Wednesday, October 4, 2017

More Precise Proton Mass Bodes Well For Nuclear Physics

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Recent, high precision measurement of the proton's mass by a team led by Sven Sturm of the Max Planck Institute  for Nuclear Physics promises to pave the way for improvements in our understanding of nuclear fusion -especially in stellar interiors.

We know, for example, the basic fusion process that occurs in  the proton-proton cycle:

1H + 1H + e- ®  2 H   + n + 1.44 MeV

2 D   + 1H ®  3 He + g + 5.49 MeV

3 He + 3 He ®  4 He + 1H + 1H  + 12.85 MeV

The top line shows two protons fusing to yield deuterium (heavy hydrogen) with a positron and neutrino (n) emitted, along with 1.44 MeV of energy. Empirical evidence of this reaction is obtained from gallium detectors, of the neutrinos given off, which are within 1-2% of what theoretical models predict. In the second fusion reaction, the deuterium combines with a proton to give the isotope helium 3, along with a gamma ray (g) and 5.49 MeV energy. In the final fusion, two helium-3 nuclei combine to yield one helium-4 nucleus, along with two protons, and 12. 85 MeV energy. Note that the two ending product protons commence the cycle anew, so that the generation of nuclear energy is ongoing.

The new result by Sturm et al is:

m p   =    1.007276466583 amu (atomic mass unit)

The precision attained was 32 parts per trillion or a threefold improvement on the precision of the currently accepted value. What's wrong with the latter? It may well be too large. By contrast, the lower proton mass suggested by the Sturm et al group may well be key to resolving two measurements of the helium -3 mass that disagree by more than three standard deviations. (And note how  3 He   enters the nuclear fusion reaction chain shown above)

How did the Planck Institute team achieve their result? Basically by using what is called a "Penning trap"  This device uses electric and magnetic fields to confine ions. The team was then able to rapidly switch between trapping protons (hydrogen nuclei) and  12C 6 thereby determining their cyclotron frequencies (W   =  qB/ 2nm) as the two ions cycled within the trap's magnetic field with induction, B.

The ratio of the two frequencies:

f (m p)/  f ( 12C 6+ )

then yields the proton mass (m p ) given the mass of the 12C   atom  is defined to be exactly 12 amu and the correction needed to calculate the  12C 6+  ion mass is well known and extraordinarily precise. 

Note here that the motion of any ion in a Penning trap can always be decomposed into faster and slower circular motions perpendicular to the B-field and an oscillation along the B-field axis.  In effect the cyclotron frequency, W ,  can  be "teased out" of the composite frequency.  The greatest source of measurement errors are associated with the inhomogenieties in the trap's magnetic field.

Once this measurement is confirmed it should provide a strong basis to improve our nuclear physics, including the fact that  m p  is used as an input  in precise determination of the neutron mass and other quantities.

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