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Gamma Analysis of Chagan “Atomsite”

August 19, 2012

Lake Chagan (“Atomic Lake”) was formed in 1965 following a thermonuclear cratering explosion on the Semipalatinsk Test Site in Kazakhstan.  More photos from my recent trip to the site are here.  Merely visiting the site does not answer some of the most interesting questions about its current state, such as the isotopic origin of the significant (1-2 mR/hr) gamma radiation.

I decided to take a more scientific look at the gamma rays emitted from Chagan’s fused rock—the glassy, vesiculated slag (“atomsite” or “kharitonchiki”) that covers the ground near the shore of the lake.  A grab sample was acquired, and transported home by means other than my own return flight from Almaty (this airport’s departure lounge is guarded by a notoriously-sensitive portal scintillator made by Aspect).  I filled a 3-ounce plastic jar with the material for counting.

My method of analyzing this unique “soil sample” is HPGe gamma-ray spectrometry.  I followed the same approach discussed in my earlier analysis of Japanese soils, involving comparison of the test specimen with an identically-shaped Cs-137 sand standard.  My germanium detector is operated via a homebrew LabVIEW program built around Mark Rivers’ EPICS interface for the Canberra 556 AIM MCA and Carsten Winkler’s CA Lab; I subsequently analyze the spectra (peak fitting, background subtraction, energy calibration) with FitzPeaks.  In this experiment I collected an 8000-second count of the slag sample and a 2000-second count of the Cs-137 sand standard.  An appropriate long-duration background was subtracted from each.  The quantitative calculation of activities relies on a single major line from each nuclide, chosen (to the extent possible) to be close to 662 keV.  Corrections for detector energy response were made by calibrating the energy-dependent photopeak efficiency in FitzPeaks to a point Ra-226 source, covering the range of roughly 200-1600 keV with a power-law model.  Corrections for material attenuation, including density variations from the standard, are NOT made from a calibration but are calculated based on an exponential attenuation model that assumes the sample has the elemental composition of concrete.  It’s probably not a bad comparison, and typically results in a correction of under 20%.  However, I expect better accuracy in the quantitative analysis for peaks that are closer to 662 keV.  Finally, no corrections are made for count losses to coincidence summing.  An Excel spreadsheet of this data and analysis may be downloaded here.

Referring to the 0-1600 keV gamma spectrum below, the first major observation is that most of the lines belong to europium isotopes, Eu-154 and Eu-152.  These isotopes were produced when neutrons from the “device” were captured by the ~1ppm naturally-abundant Eu-153 and Eu-151, respectively, which have remarkably high capture cross-sections.  These activation products are also long-lived enough to persist in significant quantity to the present day.  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.  The remaining major activity, Cs-137, is a product of fission in the bomb’s fissionable components.

Gamma spectrum of Lake Chagan atomsite

If we examine the smaller peaks in detail (click on below thumbnails), long-lived isotopes of holmium (Ho-166m), silver (Ag-108m), and barium (Ba-133) are in evidence.  Am-241 is present at a low concentration; on the basis of its 59-keV gamma line I cannot confidently estimate its concentration using the Cs-137 reference source technique.  Am-241 is the daughter of Pu-241 produced by neutron capture on plutonium in the bomb, and thus is a reliable proxy for the presence of plutonium in the sample.  The gamma radiations from plutonium itself are too weak and swamped by the spectrum’s low-energy continuum to be observed.

The chart below presents the results of the quantitative analysis.  Gamma-emitting radionuclide activity in “Chaganite” exceeds 375 Bq / g, with Eu-152 being the most concentrated.

Nuclide concentrations, July 30 2012

Chaganite versus Trinitite: when the activities are normalized to their initial values at the time of the respective explosions (1965 and 1945), a direct comparison can be made that illustrates just how much more radioactive the Chagan slag is (see beow).  The data for Trinitite is taken from Pittauerova, Kolb, et al., “Radioactivity in Trinitite: a review and new measurements,” Proc. 3rd Eur. IRPA Conference, Helsinki, 14-16 June 2010.

