The problem of the 27Al neutron spin-dependent scattering length

Early measurements of the neutron spin-dependent scattering length of ²⁷Al by neutron precession have given two values, consistent within the limits of accuracy, whose mean value is b₊?b_? = + 0.52 ± 0.02 fm. The positive sign was recently put in doubt from known values of neutron resonance parameters. New experiments were undertaken on a sample of ²⁷Al, a compound sample of ²⁷Al and ⁴⁵Sc, together with a redetermination of the sign of the ²⁷Al gyromagnetic ratio. These experiments confirmed in sign and magnitude the b₊?b_? value previously found. We have so far no explanation of the discrepancy.