Detecting Radon in the Backyard

June 24th, 2014
by Tim Koeth

This is cheating a bit, as this post is primarily composed of a set of .ppt slides that I assembled in 2013.  However, they concisely document the alpha and gamma spectroscopy of a simple environmental sampling experiment to capture radon daughter products from our own back yard. Read the slides from left to right, top to bottom:










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Bayo Canyon Debris & Dirt Analysis

June 17th, 2014
by Tim Koeth


In April, 2014 four friends (Brian, Dan, Jim, & Tim P) and myself hiked into Bayo Canyon of Los Alamos NM in search of historical radiological treasure. Bayo Canyon was the site of the RaLa (or RadioLanthanum) experiments that were conducted from 1944 to 1962 during which over 250 compression explosions were detonated. These experiments studied the compression of dense metal spherical shells in support of nuclear weapons development. Typically dense metals such as cadmium and copper were used, however, depleted uranium has been reported to have been employed as well.

Bayo Canyon has undergone extensive radiological remediation, however, despite the undoubted best efforts of the laboratory, radiological treasures continue to percolate to the surface with annual weather cycles. Many scraps of metal, plastics, coaxial cables, can still be found today. Some with persistent radiological contamination.


Most of the team surveyed with G-M detectors, primarily sensitive to charged particle radiations. However, Tim Ponter surveyed with a 2×2 NaI(Tl) detector, sensitive only to gamma rays. He located an area of general high background without a definitive point of origin. Considering that the source must be buried substantially below the ground’s surface, Jim and Dan began digging with improvised tools, occasionally stopping to measure the progress. It soon became clear that the source of radiation was the soil itself with a homogenous distribution of contamination. I guzzled my only bottle of water to offer a sample container, which Jim filled half-way with the most active soil.


Back home in the lab, I transferred the soil sample to a 150 ml Nalgene bottle, and performed a 4-hour background subtracted HPGe gamma ray spectroscopy count. The measurement surprisingly revealed a gamma ray spectrum signature closer to natural uranium than depleted uranium. To verify this, the spectra of isotopically enriched uranium 235, isotopically natural uranium, and depleted uranium standards were compared. More sophisticated analyses will compare ratios of Np239 and other decay chain products.


However, since the Pa234m (t1/2 1.2m) isotope has reached equilibrium with parent Th234 (t1/2 24 days) whose parent is U238 we can use the Pa234m 1001.3 keV gamma ray to roughly judge the Bayo Canyon soil’s uranium classification. Simply determining the U235 185 keV gamma to the U238’s descendant’s Pa234m 1001.3 keV gamma ratio of our three standards, we can make a statement about he soils enrichment, or lack thereof. The natural and depleted uranium standards were prepared comparable sized containers (though not exact). The U235 standard utilized a defunct fission chamber.

The ratio of 185 keV/1001.3 keV for the natural uranium standard was 8.8; the ratio for the U235 standard was 364; the ratio for the depleted uranium standard was 3.0. The ratio of the Bayo Canyon soil was measured as 7.2. While not exactly the same ratio of the natural uranium standard, the soil sample’s ratio came closest to matching it, coming to the conclusion, this contaminated soil find was most likely from an experiment involving a natural uranium spherical shell.



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The Tek 519 Oscilloscope

June 8th, 2014
by Tim Koeth


The Tektronix 519 Oscilloscope is one of my favorite instruments. It is generally not useful as it has a fixed vertical “gain,” and an uncommon 125 Ohm input impedance that uses special 125 Ohm (gold colored) General Radio connectors, which requires a sequence of properly impedance-matched connectors to provide a standard N or BNC connector. The motivation for the oddball connectors must have come from specifications of the then AEC as I have even noticed a chain for adapters hanging off 519s pictured in historic Tek literature.

I own two 519s, both came from the Los Alamos Black Hole surplus store for $30 each.  I drove them and a Tek 555 across country on my  way back to the east cost.  As soon as I was back home, I began to dig into them and start restoration.  Much to my surprise, neither 50+ year-old 519 required any work and both were found to still be in calibration.


Outside of its limitations, the 519 an amazing instrument in many ways. Perhaps the neatest feature boasts 1 GHz bandwidth accomplished by bringing the input signal directly to the deflection plates of the CRT. This makes the 519 the first direct reading 1 GHz scope (as opposed complicated and finicky sampling oscilloscopes). They are part of the transmission line. No middleman of an amplifier chain to limit the high-frequency response.  About one half of the cabinet houses a fixed delay line, which can be seen coiled up between the front-panel input connectors and the input to the CRT.  A ferrite loaded pickup, just behind the front panel, located at the beginning of the delay line, initiates the trigger circuitry for the CRT horizontal sweep.  The delay line, as the name suggests, delays the signal’s arrival at the CRT.

