Unexpected ¹³N concentrations in ISIS synchrotron room air

Area of Science:

• Nuclear Physics
• Environmental Monitoring
• Accelerator Science

Background:

• The ISIS facility utilizes an 800-MeV proton synchrotron for neutron and muon production.
• Understanding air radioactivity is crucial for radiation safety and environmental monitoring at particle accelerator facilities.

Purpose of the Study:

• To measure the specific activity of air within the ISIS synchrotron room.
• To identify the primary radioactive isotopes present in the air and their likely sources.

Main Methods:

• Air samples were collected from multiple locations inside the ISIS synchrotron room.
• Collected air was passed through a shielded High-Purity Germanium (HPGe) gamma-ray detector located outside the room.
• Gamma-ray spectroscopy was used to quantify the activity of different isotopes.

Main Results:

• Measurements detected the presence of carbon-11 (¹¹C) and argon-41 (⁴¹Ar) in the synchrotron room air.
• The activity of nitrogen-13 (¹³N) was found to be below expected levels, with an upper limit of approximately 0.01 Bq cm^-3.
• Typical specific activities measured were ~0.10 Bq cm^-3 for ¹¹C and ~0.03 Bq cm^-3 for ⁴¹Ar.

Conclusions:

• The measured air activity is primarily composed of ¹¹C and ⁴¹Ar.
• The results suggest that the main source of this airborne radioactivity is not the synchrotron itself, but the massive shielding monoliths surrounding the neutron targets.
• Ventilation systems drawing air through these shielded areas likely contribute significantly to the observed air composition within the synchrotron room.