Low temperature properties of transuranic metals : a study of the specific heat of plutonium, neptunium and plutonium monocarbide

The specific heat of the transuranic metals, plutonium and neptunium, has been measured below room temperature. Both these metals are strong α-emitters and are extremely toxic. As a consequence they have to be handled under special conditions, and the usual methods are either by containment in glove boxes or by encapsulation. The latter method has been adopted here and developed such that the calorimeter formed the encapsulation, and the canned specimens could then be handled in an inactive cryostat.
A further consequence of the high α-activity of these metals is their property of 'self-heating', which makes the determination of their specific heat by the conventional continuous-heating method impossible. The magnitude of the self-heat, particularly of plutonium, meant that a continuous heating method had to be evolved which could deal with the inevitable high rates of rise of temperature. A cryostat and measuring system were constructed and developed such that the specific heat could still be determined with reasonable accuracy. A control system was also developed in order that an adiabatic technique could be employed.
The self-heat is temperature independent and was utilised as a constant power supply to the calorimeter. A review of the literature values of the half-lives and decay energies of the various radioactive isotopes present in any sample of a transuranic metal, is given since the calculation of the self heat is critically dependent of these quantities.
The specific heat (CP) of α-plutonium was not found to exhibit the large and irreproducible peaks which characterised the early work of Sandenaw. CP followed a smooth 'sigmoid' curve, except for a small 'excess' between 40–70°K the magnitude of the excess (~ 2.5–3%) was not sufficient to produce a maximum on CP. CP values were generally high, rising to a value of 7.86 cals./mole/deg. at 300°K, and analysis of the data gave an electronic coefficient (γ) of 38 × 10−4 cals./mole/deg.2 and Debye theta (θD) values between 150 and 170°K in the temperature range ~ 14–300°K. Further analysis and separation of the excess, yielded a contribution to the specific heat which had the typical λ-shape of a magnetic transition, though the energy and entropy involved were extremely small.
One of the main theories proposed to explain the unusual low temperature properties of plutonium is that there is a paramagnetic-antiferromagnetic transition at ~ 60°K, and the present specific heat work would indicate that this is not an unreasonable hypothesis. Lack of affirmative evidence from neutron diffraction experiments however has necessitated a review and discussion of the alternative theories proposed, and also a comparison with the known antiferromagnetic α-manganese, to which many parallels exist.
Short term self-irradiation experiments were carried out on α-plutonium. The magnitude of the energy stored by the defects produced by storage at low temperatures, and the temperature range in which this annealed out, were found to give satisfactory agreement with the theoretical estimates and the self-irradiation damage effects previously observed on the electrical resistivity.
The specific neat of α-neptunium was found to be a smooth curve throughout the range of measurement of ~ 7.5–300°K, and the values of γ = 34 × 10−4 cals./mole/deg.2 and θ0 = 187.5°K resulted from analysis of the low temperature region. No evidence of any magnetic phenomena which must always be in mind in view of the close similarities between the actinide and lanthanide series of metals, was seen on α-neptunium in keeping with the bulk of the other known properties. Neptunium however is still only available in very limited quantities and in a state of purity that is by no means high (~ 99%).
In view of the large electronic coefficients observed in plutonium and neptunium, a discussion is given on the baud structures of the actinides in relation to their electronic properties.
The analyses of the specific beat dats are dependent in the first place on a reduction of CP to CV. Lack of sufficient data on the expansion coefficients (αL) of plutonium and neptunium, led to the measurement of αL for these two metals in the temperature range 20–300°K. A simple push-rod dilatometer technique was used, and expansion coefficients of ~ 45 × 10−6/deg. for α-Pu and ~ 20 × 10−6/deg. for α-Np were found. The Grüneisen gamma values were ~ 2.6 for α-Pu and ~ 1.8 for α-Np respectively.
The development of reactor fuel materials has caused a great deal of interest to be centred on the carbides of uranium and plutonium. Since no previous specific heat determination had been carried out below roots temperature on PuC, this was investigated down to ~ 12.5°K. Other low properties of this metallic carbide had again suggested antiferromagnetic behaviour below 100°K. No evidence in support of this was found on the specific heat results though an extremely high density of states at the Fermi surface is suggested.
The standard thermodynamic functions of plutonium, neptunium and plutonium monocarbide have been calculated and are tabulated in the appendices.