A close up, note the two lines of RF plumbing going into the right side of the CRT. At the very rear of the CRT sits a 25 keV electron gun. This is a fairly high accelerating voltage for a CRT, which is typically more like 10 keV. The signal to be measured enters the CRT in the rear coaxial line, and travels toward the phosphor end of the CRT. The 25 keV electron beam enters the deflection channel, travels parallel to the unshielded high frequency pulse/signal transmission line, and incurs a vertical kick. At the end of the transmission line/deflector, the signal propagates back out of the CRT and terminates in the 125-ohm load. After the beam has left the deflection channel, it is allowed to drift ~ 20 cm so as to permit the deflection to become pronounced. Since there is no vertical amplifier chain, what you see is what you put in…. fixed vertical scale – any scaling comes from external amplification or attenuators.


The signal enters the scope, passes through the trigger circuitry (which starts the CRT writing), then goes through the delay line (so the CRT writing can catch up), then finally enters the CRT. The following is an example of a 800 picosecond (FWHM) pulse from my Tek 109 pulse generator. The scope’s horizontal time base is set to 2 ns/div.


The above is a repetitive signal, however, because of the special blue P-11 phosphor screen which provides a light which photography is very sensitive to, single shots image captures are possible.  Sometimes you only get one pulse – consider the initial client and from whom I purchased these.  If the scopes could talk, I am sure they saw some very interesting cold-war history.  It is very difficult to obtain and process film for the Tek C-50 scope cameras today, so I did the unthinkable and broke authenticity by making my own scope camera, using the very low-light sensitive Nikon D3s.


With this camera, I was able to capture a single shot of the same 800 psec pulse:


Beyond restoring and admiring this Cadillac of Oscilloscopes, I have had the need to make fast-rise time, high voltage, single shot EMP measurements.  The sort of measurement that I would never bring a state of the art digitizing scope near, but without trepidation I was successful with my trusty 519 !  “Data Thief” is a fantastic program that permits the digitizing of images.


Each CRT was slightly different, and had to be characterized and its gained uniquely stamped on the bezel. his one had a vertical sensitivity of 9.6 V/cm.



An interesting observation, such high accelerating voltages will produce x-rays.  I was able to measure about 3,ooo CPM with the CRT at full intensity.











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Piscataway Radium Incident

May 27th, 2014
by Tim Koeth


February 1st, 1989

My fascination for radioactivity started very early, at the age of 10 I knew I needed a Geiger counter. I searched high and low for one, calling my science teachers and their friends, but to no avail.  It wasn’t until 8th grade, at the age of 13, having transferred to Alma Preparatory School in Zarephath NJ, did my long search come to an end with old civil defense Geiger counter placed into my hands. It was a CDV-700 made by Victoreen stored away in a box locked in a cabinet for forty years. I talked Mr. Cruver, my science teacher, into letting me take it home. That was it ! It never left my side. I surveyed everything. I’d go for walks around my neighborhood looking for something, anything. I found a few radium painted clocks and smoke detectors, but not much more. The afternoon of Thursday February 1st, my radiological hunting advanced to big game. That afternoon, I visited my former grade school where my aunt was teaching, and while waiting for her, my cousin and I scouted about the school with the trusty CDV-700. I still can remember the faint buzz of the high voltage oscillator bleeding through to the audio circuit, being interrupted every so often by the click of a cosmic particle or some stray background radiation. As we walked down the corridor the frequency of clicks increased, it didn’t take long to realize there was a source nearby. We poked into different rooms and halls mapping out the field rates.  Finally, we pinpointed the source location to be in what was known as the “old science closet.” Unfortunately, due to the lateness of the day, anyone who had the key for the room had gone for the day.


Thursday, February 8th, 1989

One week later, Thursday – February 8th, I returned to the school having made arrangements with the science teacher right to inspected the closest. He unlocked the door and I stepped in.  The field inside the room pegged the CDV700; piece by piece, objects had to be removed from the room in a processes of elimination to determine the source of this fantastic radiation.  It was just a little metal rod with an obvious handle at one end and a tip at the other.


(Credit: Paul Frame, ORAU Health Physics Museum)

Seeing as my CDV-700′s maximum scale was 50 mr/hr, I held the source by the handle and away from my body. Honestly, thinking the source was not that bad, only being marked “Radium Chemical Company,” I took it home. A few minutes later I arrived home (my mom was driving), coincidentally my father pulled into the driveway from his day at work. Being the proud hunter, I had to display my kill to him.  Dad wasn’t very thrilled, but he trusted me. I took the source down to my lab, which had been converted from my father’s old darkroom. I was curious to see if I could make the needle of my CDV-720 Ion Chamber deflect the slightest since I was never able to with any test sources of mine. I turned the unit on to the most sensitive setting(0-5 R/hr), and to my jaw dropping awe I saw the needle peg out on the 5 R/hr scale. Next, I set the Ion Chamber to the 50 R/hr scale and noted that the field of the device near the tip leveled off at 50R/hr. ” OH MY ! What have I done” were my precise thoughts. I knew a lot, but not enough. I had not known about the inverse square spreading law, and I thought that I had given myself a near fatal dose of radiation.