Comparison of “Chaganite” with Trinitite

The Chagan slag contained almost an order of magnitude more Cs-137 at the time of formation, but it is the rather staggering ratios of the activation nuclides that surprises me the most: 400 times as much Eu-154 in Chaganite versus Trinitite.  70 times as much Eu-152.  And 370 times as much Co-60.  Why?  One fairly obvious explanation is found in the facts that Chagan was a more powerful bomb, detonated in closer proximity to the crustal rock that its neutrons activated since it was underground.  Some further considerations may also be relevant.  According to Carey Sublette’s Nuclear Weapons Archive, Chagan “was reported to be a low-fission design, which had a pure thermonuclear secondary driven by a fission primary with a yield of about 5-7 kt.”  In contrast, the Trinity bomb was a pure fission core surrounded by a uranium tamper.  Thus, escaping neutrons with a hard DT fusion spectrum probably carried a significantly higher fraction of Chagan’s energy yield relative to Trinity’s.

There is not a statistically-different concentration of Ba-133 between the two slags.  I think most of Trinity’s Ba-133 came from the bomb’s explosives, while Chagan’s probably came from crustal concentrations of barium.

Finally, if the Trinity bomb had a fission yield more than three times larger than Chagan, why is the latter’s concentration of Cs-137 higher?  The best reason I can suggest is Chagan’s better underground containment of volatile fission products.  In a surface explosion, volatile Cs and its beta-decaying precursors exist as gases for a long time, enabling atmospheric dispersal.  In an underground explosion, volatiles are condensed rapidly near where they were formed.

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Visit to the Semipalatinsk Nuclear Test Site

August 13, 2012

Soviet Ground Zero

At 7:00 on the morning of August 29, 1949, a nuclear fireball lit up the skies over a desolate expanse of steppe about 100 km from Semipalatinsk in the Kazakh Soviet Socialist Republic.  This explosion—the culmination of a research effort personally supervised by fearsome NKVD chief Lavrenty Beria—earned the Soviet Union status as a nuclear-armed superpower to rival the United States.  Over the course of the next 50 years, 615 more nuclear explosions, as well as numerous subcritical, radiological, and reactor-based experiments, occurred on the same New-Jersey-sized reservation—the Semipalatinsk Test Site.  The STS was largely abandoned in 1991 in the turbulent prelude to Kazakhstan’s independence.

This July I had the good fortune to visit the STS and its formerly-secret support city, Kurchatov.  Physical access to the STS is minimally controlled, but given the Kazakhstani police behaviors we observed, foreigners would arouse decidedly too much suspicion traveling to the area without official sanction for their trip.  Some reactors remain operational and some testing grounds (particularly Degelen) contain proliferation-sensitive debris.  I recommend contracting with a registered adventure tour company (I hired Nomadic Travel) to handle permissions, lodging, and appropriate transportation.  Roads on the Test Site are impassible in wet weather, merely brutal when dry, and I don’t exaggerate in the judgment that some of them would be faster on horseback!

Photo selections below include Kurchatov; Soviet “Ground Zero;” the aerial bombing target for the first Soviet staged thermonuclear bomb; the Degelen Mountain underground test site; a borehole on the Balapan underground site which experienced an “emergency situation;” and finally, the radioactive crater known as Lake Chagan.  The photos provided below are all captioned with additional detail.

Links to photo galleries (or scroll down):

  1. Kurchatov, the Secret City
  2. Soviet “Ground Zero”
  3. The RDS-37 Site
  4. The Degelen Mountains
  5. Borehole 1007: “Emergency Situation”
  6. Lake Chagan, the “Atomic Lake”
  7. Reactor Facilities at STS

Kurchatov, the Secret City

Kurchatov appeared on no maps and had no name (except for a cryptic post office number) for most of its existence. It was built hastily by GULAG labor and hosted many famous (and infamous) people of importance to the Soviet nuclear weapons project. Now it has a new life as a peaceful nuclear city, with a satellite campus of Kazakhstan’s National Nuclear Center occupying new buildings in town. Meanwhile, historic structures are crumbling and the town is clearly a shadow of its former self.

Soviet Ground Zero

60 kilometers southwest of Kurchatov is the 20-km-diameter “Experimental Field” (Опытное Поле), dotted with strange and dilapidated structures, radioactive slag, and swampy craters.  Its P-1 site, shown in all the photos below, was Ground Zero for “Joe-1″, RDS-6S (the first Soviet thermonuclear bomb, named for a delightful Russian pastry), and two other successful bombs.  All bombs tested at this spot were positioned on 15-30m towers.  At least two dozen more surface tests took place elsewhere on the Experimental Field.  To watch a video of “Joe-1,” click here.  To watch a video of the “sloika,” click here.

