Sunday, December 25, 2011

Beta decay, neutron activation analysis and the r-process

A Wikipedia article on neutron activation analysis (NAA) is helpful in understanding a little of what would be involved if inverse beta decay were causing a large number of neutrons to attach to nearby nuclei.  Not only would there be transmutations not unlike those found in the r-process, a phase of rapid nucleosynthesis hypothesized to take place within supernovae that is responsible for creating elements above iron.  In addition, following this activity there would be beta decay of unstable radioisotopes to more stable isotopes which would release gamma rays at specific energy levels, long after the experiment was dismantled, along the lines that Jacques Dufour described in a note on the Widom-Larsen work referred to in an earlier post.  This gamma emission is a property of beta decay that is used in NAA to detect the relative levels of different elements (and presumably isotopes) in a sample.

The process works by bombarding the sample with neutrons from a neutron source.  Following the bombardment transmutations take place and then decay into more stable isotopes according to the half-lives of the elements concerned.  Apparently there is a body of expertise that has developed around analyzing the gamma ray spectra that result.  The neutron bombardment does not damage the sample, but it will remain radioactive at low and possibly medium levels after the analysis.

An important implication seems clear:  if there is a comparable process of electron capture that leads in turn to neutron capture and transmutations, then either
  1. The nickel or palladium used in the experiment will exhibit mild to medium levels of radioactivity afterwards; or
  2. There is some variable that has the effect of selecting transmutations of elements with very short half-lives.
I don't yet know much about the various decay chains that could be involved in such beta decay, but the second possibility could potentially require new physics if it turns out to be true.  We should start with the assumption that the first case is probably the one happening.  If the first case turns out not to be happening in some experiments, this is a good indication that some process other than inverse beta decay is taking place, although I find this scenario implausible, for the reason that it would appear to require that the measurements of the transmutations be in error.  (This conclusion assumes that possibility 2, above, is even less plausible.)

Up to now I haven't paid attention to the radioactivity of the samples that are mentioned in the LENR experiments, but I will start to keep tabs on this detail.  I remember something mentioned in connection with Piantelli, possibly, where a cathode was placed in a cloud chamber after excess heat was exhibited, and it was necessary to wait for two hours before attempting to look closely at the trajectories due to the high number of emissions coming off of the cathode initially.  In other experiments, it may be simply that the post-experiment radioactivity was overlooked.  Assuming that there is radioactivity (an assumption I will proceed with for now), I wonder if the cathodes become unsafe.

This excellent page at the University of Missouri, Columbia, Web site describes neutron activation analysis in further detail.  It mentions several parameters that have bearing on the results of a run:
  • Neutron flux
  • Irradiation time
  • Decay times
  • Measurement time
  • Detector efficiency
  • Isotope abundance
  • Neutron cross-section
  • Half-life
  • Gamma-ray abundance
The page includes a bibliography of books on activation analysis at the bottom.

The r-process is a kind of nucleosynthesis that is hypothesized to take place during supernovae and to have taken place during the Big Bang.  If I understand what I have read, it involves a sufficiently high neutron flux to push atomic nuclei along the neutron drip line to counter the rate at which they beta decay into higher elements.  As the neutrons pile on, unstable isotopes are formed which eventually undergo decay into other elements.  The r-process was set out in a landmark 1957 review paper by Burbidge, Burbidge, Fowler and Hoyle.

The r-process seems to be very similar to what is going on in the nickel and palladium hydride cathodes in the LENR experiments.  This connection was noted by Widom and Larsen in their February 20, 2006, paper.  I'm beginning to think that the spectroscopy results of the LENR experiments are like fingerprints that are unique or almost unique to each experiment and that, for a given graph, one can work backwards to deduce all or most of the important details that went into the reaction:
  • the original composition of the cathode;
  • the contents of the gas or electrolyte;
  • the energy released by the experiment;
  • the range of isotopes that were somehow given preference by the conditions of the experiment by an unknown variable; and
  • the approximate time the experiment ran.
The idea here is that the process would be similar to analyzing the radiation given off by a star, which I assume enables one to infer a number of details about the star.  I wonder if there are computational models on the Internet that can be leveraged to pursue this line of investigation further.

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