Posts Tagged ‘germanium detector’


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.


HPGe Detector, Part I: Repair

September 16, 2011

A high-purity germanium (HPGe) detector is the ultimate instrument for energy spectrometry of gamma radiation.  For the nuclear hobbyist, an HPGe opens a window into a fascinating realm of  home-accessible, low-intensity nuclear reactions that are obscured by background in other detectors lacking the superlative resolution Weak alpha sources available without a specific NRC license can be used to detectably excite (a,n) and (a,p) reactions attended by emission of gamma rays from product nuclei.  Radioactivities induced at the fractional Bq level by weak (a,n) or DD fusion neutron sources can be identified.  The downsides of HPGe detector ownership are obvious to most amateur scientists who have considered them: they’re fragile, consume liquid nitrogen, and—perhaps most significantly—require multidisciplinary knowledge to return to operation.

I was kicked into these uncharted waters when Taylor Wilson sent me an older 2″ Ortec coaxial HPGe detector in unknown condition, and I hesitantly began an effort toward making it work.  Right away I knew Lady Luck hadn’t smiled on me: the input FET was blown.  As I detail in the gallery below, I replaced it with a $2 Japanese audio FET, rigged a vacuum pumping scheme for the Dewar, adjusted the preamplifier, and—voila!—the thing works now, ultimately providing about 1.7 keV FWHM at 662 keV.  From my limited experience repairing an HPGe detector I can’t generalize too much, but perhaps other amateur nukeheads will find encouragement in the success story documented here.

Gallery 1: Teardown and Repairs (click any image for larger captioned version)


The following steps comprised my path to a working detector.  Additional details for some procedures can be found at TRIUMF’s website.  To make these repairs, you need an oscilloscope, an MCA, an electrometer, some NIM-standard electronics, and a high vacuum system.

  1. Demount and test the HV filter.  Jon Rosenstiel has found the filters to be a weak point in his repair experience.  Not only will blown resistors and capacitors in the filter prevent the detector from operating, they can cause failure of the input FET.  Make sure the filter’s through resistance is a stable high value (200 MΩ in my model).  These filters do not appear easy to replicate or repair, so if yours is bad, you can pretty much count on spending $500 for a new one.  Nice to know up front before getting too involved in the project!
  2. Test the detector’s preamplifier.  With low-voltage power applied from a NIM bin (but no HV bias), monitor the preamp output for noise on an oscilloscope or MCA.  At room temperature, there will be lots of thermal noise if the FET is alive.  If you’re lucky and your FET checks out, skip the next two steps.
  3. Replace the FET.  You can either pay hundreds of dollars for a new one specially culled by the manufacturer…or you can take a little pot luck on a $2 off-the-shelf part.  For relevant noise and capacitance information on specific commercial FETs, Amptek’s note here is a must-read.  (I initially tried a pair of 2SK152s in parallel, having made questionable assumptions about the crystal capacitance.  Later, when I tried a single 2SK152 transistor, I did not obtain a measurable difference in system noise.)  Take apart the detector head and break the main vacuum o-ring seal on the detector cap.  Solder in the transistor(s) using no flux.  Use a clip lead to ground the crystal HV electrode during this procedure to protect the FET.
  4. Check for high voltage clearance between the cap and the crystal package.  Sometimes there is a thin (0.01″) plastic spacer sheet interposed between–check it for burns or holes.  Damaged plastic sheets may be replaced with the plastic from a clear binder cover (from an office supply store), carefully washed and dried.
  5. Evacuate the Dewar.  Even if the FET is OK, Dewars tend to go soft over time…and that puts the FET in jeopardy because of low-pressure HV breakdown.  Preemptive attention to the vacuum may even be warranted.  You can buy an evacuation attachment from the manufacturer for hundreds of dollars, or you can drill a hole in the Dewar wall (carefully! slowly!) with a standard jobber drill and epoxy a vacuum fitting through it like I did.  Whatever you do, make damn sure the vacuum is good (< 10 mtorr) and will stay good.  Whether to continuously pump or seal off is your choice, but I do the former.
  6. Remount the HV filter and preamp components.  Supply power to the preamp (but not the HV bias!).  If you have an Ortec detector, adjust the preamp charge loop per these instructions.  Failure encountered in this procedure probably indicates a blown FET, but I am told the hybrid ICs on the Ortec preamps go bad sometimes too.  Leave the cover off the preamp; the charge loop procedure (and PZ procedure) will have to be revisited once the detector is cold.
  7. Obtain liquid nitrogen.  Pricing in small quantities is ~$1.20 / liter, so don’t get ripped off by opportunistic asshats at the welding shop who smell teh noob.  Some dealers freak if they see you driving a Dewar around in your passenger car.  If you do take a Dewar in your car, make sure it is strapped in so it can’t spill, and roll the windows all the way down for ventilation.  My 30-liter supply Dewar weighs 83 lb full and sits very nicely in the back seat of a sedan.
  8. Wait several hours after filling the detector Dewar for the detector to be operable.  You can observe the decrease in thermal noise from the preamp output as the detector and FET cool down, and you can periodically readjust the charge loop circuit to track 0 millivolts per the above instructions as the temperature drops.  This adjustment will stabilize when the FET gets cold.  In my system, the process takes just under 1.5 hours.  I recommend waiting several hours before applying bias.
  9. Give it a try: Turn on a variable HV bias supply set at 0V initially.  Use an oscilloscope or MCA to monitor the preamp output.  Approach a radioactive source to the detector head.  Counts should appear even with bias at 0V due to the photovoltaic effect.  Raise the bias to ~100V.  Noise should decrease dramatically.  Keep pushing the voltage while collecting spectra from your favorite gamma source.  At this point, hopefully you’re witnessing your new toy’s sick resolution.

Gallery 2: Testing and Initial Operation (click any image for larger captioned version)


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