Posts Tagged ‘gamma radiation’

h1

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.

h1

HPGe Detector, Part III: Gamma Rays From (A,P) Reactions

October 6, 2011

Here are a series of experiments involving alpha particle transmutation of light elements and detection of the resulting gamma radiation signatures.  Such reactions, mostly of the (a,p) type (i.e., the alpha particle is captured and a proton is ejected), stand out for their remarkable accessibility: No particle accelerators, no vacuum environment, no dangerous and specifically-licensed radioactive sources are required.  All you do as a member of the interested public is lease a ~$150, 5-millicurie Po-210 source (the “Nuclespot” static eliminator from NRD, Inc.), stuff some test materials in front of it, and pop it in front of an HPGe detector or other gamma spectrometer.  The radiation detected can be attributed to short-lived excited states of the product nuclei.  I demonstrate the Na-23(a,p)Mg-26 reaction in table salt, the F-19(a,p)Ne-22 reaction in sodium fluoride, and the B-10(a,p)C-13 reaction in elemental boron.  Please see the earlier posts in this series describing the repair of the HPGe detector and its use in neutron activation experiments.  A PowerPoint presentation from HEAS’11 about these (a,p) reactions can be downloaded here.

♦♦♦♦♦♦♦♦♦♦

Chapter 1: Na-23(a,p)Mg-26 Generate the signature of plutonium-processing waste in the comfort of your kitchen.

This experiment is easy to carry out, as shown to the left.  All you do is fill the Nuclespot grill with table salt (NaCl), contain the salt with a piece of packing tape across the face of the source, and place the salted source in front of an HPGe detector for a few hours.  “Background” from the standpoint of this experiment is the spectrum taken from the unsalted source.  Subtracting the background thus leaves only the peaks contributed by reactions of alpha particles on the nuclei in salt.  Watch a discussion of this experiment on YouTube:

Results are shown in the gallery below.   The first 0-to-3-MeV spectrum is the result of collecting data for about 7 hours, “background” not subtracted.  The most prominent feature of the spectrum is the Po-210 gamma line at 803 keV.  However, the 1809-keV radiation from the decay of the first excited state of the Mg-26 nucleus is obvious too.  The second image shows a closer view (850-3000 keV) with attribution of peaks to the likely-responsible nuclides.  Many background peaks from natural K/Th/U decay are eliminated by background subtraction, which results in the third spectrum below.  The 1133-keV peak from the decay of the second excited state of Mg-26 is apparent in addition to the large Mg-26 peak at 1809 keV.  I attribute the slight negative peak at 2236 keV to a bit of excited Si-30 formed by the (a,p) reaction on Al-27 in the background count.  In this situation, the un-salted alpha source faces the detector’s aluminum can.  Click any image for a full-size view.

The table salt reaction just demonstrated actually has an application in the nuclear industry: assaying the actinide activity in salt wastes from plutonium processing.  The spectrum at left is taken from Sher and Untermeyer, The Detection of Fissionable Material by Nondestructive Means (ANS, 1980), and the Mg-26 gamma is a prominent feature.

♦♦♦♦♦♦♦♦♦♦

Chapter 2: F-19(a,p)Ne-22 Strong gamma rays at 1275 keV detectable in seconds.

Here, instead of salt, the material placed into the Nuclespot grille is sodium fluoride (NaF) powder.  Otherwise, procedure is the same.  If I had to recommend one of these three reactions to a beginner, or to someone using a low-resolution detector like a NaI(Tl) scintillation detector, this would be it.  The reason why is fairly evident from the spectra below—the peak at 1275 keV from the decay of Ne-22’s first excited state with a half-life of 3.63 ps is a very large and noticeable peak without much company in that region of the spectrum.  Of course, this is sodium fluoride, so some of the previously-discussed Na-23(a,p)Mg-26 reaction can also be detected by way of its 1809-keV peak.

