Neutron activation with a hydrogen bomb

[Carl Willis]

Neutron activation analysis is normally thought of as a non-destructive method of testing. As has been demonstrated many times using various compounds and elemental samples, a fusor is an adequate neutron source for inducing detectable short-lived activities. It's not a particularly intense neutron source, however. Sometimes one has urges to "kick it up a notch."

The most intense neutron sources at our disposal (and of course I don't mean we as hobbyists, but as society most generally) also happen to be fusion devices. To use one of these "devices" for activation, you explode one near your sample. A subtle distinction that should be emphasized is that this is not a non-destructive test: following the experiment, the sample must be retrieved in the form of a slag. Following sample retrieval, the analysis instrumentation and methods are the same as with a fusor or any other hobby neutron source. Typically I've favored gamma spectroscopy for this task.

A few weeks ago I had the great privilege of traveling to the Soviet Union's Semipalatinsk Nuclear Test Site in Kazakhstan. One of the tests there, involving a 140-kiloton hydrogen bomb, resulted in the crater lake known as "Lake Chagan" in 1965. The interior of this crater is composed of vesiculated slag (atomsite, or "kharitonchiki" as such slags are known in Russian). I show a photo below of an energy-compensated GM tube being used to measure the exposure rate on top of some such slag on the shore of the lake; it's about 1-2 mR / hr in most places. There are also photos showing a close-ups of this slag and a 3-ounce container filled with it, which I used for the subsequent analysis on my HPGe gamma spectrometer.

Looking at the gamma spectrum, the first major observation is that most of the lines belong to europium isotopes, Eu-154 and Eu-152. These isotopes are produced when neutrons from the "device" are captured by the ~1ppm naturally-abundant Eu-153 and Eu-151, respectively, which have remarkably high capture cross-sections. I offer more discussion about activating europium here (albeit with a smaller and somewhat gentler neutron source):

viewtopic.php?f=13&t=5822#p34266

The other major long-lived gamma-emitting activation nuclide is Co-60. Some of this cobalt could be from metal in the bomb's well casing, but it could also be from activation of crustal mineralization.

If we look down into some of the smaller peaks in detail, long-lived isotopes of holmium (Ho-166m), silver (Ag-108m), and barium (Ba-133) are in evidence. One observes a lot of Ba-133 in American Trinitite (the slag from the Trinity test), since barium nitrate was used in the explosive in that bomb. The amount of barium seen here in the Chagan slag is much smaller--I'm more inclined to attribute it crustal composition.

Lastly, there are some gamma energies associated with the bomb itself. Most notable is Cs-137, a fission product. Am-241 is present at a low concentration; I cannot reliably estimate its concentration using the Cs-137 reference specimen I use to calibrate my detector. This is the daughter of Pu-241, produced by neutron activation of plutonium in the bomb. It is a reliable proxy for the presence of plutonium as well, but the gamma radiations from plutonium are weak and swamped by the spectrum's low-energy continuum.

Enjoy!
Carl

[Brian_Gage]

Carl, though I hate travelling or flying long distances, I'm almost envious of your trip to the Russian test site. Thanks for all the text info on your experience. Made for real interesting reading.

[Richard Hull]

Carl, Fabulous data presentation with a great writeup! I am sure we will talk about this at length in future, probably at the HEAS 2012 in October. You are, of course, expercted to talk about this at our evening talk sesssions.

I am glad you made it back save and sound and had an interesting trip.

Richard

[Doug Coulter]

Nice work indeed, Carl. I admit to a certain amount of gamma-spec envy...

[Jon Rosenstiel]

Great report, Carl. I bet you were pretty excited when those Ag and Ho lines showed up in the spectrum.

Jon Rosenstiel

[Carl Willis]

Hi Jon,

Ag and Ho were surprises. My FitzPeaks software correctly identified both, although at first I was skeptical. I had to see multiple lines from each before I believed it. At first I thought the program might be mistaken on the holmium and the 184 keV line might be from uranium (U-235) instead. But the energy calibration is really very accurate on this thing, and that would have been a stretch given how accurate all the others were.

There's the tiniest nub of what appears to be Eu-155 in this spectrum. I wish I had a longer count with resolution still under control, but that's hard to do. Maybe more oddballs would pop up?

