  The two-nucleon momentum distributions have been calculated for nuclei up to
A=40 and various values of the relative and center-of-mass momenta and angle
between them. For complex nuclei a parameter-free linked-cluster expansion,
based upon a realistic local two-nucleon interaction of the Argonne family and
variational wave function featuring central, tensor, spin and iso-spin
correlations, has been used. The obtained results show that: 1) independently
of the mass number A, at values of the relative momentum k_rel> 2 fm^{-1} the
proton-neutron momentum distributions for back-to-back (BB) nucleons (K_cm=0)
exhibit the factorization property n_A^{pn}(k_rel,K_cm=0)=C_A^{pn} n_D(k_rel)
n_{cm}^{pn}(K_cm=0), where n_D is the deuteron momentum distribution,
n_{cm}^{pn}(K_{cm}=0) the momentum distribution of the c.m. motion of the pair
and C_A^{pn} the nuclear contact measuring the number of BB pn pairs with
deuteron-like momenta; 2) the values of the proton-neutron nuclear contact
C_A^{pn} are obtained in a model-independent way from the ratio
n_A^{pn}(k_rel,K_cm=0)/n_D(k_rel) n_{cm}^{pn}(K_cm=0); 3) also the
K_cm-integrated pn momentum distributions divided by the deuteron momentum
distribution exhibits a constant behavior equal to C_A^{pn}, but only at very
high values of k_{rel}> 3.5fm^{-1}, where the relative momentum distribution is
entirely governed by BB short-range correlated nucleons; 4) the absolute value
of the number of pn and pp short-range correlated pairs is calculated,
illustrating that the high values (K_cm>1 fm^{-1}) of the pair c.m. momentum
appreciably reduce the dominance of the pn over pp pairs produced by the tensor
force when K_cm=0; 5) calculations are in good agreement with the VMC
calculations for light nuclei and with available experimental on the processes
A(e,e'pn)X and A(e,e'pp)X.
