Reactor Parameters by Pulsed Neutron Beam

It is clear that although the exponential pile and criticality experiments would be required ultimately in any large-scale reactor development program, it would be desirable to obtain some preliminary experimental verification of reactor calculations by means of other more modest tests. One experiment which appears to be eminently suited to this purpose is based on the pulsed neutron-beam technique. This technique has been applied by several investigators to the determination of the thermal-diffusion coefficient and macroscopic absorption cross sections of reactor materials.More recently, it has been used by E. C. Campbell and P. H. Stelson in the study of short-lived isomers and for the measurement of reactor parameters of multiplying media. The experiment con- 

Briefly, the experimental procedure consists of bombarding a target of, say, triton with a beam of protons from an accelerator (see Fig. 9.1). The ensuing reaction, as indicated in the above equation, produces high-energy neutrons (in the order of 1 Mev). If the proton beam is applied intermittently, the target will yield bursts of fast neutrons, and some of these in turn will produce neutron-flux transients in a test specimen placed in the vicinity of the target. The time variations in neutron 

Thus our immediate problem is to determine the thermal-flux distribution 4 (r,0 in an arbitrary geometry, given the distribution 4 q t) at i = 0. For simplicity, both analytical and experimental, we assume that the specimen is a bare homogeneous system and of some elementary shape such as a parallelepiped, cylinder, or sphere. The differential equation for such a system which contains fissionable material is given by [cf. Eq. (9.10)1 

The solution may be obtained by the separation of variables. Thus take = F t) G(t). Equation (9.57) may then be written 

The solutions to the wave equation (9.596) were developed previously in Secs. 5.4a, b, and e. For our present purposes it is convenient to write these solutions in the form 

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