Preliminary Coincidence Experiments

Since my last post, I have been working through a lab on nuclear spectroscopy to develop an understanding of the kind of equipment and techniques I will use later to measure the lifetimes of positrons.  This involved setting up two of the detectors described in my last post and taking data on the photons emitted from the decay of  the radioactive isotope 22Na.

Radioactive isotopes like 22Na are constantly emitting radiation as they decay in the form of gamma rays, positrons, electrons (Beta – particles), and helium nuclei (alpha particles).  Using the detectors and software described previously I was able to measure the number of photons emitted at a various energies as 22Na decayed.  An initial spectra revealed two distinct peaks, indicating that the sample was emitting radiation at two different energies.  Consulting a reference on the known decay scheme of 22Na, I found that these peaks represented a 0.511 Mev gamma ray emitted from the annihilation of a positron, and a 1.28 MeV gamma ray emitted as part of the normal radioactive decay of the isotope.  This spectra is shown below, and though the x axis is not yet calibrated for energy, the 1st peak corresponds to the 0.511 Mev gamma ray, and the 2nd to the 1.28 MeV ray.


To further investigate the decay, I turned to a more in depth measurement – coincidence spectra.  Coincidence spectra determine whether two events occur at the same time.  In this case, I wanted to know what was found in detector #2 at the same instant one of the 0.511 MeV gamma rays from positron annihilation was detected in detector #1.  To accomplish this I used an SCA timing device and a gate to signal detector #2 to let measurements through only when a photon within a certain energy range (about 0.511 Mev) entered detector #1.  This way I could see what else was emitted when the positron annihilation occurred.

Na22 in the 180 degree orientation

When the detectors were aligned at 180° to each other (facing one another), with the 22Na sample in the center, spectra from detector #2 showed a peak at 0.511 MeV.  This tells me the 0.511 Mev gamma ray has at least one partner gamma ray emitted during positron annihilation, which is what theory predicts.  Before annihilation, the electron and positron (basically) have zero total momentum, and conservation of momentum requires that the system maintains this zero momentum after annihilation.  To accomplish this, two gamma rays must be emitted with equal energies, in opposite directions – so that the sum of their momenta is zero.  We should therefore expect to find the partner gamma ray at 180° relative to the detection of the first detected ray, and none in other directions.

Na22 at 90 degree orientation

However, turning the detectors so that they are at 90° relative to each other, with the sample in the center, we actually find a very small peak at 0.511 MeV (shown above). This is not an indication of the breakdown of conservation laws, but that there is actually a very small probability that during the brief time the gate lets signal pass in response to detection of the 1st 0.511 MeV gamma ray, a second positron annihilation occurs that happens to land in detector #2.  We also find a small peak at 1.28 MeV, since there is also a chance of non-positron related emission during the time the gate is open.

I’m still working up to taking measurements for the core research, but after some more training oscilloscopes and other lab equipment coincidence spectra to measure the time between the birth and death of positronium.


September 18, 2009. Physics, Positronium Research.

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