A Little Bit of Fission

One of the fun things you can do with uranium is to turn big atoms into little atoms.  All natural heavy nuclei will undergo fission after a hard enough kick (for instance, protons accelerated to around 50 MeV will fission gold or bismuth), but to split uranium, all you need are some household-variety neutrons.  Offering a neutron to a U-235 or U-238 nucleus is like giving Mr. Creosote his “wafer-thin mint” in the infamous Monty Python sketch: the recipient is violently blown to chunks and the surroundings drenched in postprandial gibbage!  Maybe I’ve gone overboard with that metaphor.  Anyhow, uranium fission residues include a long list of mostly-radioactive lighter nuclei, additional prompt and delayed neutrons, and some gamma rays.

25 grams of uranyl peroxide in a Nalgene bottle, ready to be irradiated with neutrons.

The 2-4 Ci PuBe source used to irradiate the uranium sample.  A string is provided for safe handling.

The experiment described here relates to the question of what specific fission product gamma signatures a nuclear hobbyist, equipped with typically limited resources, is likely to observe pursuant to neutron irradiation of some natural uranium.  Preliminary considerations suggest we’ll only notice products that emit strong gamma radiation, have a half-life comparable to or shorter than the irradiation period, and have high fission yields.  Uranium’s natural radioactivity causes additional complication, probably blinding us to fission products that emit at energies near the major features of the Pa-234 spectrum.  Beyond these generalities, predicting what we might see is a nontrivial task, so the question can really only be addressed convincingly by experiment.

Neutron source and uranium are lowered into a wax moderator.

_ at the University of _ kindly offered his HPGe detector for use in this experiment.

I irradiated 25 grams of natural uranyl peroxide, freshly prepared from Utah pitchblende ore, overnight with a ~5E+06 n/s PuBe neutron source.  This source intensity is comparable to contemporary hobby fusion neutron sources, like well-constructed Farnsworth fusors.  After irradiation, a 2.5-hour gamma spectrum of the sample was collected with an HPGe detector.  25g of non-irradiated uranyl peroxide in an identical container served as a control, the spectrum of the control being subtracted from the spectrum of the irradiated sample to eliminate most features belonging to uranium or its own decay daughters.  What we’re left with is a difference spectrum containing features attributable to the nuclear transmutations in the irradiated sample.  Here’s that gamma spectrum, in three graphs, encompassing the range of 200-1500 keV.  I have labelled the identified peaks.

So what did we make?  Here’s a summary of the nuclides contributing peaks found in the gamma spectrum, with my comments on a few.  All are short-lived, having half-lives between 30 minutes and 2.4 days.

• Np-239: The largest new peaks in the above spectra are the ones at 229 and 278 keV belonging to Np-239, which is formed not by fission but by (n,g) neutron capture on U-238 followed by beta decay of U-239.  Np-239 is the parent of the important fissile isotope, Pu-239.
• Sr-92: Although not the largest new activity, Sr-92’s peak at 1384 keV is the most prominent above background.
• I-135 and Xe-135: These are high-yield fission products, Xe-135 being the daughter of I-135, having huge neutron cross-sections, responsible for effects known variously as “xenon poisoning” and the “iodine well” in nuclear reactor behavior.
• Sr-91 and Y-91m: Sr-91 is a high-yield fission product; Y-91m is not, but grows in as Sr-91 decays.
• Zr-97, Nb-97, and Nb-97m
• I-133
• I-134
• Cs-138
• La-142

If you’re doing a fission experiment with a very weak source of neutrons, and your irradiation time is on the order of at least a few hours, I recommend you first set your sights on Sr-92’s whopping peak out at 1384 keV.  If you can’t see that one, you probably won’t see anything else.  Xe-135 and the iodine isotopes might be easy to separate from an aqueous uranium solution by solvent extraction with corn oil or some similar nonpolar medium, improving their visibility against background.

More writeup on this experiment here.

Many thanks to _ at the University of _ for the use of some of his resources.

More radioactive goodies from Bayo Canyon

I’ve written about this place twice before, and a bumper crop of radioactive souvenirs from a February visit compels my new assessment that Bayo Canyon, New Mexico is simply unmissable for any hardcore nuclear tourist.  Of course, there’s the historical dimension:  the radiolanthanum experiments that commenced here in 1944 provided crucial insight into the implosion weapon design validated in 1945 by the Trinity test (and embodied later by “Fat Man” and virtually all successive bombs).  But what makes Bayo so special is that the history here is tangible, collectable, and detectable provided you come with a Geiger counter.

