The primary objective of the Reactor Physics Constants Center (RPCC), Argonne National Labora-
tory, is the periodic compilation and publication of the latest and best values of the constants, recipes,
formulae, etc., which are necessary to calculate reactor characteristics. In addition to providing a
unified compilation of pertinent data, the precepts of the RPCC are: (1) to encourage critical compari-
son of experiment with theory; (2) to encourage critical analysis of experimental techniques; (3) to en-
courage critical evaluations of contradictory data; and (4) to call attention to areas where further ex-
perimental and theoretical work will be most profitable; (5) to promote standardization in definitions
of reactor physics parameters; and (6) to serve as a clearing house for reactor physics constants.
In this second edition of Reactor Physics Constants, the editors have assumed the prerogative to a
somewhat greater degree than in the first edition (1958), of evaluating the data selected for publica-
tion. However, the absence of a particular set of data is not to be construed as an adverse judgment by
the editors; in many instances, the editors were unaware of the existence of such information, or they
may have decided that the information did not fit properly into the framework of the compilation. Fur-
thermore, because of the wealth of material evaluated, and owing to space limitations for each sub-
ject, certain data are included and comparably good data are excluded. In such cases, the included data
are intendod to exemplify some facet of reactor physics.