Posts Tagged ‘radiation detector’

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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.

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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.

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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.

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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.

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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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