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Lead Radius

Physics program > Standard Model tests

The doubly-magic nucleus 208Pb has 44 more neutrons than protons. Some of these extra neutrons are expected to be found in the surface, where they form a neutron-rich skin. The PREX experiment performed for the first time a clean measurement of the neutron radius of 208Pb and as consequence of the lead neutron skin, measuring the parity violating asymmetry Apv, that is the fractional difference in cross sections for positive and negative helicity electrons elastically scattered from a lead target. Apv is sensitive to the existence of the neutron skin.  Measuring the neutron radius of 208Pb constrains the equation of state (EOS) of neutron matter, that gives the pressure as a function of density. EOS of neutron-rich matter is on the other hand related to the symmetry energy S, the energy cost for having a different number of protons and neutrons. The value of the neutron radius of 208Pb has very interesting implications for the physics of neutron stars. A larger neutron radius in 208Pb would imply a stiffer EOS and consequently a larger neutron star radius. If some recent measurements suggesting a very small value for the neutron star radius were confirmed, a large value of the neutron radius of 208Pb would imply a softening of the EOS at high density and a transition to an exotic phase of QCD in neutron star core.

In addition, a thicker neutron skin in Lead means that energy rises rapidly with density and this quickly favors uniform phase and a thinner neutron star crust. At last, a large value of the neutron radius of  208Pb would imply a large value of the symmetry energy S at high density and hence a large proton fraction in neutron stars, that would allow the direct Urca process of rapid neutrino cooling in neutron stars and would explain why some neutron stars appear to be unexpectedly cold. The experiment PREX used a 50 to 70 µA beam of longitudinally polarized 1.06 GeV electrons and a 0.55 mm thick  208Pb target foil. Electrons were detected at a scattered  angle of about 5°. A value of A pv equal to 656 ± 60 (stat) ± 14 (syst) ppb was obtained. This value corresponds to a value of the neutron radius of  208Pb equal to

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