Thermal diffusion length, effect experiment

The deterioration of the grating (shown in Fig. 91) with successive pulses cannot be attributed to thermal effects, since in an experiment using a mask and projection technique (1 pm slit pattern and multiple pulses with 200 mj cm-2) sharp contours remained. Additionally, the thermal diffusion length for the laser pulse corresponds only to 30-40 nm for commercial polymers such as poly(ethylene terephthalate). 

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

The first group in a rather arbitrary ordering of the experiments is devoted to studies of the behavior of neutrons from fission or source energy to thermalization. Moderation and diffusion properties, diffusion length, and Fermi age are measured in a water tank facility at a reactor. Distribution of thermalized neutrons can be measured in several uraniumbearing exponential facilities in which neutron multiplication occurs and from which material buckling and critical reactor size can be inferred. In another exercise, the effective neutron temperature in two of the critical training reactors is measured by several methods. 

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