  A new critical survey is presented of all half-life, decay-energy and
branching-ratio measurements related to 20 0+ to 0+ beta decays. Compared with
our last review, there are numerous improvements: First, we have added 27
recently published measurements and eliminated 9 references; of particular
importance, the new data include a number of high-precision Penning-trap
measurements of decay energies. Second, we have used the recently improved
isospin symmetry-breaking corrections. Third, our calculation of the
statistical rate function now accounts for possible excitation in the daughter
atom. Finally, we have re-examined the systematic uncertainty associated with
the isospin symmetry-breaking corrections by evaluating the radial-overlap
correction using Hartree-Fock radial wave functions and comparing the results
with our earlier calculations, which used Saxon-Woods wave functions; the
provision for systematic uncertainty has been changed as a consequence. The new
corrected Ft values are impressively constant and their average, when combined
with the muon liftime, yields the up-down quark-mixing element of the
Cabibbo-Kobayashi-Maskawa (CKM) matrix, V_{ud} = 0.97425(22). The unitarity
test on the top row of the matrix becomes |V_{ud}|^2 + |V_{us}|^2 + |V_{ub}|^2
= 0.99995(61). Both V_{ud} and the unitarity sum have significantly reduced
uncertainties compared with our previous survey, although the new value of
V_{ud} is statistically consistent with the old one. From these data we also
set limits on the possible existence of scalar interactions, right-hand
currents and extra Z bosons. Finally, we discuss the priorities for future
theoretical and experimental work with the goal of making the CKM unitarity
test even more definitive.
