Low temperature properties of transuranic and oter heavy metals.

The transuranic metals, neptunium and plutonium, are strong alpha emitters and have to be handled under special conditions to safeguard the health of those working with them. The most common method of containing these metals is to use glove boxes but in certain circumstances it is possible to encapsulate specimens for experimental purposes. The relative merits of the use of glove boxes and sealed capsules for cryogenic experiments are discussed in this thesis.
A further consequence of the high alpha activity of these metals is their property of self heating. This self heat is a result of the absorption by the crystal lattice of the kinetic energies of the alpha particle and the recoil nucleus. At room temperature this has little or no effect on the physical properties of these metals but at low temperatures it is possible for the defects produced in the lattice to remain in equilibrium with their surroundings.
The resistivities of alpha and delta stabilised plutonium, neptunium and uranium 233 are found to increase when these metals are held at low temperatures for long periods of time. Little work on the self-irradiation of uranium 233 and neptunium has been undertaken because of their low self heats and the consequent long self-irradiation times necessary to cause a significant increase in their low temperature resistivities. The storage and annealing of defects in alpha and delta stabilised plutonium have however been studied in considerable detail.
The resistivity of alpha plutonium has an extremely anomalous temperature dependence. As the temperature falls from room temperature the resistivity first rises slowly to a maximum at ~ 195°K, then drops sharply as the temperature is further lowered to zero. Its residual resistivity is high and very sensitive to small impurity concentrations. It is found that over a period of about 5,000 hours at ~ 5°K the resistivity of alpha plutonium increases from between ~ 20 and ~ 5 μΩ cm. to approximately 140-145 μΩ cm.; some 15-20 μΩ cm. lower than the value of the resistivity maximum and ~ 105°K. Furthermore the additional resistivity caused by self-irradiation at ~ 5°K does not obey Matthiessen's rule and it is found that after long periods of self-irradiation at ~ 5°K the resistivity measured at ~ 77°K has decreased by about 15-20 μΩ cm. More detailed investigation of the increase in resistivity during self-irradiation at ~ 5°K shows that the additional resistivity decreases steadily with increase in temperature, becoming zero at ~ 55°K and then negative above ~ 55°K. In contrast, self-irradiation at ~ 77°K causes an increase in the resistivity both at ~ 5°K and ~ 77°K.
The resistivity of delta stabilised plutonium also increases when held at low temperatures for long periods of time. The initial rate of resistivity increase is however much lower than that of alpha plutonium and is found to be dependent on the concentration of aluminium used to stabilise the delta phase. Although the resistivity of delta stabilised plutonium increases initially at a lower rate than alpha plutonium it does not saturate so quickly and eventually rises to values considerably in excess of that of its original resistivity maximum.
The additional resistivity produced in alpha plutonium is found to anneal in two distinct stages. The first stage is centred about 80°K and analysis shows that the activation energy of the recovery process is approximately 0.2 eV. At 90°K the anneal is found to be characterised by second order reaction kinetics but it is difficult to analyse simply anneals carried out at ~ 77°K.
The second annealing stage in alpha plutonium is centred at ~ 160°K and gradual annealing occurs up to room temperature where all but a few percent of the original resistivity increase have annealed. It is found that annealing in delta stabilised plutonium sets in at a lower temperature than in alpha plutonium, that the first annealing stage in delta plutonium is enhanced at the expense of the second stage and that a larger percentage of the resistivity increase remained unannealed after warming to room temperature.
None of the theories proposed to explain the low temperature properties of plutonium has as yet gained universal acceptance and it is therefore difficult to give an unequivocal explanation of the results presented in this thesis. The various theories of the low temperature properties of plutonium are reviewed and discussed and it is shown that the behaviour of alpha and delta stabilised plutonium during self-irradiation may be best explained by assuming that they are both anti-ferromagnetic at low temperatures. Such an assumption is not unreasonable in view of the close similarity between the actinide and lanthanide series of metals.