Unexpected ¹³N concentrations in ISIS synchrotron room air

The specific activity of air in the large open room housing the 800-MeV proton synchrotron of the ISIS Spallation Neutron and Muon Source has been measured. Air from several positions within the ISIS synchrotron room was sucked through a long flexible tube, and run past a shielded HPGe gamma-ray detector outside the synchrotron room. In spite of an expectation that ¹³N should be the largest component of the overall activity in the air, the results of the measurements are consistent with the presence in the air of ¹¹C and ⁴¹Ar only, and suggest that the activity in the air is mostly created not in the synchrotron room itself but in the massive shielding monoliths around the neutron-producing targets, monoliths through which ventilation air is drawn into the synchrotron room. Typical specific activities of ¹¹C and ⁴¹Ar in the air in the synchrotron room are ∼0.10 and ∼0.03 Bq cm^-3 respectively, the upper limit for ¹³N being at most ∼0.01 Bq cm^-3.