Dark Current

["Scott O. Schwahn" <schwahn@CEBAF.GOV>]

Although I cannot specify for you what levels of radiation you might
encounter from purely dark current sources, I can say that we have seen
dark current in our injectors (100-500 keV) in many cases.  For us, it
depends on how the beam is steered, as there are thin and thick parts along
the beamline.

I would make the following two minimum recommendations for you:

1) ALWAYS enter the room where such a device has been used with a live,
calibrated and checked, radiation monitor appropriate for your source term
(energy and rep rate).  I am guessing, but I would bet over 99% of all
accidents related to radiation generating devices could have been prevented
by this step.  It is policy here despite over 10 years of operating
experience with flawless (w.r.t. radiation production) interlocks.  It is
always best to assume all interlocks have failed.

2) Your interlocks and shut-off devices required prior to
approaching/maintenance should ALWAYS shut off high voltage (with a
fail-safe mechanism) as well as current.  If there is a need to be there
with HV on for some maintenance activities, keep the above-mentioned
radiation monitor or personal alarming ion chamber (again, appropriate to
energy) dosimeter with the worker/s at all times.



Scott O. Schwahn, CHP
Thomas Jefferson National Accelerator Facility
Newport News, VA
schwahn@jlab.org