Silver Activation From Hell - 18MeV Clinac.

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RobertTubbs
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Silver Activation From Hell - 18MeV Clinac.

Post by RobertTubbs » Mon Feb 06, 2012 10:16 am

I took the day to play with a few Clinacs in town and thought I'd share what is probably the hottest silver activation on record here.

The procedure was to sandwich a 1 troy-ounce .99 fine Vienna Philharmonic between two wax bricks and nuke it from orbit with the 18MeV Linac in photon mode.

Activation after 10 minutes was beyond the scope of the Ludlum-12 Ratemeter/thin window NaI so I numbed it alittle with a generic 6993 GM tube which still pegged initially. According to the dose calibrated (for Cs-137) Eberline on hand at the hospital the coin was reading 50-60mRem/hr just before the Eberline bought the farm.

After 2 hours it was ~2,000 CPM on the surface with the 6993 tube.

After 4 Hours it was ~200 CPM on the suface with the 6993 tube.

Here's a video of the activation: http://youtu.be/wsiclNmrD7U

Here's a video of the coin at 2 hours: http://youtu.be/1V7-ZDGg8LM

Here's a video of the coin at 4 hours: http://youtu.be/rf0ZT9BrNCk

It's been almost 5 hours and it's still singing....

[Pictured is a different machine, one that was getting some TLC.]

Enjoy,
RT
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Steven Sesselmann
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by Steven Sesselmann » Mon Feb 06, 2012 10:43 pm

Robert,

For those of us who are less familiar with this kind of Linac, would you like to share what this machine is used for, how you get access to play with it, and what nuclear reaction is taking place between the machine and the silver coin to make it radioactive.

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

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Carl Willis
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by Carl Willis » Tue Feb 07, 2012 1:07 am

Interesting experiment, Robert. That's some quite-hot silver.

Steven, Robert will have to fill in the details particular to his equipment, but broadly these are electron linear accelerators made for external-beam radiotherapy, one of the leading methods for treating cancer. The electron beam is usually a few dozen microamps accelerated through an RF coupled-cavity waveguide to energies as high as 25 MeV. Most treatments use the poly-energetic bremsstrahlung beam created by the electrons striking a high-Z target as in a diagnostic x-ray tube, and I suspect that's in place for Robert's experiment. Photons of such high energy can break apart nuclei in what are referred to as photonuclear reactions. The (g,n) reactions are particularly common. 1.7-MeV photons will blow a neutron out of beryllium; 2.2-MeV photons will blow a neutron out of deuterium; 6-7 MeV photons will blow a neutron out of most anything. At 18 MeV, there are many photons with energies exceeding the photoneutron threshold in the accelerator's target and collimator. Most of the neutrons are probably born in the head of the linac for this reason.

Illy Sommer posted on a similar activation experiment involving simply a Geiger counter:
viewtopic.php?f=13&t=6248#p34692

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

Post by Steven Sesselmann » Tue Feb 07, 2012 1:19 am

Carl,

Are you suggesting that neutrons are a byproduct in this machine, and that high energy gamma rays are what is required for the cancer treatment?

Further you say that 1.7 Mev photins will dislodge neutrons from Beryllium, is this an energy specific reaction, or are you saying that anything over 1.7 Mev will do it?

Steven
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https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Carl Willis
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by Carl Willis » Tue Feb 07, 2012 2:45 am

Hi Steven,

Yes, neutrons and radioisotopes are undesirable byproducts, although their contribution to the overall healthy-tissue dose in a typical treatment is small.

The energies I mentioned are thresholds, below which that particular photonuclear reaction won't happen and above which it is kinematically allowed (though the cross-section tends to be very low just above the threshold).

-Carl
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RobertTubbs
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by RobertTubbs » Tue Feb 07, 2012 3:12 am

Carl kindly covered everything of significance in this context, the only thing I'd stand to say is that in photon mode these machines are accelerating beam currents between 20-50mA resulting in a dose of approximately 700Rads/minute. I don't know exactly what the continuum of X-Rays looks like on these machines but I imagine they've been engineered to produce X-Ray energies as homogeneous as possible depending on what energy mode they're locked into.

RT

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Re: Silver Activation From Hell - 18MeV Clinac.

Post by George Schmermund » Tue Feb 07, 2012 4:04 am

Why is there a rabbit in the background?
Anything obvious in high vacuum is probably wrong.

RobertTubbs
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by RobertTubbs » Tue Feb 07, 2012 4:16 am

Haha, Tyler had to email me a photoshopped picture to really see it. The object in the screen of the Clinac 6EX is an arm holding a camera.

I can assure you no rabbits were harmed in the process of yesterday's shenanigans.

RT
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Steven Sesselmann
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by Steven Sesselmann » Tue Feb 07, 2012 6:28 am

Robert,

700 rads converts to 7 Gray, which is 7 joules/kg

If the rabbit weighs around a 1 kg, and to fully cook it, you need to raise it's temperature to 100 C˚.

Take 100˚ less its normal body temperature 35˚ = 65˚ heating required.

The energy needed to heat one gram 1˚ is around 4.2 Joules, ergo to cook the rabbit we need.

1000 grams x 4.2 J x 65˚ = 273,000 joules

273,000 J / 7 J / 60 min = 650 hours

Have I made a mistake, or is this a really expensive way to prepare a rabbit?

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

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Carl Willis
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Re: Silver Activation From Hell - 18MeV Clinac.

Post by Carl Willis » Tue Feb 07, 2012 8:26 am

Hi Steven,

Rabbit is recommended to be cooked to an internal temperature of 82 deg. C, although the tender breast is often prepared at a slightly lower temperature.

One of the difficulties facing the chef in this unique situation is the fact that rabbits are too small to achieve charged-particle equilibrium for such high-energy photons. The depth dose profile will peak toward the distant side of the bunny, but a lot of energy will still be wasted in the surroundings. If this were my task, I would first cram the animal into a vacuum-insulated Thermos bottle in order to curb heat loss. I would then surround the Thermos on the outside with a "buildup cap" of sorts, made out of any convenient material, perhaps accomplished by sinking the Thermos in a larger pot of water, in order to place the depth dose peak right in the middle of Peter Cottontail's wee rib-cage.

After cooking and before eating, it would be wise to allow the slow-roasted lapine to stand for about an hour so that N-13 produced in the flesh could decay away.

-Carl
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