♦♦♦♦♦♦♦♦♦♦

Chapter 3: B-10(a,p)C-13 A  remarkable demonstration of the nuclear Doppler effect

Finally we arrive at the most complex and interesting of these three (a,p) reactions.  This one results in a multitude of high-energy gamma rays from three excited states of C-13, two of which are very short-lived.  So short-lived, in fact, that the energetic C-13 nucleus decays before it has a chance to appreciably slow down, and hence the corresponding gamma peaks are distorted by Doppler broadening and a considerable blue shift that is likely linked to the experimental geometry.  The method follows the two previously described, except with boron powder in the Nuclespot’s grille, but I should emphasize that when the loaded source is placed against the detector to be counted, it is with the grille (boron) side toward the detector.  I predict that turning the source around will cause a notable red shift rather than blue shift as seen below.  Anyway, let’s have a look.  First up, the spectrum without background subtraction.  Second, a close-up of the high-energy range with background subtracted.  Detector physics adds some complexity to the spectra through prominent single-escape (SE) and double-escape (DE) peaks associated with the full-energy photopeaks of interest.  This is because at such high energies, the dominant mode of interaction for gamma rays is by pair production.  More discussion about these results after the gallery:


In seeking an explanation for the rather odd peak shapes that show up in the above gamma spectrum, it helps to first consult a listing of the energy levels of the C-13 nucleus, where it is notable that the two lower states decay with half-lives of about 1 fs (i.e. 1E-15 s).  A plot (left) generated with data from the freeware program SRIM shows the velocity of the C-13 nucleus traveling in boron as a function of time post-reaction.  Obviously, 1 fs is insignificant on this time scale; these excited states will decay before the C-13 comes to rest in essentially every instance.

Three scenarios are possible for describing the relative motion between the C-13 and the HPGe detector.  The C-13 can be moving toward the detector when it de-excites (Case 1 at left), leading to a Doppler blueshift; it can be moving away from the detector (Case 2), leading to a redshift; or it can be either stationary or moving transverse to the direction of gamma emission (Case 3), in which case no Doppler shift is expected.

Inspection of the actual spectrum collected in this experiment shows a pronounced blue shift in the gamma peaks corresponding to the lower excited states.  This is not surprising in light of the fact that most of the C-13 nuclei carry momentum in the direction of the reactant alpha particles, and we have a surface alpha source aimed toward the detector.  Presumably, flipping the source around would produce a peak with significant redshift instead…I am still waiting to try this experiment.  Our observed blue shift is consistent with the kinematics of this reaction.  I’m hard-pressed to offer mathematical detail in WordPress, but my PowerPoint linked at the top has some detail.  Finally, the higher excited state at 3853 keV is long-lived relative to the slowing-down time of the C-13 and exhibits little Doppler effect as expected.

h1

HPGe Detector, Part II: Neutron Activation with a Weak AmBe Source

September 21, 2011

Activation of antimony, indium, and aluminum is possible using a homemade neutron source containing ~5.6 millicuries of Am-241 in sealed smoke detector sources pressed against beryllium.  Although the source is weak (at best about 2000 n/s) and the activities induced by it even weaker, an HPGe detector can convincingly sniff out the telltale signs of neutron exposure in certain materials that have been nearby.  (Read Part I in this series for details about the HPGe detector.  Successful detection hinges on the high resolution of the HPGe in these three cases, but see my video here for an example of activation detected by a NaI:Tl scintillator.) 

Below are the three test specimens: 200-mesh antimony powder, an old piece of 0.01″ indium foil, and some little scrap pieces of 6061 aluminum:

When manufacturing artificial radioactivity by neutron capture, it’s important to optimize both the irradiation conditions and the detection conditions according to the physics of the experiment—tailoring the neutron energy spectrum to the reaction cross-section with moderators, choosing irradiation and counting durations according to product lifetime, and using an appropriate detector for the expected activities (in these cases, all expected products are strong gamma emitters).  My goal with the antimony is to detect Sb-122 from radiative capture of low-energy neutrons on the natural isotope Sb-121, i.e. Sb-121(n,g)Sb-122.  With indium, I’m seeking the In-115(n,g)In-116m1/In-116m2 reactions, also favored by low-energy neutrons.  The aluminum presents an opportunity to perform a fast neutron reaction, Al-27(n,p)Mg-27.  I will discuss the challenges particular to this reaction in more detail later.

