Archive for September, 2011

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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
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A Simple Spark Detector for Alpha Particles

September 17, 2011

Back in May, Explora!, the local science center for which I occasionally volunteer, referred me to the local public TV station to lead a weekend “Science Cafe”.  The discussion subject was lightning and its connection with cosmic rays.  Trying to augment my usual hands-on electrostatics program with something perhaps more topical, my mind wandered back to a fascinating radiation detector that I’d first encountered in an embodiment built by the consummate craftsman Tim Raney of Richmond, Virginia: an open-air spark counter for alpha particles.

In this type of detector, thin negatively-charged wires are strung through atmospheric air above a planar anode, and sparking occurs when strongly ionizing radiation like alpha particles passes through the high-field region near the wires.  The concept was first described by Chang and Rosenblum in Phys. Rev. 67 (1945).  Click to download the paper.  My version is pictured above, the left hand photo showing its response to a radium source from a Walkie Record-All and the right hand photo the response to a Nuclespot 5-mCi Po-210 source.  Note that this is not a traditional spark chamber; it’s much simpler than a spark chamber because it is self-triggering. It also only responds to alpha particles—no beta or gamma sensitivity at all.  (I should also mention that it is not closely representative of the runaway relativistic breakdown mechanism postulated to trigger lightning, although it does obviously exploit the ionization effects of radiation to trigger avalanche breakdown.)

Construction and operation details are discussed in the video below:

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Videos from my recent trip to Chernobyl

September 17, 2011

Two videos from my most recent radioactive scavenger hunt in Ukraine’s Chernobyl exclusion zone are now on YouTube.  One features a pinhead-sized piece of spent nuclear fuel (pictured at left) that was carefully excavated from under about six inches of soil with the aid of a CDV-700 Geiger counter probe, taken back to our hotel through Checkpoint Lelev (where the scintillation portal monitor was conveniently out of service), and analyzed using a scintillation detector and Marek Dolleiser’s “PRA” software—a clever MCA emulator that uses one’s computer audio device as a nuclear ADC.  Check it out (I recommend selecting the HD format at the bottom of the window):

The second video illustrates some environmental radiochemistry at work, namely the affinity of the beta emitter Sr-90 for the phosphate matrix of deer antlers.  In this video I show that although the gamma activity (i.e. Cs-137 activity) in a pair of shed antlers is no different than local background, the beta activity is much higher.  The reasons for Sr-90’s notoriety are tangibly apparent: a decades-long half life that keeps it cracklin’ long after the accident, and alkaline-earth chemistry that favors uptake in bone.

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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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Visiting Vogtle and Bellefonte Nuclear Power Plants

September 6, 2011

The control room in Unit 2 at Plant Vogtle, August 12, 2011, just before 5:00PM. Reactor controls at left, balance of plant on the right. The reactor is at full power. A routine maintenance and refuelling shutdown is planned for the Fall.

The American South is widely seen as the most viable US market for new nuclear power plants.  Although the “nuclear renaissance” faces serious obstacles in the post-Fukushima world, if reactors are to be put on the grid then the South is almost certainly where it will happen first.  Dominion’s North Anna plant, which I visited in 2009, plans to add an ESBWR. This August 12th and 15th I accompanied Atlanta fusion hobbyist Chad Ramey, his father, and friend Steven Shaw to two other southern nuclear nurseries. Plant Vogtle (pronounced “VO-gel” in local dialect) is an operating two-unit Westinghouse PWR plant of recent vintage that is adding two additional Westinghouse AP1000 reactors.  Bellefonte Nuclear Generating Station, by marked contrast, is a 37-year-old never-completed Babcock and Wilcox PWR plant with two units, one of which TVA elected to complete by unanimous vote of its board on August 18.

Nuclear power plants are some of the most uptight and inaccessible places on the planet unless you work there, so I’m grateful to Mike McCracken at Plant Vogtle and to Chris Griffin at TVA for accommodating us.  I’m especially indebted to Mike for all the photos from Plant Vogtle.   (Unfortunately there is a strict no-photography policy in place at Bellefonte, so my gallery contains just two exterior shots.  However, we visited the reactor vessel head, a steam generator, spent fuel pools, a cable spreader room, and the well-preserved ’70s-vintage control room, among many other parts of the plant.)  Click any image below for a larger version with caption.

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