The RDS-37 Site

On Nov. 22 1955, the Soviet Union’s first multi-stage hydrogen bomb (embodying what is known as the “Teller-Ulam” configuration in the US, credited as Andrei Sakharov’s “Third Idea” in the Soviet Union) was dropped from an airplane toward a target designated by an 800-meter-diameter chalk circle on the Experimental Field about 3 km southwest of the P-1 site.  The bomb detonated at an altitude of 1.6 km with an unexpectedly-high yield of 1.5 megatons, killing a number of people in the region (including a 3-year-old girl). What remains today are faint traces of the target markings. Like the Nazca Lines, these are easier seen from space (see the Google satellite pic). Radiation levels at the site are modest, no more than about twice regional background.  There is no notable “atomsite” slag on the surface of this site.  Watch video of the RDS-37 blast here, which shows some footage of the event as seen from Kurchatov at the end.

The Degelen Mountains

“There’s plutonium in them thar hills!”  The Degelen Mountains were used for hundreds of underground nuclear tests carried out in horizontal adits in the rock. These adits are now “prohibited areas” because many tests were subcritical and chunks of plutonium remain in the residues that the Soviet Union neglected to clean up.  According to William Tobey’s sources, “hundreds of pounds of weapons-grade fissile material was ‘readily recoverable’ in the tunnels” at Degelen, enough to make quite a number of bombs.  The mountains themselves are hauntingly beautiful, and the surrounding foothills dotted with military ruins.

Borehole 1007: “Emergency Situation”

Borehole 1007 at the Balapan site was supposed to contain a routine underground nuclear test in February of 1972. But the bomb was a little too feisty, and ended up blowing the top off the well. A piece of the well casing (quite radioactive, I should mention) is now displayed in the STS Museum in Kurchatov. The rest of the well, and all its radioactive ejecta, is right here where we found it on the steppe.

Lake Chagan, the “Atomic Lake”

An idyllic and suspiciously-round lake of some 10 million cubic meters capacity graces the left bank of the Chagan River. It owes its existence to a 140-kiloton “peaceful” nuclear explosion carried out on January 15, 1965. The stated objective was to experiment with changing the course of rivers. Chagan was a filthy test, heavily contaminating the surroundings with radioactive byproducts. Like the American Operation Plowshare, bomb developers found that these peaceful uses worked after a fashion, but resulted in contamination that tended to preclude practical use. Lake Chagan would make a great picnic spot, but we were not able to enjoy some nourishment here ourselves because we were required to wear respirators over our pie-holes. The banks of Lake Chagan are strewn with this bomb’s unique slag, a sort of foamy, pumice-like rock.  Hottest spots on the bank now seem to be about 2 mrem / hour.  Click here to watch a video of Lake Chagan’s creation, including footage of swimmers in the water.

Reactor facilities

The Semipalatinsk Test Site contains more than just old nuclear weapons tests; it is also home to some working nuclear facilities that are quite fascinating. We didn’t make it inside the Baikal and IGR complexes, but I grabbed some photos in their general direction.

For more photos, including photos from the Tien Shan Mountains, Astana, Almaty, and other cities in Kazakhstan, please see my Facebook page.

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A Little Bit of Fission

June 3, 2012

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.

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Inside Peach Bottom Atomic Power Station

May 26, 2012

The two boiling water reactors at Peach Bottom Atomic Power Station are of the BWR/4 product line from General Electric and are housed in Mark I (“lightbulb”) containments.  They share a common turbine building and a common control room.  Electrical output is about 1200 MW each, leaving the station at a respectable 500 kV to feed the power-hungry metropolitan areas of the northeastern United States.  Condenser waste heat is rejected to the Susquehanna River, supplemented during particularly hot weather by some small forced-draft cooling towers.  Peach Bottom’s official name harkens back to 1958, when “atomic power” was a celebrated novelty, and construction began on a unique gas-cooled reactor at the Peach Bottom site.  The GCR operated until 1974.  Units 2 and 3 came on line that same year on a site on the right bank of the Susquehanna River just north of Unit 1.