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The next four photos at left show pieces of blast debris that were scattered across the surface near the escarpment under Point Weather (where I am standing, 2nd photo above), along with readings in counts per minute on a Ludlum 44-9 pancake GM tube.  While the great majority of findings are not detectably hot, there is so much debris available that the prospects for major finds here are good.  This is my second piece of radioactive cable, and the other two pieces appear to be aluminum metal.  For comparison, local background is about 60 CPM.

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There is sufficient gamma radiation to identify uranium in one of these samples by scintillation spectrometry and to estimate its present activity.  The piece of cable was my choice for this test, owing to easy source-detector geometry and negligible self-absorption.  The last image is the 2000-second NaI:Tl gamma energy spectrum.  The peaks are consistent with the prominent decay radiation of U-235 at 185.72 keV (emitted in 57.2% of decays).  Assuming a geometric efficiency of ~50% and an intrinsic photopeak efficiency of ~75%, the piece of cable contains about 8 mg of uranium if the uranium has its natural isotopic ratio, or about 20 mg if it is depleted. (Both DU and natural U were used in the Bayo experiments.)

Nuclear Collection (Part V)

Today’s long menu includes more radioactive pottery, more radioactive vacuum tubes, smoke detectors, a couple lesser-known radioactive elements, and a few interesting odds and ends. As always, if you have something radioactive and in need of a good home, I buy and trade all the time.  Enjoy!

Uranium-glazed artistic pottery is hard to come by, in contrast to the mass-produced (and mass-collected) Fiestaware and similar.  Here are two examples of handmade ceramics.  Especially interesting is a vase made in 2010 (left) that is representative of the work of crystalline-glaze artist William Melstrom, who has a studio in Austin, Texas (photo courtesy of Mr. Melstrom).  Melstrom is one of very few contemporary artists who have gone to the lengths required nowadays to work with uranium.  His adventuresome report on obtaining uranium compounds in France to formulate his glazes is a must-read.  The fluorescent light yellow glaze on this vase clocks in at 2200 CPM on a 2″ pancake GM tube.  At right is a hand-thrown and hand-glazed  decorative bowl from an unknown artist containing a typical “uranium red” glaze.  It registers 38,000 CPM on a 2″ pancake GM tube, making it among the hottest pieces of pottery in my collection.

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These raw ceramic underglazes containing uranium are a gift from William Melstrom, who made the vase pictured above.  Before Melstrom owned them, they were in the possession of a radiation safety officer at the Texas Department of State Health Services, slated for official disposal as radioactive waste.  Because so few artists use or even know about uranium glazes now, old bottles such as these sometimes present surprise disposal problems when studios are cleaned out.  Both are products of Thompson Enamel and both read about 12,000 CPM on a 2″ pancake GM tube.  At left is a “531 Burnt Orange” (when fired, of course), and at right is a “108 Forsythia.”

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This is a 6″ Corning uranium-glass optical filter I recently obtained on eBay.  The uranium concentration is through the roof: it emits 11,000 CPM into a 2″ pancake GM tube, making it more than twice as hot as the hottest decorative vaseline glass items I own.

Some other interesting properties of uranium glass are dramatically demonstrated with this example.  In the second photo, ultraviolet light from a distant Sun-Kraft lamp (an electrodeless quartz-mercury discharge tube) excites the uranium glass, provoking the characteristic green fluorescence.  Based absorption of the  lamp’s harsh 254-nanometer UVC radiation, it’s easy to distinguish a quartz crucible (casting the central shadow) from the nearly-opaque borosilicate tube (left) and soda-lime glass vial (right).

Uranium glass is also apparently a fair scintillation medium.  In the lower photo, a thin face of the Corning filter abuts the output window of a commercial x-ray machine, where exposure rates are on the order of 1000 roentgen / hour.  The glass glows its characteristic green color as the x-ray beam expands across its surface.

