Neutron activation... of a geiger counter. Oops.

[bionerd23]

Hey folks,

I just recently did a neutron activation experiment by accident - it was quite unexpected to be honest!

I knew that parts in a linear accelerator (LINAC for treating cancer) would turn radioactive, but for large-scale activation, I had only heard about this happening to parts in the main beam, i.e. the multi leaf collimators used to "shape" the beam, filters inserted into the beam, etc.

The setup was as follows:

18 MeV Photon beam
20x20cm field
1 Gy total dose in 1 meter distance
~30 seconds irradiation time (until 1 Gy target dose was archived)
1 meter distance of my gamma scout to the center of the field / beam (horizontal position 90 degrees to the beam, so not in the direct beam)

...While the gamma scout was on a carbon table - and the beam would strike through the carbon table and hit the opposing floor straight (0 degrees rotation of gantry, approx. 2 meters distance from collimator to floor).

The result was activation of my poor gamma scout, which was reading over 2 uSv/h as "background" after the irradiation had stopped, and even after about a minute when I took the gamma scout out of the room to see if it was really the device itself that was activated, it was still radioactive with over 1 uSv/h of radiation coming from itself. Wow.

The metal in the machine's head was radioactive, too, but yeah, that was slightly expected. However, I didnt expect to activate my gamma scout with...

... a nice high energy photon beam but
... no great high-Z material as a target for photonuclear effect and
... no moderation of the resulting neutrons and
... a short irradiation time for a dose of 1 Gy on
... my gamma scout, which doesnt exactly contain material with great neutron cross sections, and was quite far away from the beam.

Just wow, so I thought I'd share.

In addition to the graph (IMP / 10s, red shows LINAC operating time), here's a video report:
http://www.youtube.com/watch?v=VWpx2JaFu5g


EDIT: Considering my gamma scout never turned radioactive at all from Jon's wonderful fusor neutrons, no matter if moderated or unmoderated, this is even more astonishing. I wonder how much of a neutron dose I was dealing with in that room.

[Carl Willis]

Hi Illy,

Nice report. 18 MV is at the upper end of the treatment range on modern therapy linacs, and is high enough to induce a wide range of photonuclear reactions of the (g,xn), (g,xp), (g,xa) types in the light nuclides. It might be difficult to pin down just what reaction is responsible for the activity in the Geiger counter, although I do suspect it's due to neutron activation from photoneutron emission in the target rather than some of these other reactions in the Geiger counter itself.

Entertaining video too!

-Carl

[bionerd23]

Hey Carl,

interesting stuff. I should consult some books for more photonuclear reaction details; I wonder what e.g. the energy required for photodesintegration is.

...So, is it possible that the photon-struck atoms are "metastable", only emitting e.g. a proton after a given time? I never thought about that before. Somehow, I always imagined it to happen "instantly" (as per definition no production of metastable isomers).

Thanks for inspiration to more fun stuff to study.

[Carl Willis]

Hi Illy,

Here are a few cross-sections for photonuclear reactions between 0-20 MeV, from the JENDL-3.3 library at www.nndc.bnl.gov (the National Nuclear Data Center in the US).

The most prominent are the N-14(g,p) and N-14(g,n) reactions, the latter of which results in a radioisotope (10-min. positron emitter N-13, which actually looks ballpark consistent with your decay data on first blush). Actually, I assume that N-14(g,xn) = N-14(g,n) for exampe, which may not be good at the higher energies; there is enough photon energy available at ~20 MeV to blow off two or more nucleons at the same time, so some (g,2n) etc. will be in the mix there too.

Just to be clear, these photonuclear reactions happen nearly instantaneously for all practical purposes. But the products may be radioactive, and of course the neutrons induce activity upon capture.

-Carl