Archive for June, 2014

Detecting Radon in the Backyard

June 24th, 2014

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:










Posted in Radioactive | Comments (0)

Bayo Canyon Debris & Dirt Analysis

June 17th, 2014


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.



Posted in Radioactive | Comments (0)

The Tek 519 Oscilloscope

June 8th, 2014


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.











Posted in Tektronix | Comments (0)