Nuclear power plants have understandably committed unprecedented attention to safety and security in the last decade or so.  An unfortunate side effect has been that those of us who don’t work in these facilities have scant resources to help wrap our heads around their scale, layout, equipment, and operations.  With that in mind, I’m profoundly grateful to Exelon Corporation’s Peach Bottom staff, and in particular Jim Kovalchick, director of security, for allowing the comprehensive tour on which these photos were taken in April 2012.

To see pics with my descriptive captions, you must click “permalink” in the slideshow view after clicking the thumbnails below.  Sorry that’s not obvious, but WordPress.com has gone all knuckle-head in the tech department this year.  If you want to see the FULL SIZED photo: (1) click the thumbnail; (2) select “permalink”; (3) click the larger photo.  Whew!

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U.S. Radium, Then and Now

May 14, 2012

Many people know the tragic story of the “radium girls,” the luminous-dial painters of the flapper era who tipped their paintbrushes in their mouths, became sickened from internal radiation exposure, and had to fight for workers’ compensation as they died.  Although a large number of radium paint factories existed, one in particular is identified with this infamous episode: the United States Radium Corporation, sited on two acres at the southwest corner of High and Alden Streets in Orange, New Jersey.  This factory was built in 1917 for the combined purposes of radium extraction, purification, and paint application.  Two original buildings—including the paint application building—remained standing until the US EPA had them torn down as part of a Superfund remediation project in 1998.  Today, the site is a barren, fenced-in, field with no hint of radioactivity betraying its former capacity.  In this post I’ll share a few photos from my trip this month, from the Library of Congress’s archive of the recent past, and even one from the plant’s heyday.  I’ll share some quotes about the technical operation of this facility, and a pic of my samples of its product, Undark.

The former U.S. Radium site viewed from the southeast corner in 2012. A railroad track once paralleling the confined Wigwam Brook brought 100-lb sacks of carnotite from Paradox Valley, CO, as well as soda ash, to a siding here. Radium was extracted in a long-since-demolished building at this corner of the property before going to the crystallization lab and ultimately the paint shop on site.  Hydrochloric acid, the main extractive lixiviant, was stored in a tank on the opposite side of the property.

______________________________

Paint Application Building, exterior: About 300 dial painters, virtually all of them young women, came to work here between the years of 1917 and 1926.

South-easterly view of U.S. Radium’s paint application building from Alden Street, mid-1990s (public-domain photo from the Library of Congress). Grace Fryer and her dial-painting cohort probably ingested their fatal doses of radium on the second floor of this building.

A similar view today (2012): all that’s here now is an empty field behind a fence. A scintillation counter measures nothing above background levels of gamma radiation.

……..

……..

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Paint Application Building, interior: “Dial painting areas had four parallel rows of work benches, aligned with the building’s longer axis.  Both floors included large wooden, double-hung, triple windows, and at least one section of the upper floor appears to have skylights.”

Second floor of the Paint Application Building, interior view to the southeast in this 1922 photo belonging to Argonne National Laboratory. Note the open skylights.

The same room, late 1990s, Library of Congress photo. The skylights have been filled in, but their recesses and original plumbing are still visible.  The floor has been replaced.

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Crystallization Laboratory: From the element’s discovery well into the 1950s, the only practical chemical technique for separating radium from barium was arduous multi-stage fractional crystallization.  U.S. Radium used a chloride and bromide system, as described by Florence Wall, plant chemist: “…in the crystallization laboratory, large quantities of radium chloride solution from the plant progressed in stages from silica tubs, three feet in diameter and about a foot deep, into smaller evaporating dishes until, after conversion, the product appeared as a few crystals of radium bromide in a tiny dish, 1/2 inch in diameter.” 

The one-story crystallization lab as it looked from the northwest, in this mid-1990s Library of Congress photo. Behind it is the Paint Application Building.

In 2012, the grass covers all. (The same house can be seen in the background in both images.)

……..

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The Product: U.S. Radium named its radioluminous paint Undark.  An article that was painted with this product was said to be “Undarked.” The formula of Undark varied with application and was a trade secret.  At the time of the “Radium Girls” poisoning, a single employee named Isabel manufactured a zinc sulfide base activated with trace quantities of cadmium, copper, and manganese.  Another employee, originally company founder S. A. von Sochocky, added a measured amount of radium to the base and fixed it in its insoluble sulfate form: “[D]epending upon the type of work the material is to be used for the element of radium varied from one part of radium element to 140,000 parts of the base—zinc sulphide, to one part of radium element to 53,000 parts of the base [about 20 microcuries per gram].  The radium element when added to the zinc sulphide [...] is in an aqua solution.  When that is added to the zinc sulfide which is in the form of a dry powder, it becomes like a paste.  The radium element when mixed with the sulphide powder is soluble.  In order to make certain that it will become insoluble and also that it will be equally distributed in the paste and also to prevent the radium element from being dissolved later when water is applied to it, I converted the radium into radium sulphate which is insoluble by adding amount of ammonium sulphate also in an aqua solution.” 

