Analysis of Soviet smoke detector plutonium

Plutonium is a practical and versatile substance, having applications that range from planetary extinction to routine fire protection, depending on the user’s fancy.  The element has been mass-produced in nuclear reactors since World War Two, and occurs in various isotopic compositions in the discharged reactor fuel, reflecting variables such as fuel burnup, initial uranium enrichment, and neutron spectral characteristics of the reactor design.  The Soviet Union cooked up more plutonium than any other nation.  Most of this was slated for the noble purpose of containing capitalist imperialism, but some found its way into commercial ionization smoke detectors like the KI-1, RID-1, and RID-6M.  (The bourgeois warmongers themselves preferred, and still prefer, americium-241 for this application.) Occasionally, people in the former USSR try to peddle their old smoke detector plutonium on the nuclear black market, thinking that it may attract top dollar from terrorists with an appetite for nuclear warfare.  We’ll examine that possibility in more detail shortly.

Since I was curious about the technical characteristics of Soviet smoke detector plutonium, I picked up an old KI-1 smoke detector and sacked it for the source.  The source design bears much resemblance to the “lipstick” sense chamber sources in early Pyrotronics detectors made in the USA.  This one is brass and a bit wider than its Pyrotronics analogue.  An internal axial thread positions a cup-shaped alpha particle shield around a band containing the active deposit, thereby regulating the amount of ionization produced by the source in the chamber and controlling the sensitivity of the detector.  The sections below describe my analysis of the gamma and alpha radiations emitted by this source, and my conclusions about the plutonium’s age, activity, mode of production, and suitability for nuclear combat.

A beat-up old KI-1 smoke detector found in an industrial facility in the former USSR dating from the mid-1970s.

Cute, isn’t it? This is the ~1-milligram plutonium source from a Soviet KI-1 smoke detector.

Plutonium source from the Soviet KI-1 smoke detector contains about 1 mg of Pu, deposited on the grayish ring in the middle. The position of the umbrella cap is adjustable.

Plutonium isotopics by gamma spectrometry

High-resolution gamma spectroscopic measurements allow direct determination of the relative concentrations of Pu-238, Pu-239, Pu-240, and Pu-241 in a plutonium sample.  In such measurements, Pu-242 is customarily inferred from heuristic correlations to the other isotopes; it can be directly measured only with costly and destructive mass spectrometry.  Additionally, the ratio of daughter Am-241 to parent Pu-241 can be used to date plutonium.  The basic methodology is discussed in good detail in Sampson, T. E., Plutonium Isotopic Composition by Gamma-Ray Spectroscopy (1986).  I employed the multiple linear regression (MLR) formula of Sarkar, Shah, et al. (2014) to estimate Pu-242.

My gamma detector is a PGT n-type coaxial HPGe detector that lives in the guest bedroom of the home (the least radioactive room, as it should be), shielded with lead bricks and a graded inner shield of copper and tin sheet.  One preparation that is almost essential with plutonium is selective attenuation of the 59-keV gamma radiation from Am-241, as discussed in Sampson’s article above.  If you don’t do this, then the pileup and sum peaks caused by the intense Am-241 radiation will swamp the rest of the plutonium spectrum.  To hold the “lipstick” source, I made an attenuator out of rolled cadmium sheet and endcaps stuffed inside of a piece of copper water pipe with copper endcaps.  Such an arrangement works by strategically situating the K-edge energy of the absorber materials close to the energy of the offending radiation.  In quantitative gamma spec measurements, another important point is to avoid getting the source too close to the detector.  Otherwise, coincidences will distort the spectrum.  With the right attenuation and geometry, all that remains is to gather a statistically-useful number of counts in the spectrum–in this case, about 42 hours of counting.

The “lipstick” plutonium source from a KI-1 smoke detector is wrapped in cadmium and then stuffed in a copper pipe. These metals selectively attenuate the 59-keV Am-241 gamma energy.

Graded attenuator capsule (copper pipe) holding the smoke detector source is placed in front of the HPGe detector. Note the lead shielding and the tin-copper graded liner.

HPGe detector (right, on floor) and associated NIM electronics.

The gamma spectrum is shown, annotated, in the gallery below.  It can be downloaded in ASCII format as an Excel spreadsheet here.  (Note that there are no channel numbers or energy calibration in the ASCII format, so you will have to add them.) As can be seen, Am-241 and Pu-239 peaks are scattered throughout, while Pu-238, Pu-240, and Pu-241 are represented by a single good peak each in the 150-keV neighborhood.  Am-241’s granddaughter Pa-233 is also in evidence, attesting to the unseen Np-237 daughter.  U-237 is a product of the minor alpha decay branch of Pu-241, and it interferes with some lines in the Am-241 decay spectrum as both nuclides decay to Np-237.  Those energies subject to interference cannot be used for quantitative analysis.   Click any image for the larger original:

Calculating relative activities from the peaks in the spectrum involves the following:

• Measuring counts in each peak by peak-fitting algorithms.  I use the free software Hypermet-PC 5.12 to do this. Its algorithms are old, but well-known and still widely used.  Modern users will need to run it in DOSBox.
• Correcting measured counts by an efficiency function of energy.  I fit this function in Hypermet-PC using a sealed Ra-226 source that can be placed in the same graded attenuator (and the same counting geometry) as the “lipstick” plutonium source.
• Calculating relative activities from efficiency-corrected counts using the tabulated yields per decay of each radiation.  I used this website for my data.
• Estimating Pu-242 activity using a suitable model.  My reference is here.