“Time to tell DAD, gulp.”

He actually took it pretty well. He called the local authorities first – they were useless, then he called the NJ Dept of Health – they were even more useless, for they thought my dad was cranking them when he told them the dose readings that I had found. My dad gave up and waited till the next day to call around some more. Meanwhile, I took the source and, using pliers, bent the rod, near the tip, to decrease the linear length of the device. Then placed it into a 1 liter Nalgene jar that was completely filled with cement with the exception of a small tube along the central axis of the container. I placed the cap on the jar, and then placed the jar in a 33 gallon drum of similar description. Except the drum had a 4 inch PVC pipe down the center, and the rest was also filled with concrete. I had made these for fun, never with the intention to use them, but they were quite handy for shielding. They greatly reduced the field of the source, but not to background. With the source shielded as best as I could, I called it a night !


The next day. Friday, February 9th, 1989

I went to school as usual, worrying all day about my exposure. I kept tugging on my hair to see if it was starting to fall out. I pulled so hard I think it began to come out. My mom picked my up from school at 2:50 that afternoon, and I immediately asked about dad and if he was able to contact anyone. She laughed, laughed just that certain way; “uh oh” as my throat sank into my gut. She said that several official were already at the house and mentioned that they were going to need to take our house away in 55 gallon drums because it was heavily contaminated. Oh no, oh no! She continued to say that the task force was on their way to our house as we were also on our way home. You must understand that my mom has seen one too many movies, and likes to be dramatic. But this time, she was not embellishing; sure enough, the task force was on their way over. About ten DEP and EPA Health Physicists were enter the house before that long evening was over.

As I arrived home from school that Friday afternoon, I was greeted by a health Physicist who proceeded to ask me many questions over and over and over at the kitchen table. Meanwhile a small group of Health Physicists were gather in my garage where the 30 gallon drum had been wheeled. It was a rainy and dismal day. I remember wanting to see what was going on in the garage so badly, that finally the Health Physicist interviewing me let me watch from a great distance (in an attempt to keep my dose where it was). After wrapping the drum several times with thick lead sheet, they were ready to pull the source out. A couple of technicians from Teledyne Isotopes arrived with a van containing a drum lined with depleted Uranium – very dense material, great for shielding. They backed the drum up to the garage and transferred the source to their superior container. Returning to the kitchen table I faced lots of questions from lots of people. At one point one of the health physicists came up from the basement with an astonished look on his face. “Why he has….” and proceeded to list most of my surplus scientific equipment that I had gathered in my lab. After more questions, and recreating the trip home with the source to the point of where I sealed it in the drum in the back yard, the team of health Health Physicists estimated my dose to be 275 millirem, about one years dose for an average member of the public. Substantially less than what I thought that I had given myself.

As the Teledyne technicians drove away all the needles on all the meters fell to zero, there was no contamination. All the material was contained in the tip. This I am very lucky of.  As Radium decays it emits alpha particles which are just energetic Helium nuclei. After a few interactions, the alpha particles lose their energy, then they pick up free electrons (from the metal case, etc) and form Helium gas. This gas builds up in the tip and wants to release. The pressure on the case coupled with the case’s radiation damage of 60 years of direct exposure, it is not uncommon for these caps just to pop open and spread their material all over. This would have been extremely bad ! I am so thankful that this did not happen.

The spray paint markings on our garage floor where the Health Physicists took measurements in order to determine the amount of activity that was in the tip persisted for years. The estimated amount was found to be 100 millicuries of Ra-226.  In the 1920′s that was worth 11,300 dollars. ($113/mgrm of radium).  This also confirmed my readings of 50 R/hr.

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CP-1 Memorabilia

May 26th, 2014
by Tim Koeth

I was visiting a friend, stopped by his office, and found a graphite lithograph sitting on the floor. It was framed, but the glass was broken. He saved it from his father’s estate. I made such a fuss about it, he gave it to me.


Below it is a piece of CP-1 Graphite set in an acrylic block. These came from different owners, but were all components of a gift set awarded to accomplished Argonne National Lab employees upon retirement. The gift set also included several post cards and envelopes, as well as a pencil, whose graphite, as you might have guessed, also came from CP-1 moderator.


