Molybdenum Activation From Hell - 18MeV Clinac

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RobertTubbs
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Molybdenum Activation From Hell - 18MeV Clinac

Post by RobertTubbs » Fri Mar 02, 2012 2:27 pm

Took an hour out of this evening to activate an ounce of Molybdenum with an 18MeV Varian Clinac in photon mode. There were some shorter-lived products (notably Mo101 15-minutes) accounting for the greater activity within the first hour, however, I was more interested in the thermal neutron absorption of Mo98 and the photonuclear reaction of Mo100 to produce the longer lived Mo99 (~66-hours) and it's daughter product Tc99m (~6-hours.)

The procedure was to sandwich an ounce of Molybdenum bullion between two blocks of acrylic and nuke it from orbit with the accelerator in photon mode at 18MeV.

After ~1 hour of activation under the accelerator the coin was reading 100,000CPM on the surface with my 6993 GM tube and dropped off sharply over the next hour or two. By 4 hours it was hovering around 150CPM and by 8 hours it was just under 150CPM.

The production of Tc99m seems to have more or less entered equilibrium so I'll keep an eye on it for the next several days. It'd be nice to gamma-spec the thing but I lack a working scintillator at the moment. For completion sake, I reserve the right to resurrect this thread later down the road once I get a scintillator and zap the coin again.

Here's a video of the coin just after activation: http://youtu.be/eSt5GZ6I-JE

Here's a video of the coin at 4 hours: http://youtu.be/tzRU1C8cDaA

Here's a video of the coin at 8 hours: http://youtu.be/ASAQ42VtEXw

Enjoy,
RT
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Carl Willis
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by Carl Willis » Fri Mar 02, 2012 7:08 pm

Hi Robert!

Nicely done. If you want to mail any such activated items my way, I will have an HPGe ready for them. In this case, the half-life would be long enough to permit such a procedure.

It would be interesting to see how much Mo-99 results from the various processes that form it:
Mo-100(g,n)Mo-99 vs. Mo-98(n,g)Mo-99. You could probably boost yields considerably in the latter case by wrapping the moly in a moderator with some deuterated or beryllium-containing materials, and in the former case by taping that bad boy right on the gantry head.

Fun stuff.

-Carl
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Steven Sesselmann
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by Steven Sesselmann » Sat Mar 03, 2012 12:11 am

Robert,

What if any activation occurs in the materials of the linac itself?

As Carl suggests, a layer of Be sandwiched with the coin should boost activation.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

RobertTubbs
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by RobertTubbs » Sat Mar 03, 2012 12:45 am

Thank you Carl, I'll definitely take you up on your offer and send you a fresh piece when next I zap it - I really need to get my hands on a HPGe setup at some point.

In regards to Deuterium/Beryllium/Uranium neutron/photoneutron amplification, a little hybrid neutron-howitzer is high on the to-do list but I need to find more hours in the day to fumble that together, however, for now the roughly 10E12-10E14 is nothing to sneeze at.

As far as other activation products in the room go, there's the photonuclear interaction with Copper and Tungsten in the beamline, target and collimator leafs that account for the bulk of the neutrons aswell as residual radiation - there is also some Oxygen-15 produced although that decays off fairly quick. After an hour of running the machine itself is reasonably hot but the freshly made Mo101 (15-minutes) present in the coin accounts for the vast majority of counts seen in the video.

-RT

Jerry Biehler
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by Jerry Biehler » Sat Mar 03, 2012 10:47 am

Pretty cool. Where did you get a moly coin?

On another note, please hold the camera in landscape. It totally wastes the resolution on the camera when it gets cropped for a normal screen.

RobertTubbs
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by RobertTubbs » Sat Mar 03, 2012 12:16 pm

Jerry,

Ebay.

Beggars can't be choosers.

-RT

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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by Jerry Biehler » Sat Mar 03, 2012 10:38 pm

Its not begging, it is proper videography knowing the aspect ratio of the intended audience.

RobertTubbs
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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by RobertTubbs » Sat Mar 03, 2012 10:51 pm

You're welcome to make your own.

-RT

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Re: Molybdenum Activation From Hell - 18MeV Clinac

Post by RobertTubbs » Mon Mar 05, 2012 2:14 am

An update on the activity, it's been 66-hours (1 half-life) since the activation and it's indeed gone down by about half. I feel compelled to mention that for all the previous videos I left the plastic cover the coin came in on which is acting as a bit of a shield to the Mo99 beta decay.

With the cover on it's down to about ~50-60 CPM.

However, with the cover off, it's still over 200CPM!

Video of activity at 66-hours: http://youtu.be/9OdClAhax8U

Next one's all you Carl, I look forward to seeing the Gamma-spec.

-RT

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Carl Willis
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Re: Molybdenum Activation From Hell - 18MeV Clinac (gamma spectra)

Post by Carl Willis » Sun Mar 18, 2012 4:31 am

At Robert's request, I am adding some gamma spectra of a molybdenum coin that he irradiated at 10PM Pacific time last Sunday, and which I analyzed at 8:25PM last Thursday on my HPGe detector.

The spectrum is shown below, followed by some photos of the HPGe detector (recently mounted on an aluminum frame and surrounded by half a ton of lead). The coin can be seen getting mounted behind a Uline polystyrene jar that fits snugly into the cap on the detector crystal.

The spectrum shows the presence of a number of isotopes. Most prominent are 66-hour Mo-99 and its daughter Tc-99m. The Mo-99 results from the reaction Mo-100(g,n)Mo-99 and possibly from Mo-98(n,g)Mo-99 from ambient photoneutrons. Niobium isotopes (Nb-91m, Nb-95, Nb-95m, Nb-96) can be attributed to various (g,p) reactions, e.g. Mo-97(g,p)Nb-96. Finally, a little surprise in the form of Y-89m, a decay product of the positron emitter Zr-89 made via Zr-90(g,n)Zr-89. Evidently the molybdenum in this coin has a zirconium contaminant. Two major sum peaks have also been labeled.

Tc-99m is a medically-important nuclide that occasionally falls in short supply when one of the few reactors that produce it goes offline. So the literature is full of suggestions for making it by all kinds of methods with the aim of securing a more reliable supply. Indeed, the photonuclear / electronuclear methods as illustrated in this experiment are high in the running as viable alternatives. But when natural molybdenum is irradiated by photos there are other byproducts that must be chemically excluded from the drug, and doing that probably requires a new radiopharmacy approach that hospitals would be loathe to get onboard with.

-Carl
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Carl Willis
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