The primary techniques used to characterize the behavior of He and He bubbles in materials include TEM, small-angle neutron scattering (SANS), posi­tron annihilation spectroscopy (PAS), and thermal desorption spectroscopy (TDS). All of these techni­ques, and their numerous variants, have individual limitations. Complete and accurate characterization of He transport and fate requires a combination of these methods; however, such complementary tools are seldom employed in practice. Note that there are also a variety of other methods of studying helium in solids that cannot be discussed due to space limitations.

TEM, with a practical resolution limit of about 1 nm, is the primary method for characterizing He bubbles. Bubbles and voids are most frequently observed by bright field (BF) ‘through-focus’ imaging in thin regions of a foil. The Fresnel fringe contrast changes from white (under) to black (over) as a func­tion of the focusing condition. The bubble size is often taken as the mid diameter of the dark under focus fringe. Two critical issues in such studies are artifacts introduced by sample preparation, which produce similar images and determine the actual size, especially below 2 nm.66-68 Electron energy — loss spectroscopy (EELS) can be used to estimate the He pressure in bubbles.69,70

SANS provides bulk measures of He bubble microstructures. In ferromagnetic steels, both nuc­lear and magnetic scattering cross-sections can be measured by applying a saturating magnetic field («2T) perpendicular to the neutron beam. The coherent scattering cross-section variations with the scattering vector are fit to derive the bubble size distribution, with a potential subnanometer resolu­tion limit.71 The magnitude of the scattering cross­section is proportional to the square of the scattering length density contrast factor between the matrix and the bubble times the total bubble volume fraction. Since the magnetic scattering factor contrast is known (He is not magnetic), the bubble volume fraction, and corresponding number densities, can be directly determined by SANS. The nuclear scattering cross­section provides a measure of the He density in the bubbles. Thus, the variation in the ratio of the nuclear (He dependent) to magnetic (He independent) scat­tering cross-sections with the scattering vector can be used to estimate the He pressure (density) as a func­tion of the bubble size.71,72 Some studies have shown that SANS bubble size distributions are in good agreement with TEM observations,73’74 while others show considerable differences for small (<^2 nm) bubbles.72 Limitations of SANS include distin­guishing the bubble scattering from the contributions of other features; note that, in many cases, these features may be associated with the bubbles. Other practical issues include measurements over a suffi­cient range of scattering vectors and handling of radioactive specimens. Note that small-angle X-ray scattering studies can also be used to characterize He bubbles, and this technique is highly complementary to SANS measurements.

PAS is a powerful method for detecting cavities that are smaller than the resolution limits of TEM and SANS. Indeed, positrons are very sensitive to vacancy type defects, and even single vacancies can be readily measured in PAS studies.75,76 PAS can also be used to estimate the He density, or He/vacancy ratio, in bubbles.77 In the case of He-free cavities, the positron lifetime increases with increasing the nano­void size, saturating at several tens of vacancies. How­ever, in the case of bubbles, the lifetime decreases with increasing He density. In principle, positron orbital electron momentum spectra (OEMS) can also provide element-specific information about the anni­hilation site.78 Thus, for example, OEMS might detect the association of a bubble with another microstruc­tural feature. Limitations of positron methods include that they generally do not provide quantitative and unique information about the cavity parameters. The application of PAS to studying He in steels has been very limited to date.

TDS measures He release from a sample as a function of temperature during heating or as a function of time during isothermal annealing. The time-temperature kinetics of release provides indi­rect information about He transport and trapping/ detrapping processes. For example, isothermal anneal­ing experiments on low-dose (<2appm) a-implanted thin Fe and V foils showed that substitutional helium atoms migrate by a dissociative mechanism, with dissociation energies of about 1.4 eV, and that dihe­lium clusters are stable up to 637 K in Fe and up to 773 K in V.79 At higher concentrations in irradiated alloys, He can be deeply trapped in cavities (bubbles and voids); in this case, He is significantly released only close to melting temperatures.80, 1 Given the complexity and multitude of processes encountered in many studies, it is important to closely couple TDS with detailed physical models.82,83 Techniques that can quantify He concentrations at small levels

used in TDS can also be used to measure the total He contents in samples that are melted.81

In summary, a variety of complementary techni­ques can be used to characterize He and He bubbles in structural materials. A good general reference for these techniques and He behavior in solids can be found in Donnelly and Evans.84 TEM and SANS can measure the number densities, size distributions, and volume fractions of bubbles, subject to resolution limits and complicating factors. The corresponding density of He in bubbles can be estimated by TEM-EELS, SANS, and PAS. TDS can provide insight into the He diffusion and trapping/detrapping processes. Unfortunately, there have been very lim­ited applications in which various methods have been applied in a systematic and complementary manner. Major challenges include characterizing subnan­ometer bubbles in complex structural alloys, includ­ing their association with various microstructural features.