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Catching Cosmogenic Dust

May 26th, 2014
by Tim Koeth

Along with life-giving photons, our sun emits energetic particles, electrons and protons. The earth’s magnetic field does a good job at protecting us (and all life) from the continuous bombardment of these particles. However, some still make it through, and our atmosphere is the next line of defense. The electrons and protons scatter, ionize, react with atoms of the upper atmosphere expending their harmful energy well above the surface of the earth. One method of interaction is the spallation (fancy way of saying blowing to smithereens) nitrogen and oxygen, leaving behind smaller nuclei. One particular nuclide is Be-7, and it happens to be radioactive. It has a half-life of about 53 days, so it sticks around long enough to work its way down from the upper atmosphere. Here are photos of a very simple experiment that needs only a surplus air sampler, a low-background gamma ray spectroscopy system, and nearly infinite patience.


This is a Staplex high-volume air sampler, with a 4-inch diameter filter paper mounted in it. I ran it outside for 2 hours a few weeks back. I purchase this air sampler at Surplus City in Albuquerque for $50, new in the box

Humorously, Staplex makes electric staplers & high speed air samplers. I really don’t see the synergistic connection.

It started out drawing a little more than 20 CFM of air through the paper.

After two hours, I removed the filter paper, folded it up and placed it on my HPGe for days, and days…


I counted it for five days, waiting until the radon daughters die away. Then I launched into an extremely long count on the filter paper, and an even longer background count.


This is the result of a 221,000 second count (2.55 days) on the filter paper.


This is the result of a 368,818 second (4.25 day) background count. Note all the peaks despite being a “low background” – these are all from the building materials.


Performing a scaled comparison (filter paper in red, background in black) a single lone-red peak appears.


Subtract the background spectrum from the filter paper spectrum and a single 477 keV gamma line remains: Be-7.
The Be-7 concentration varies with the seasons, as thermals lift and descend the isotope. Note that there were only about 400 counts captured under this peak in 2.5 days.


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Radioactive Mystery Solved !

May 26th, 2014
by Tim Koeth

During a trip to Cape May NJ while visiting my in-laws over a year ago, the whole family spent an afternoon at the Naval Air Station Wildwood Aviation museum. It is a very hands-on, climb-in, bump-your-head-on kind of museum. My father-in-law and I climbed into a Vietnam era aircraft and I was happy to see a fully instrumented control panel loaded with authentic radium bearing meters and dials. Anxious to demonstrate the large gamma field that I could almost feel, I went back to the car to retrieve my micro-R meter. I re-entered the museum armed with the radiation meter, but never made it back to the airplane. Rather, something else in the hangar-based museum was spicy and diverted my attention! The counter quickly led me to a jet engine sitting on display some twenty feet away. There was a clear point of maximum gamma activity on the order of 5 mr/hr on contact. I asked around, but no one was aware of what might be radioactive in the engine. I vowed that someday I would return with a gamma ray spectrometer to get to the bottom of this.
This Memorial day weekend was the promised return, plus a nice weekend spent with family and friends. Several days of planning, gathering the detector and necessary supporting electronics, a 30 liter Dewar of liquid nitrogen, a power strip, and an extension cord. The ‘hot’ engine.

Michelle, her mom and dad, and I returned on Sunday to the Aviation Museum, but to my horror found that the museum floor had been re-arranged, and the engine that I recalled could not be seen anywhere. Cool thinking prevailed and I again returned to the car to get the portable radiation meter. Much to my delight, the meter now unveiled two radiation emitting engines, neither of which I remembered seeing the previous year. This could be detected up to 20 feet away.


The maximum reading was 1 mr/hr in between exhaust manifolds.


There were no identifying marks on the first engine, so I can not tell you what type it is, however with the High Purity Germanium (HPGe) detector, I can now tell you what radioactive material it is comprised of. Several ten minute live-time counts were performed. The detector was set close to the engine and then as far away as possible to attain a background count, and another with three known standard sources for energy calibration.

Collecting data with the detector on the floor the dead time was still 6%. And I attempted a background count, but not very successfully.


Analysis of the spectral data revealed several strong lines, the most prominent at 238.62, 338.35, 583.25, and 969.04 keV. There is strong agreement with the Thorium-232 spectrum library. Further confirmation is gained by comparing the relative counts between the measured 911 and 969 keV peaks. In the Th-232 decay chain the Ac-228 decays into Th-228 with the two gamma rays of relative branching ratio at 64% which perfectly matches my measured to be 64%.

On to the second radioactive engine. I only took a single 10 minute count at the second engine, labeled as a Cobra helicopter jet engine. Its energy spectrum was identical to the first engine, so at that point it became clear the material was common to the construction of various jet engines.

I have since learned that these engines use an alloy called MagThor – which is a high temperature and high strength material. Indeed the thorium is responsible for the detectable radiation.


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