I elected to use polyethylene as a moderator and reflector for all three irradiations.  The AmBe source emits a broad spectrum of fast neutrons with a mean energy near 5 MeV, so the indium and antimony activations benefit from having those neutrons slowed down.  The Al(n,p) reaction does not benefit from slow neutrons; however, the mean free path of fast neutrons in Al metal exceeds the thickness of my pieces, and the plastic will serve to reflect many neutrons back into the sample that would otherwise be wasted.  Below are a couple ad-hoc contraptions to surround samples with plastic.  At left is my black HDPE “neutron oven,” originally part of a “Snoopy” neutron detector.  At right is a more versatile concept—HDPE bricks.  For no particular reason the antimony and aluminum went in the “neutron oven” and the indium was cooked inside the stack of bricks.

The half life of Sb-122 is 2.72 days, so ideally the antimony would cook next to the neutron source for more than a week to bring it up toward saturation activity.  I’m not that patient, so it cooked for only two days.  In-116m1 (half-life 54m) saturates in a few hours, so I cooked the indium in contact with the neutron source for two hours.  Mg-27 from the Al(n,p) reaction (half-life 9.5m) is expected at exceedingly low activity and a special technique of repetitive irradiating and counting was adopted: irradiate (1800 s), count (700 s), wait (700 s), count background (700 s), and repeat this sequence two more times.

After irradiation, I counted the samples with the HPGe detector (left, its electronics at right).  My approach is generally to count for at least one half-life if practical, but no longer than two.  The count times (detector “live times”) are noted in each collected spectrum in the gallery below.  Background spectra for subtraction are best obtained by allowing the sample to remain in position near the detector for several half-lives to decay, then counting again for as long as possible.  I used this general approach with indium and aluminum, but with the 2.7-day antimony, it would have been somewhat impractical.  An alternative is to simply remove the sample entirely and count as I ended up doing; a better alternative would have been to count the background before irradiation.

Each experiment resulted in radioactivity of the type expected.  Or did it?  Check out the results (click any image for full size):

The radiation from antimony has a simple spectrum that matches expectations of a 564.2-keV gamma ray accompanying 71% of decays.

The indium spectrum offers a robust and distinct fingerprint of multiple gamma energies that are a textbook match to the expected values from In-116m1; other possible radiations from other isomers of In-116  or In-114 are not in evidence.

And then we come to a genuine interpretive challenge with the aluminum.  I expect a major peak at 843.8 keV (72%) and a minor peak at 1014.4 keV (28%) from Mg-27.  Obviously there’s a peak in the immediate vicinity of the former value—its centroid is calculated to be 841.9 keV based on calibration of the energy scale with Cs-137 and Co-60.  Close enough to be conclusive?  Well, there’s a hitch.  Manganese is an important contaminant in 6061 alloy.  Its (n,g) reaction has a high cross section and results in 2.6-hour Mn-56, which emits an 846.8-keV gamma ray.  There are just two channels of separation between 846.8 keV and 843.8 keV in my pulse height spectrum.  Further infusing doubt into the Mg-27 hypothesis is the absence of a significant peak at 1014.4 keV, although only 12±4 counts are expected there—well into the noise.  But if, on the other hand, Mn-56 is responsible for this peak, we can look to additional evidence to corroborate that hypothesis.  First, Mn-56 emits an 1811-keV gamma ray (27%) in addition to the 846.8-keV gamma ray (99%).  There is the noisy suggestion of a peak at 1764 keV that could be Mn-56’s 1811-keV radiation if the detector’s energy calibration is poor, but this could also plausibly be Al-28 (1779 keV), or radon daughter Bi-214 (1764 keV).  Second, Mn-56’s half life is much longer than Mg-27’s and longer than the duration of the experimental sequence, so the “background” spectra for this experiment should show  many counts at this energy if Mn-56 is the culprit.   In fact, the background has just 8 counts in this channel.  In conclusion: I’m willing to put faith in my three-point linear energy calibration and attribute the 1764-keV peak to ambient Bi-214, and from examination of the background spectrum in this experiment, attribute the 841.9-keV peak to Mg-27.  Sometimes these analyses aren’t straightforward!

Reference information from the National Nuclear Data Center:

  • Energy-dependent cross-section for the (n,g) reaction in Sb-121; decay radiation from Sb-122
  • Energy-dependent cross-section for the (n,g) reaction in In-115; decay radiation from In-116
  • Energy-dependent cross-sections for the (n,g) and (n,p) reactions in Al-27; decay radiation from Mg-27, Al-28, Mn-56, Bi-214
h1

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)

 

%d bloggers like this: