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Applying the pulsed ion chamber methodology to full range reactor ...

Applying the pulsed ion chamber methodology to full range reactor ...

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charge distribution in an argon filled chamber for low. and high ioniza- tion source rates, respectively. The severe flattening of the distribu- tion of n at the higher source rate is clue to volume recombination, figures 2-4 and 2-5 depict the theoretically determined v(t } versus S for argon and neon gas filled miniature fission, chambers. In both cases the response is at first, linearly controlled by free diffusional losses. At approximately 10" ion pair/sec, ambipolar diffusion becomes the con- trolling factor and there is a lateral shift in the linear response due to the more rapid ambipolar diffusional loss mechanism. Above 10 8 ion pairs/sec volume recombination becomes the controlling loss mechanism and the response becomes second order. These results were, to some extent, experimentally validated by Markwell . Figure 2-1 shows measurements taken in the University of Florida Training Reactor (UFTR) with a PIC system. Over seven decades of reactor flux were measured. Both first and second order gas kinetic response region are observed, as was predicted, A milliwatt was the minimum measurable reactor power, due to the noise and time jitter of that PIC system The upper limit of 10 Kw was then the maximum output of the UFTR. Note that care must be taken in directly relating the experimental and numerical data shown, since the former deals with the collection of electrons and the later with the collection of ions. The effective replication of the characteristics of the sets of data, resulting from either gamma or neutron caused ionization, indicates the basic dependence of the PIC operation on the ionized gas kinetics, regard- less of which radiation induced the ionization. Thus, although a neutron sensitive chamber results in a signal which is comprised of both neutron