Direct Measurement of the Internal Pressure of Ultrafine Bubbles Using Radioactive Nuclei

The internal Xe gas pressure of ultrafine bubbles has been directly measured for the first time by using the γ‐γ angular correlation technique, a well‐known measurement technique in nuclear physics. Radioactive 125 Xe ( T 1/2 = 16.9 h) was induced inside natural Xe‐ultrafine bubbles in water, having an average diameter of 200 nm, by thermal‐neutron irradiation. The asymmetry factor for the 55–188 keV γ‐γ cascade in 125 I, followed by the β decay of 125 Xe, was used as a probe for the gas pressure in ultrafine bubbles. The internal pressure of the ultrafine bubbles was determined to be × 10 5 Pa, lower than those for ultrafine bubbles of N 2 , CH 4 , and Ar by Ohgaki (6 × 10 6 Pa) or that from the Young‐Laplace equation (1.6 × 10 6 Pa).