Sadly, plutonium is too hard to see on my detector (P-type, no window) at the low concentrations in which it is present. With a suitable separation of the europium from Pu on an ion exchange column, it could presumably be detected.

-Carl

[Richard Hull]

I was rather surprised at the Co60 peak after all these years. The short half-life would indicate a significant amount of cobalt to be present to have survived so long at the level detected.

Do you remember the supposed and never tested cobalt bomb scare of the 50's and early 60's? I wonder who talked of this first into the news media and when in relation to this Russian test?

The .15mt H blast was very weak for an H blast. (maybe a simple boosted A blast?)
Regardless, could they have very lightly salted this bomb or inadvertently or deliberately used a high cobalt alloy within the bomb?

Could we have detected the heavy Co signature in the 50's in one of our countless near USSR "air sniffing" flights? And, if this be the case, figured that the Russians were salting their bombs and, in an effort to scare the pants off them, proposed the theoretical cobalt bomb idea openly, hoping they would shut the effort down? Anyway, this apparently worked as no one is on record as having tested a heavily salted cobalt bomb.

Can you begin to imagine this Co signature level shortly after the blast!!! It was probably the number one signature, especially after the very short lived isotopes decayed! A little back figured math ought to give some idea. Musta' been one nasty place there for several years after the blast.

Just throwing this out there for musing and not to say it happened. It's just that there is a lot of cobalt still in that debris after 50-60 years.... 10+ half lives later. Was the tower steel or ground that cobalt rich?? I would be amazed if it was.

Richard Hull

[Brian_Gage]

As recently as last year, I recall reading that one of the British bomb tests in Austrailia had a jacket or sections made of cobalt. I'll start searching for the document.

[Brian_Gage]

Okay, found a bit of what I'd read on cobalt salted bombs. This from Wiki-
"As far as is publicly known, no cobalt bombs have ever been built. The Operation Antler/Round 1 test by the British at the Tadje site in the Maralinga range in Australia on 14 September 1957 tested a bomb using cobalt pellets as a radiochemical tracer for estimating yield. This was considered a failure, and the experiment was not repeated."

I recall another article about claims that cobalt fallout from this test affected military personnel, IE. cancer claims.
Regards

[Richard Hull]

I wonder if the British Co pellet bomb was a failure in both the sense that the intended yield report failed to meet expectations and perhaps also left the area highly contaminated, longer term, compared to standard past bomb test areas. Also, the fallout might have been nasty too. What year was the pellet bomb shot?

Richard Hull

[Brian_Gage]

Richard, it looks like 1967 was the year of the test involving cobalt pellets. It is mentioned below. I saved the text from a search, but couldn't find the correct web address in the article. This was a Google search.
.
"About Christmas Island (Kiritimati) - and Australia - Bomb Tests - Part II"
<following exerpt>
"There was also the problem of Cobalt 60, a powerful gamma emitter. It was not
until British records showed that Cobalt 60 pellets were found scattered at
Maralinga that it became known that cobalt had been tested as a bomb
component. One of the British clean-up team at Brumby remembers
'hand-scavenging' the pellets., which involved locating the pellets and
scooping them up on a trowel of sand and placing them in a lead tin under the
supervision of AWRE scientists. 'Although we all started keenly enough and
aware of some danger, after a while things started being rushed and our boffin
friends seemed homesick! This is when to my mind things got skimped. I know we
did not recover all the pellets before the site was taken as cleared. I hope
no Aborigine ended up carrying a pellet of cobalt between his toes one day!"
Another veteran remembers hand-picking the still radioactive material that had
been fused into glass by the heat of the atom bomb at the One Tree site. They
were given protective clothing for the job but because of the temperature in
the 120s they wore just army shorts and boots and dispensed with their
respirators. The details of the 1967 clean-up operation were all carefully
catalogued in a report by AWRE scientist, Noah Pearce. The Australian Weapons
Safety Committee said they were happy with the operation and the 'Pearce
Report' was subsequently classified by the British Government. Britain told
Australia that she had no further use for the site, which remained under
Federal control pending a survey and to 'return' to the South Australian
Government. A permanent police presence was established at the site, located
in the former cook-house, and a perimeter fence was built around the
prohibited area."

KIRIBATI