Spin-wave theory of a Heisenberg antiferromagnet predicts a zero-point spin deviation η from the assumed ground state, namely, the Néel state. Measurements of η using NMR in a number of compounds give values substantially smaller than that given by spin-wave theory. The object of this paper is to present the theory necessary to allow the measurement of η by magnetic scattering of thermal neutrons from an antiferromagnet containing a weak concentration c of nonmagnetic impurities. The diffuse elastic cross section for such a system is dσ/dΩ=(γe2/2mc2)2(1−̂z2)Nc(1−c)F2(κ) 〈Sz〉2 | Q(κ−τ) | 2,where 〈Sz〉 is the thermal average of the unperturbed host spins, Q(κ) the Fourier transform of the deviations in the z components of spin from 〈Sz〉 due to an impurity and all other symbols have their usual meaning. It is evident that η may be measured provided Q(κ) is known. We have calculated Q(0) for a body centered antiferromagnet of arbitrary spin in the linear spin-wave approximation. This has been achieved using thermal Green functions which also permits the dynamics of the system to be investigated.