Undark, dated 1940, made to Army Specification 3-99D, packaged in 1g vials. Each produces a gamma exposure rate of about 40 mR / hour on contact, broadly consistent with about 20 microcuries of Ra-226 activity per gram.

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The Waste: Anything that was not radium—i.e. the vast majority of the ore that entered the plant—was waste and had to find a new home!  This included the uranium content of the ore; preceding the discovery of fission, uranium was effectively worthless.  One common application for U.S. Radium tailings was infill for construction projects in nearby Glen Ridge, Montclair, and Orange.  Contaminated fill was identified, dug up, and replaced throughout the 1990s.

Carteret Park (e.g. Barrows Field), located in Glen Ridge, was originally filled with waste tailings from U.S. Radium. Third base was rumored to be particularly “hot.” The entire ballfield was dug up, trucked away in drums, and restored with clean fill in 1998.

The hottest spots at Barrows Field today are along the concrete fence wall. Whether the minor detected radioactivity is owing to natural occurrence in the concrete materials, or un-remediated residues from U.S. Radium, is impossible to say.

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References:

Historic American Engineering Record HAER NJ-121, National Park Service (1996)  (All quotations in italics above are from this source.)

Photographs from above record by Thomas R. Flagg, Gerald Weinstein, 1995-1996, at the U.S. Library of Congress

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Gamma activity measurements of Tokyo-area soil samples

November 4, 2011

Three nuclear reactors melted down at the Fukushima-I Nuclear Power Plant following the Tohoku Earthquake of March 11 this year, resulting in the release of volatile fission products in what is widely regarded as the worst nuclear accident since Chernobyl.  Radionuclides were carried by air currents across eastern Japan.  Areas closer to the stricken plant suffered heavier contamination, but even densely-populated Tokyo, some 150 miles distant, received significant fallout.  Last month, I received a set of six soil samples from the Tokyo region, and, using my HPGe gamma detector, I have attempted a quantitative analysis of the two predominant gamma activities in these samples, Cs-137 and Cs-134.  I am grateful to Jamie Morris for the specimens, and to Dr. Steven Myers, Los Alamos National Laboratory, for his helpful communications about technique and analysis.

Jamie collected six soil samples of about 5 fl. ounces apiece, three from roadside gutters and three from nearby garden areas in the greater Tokyo region, and sent them to me in Ziploc baggies by regular airmail declared as “soil samples.”  He documented his collecting spots with geotagged photos (below).

Upon receipt of Jamie’s samples, I packed them into 3-oz clear plastic wide-mouth jars (Uline S-17034), weighed the contents, and Superglued the lids on to prevent spills.

It is important to control the source-detector geometry in quantitative measurements.  To that end, I lathe-turned a holder for the jars out of acrylic that fits onto the HPGe detector’s cap.  The jars press-fit into this holder until the lip of the cap thread contacts the front face of the acrylic piece.  Held thusly, the bottom of the sample jar is nominally one inch from the end of the HPGe cap.

A standard source, consisting of a known quantity of Cs-137 in a matrix and geometry approximating those of the samples as closely as possible, will be used as a reference against which to compare the activity in the samples.  Although commercially available, such sources are astronomically expensive and companies making them are reluctant to sell to individuals who just want to fool around.  So I’ll produce my own from the following supplies, using the procedure recommended on Slide 23 of this IAEA presentation:

  • Play sand (Lowe’s)
  • Liquid Cs-137 source (25µl / 0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Sealed Cs-137 disk source (0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Nitric acid
  • Beakers, syringe, stirring rod
  • Geiger counter (or scintillator)
  • An oven

Basically, the Cs-137 is mixed with sand and put in a Uline jar.  Click any photo below for a caption describing relevant details from the process.

