1.06.2.1 Single, Dual, and Triple-Beam CPI

Single (He), dual (typically heavy ions to produce dpa and He), and triple (typically heavy ions, He and H) beam CPI have been extensively used to study He effects for a wide variety of materials and conditions. The number of facilities worldwide, both current and historically, and the large resulting liter­ature cannot be fully cited and summarized in this chapter, but some examples are given in Section

1.06.1 . A more complete overview of these facilities can be found in a recent Livermore National Labo­ratory Report.24 Extensive high-energy He implan­tation studies of creep properties were carried out at Forschungszentrum Julich using a 28 MeV He cyclo­tron.2 Major dual — and triple-beam studies were previously carried out at Oak Ridge National Labo­ratory (180 keV H, 360 keV He, 3.5 MeV Fe)26 and many other facilities around the world.24 The new JANNUS facility at Saclay couples a 3 MV Pelletron with a multicharged ion source and a 2.5 MV single Van de Graaff and a 2.25 MeV tandem accelerator.27 Another multibeam facility at Orsay couples a 2 MV couple, a tandem accelerator, and a 190 kV ion im — planter to a 200 kV transmission electron microscope (TEM) to allow simultaneous co-irradiation and observation.27

The advantages of He implantation and multibeam ion irradiations include the following: (a) conditions can be well controlled and in many cases selectively and widely varied; (b) high dpa, He, and H levels can be achieved in short times; (c) the specimens are often not, or only minimally, acti­vated; and (d) in situ TEM observations are possible in some cases. The disadvantages include the follow­ing: (a) highly accelerated damage rates compared with neutron irradiations; and in the case of
multibeam ion irradiations, (b) shallow damage depths and the proximity of free surfaces; (c) non­uniform damage production and the deposition of foreign ions; and (d) inability to measure bulk prop­erties. High-energy He implantation can be used on bulk specimens tested, either in situ or post­implantation, to measure tensile, creep, and creep rupture properties. The corresponding disadvantages are that He implantation results in high He/dpa ratios («6000 appm He/dpa).2 The differences be­tween CPI and neutron irradiation can significantly affect microstructural evolution.

Thus, it must be emphasized that He implantation and multibeam CPI do not simulate neutron irradia­tions. Although it has been argued that CPI reveal general trends and that corrections, like temperature adjustments, allow extrapolations to neutron-irradiation conditions, both assertions are problematic. The proper role of He implantation and multibeam CPI is to help inform and calibrate models and to identify and quantify key processes based on carefully designed mechanism experiments.