nucleus after (difference in binding energy). As I am sure you are well aware, proton to neutron decay does NOT occur in isolation.
 
Thanks for the reply Mathman,

Mass of neutron = 1.675 * 10^-27 kg
Mass of proton = 1.673 * 10^-27 kg
Mass of positron = 9.1 * 10^-31 kg

This is what I can't understand. In Beta-plus decay, a proton decays into a neutron where a positron is also emmited.

(1.675 * 10^-27) + (9.1 * 10^-31) = 1.67591 * 10^-27

(1.67591 * 10^-27) - (1.673 * 10^-27) = 2.91 * 10^-30

Somehow 2.91 * 10^-30 kg of mass appeared.
 
I guess you don't fully appreciate the point that I was trying to make. Positron emmssion is a nuclear decay process, not a proton decay process. For example, PET devices may use F18, which decays into O18.

The following table should help:

nuclide mass (amu) binding energy (kev)

O18 17.9991604 139807.0
F18 18.0009377 137369.2

energy difference 2437.8

Since a positron hass a mass equivalent to 511 kev, there is plenty left over for momentum.
 
Originally posted by repugno
Thanks for the reply Mathman,

Mass of neutron = 1.675 * 10^-27 kg
Mass of proton = 1.673 * 10^-27 kg
Mass of positron = 9.1 * 10^-31 kg

This is what I can't understand. In Beta-plus decay, a proton decays into a neutron where a positron is also emmited.

(1.675 * 10^-27) + (9.1 * 10^-31) = 1.67591 * 10^-27

(1.67591 * 10^-27) - (1.673 * 10^-27) = 2.91 * 10^-30

Somehow 2.91 * 10^-30 kg of mass appeared.

What you have done is look at the isolated case of a particle decay.
Mathman has already said that this isolated process does not occur for the exact reason you have shown, the proton does not have enough energy to produce a neutron and a positron. What is happening in the systems where this happens is that it is more energetically favorable to have a proton turn into a neutron and positron. The system, as mathman said, are nuclei where there is a large proton to neutron ratio. Because of the odd things that happen in nuclei, the lower energy state is obtained by this new configuration. So in the whole system there is enough energy to create the positron and the neutron.
Hope this helps.
Cheers,
Norm
 
Positron Decay...



One of the lightest nuclei that produces positronium decay:

^7_5B \to ^7_4Be + e^+(1.4 Mev)

proton/neutron ratio: 5/2

Fusion produced Deuprotium positronium decay:
^2_2He(+5.294 Mev) \to ^2_2D + e^+(.24 Mev)

proton/neutron ratio: 2/0

 
Last edited:

Similar threads

Undergrad Can proton/neutron decay be avoided in some conditions?
  • · Replies 3 ·
Replies
3
Views
4K
High School Beta Plus Decay: What's Left for the Electron?
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad How Does Beta Decay Relate to the Role of W Bosons in Weak Nuclear Force?
  • · Replies 4 ·
Replies
4
Views
5K
Undergrad From what locations do radiated electrons start their movement in Beta decay?
  • · Replies 21 ·
Replies
21
Views
2K
Graduate "Reversal" of Nuclear Decay in Beta Emitters
  • · Replies 4 ·
Replies
4
Views
2K
Graduate Beta Decay, why did they have to resort to Neutrinos?
  • · Replies 32 ·
2
Replies
32
Views
4K
High School Where does the mass of a W boson come from in beta decay?
  • · Replies 3 ·
Replies
3
Views
3K
High School Explaination of beta + decay in layman language
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad On the weak force and beta decay
  • · Replies 27 ·
Replies
27
Views
5K
Graduate Pandora's Box: Carbon-14 vicious cycle from transmutation
  • · Replies 1 ·
Replies
1
Views
3K