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Technical Discussions
Luitpold A. Vogels
Luitpold A. Vogels
20:09 Apr-11-2017
beta decay

ASNT level iii study guide: Neutron yields Proton + Electron + Antineutrino

In my ASNT exam:

Neutron decays to
1. proton & positron
2. proton & positron and neutrino
3. proton & electron and neutrino
4. proton & beta

I chosen 3 but why it was not antineutrino?

Hamid Reza
Canada, Joined Oct 2016, 68

Hamid Reza

Joined Oct 2016
22:46 Apr-11-2017
Re: beta decay
In Reply to Luitpold A. Vogels at 20:09 Apr-11-2017 (Opening).

Dear Luitpold,

Neutron decays to ----> Proton + Electron + Electron Antineutrino

But you answered it right. Some texts still show this as Neutrino. I have some elementary knowledge of particle physics and I will try to explain it to you in simple terms.

* Short answer:

In 1930s Wolfgang Pauli, predicted that there was something missing in beta decay equation:

Neutron --> Proton + electron + ?

In simple terms, there was something needed to be there to keep the total energy of the system constant, before and after the reaction . In other words, the law of conservation of energy must hold true. This unknown particle shown above with ? was responsible for the missing energy along with linear and angular momentum. Pauli called this particle "Neutron". Later the famous Fermi renamed it to Neutrino.

In 1956 Cowan and Reines were able to prove its existence experimentally which lead to 1995 Nobel prize. It was called an "electron-Neutrino":

Neutron ---> Proton + electron + electron-Neutrino

At the same time another scientist was also trying to find these particles but was unsuccessful. Later he hypothesized that one of the reasons that he was not able to find "neutrino" was that there might be also another anti-particle called anti-neutrino. He later proved that the particle that Cowan and Reines found was actually an Electron-Antineutrino. He himself later detected Electron-Neutrinos (real electron-neutrinos and not the electron-antineutrino) from the sun and won the 2002 Nobel prize.

I have seen two books written for beginners mentioning this fact by saying things like:

neutrino (in fact, in today’s parlance, an antineutrino).

* A little more detailed answer:

In the world of particle physics, there are some other conservation laws that must hold true for certain reactions... among these are conservation of baryon number, conservation of electron number and various other conservation of quantum numbers.


Electrons and Electron-Neutrinos have electron number = +1
Positrons and Electron-Antineutrinos have electron number = -1
All other particles have electron number = 0

Now: N ---> P + e + electron-antineutrino
0 ----> 0 + 1 + -1

There is another law called Crossing:

N ---> P + e + electron-antineutrino equals to Neutrino + Neutron = Proton + Electron
P ---> N + positron + electron-neutrino equals to electron-antineutrino + Proton = Neutron + Positron

There are a lot of other equations that were statistically and tentatively found that must hold true. In simple terms, they found out that using an neutrino instead of antineutrino (a particle instead of an antiparticle) violated some of these laws. They found out that the >>> ? >>>needed in the beta decay equation was the antineutrino and not the neutrino. So now they have corrected it. Apparently ASNT didn't.

* Extra information: Since you mentioned that you have taken the ASNT Level III examination I believe you are familiar with pair-production and positron-electron annihilation phenomena. They found the antineutrino with the above crossing law:

electron-antineutrino + Proton = Neutron + Positron

When a positron hits an electron it annihilates and two gamma rays are emitted. Also a free neutron is captured by cadmium chloride and emits a gamma ray. Cowan and Reines used a special water tank with solved cadmium chloride embedded in scintillator screens near a nuclear reactor (source of electron antineutrino) to detect the radiation. The interaction of electron-antineutrino + proton of the Hydrogen atom resulted in 3 gamma rays. Two happened simultaneously (electron-positron annihilation) the other one with a very short delay (neutron capture). Three times per hour, 9 gamma rays in total... This way they proved the existence of neutrinos.



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