Gamma spectra are collected from each sample and from the standard in my Canberra NIM MCA, using Mark Rivers’ open-source “mca” application for EPICS and my own LabVIEW interface.  8192 channels of memory are used, with the gain set at about 0.2 keV per channel.  I process the spectra to subtract background and find peak areas in the free evaluation version of FitzPeaks (note: does not work on 64-bit Windows 7).  Spectra for each sample are displayed below (click any image for a full-size version).

Activities are estimated by comparing net counts in the relevant peaks in the sample spectra with net counts in the 662-keV peak of the standard source.  Count rates are scaled to account for gamma emission probability of each nuclide.  A simple exponential attenuation mode is used to correct for matrix density variations; better accuracy can be expected for samples that most closely resemble the standard (i.e. the gutter debris samples).  I use only the 605-keV peak to estimate Cs-134 activity, since it lies closer to the 662-keV calibration energy and the systematic errors involved with energy and matrix density corrections will be smaller than for the 796-keV peak.  Ultimately, the values of interest—specific activities, becquerel per kilogram—are obtained, along with uncertainty propagated through the calculations.  These values are illustrated below:

Download the data and analysis spreadsheet (Excel 2010 format) here.

In conclusion: The synthetic fission products CS-137 and Cs-134 dominate the natural gamma radioactivity (K-40 and U / Th daughters) in all six samples.   Cs-137 is present at levels at least 1-2 orders of magnitude above levels expected from older atmospheric weapons tests and the Chernobyl accident in every one of these samples.  Total activity is roughly evenly divided between Cs-137 and the shorter-lived Cs-134 at this time; the Cs-134 will decay to irrelevance in the span of 5-10 years.  Together, high concentrations of Cs-137 and Cs-134 point to the recent Fukushima accident as the source of virtually all of this activity. The gutter debris sample from Chiba (#C) has the highest activity, and depending on how representative this sample is of the surrounding soil, MAY be indicative of significant enough cancer risk to human residents to encourage alternate patterns of occupancy or land use.  More information would be needed to quantify the severity of this kind of risk from external exposure and various routes of possible internal exposure.   Sample #C is also easily detected with small consumer-grade and homebrew Geiger and scintillation counters.   It should be noted that various physical / chemical mechanisms (e.g., runoff of soluble Cs into road gutters) tend to increase the activity of some of these particular samples relative to the surroundings.

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Trick or Treat!

October 30, 2011

What lurks inside these beguiling treats?

A razorblade, perhaps?

Or a sewing needle?

Time for another Halloween in America, and you know what that means: some good ol’ fearmongering about dangerous strangers who are hell-bent on kidnapping, raping, and chainsaw-murdering your darling little moppets! Oh joy. Perhaps no trope of the “stranger danger” variety is more firmly ensconced in the contemporary American Halloween lore than the idea of the devious misanthrope who slips razor blades into apples or needles into candy bars before passing them out to trick-or-treaters.  Concern persists despite scant evidence for such activity.  The fear had reached its zenith by the mid-’80s following the Chicago Tylenol Murders of ’82.  By 1988, the city of Reno, NV was spending about $1630 per annum to x-ray trick-or-treaters’ loot in the radiology wards of its three hospitals, as an article by J. Calvanese in Veterinary and Human Toxicology reported. “No films were positive for radio-opaque foreign bodies.” Despite a near-zero incidence of such tampering, it is still common for establishments with x-ray equipment to operate it for paranoid Halloweeners.

Tonight I offer you a couple low-energy radiographs of compromised “treats” prepared with great care in my kitchen (click any image for a high-res version). Typical dental and medical x-ray equipment operating in the 80-120 kVp range has difficulty producing high contrast for the objects pictured here, so I recommend a mammography or extremity type of tube operated at a low voltage. These photos were taken at 26 kVp with the tube and screen shown.  The screen was 5 feet downstream of the tube, where the exposure rate was about 100 roentgen / hour. The photos of the screen were made with my Panasonic LX5 point-and-shoot at ISO200 / f2.1 / 30-40 seconds. Happy Halloween…and remember, your kids are far more likely meet a grisly end in a traffic accident driving to the hospital / police station / courthouse to x-ray their candy, than they are from the candy itself.

At top: beryllium-window x-ray tube used in these images.

At bottom: the diverging x-ray beam from the tube’s window impinges on the CdZnS fluoroscopy screen used to take these images.

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