User:Samnotwil/Goldhaber Experiment

The Goldhaber experiment was a nuclear physics experiment performed in 1957 by Maurice Goldhaber, Lee Grodzins, and Andrew Sunyar that directly measured the helicity of the neutrino. Neutrinos were shown to exist only as left-handed particles or right-handed antiparticles. The experiment was first performed at Brookhaven National Laboratory.

Background

In 1956, the year prior to this experiment being performed, the weak interaction was shown to violate parity, in the Wu Experiment.

Experimental Technique

It is not practically feasible to directly measure the helicity of the neutrino, due to its minimal level interaction, so a scenario was proposed in which a gamma ray would only be measured if the neutrino had a specific helicity.

The experiment begins with a sample of $_{93}^{152m}$Eu, a metastable state. Approximately 20% of $_{93}^{152m}$Eu decays are electron capture. Electron capture is almost always of a k-shell electron, which has no orbital angular momentum. The nuclear equation that describes this process is

${}_{93}^{152m}{\text{Eu}}+e^{-}\rightarrow {}^{152*}{\text{Sm}}+\nu _{e}$

Alternatively, this could be written:

${\begin{array}{ccccccccccc}\Leftarrow &&\Rightarrow &&\Leftarrow \Leftarrow &&\Rightarrow &&\Leftarrow &&\Rightarrow \Rightarrow \\{}^{152}{\text{Eu}}&\longrightarrow &\nu _{e}&+&{}^{152}{\text{Sm}}^{*}&\quad {\text{or}}\quad &{}^{152}{\text{Eu}}&\longrightarrow &\nu _{e}&+&{}^{152}{\text{Sm}}^{*}\\&&\longleftarrow &&\longrightarrow &&&&\longleftarrow &&\longrightarrow \\\end{array}}$

This decay results in a nuclear excited state of samarium, ^152*Sm. The samarium nucleus will recoil. The initial states, the Eu and the electron, are at rest; in order to conserve momentum, the neutrino and the samarium nucleus must travel in opposite directions. So, knowing the direction of momentum of the samarium nucleus, the neutrino momentum direction can be inferred.

The energy of this nuclear recoil is approximately 5 eV.

This nuclear excited state of samarium has a lifetime of approximately 10^-13 s.

The linewidth of the Samarium excited state is small, of order 0.01 eV, however, due to doppler broadening, the line becomes approximately 0.5 eV wide.