Once relative activities were established, I estimated total activities by comparing the gamma count rate on a Geiger counter between the KI-1 source and the ~60 microcurie Am-241 source from a Pyrotronics F-3/5A in the same counting geometry.  The overwhelming majority of the gamma rays emitted by both sources are 59-keV photons from Am-241.  These estimates are limited by the uncertainty surrounding the total activity of the Pyrotronics source.  The relative activities are known to much higher precision.  (I should note that the uncertainties given in the table relate to the relative measurements.)  As the table below illustrates, the KI-1 source contains a total activity of about 700 microcuries today, most of which is the 14-year weak beta emitter Pu-241.  The runner-up is 88-year alpha emitter Pu-238.  On an activity basis, the other nuclides are lower in the lineup.  The plutonium mass can be calculated, and it is about 1 mg.

The alpha spectrum

Alpha spectroscopy of plutonium is confounded by the fact that Pu-239 and Pu-240, and Pu-238 and Am-241, emit alpha particles with very similar energies.  The general technique is also laborious, involving chemical preparation of samples in virtually all cases.  Like Pyrotronics sources, there is some removable contamination on the KI-1 detector source.  I wiped a tissue on the source surface, ashed it, dissolved the residues in nitric acid, and evaporated them onto a stainless steel disc to make the spectrum shown below using an Ortec solid-state detector.  Despite this effort, it is not of great technical quality compared to what one could expect with a rigorous radiochemical technique.  All that said, though: the spectrum confirms the expectation of two main alpha energy groups, the larger at 5.4-5.5 MeV (Pu-238+Am-241) and the smaller at 5.1-5.2 MeV (Pu-239+Pu-240).

Dating plutonium using the Am-241:Pu-241 ratio

The Am-241:Pu-241 atom ratio is a daughter-parent ratio, a clock that allows us to date the plutonium.  More specifically, the method determines when Am was last chemically separated from the Pu, assuming that all the material in the source traveled together through the same process.  (The assumption may not be very good if multiple batches of Pu were mixed.)  A graphical solution of the coupled Bateman equations modeling Am and Pu ingrowth and decay is shown below.  The sample age is the point on the horizontal axis where the solution intersects the measured value of Am-241:Pu-241, represented by the one-standard-deviation band between the red and blue lines.  This plutonium appears to be 44.9 ± 0.4 years old, meaning it was probably processed in 1972.

Other dating ratios

Another member of the Pu-241 decay chain, Pa-233, can also be used for dating.  In its ratio with Am-241, we get an estimate of 55.4 years; in its ratio with Pu-241, we get an estimate of 48.2 years.  The Am-241:Pu-241 method above predicted 44.9 years.  These three ages would be harmonized if there were a bit more Am-241 in the mix, specifically about 18% more, suggesting that some may have been removed in the earlier history of the sample.  The removal may have coincided with initial fuel processing delayed appreciably after fuel discharge from the reactor, or it may have been undertaken some time after the initial processing.  I am in favor of a view that americium was last chemically separated about four years after fuel discharge, the fuel itself being about 49 years out of the reactor (discharged in 1968), and that the separatory chemistry in the early 1970s was selective for Am and largely left ingrown Np-237 (parent of Pa-233) with the Pu.  This hypothesis harmonizes all three age estimates.

Original plutonium composition

Armed with an age estimate and current activity ratios among all the Pu isotopes, the calculation of mass composition at the time of preparation is straightforward using tabulated values of the half lives (or decay constants) of the isotopes.  Once again, there are assumptions in this calculation and in the conclusions derived from it.  The most important is probably that the plutonium was “fresh” when it was processed (or, more specifically, that the time difference between when irradiation stopped and when processing occurred was small enough to be insignificant to the isotopics).  Is that a good assumption?  Because the half-life of Pu-241 is only 14 years, and because the logistics of nuclear fuel processing usually dictate several years of cool-down during which time the fuel is in storage, transit from the reactor, and standing in queue for processing, this number is perhaps most suspect–and we would expect its calculated value and that of the correlated Pu-242 estimate to err on the low side.  Keeping this caveat in mind, here is the composition of the original KI-1 smoke detector plutonium as calculated from the Am-241:Pu-241 age:

What if the plutonium is actually four years older (1968) and was just processed in 1972, as the Pa-233 dating methods hint?  Then, the composition looks like the table below.  I believe this is more accurate:

Conclusions: Low-burnup, reactor-grade plutonium from 1970 is nothing to fear

With original Pu-240 concentration near 20%, the ~1 mg of plutonium used in this Soviet KI-1 smoke detector falls into the “reactor grade” classification rather than “weapon grade.”  The classification convention distinguishes plutonium compositions on the basis of Pu-240 content because of this isotope’s high spontaneous-fission neutron yield and its consequences for pre-initiation in nuclear weapons.  However, weapons made from reactor-grade plutonium are known to work.  Their yield may not be statistically reliable or as high as could be expected with weapon-grade fissile material, but they are useful weapons nonetheless.  The real barrier to would-be proliferants hoarding Soviet smoke detectors is the sheer number–millions!–of the motherfuckers they would in principle need to acquire through the typical nuclear smurfing networks.  (The entire output of the Soviet smoke detector industry is unlikely to have involved more than one formula quantity of plutonium.)

Now that we can sleep easily on the nuclear holocaust issue, I’ll add a few more observations about this plutonium.  Although reactor grade, its high fraction of Pu-239 and low fractions of Pu-241 and Pu-238 suggest moderately low burnup, probably not in excess of 5 GWd/t, in a reactor amenable to such light utilization (e.g. an isotope production reactor or online-refuelable type).  The measured dates of production (1968) and last separation (1972) rule out VVER and RBMK power reactors as sources.  Some of the RBMK’s graphite-moderated, low-enrichment-fueled predecessors designed for isotope production and co-located with processing plants (such as the ADE types) are likely origins.  These reactors also turned out a weapon-grade stream as the USSR frantically raced for nuclear parity with the Yankee imperialists.

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.