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Lanthanum and lutetium are two of the lesser-known natural radioactive elements.  Although there are other natural, primordial radioelements (e.g. V-50, Rb-87, Sm-147, Re-187, In-115), these two stand out (along with good old potassium) for their usefully high gamma activity.   Both could be used as check sources or energy calibration sources for scintillation detectors.  La-138 (0.09% abundance, T1/2 = 1.02E+11 y) decays by electron capture or beta emission, unleashing gamma rays in either branch.  A ~50-g specimen of the metal (inset, left) racked up 7.2 counts / sec above background into a 2″ NaI:Tl detector.  Lu-176 (2.6% abundance, T1/2 = 3.78E+10 y) undergoes beta decay with a high yield of several gamma energies, most notably at 202 and 307 keV.  The peak at 509 keV in the spectrum is not a real gamma energy, but rather a “sum peak” caused by 202- and 307-keV gammas simultaneously entering the detector (this happens to be an “anomalous” sum peak, larger than would occur by random summation, precisely because the two radiations involved are frequently part of the same decay sequence).  The 23-g chunk of lutetium in the right inset veritably boils a 2″ NaI:Tl detector with more than 120 counts / sec above background.

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More radioactive vacuum tubes. At right are three similar radar TR switches and their packaging (left to right: Bomac JAN-CBNQ-5883 from 1961 originally containing 0.3 µCi of Co-60; a Westinghouse 1B37 from 1952 containing several µCi or Ra-226; a GE 1B35 containing a small amount of Co-60.  At left, a spark gap (in hand) originally with 5 µCi of Cs-137 and a dual TR switch originally containing less than 0.7 µCi of Co-60.

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Ionization smoke detectors contain an alpha emitter, typically Am-241.  The left-most pic shows industrial smoke detectors from ca. 1960, each containing a total of 80 microcuries of Am-241.  These detectors measured the current imbalance between an exposed “sense chamber” and a sealed “reference chamber,” both of which contained alpha sources.  In front of the detectors are examples of their sense-chamber sources, which hold the greater amount of activity (~60 microcuries).  Left is a Pyrotronics F5-B4 with its annular source holder bearing six thin sealed sources; at right is an F3/5A and its pedestal source, containing a single foil covered by a screw-adjustable bonnet.  More modern detectors are shown in the upper-right image: At left is a Simplex 2098-9508 with 4.5 µCi of Am-241, manufactured in 1980, and at right a run-of-the-mill modern detector with the typical  1-µCi source.  The lower right photo shows a Ra-226 foil source from a batch of smoke detectors, make unknown, that was intercepted on its way into a Pennsylvania junkyard.  Approximate activity is 1 microcurie.

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Tritium glow-in-the-dark devices include emergency exit signage and the button at right.  Self-luminous exit signs are undoubtedly the most radioactive items in peoples’ everyday experience, but few probably realize it.   They can contain up to 20 curies of H-3 (tritium) gas in the glowing phosphor-lined tubes, as does the example shown here.  They are regulated under a General License by the Nuclear Regulatory Commission (see yellow sticker in right image).  Though initially costly, these self-powered signs easily deliver value over the life of a building by eliminating the need to conduct tests and change light bulbs.  Numerous outlets sell them on the Internet; they can also frequently be found at bargain prices on eBay (when the NRC isn’t looking).   The lower pic shows an old luminous button that originally contained 0.1 Ci of tritium.  This item replaced more hazardous predecessors containing radium.   Common consumer goods containing tritium today include “Traser” keychain lights (technically illegal in the USA as a “frivolous use” of radioactive material) and Trijicon gun sights.

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Kodak 8-mm film projector (left) and camera (right) with radioactive thorium lenses. High refractive index and low dispersion justified the use of thoria in optical glass formulations.  The film projector’s 22-mm, f/1.0 Projection Ektar lens clocks in at 1200 CPM on contact with a 2″ pancake GM tube, while the camera’s lens only reads about 250 CPM.

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Radium postcard, ca. 1930, from Luther Gable quack outfit. Ah, the good old days when you could just send loose radioactive contamination through the freaking mail! This postcard bearing a dollop of glow-in-the-dark radium paint (11,000 CPM on a 2″ pancake GM tube) promoted Dr. Luther Gable, the man responsible for the notorious Gable Ionic Charger.  A number of these cards were found in a collection of magician’s tricks.

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The “Becquerel Chemicals” educational kit manufactured by Damon contains six small plastic boxes labeled A through F.  The contents of three are yellow powders, the contents of the other three are white crystals.  Students were intended to exploit physical and chemical properties—including radioactivity—to identify these unknowns from a list consisting of uranyl sulfate, sodium sulfate, uranyl nitrate, sodium nitrate, thorium nitrate, and sulfur.

