Because of the presence of cryogens (for the super conducting magnets of the (High Resolution Spectrometer) HRS, the Moller polarimeter and for cooling the cryogenic targets) Hall A is listed as an Oxygen Deficiency Hazard (ODH) area of Class 0 (with the exception of the area above the crane railing which is ODH-2. This rating requires that those wishing to have unescorted access to the hall must take the TJNAF ODH training course once every two calendar years. This course is typically taught once monthly by a representative of the EH&S group. There are ODH alarms in the hall and blue lights. The alarms sound and the lights will flash if there is a potential ODH hazard. Further, those working above the crane railing must have more extensive ODH training as documented in the TJNAF EH&S manual.
Aside from the more or less global concerns about rapidly expanding cryogenic gases displacing the air there is another way in which one could encounter an ODH situation in connection with Hall A operations. The problem is that of work in confined spaces where the atmospheric composition maybe different from that of the hall at large. Examples of such spaces are the main spectrometer vacuum vessels, which are evacuated during normal use (and is usually brought back to ambient pressure by filling it with dry nitrogen gas), the HRS EA (Electron Arm) gas Cerenkov detector which is normally filled with an Oxygen free gas mixture and the HRS HA (Hadron Arm) gas Cerenkov detector. Before work can be done in any of the above areas the oxygen contents of the atmosphere in the closed vessel should be verified by EH&S personnel (see attached sheet and figures).
Personnel are also reminded that the Shield Houses in Hall A are equipped with a clean agent gas fire suppression system. On actuation of this alarm, personnel are reminded to exit the Shield Houses.