Using
EELS
analysis in
STEM
to investigate the helium content in irradiated materials
Authors: Prikhodko Kirill , Emelyanova Olga 
Published: December 2016

Helium very often is forming inside the reactor structural materials during neutron irradiation due to different nuclear reactions. Because of the helium‐vacancy interaction effect the later may be stabilized by this elements. The temperature vacancy mobility leads to formation of the pores inside the materials that take part in the swelling effect. It is very important for steels under high fluence fast neutron applications, internal parts of pressure vessel nuclear reactors and also for fusion reactors. Nowadays researchers are performing experiments at accelerators to simulate under ion irradiation the materials atomic damage production and understand the possible radiation stability of the new designed materials. He ions irradiation can be used also to simulate above mentioned helium production.

Pores inside irradiated steels were visible for many years because of the typical uderfocus/overfocus TEM contrast changing. Many pores were characterized by the faceting shape that means the vacancy nature of these objects, but the presence of helium inside can be proved using analytical energy loss technique only. In this work, we show the implementation of EELS technique in STEM mode to find out the helium content inside the pores in the 40 keV He ion beam irradiated up to 5*10
20
m
−2
at 650°C Eurofer ODS steel. Because of the low value of projected range of these ions the best way was to make cross‐section samples using FIB technique.

Figure 1 shows the STEM dark field image of the steel cross‐section sample under irradiation at the ions projected range region. The thickness of the sample at interesting area was about 0.7*λ. We can see that pores with different sizes and shapes were formed under high temperature irradiation. Some of pores were faceting type that indicates the vacancy nature of their structure.
To understand the helium content inside the pore we took low energy EELS spectra from the center of the pore (probe position was indicated at figure 1) in STEM and the corresponding spectra from matrix close to the pore position. After background subtraction we normalized the intensities of two spectra by fitting the intensities of the second peak (after plasmon peak) at 57.7 eV position (see figure 2). As was shown at figure 2, the difference between these normalized spectra appeared to identify the He core‐loss line with maximum position at 22.8 eV. Experimental measuring of the He peak intensity (I) together with the measuring of elastic peak intensity (Io) allowed us to calculate the amount of helium atoms in the pore [1]:

N=I/(I
o
*σ*d), where σ is the He cross section and d is the pore diameter.

It is known [1], that the value of energy shift between the positions of the He core‐edge in the pore inside the solids and for free molecular gas (21.218 eV) is correlated with the pressure of the gas inside the pore and its He density. We have got densities of ∼19 at.He/nm
3
for pores with 20 nm diameter and ~70 at.He/nm
3
for 3.4 nm pores respectively that is close to the literature data [2]. Thus, ELLS technique is very useful for irradiated materials helium content analysis.