Virginia Uranium

This undulating farmland is situated about 20 miles north of Danville, Virginia (best known as the last Confederate capital).  From the surface, it could be anywhere in rural Southside Virginia: cattle graze in pastures surrounded by pine forest, and stately homes—some more than two hundred years old and occupied by descendants of the settlers who built them—stake out the high ground.  What’s so exciting about this particular piece of land is what lies beneath the surface: uranium!  And this is not just your minor pegmatite outcropping or localized radioactive anomaly.  With an estimated 110 million pounds of U₃O₈ reserves  (at a cutoff grade of 0.025%),  it’s considered the largest uranium deposit in the United States.

The Coles Hill Uranium Deposit, as it is known, contains resources currently worth billions of dollars.  That’s a lot of money.  So naturally, there is a commercial venture aiming to profitably extract the uranium.  Virginia Uranium, Inc. is owned mostly by the families under whose land the deposit is located.  Last January, Chief Geologist Joe Aylor (second from right) was kind enough to show us core samples and discuss the geology of the deposit at the firm’s office in Chatham before taking us out to an exposure of the radioactive ore along Coles Hill Road.

The 500-million-year-old Leatherwood Granite harbors the uranium at Coles Hill.  Specific uranium minerals are reported to be poorly-soluble phosphates, e.g. barium uranyl phosphate; the mineralization is of epigenetic origin—perhaps hydrothermal activity or  transport out of newer Triassic sediments.  In any case, our scintillation detectors were pleasantly excited by Dr. Aylor’s cores, and we couldn’t wait to hit the field to collect our very own specimens.

Here we are, atop America’s largest uranium deposit! We parked our cars in a spot where the scintillators indicated an exposure of the radioactive orebody, walking distance from Mr. Coles’s farm (see below).  Ordinary rockhounding is virtually impossible in the snow, but snow does not stop gamma rays.

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Richard Hull (left) excavates the berm beside the freshly-bladed road for a spicy chunk of rock.  At right is Richard’s bagful of  radioactive granite chunks, all located with his trusty TSA plastic scintillator.  These would be sparkling, gleaming things if only we could see gamma radiation.  But since we can’t, the ore appears mundane and unappealing, clad as it is in a tenacious layer of pasty red clay.

The hottest pieces of radioactive saprolite I collected read about 5000 CPM on a Ludlum 44-9 pancake GM tube.  This is respectable to be sure, but it doesn’t hold a candle to the more concentrated ore that can be found near Moab, Utah.  Richard Hull pulverized some average ore collected at Coles Hill and compared the gamma radiation countrate with that from samples in a calibrated ore grade test kit.  He arrived at an equivalent concentration of 0.253% U₃O₈.  Richard also noted UV fluorescence along fractures in the rock.

This is the Coles Hill farmhouse, just north of where we found our specimens.  The Coles family has lived on this land since 1785.  If uranium mining goes forward, Walter Coles says he’ll stick around to watch, and any mining will leaving the historic home intact.  But the big question of whether mining will occur is unresolved; Virginia has had a moratorium on uranium mining since 1981.  Coles Hill has become a focal point for contention over the moratorium, as well as the broader nuclear energy debate.  Virginia Uranium claims the positive impacts of mining will include the creation of 300-500 new jobs and an annual gross revenue of $300 million, while providing a valuable domestic energy source (most uranium for nuclear fuel is currently imported).  In 2008, then-presidential-candidate Barack Obama offered a cautious endorsement of sorts: “Virginia has the potential to be a national leader in uranium mining, and development of uranium resources in Pittsylvania County could create hundreds of jobs in that part of the state.”  Opponents, such as the Southern Environmental Law Center, cite the threat of water pollution, the industry’s poor environmental legacy in states where mining has occurred, and the high population density of Virginia as reasons to continue the moratorium.

Albuquerque, Ground Zero

May 27, 1957. N34.99°, W106.57°.  A lone steer was grazing this windswept expanse of mesa five miles south of the Albuquerque airport under the noonday sun.  Overhead, a B-36 “Peacemaker” churned toward the runway, ferrying a Mark 17 nuclear bomb from El Paso to Kirtland Air Force Base for service.  Such routine Cold War traffic would not normally be sufficient to jar the animal from his bucolic reverie.  But on this day, owing to a freak accident (the cause of which officially remains unknown), our bovine was about to receive airmail of a highly disruptive nature.

