Quantum diffusion in hydrogen, studied by calorimetric methods

Two methods, both employing an adiabatic calorimeter, were used experimentally to study dilute solid solutions of hydrogen over the temperature range 0.4–3 K, with ortho concentrations of x = 0.10, 0.21 0.50, and 1.10 at.%. The time-dependent specific heat was utilized to characterize the configurational relaxation time τ in the ortho subsystem, as well as the temperature dependence of the specific heat of the ortho subsystem for the equilibrium cluster concentration characteristic of the temperature of the experiment, which does not vary during a measurement of the specific heat. We compare the experimental results with values for the specific heat of the ortho subsystem calculated in terms of a simple statistical model. We propose and demonstrate a means of determining τ from data on the energy given up in solid hydrogen per unit time due to conversion and cluster formation. From the calorimetric results we find that within the limits of experimental uncertainty, the value of τ is independent of concentration. The temperature dependence of τ for T < 1 K is described by the expression τ ∼ T−1.

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