Gas absorption during ion-irradiation of a polymer target

The presence of a high partial pressure of deuterated water in the vacuum of the scattering chamber of an MeV ion accelerator system was shown to promote the incorporation of deuterium into a thin polyimide target subjected to 1.3 MeV ⁴He ⁺ irradiation. When the background pressure in the chamber was low, 2.7 × 10 ^-5 Pa (2 × 10 ^-7 Torr), the polyimide was shown by forward recoil spectrometry to evolve hydrogen when ion bombarded. This was an expected, and commonly observed, phenomenon. However, when the pressure in the scattering chamber was raised to 1.3 × 10 ^-2 Pa (1 × 10 ^-4 Torr) by the addition of D ₂O vapor through a leak valve, deuterium was readily incorporated into the target in the ion-irradiated area. The degree to which the polyimide film absorbed gas from the vacuum of the scattering chamber was unexpected. The deuterium/hydrogen atomic concentration ratio reached 0.235 after a dose of 3.5 × 10 ¹⁵ ions/cm ². The absorption of deuterium was accompanied by blistering in the polyimide at high ion fluences.