We have studied thermalization of binding energy released as an instantaneous, internal heat Pulse in a sphere. This resembles transient heat transfer in laser flash experiments, where thermal diffusivity is determined from temperature evolution measured oil front or rear surfaces of thin films. Here, instead, we are interested in the long term behavior of sample temperature when it approaches thermal equilibrium. The method is applied to a nucleus. Therefore, not only energy source, time scale, and sample geometry but also materials, properties, and boundary conditions in the present article are completely different from conventional matter and standard laser flash methods. However, the principle by which determination of diffusivity is made is conserved. diffusivity, K, of the nucleus is estimated from kinetic gas theory to be kappa/[10(-8) m(2).s(-1)] = 9.35 +/- 2.97. Second, numerical simulation yields the time, t(E), needed for thermalization after the disturbance. From comparison of t(E). with lifetime resulting from the uncertainty principle, the diffusivity can be extracted. Both results for K agree within 10 %, but K depends oil energy level density (or excitation energy), a dependence that is not reported in previous literature. Thermalization in nuclear matter is confirmed to proceed by diffusion. The new internal source method Could be transformed to experiments on a laboratory scale.