Experimental determination of the buckling for unusual geometrical shapes using paraffin

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Virginia Polytechnic Institute

The buckling constant for neutron moderators in the shape of an elliptic cylinder, hemisphere, and"piggy-back" cylinder has been determined by experimental measurements using paraffin as a moderator.

The pulsed neutron sources technique was used throughout the work; a fast neutron burst of short duration was injected into regular paraffin shapes (cylinders with diameter and height ratio equal approximately to one). An empirical curve of decay constant of the neutron population versus the buckling was obtained. The measured decay constant, λ, was fitted by the method of least square to a parabola in B² of the form:

λ=Σav+ B²D - CB⁴


λ= the decay constant

B² = the buckling constant

Σa = the macroscopic absorption cross section

v = the neutron velocity

D = diffusion coefficient

C = diffusion cooling coefficient

The resulting values of the diffusion parameters are:

Σa v = 4858 ± 162 sec⁻¹

D = 25911 ± 202 sec⁻¹ cm²

C = 1186 ± 2558 sec⁻¹ cm⁴

By measuring the decay constant, the buckling of the moderators with irregular shapes were determined from the above parameters. The result was in general accord with the theoretical approximations for such shapes.