The 42,000-pound two-stage H-bomb–historically the largest nuclear weapon in the American arsenal–was dropped by mistake as the B-36 descended through 1700 feet.  Though the plutonium pit was not on board for safety reasons, the bomb did contain its fissile second-stage “spark plug” made from either plutonium or enriched uranium, as well as the tamper (probably uranium).  It plunged nose-first into the cow-populated mesa, whereupon the shock wave from 300 pounds of detonating high explosive puréed the unfortunately-situated ruminant with inconceivable violence†.  Thunder pealed off the distant hills; Burqueños gaped in awe at the fireball rising in the southern sky.

The acrid fog of charred cow pulp had barely settled when the crack AFSWP (Armed Forces Special Weapons Project) team from Kirtland arrived to discreetly liquidate the consequences of the “broken arrow.” They encountered a 25-foot crater with gamma exposure readings of 0.5 mR / hr at the rim.  Although the Army filled in the crater and recovered most of the weapon, to make a clean sweep of the several square miles peppered with debris would have been a Herculean task.  They did a job that was good enough for government work–in other words, plenty of radioactive H-bomb components still litter the desert for the interested public to collect.  That’s the good news, and I’ll discuss my collection of bomb chunks shortly.

There’s some bad news, however. The inexorable tide of urban sprawl has engulfed just about anything resembling a “windswept expanse of mesa” in the Albuquerque vicinity, and such is the imminent fate of this one.  Forest City Covington NM, LLC has begun marketing the land as a master-planned development called “Mesa del Sol.”  Now it would be a crying shame if this unique venue for radioactive  material collectors got overrun by banal New-Urbanist homes, schools, and shoppes.  Let me make a plea to you, dear reader: If you respect the history of this place, and believe that the wonderful actinide-laden goodies in the topsoil ought to remain accessible to the collecting public rather than gumming up lawnmowers in the front yards of yuppie-stuffed townehomes, please send your thoughts to the developers by clicking here.

_{†Note: some creative license has been taken with this description of cow’s demise}

Links to further historical information:

• Oskins, J, Maggelet M.  Broken Arrow: The Declassified History of U.S. Nuclear Weapons Accidents. pp. 88-93
• Les Adler’s 1994 column for the Albuquerque Tribune
• “The X-Hunters” website, with details about the accident and photographs of the site

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Collecting nuclear weapon parts is fun and suitable for the whole family.  Both radioactive and non-radioactive components of the Mark 17 bomb may be obtained on the site (which is publicly accessible on dirt roads with a high-clearance vehicle, or by passenger car with some caution).  I am greatly indebted to Taylor Wilson for bringing my attention to this location.  He has a very nice summary of his findings at his website.  The mesa is devoid of large vegetation, so prepare for wind and weather.  Bring water.  Do not enter the Sandia shooting range to the north of the  bomb site, or approach the Sandia fenceline on the east.   Activity is almost entirely alpha and beta radiation; charged-particle spectroscopy is pending to identify the nuclides responsible.  A pancake Geiger counter is my preferred field instrument.  Shown here is a 15-pound sheet of lead with a surface reading of about 1300 cpm.

Example components of the bomb (click thumbnail for numbered image), relative to calipers set at 2 inches for scale.  Some pieces have identifiable function, others are more mysterious.  Details about the Mark 17 construction remain classified.  Any readers with a better technical eye for these components please feel free to correct my guesswork in the comments, and I will update the list accordingly:

1. Laminated cork composite from bomb liner.  Cork is very abundant, but never radioactive.
2. White solid plastic resembling polyethylene, perhaps from interstage.  Most shows signs of melting and charring.  Frequently radioactive.
3. Black plastic or composite.  Brittle, unlike #2 material.  Never radioactive.
4. Aluminum casing components, still retaining the greenish-yellow exterior paint.  Never radioactive.
5. Part of a wiring harness, containing remnants of wires.  Only example found.  Not radioactive.
6. Fabric sheath / strap material.  Only example found.  Radioactive.
7. Gear (not radioactive).
8. Aluminum sheet (no exterior paint).  Not radioactive.
9. Steel.  Rarely radioactive.
10. Lead metal, probably from the bomb’s radiation reflector.  Sometimes radioactive.

My spiciest findings are shown at left.  The most radioactive is a small piece of lead, one surface of which registers 13,000 CPM on a 2″ pancake Geiger counter.  The most radioactive plastic piece registers about 7,000 CPM, apparently due to a small embedded object.

My heaviest finding is a contorted piece of lead tipping the scales at almost 30 pounds.  (Sadly, the behemoth is not radioactive.)