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