The a-decay of the actinides taking place in the crystal lattice creates an alpha particle and a recoil atom. The recoil atom produced has a range of about 12 nm and causes a dense collision cascade with typi­cally about 2300 displacements (Frenkel pairs) within a short distance, around 7.5 nm in size. The a-particle has a path of about 10 pm, with a cascade of about 265 displacements at the end of its range.1 Although recombination will take place, point defects and eventually extended defects (dislocations, dislocation loops) will survive in the crystal lattice, resulting in changes in the properties of the materials. Computer simulations of the radiation effects in fcc plutonium have shown that the defect recombination stage is much longer than that in other metals and that the vacancies do not seem to form clusters.19 In addition to the radiation damage, helium ingrowth takes place.

As discussed by Hecker and Martz,20 the expan­sion of the lattice of a-Pu is significant due to

2500

2000

2Г

1500 l-T

1000 500

self-irradiation, when held at cryogenic tempera­tures, saturating at about 10vol.%. In contrast, the (Ti-stabilized) p-phase shows a slight contraction and the (Al-stabilized) 8-phase a substantial contrac­tion, the latter saturating at 15 vol.%. Of course this is also reflected in other properties such as electrical resistivity.21,22 The radiation effects recover upon annealing to room temperature, a few percent of the damage remaining. Gorbunov and Seleznev23 observed that a-Pu containing predominantly 239Pu retains its crystal structure after prolonged storage at room temperature. A sample of predominantly shorter lived 238Pu (t1/2 = 87.74 years) contains both the a — and p-forms at immediate examination and additionally the g-, Z-, and e-phases after a similar storage period. Chung et a/.24 showed by X-ray diffraction and dilatometry measurements on 238Pu-doped 8-phase plutonium samples that the lattice expansion by self-irradiation appears to be the primary cause for dimensional changes during
the initial 23 years of aging. Following the initial transient, the density change is primarily caused by a constant helium ingrowth rate as a result of particle decay. The two effects were combined in an equation for the expansion AL/L with an exponential (radia­tion damage) and a linear (helium ingrowth) part:

AL/L ffi A[1 — exp(-Bt)] + Ct [1]

where A, B, and C are constants and t is time.

The self-irradiation is one of the main causes that complicates the study of the heavy actinide metals. For example, berkelium metal (t^2 = 314 days; ^0.2% 249Cf growth per day) shows signs of amorphization (weak and diffuse X-ray spectra) at room temperature, which improved after annealing and thermal cycling, and the samples were found to contain two crystallo­graphic structures at room temperature, double hexag­onal close-packed (dhcp) and fcc, ofwhich the former is the stable form.25 An extreme case is Es; its crystal structure has been resolved only by rapid electron diffraction of thin film material due to the very short half-life of the isotope used.26