Highly convenient rules are given for the general term in the time-independent perturbation-theory expansion for the self-energy operator of quantum statistical mechanics. The rules are derived by starting from the usual formalism involving time-dependent Green's functions. The well-known formulas for thermodynamic quantities in terms of the self-energy operator are included for completeness.

The possibility of improved detection discrimination between selected materials; the high absorption of several 1ight materials such as hydrogen, lithium, and boron; and the relatively low absorption of many heavy materials are some characteristics which make neutron radiographic inspection attractive as a complementary technique to xradiography. A study of neutron radiography is reported in which the emphasis has been placed upon determining many of the characteristics of various techniques for detecting neutron images. Characteristics of photographic detectors for thermal neutron images are outlined. (R.J.S.)