= Solution
The functional arrangement below separates the polarization analysis from the resonance selection. The shield prevents the direct source beam from overwhelming the detector; the off-axis detector accepts <gamma rays> re-emitted by the <samarium> scatterer.
\Image[/past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2016-goldhaber-helicity-apparatus.png]
{title=Functional Goldhaber helicity measurement and the selected spin geometry}
The <europium> source emits into magnetized iron, whose magnetization is alternated. A <samarium> scatterer selects nuclear <nuclear resonance fluorescence>, favouring <photons> emitted along the daughter recoil. A shielded NaI(Tl) <scintillation counter> records the scattered nuclear line. Compare the background-subtracted line rates for the two iron magnetizations, and use the calibrated sign of the analysing power to identify the <photon helicity>.
The measured circular polarization was negative, compatible after angular acceptance corrections with negative <neutrino helicity>. By the <angular momentum conservation> argument, the selected <photon helicity> and <neutrino helicity> have the same sign. Thus the experiment concludes
$$
\boxed{\mathbf S_\nu\cdot\widehat{\mathbf p}_\nu=-\frac12}
$$
for the emitted relativistic <electron neutrino>. Finite acceptance dilutes the measured polarization; it does not justify equating the raw count asymmetry to $-1$.<Helicity of Neutrinos by Maurice Goldhaber and others>[Pages 1016–1017, apparatus, reversal calibration and negative circular polarization]{ref}
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