Fun with a 1.5 MeV Deuteron LinacApril 11, 2008
Tonight, at Linac Systems LLC, we had “first light” on a 1.5 MeV deuteron radiofrequency quadrupole (RFQ) accelerator. The work at Linac Systems involves particle beams, radiation, high voltage, and hundreds of kilowatts of RF. Needless to say, this job keeps me entertained!
One of the hats I wear in the business is Assistant Radiation Safety Officer, and in that capacity I stayed after hours today to turn on the full system for the first time and characterize the neutron radiation hazard around it. Anyone familiar with Farnsworth Fusors knows that even at 50 keV energy, deuteron beams can be prolific sources of neutrons by the 2H + 2H → 3He + n (Q = 3.3 MeV) fusion reaction. Well, at 1.5 MeV, this reaction is hotter than a greased Jesus! A 1.5 MeV deuteron beam can generate serious quantities of neutrons merely by interacting with background neutral deuterium in the vacuum chamber or deuterium implanted in the Faraday cup at the end of the linac. Below, I present a little video of neutrons being detected at the end of our RFQ.
I chose BTI bubble dosimeters for sensitive, whole-body dose-equivalent response to the pulsed fast neutron field expected around this RFQ. We use our BTI dosimeters as both temporary area monitors and personnel monitors. In the interests of safety, we conducted this evening’s full test of our RFQ at only 0.83% duty (a 100 μs beam pulse at 83.33 Hz) and increased the RF accelerating field to design strength very carefully, taking neutron dose measurements along the way. We accelerated only 6 mA of beam.
Results are shown in the photo at left and this video (AVI format, 6.5 MB). The photo shows 213 bubbles collected at the surface of the Faraday cup in 11 minutes. This is 19.4 bubbles / minute, or 34 mrem / hour for this particular dosimeter. What if we ran a continuous 6 mA beam instead of 0.83% duty pulsed beam? We’d have measured 4.1 rem / hour at the cup from neutrons, definitely not a negligible hazard. In the video, we see about 30 bubbles accumulate in a detector held by hand against the Faraday cup for one minute (and the bubble dosimeter in the photo can also be seen taped to the Faraday cup).