There are a number of gammas created at lower energies (<750 KeV) that are due to excitation of iodine atoms in the sodium iodide detector. It is not essential to understand (or measure the energy or count rates of) these gammas as part of the mass of the neutrons experiment, but students (and TAs) are often curious about the peaks which appear in the spectrum.

The only naturally-occurring stable isotope of iodine is iodine-127. The incident neutrons interact with iodine-127 in one of the following two ways, and subsequently emits gammas.

1. A neutron inelastically scatters off of iodine-127, transferring energy which excites iodine-127 into one of its possible excited states.
   • A range of possible gammas are emitted as the excited nucleus relaxes back to ground.
2. Iodine-127 absorbs a neutron to become iodine-128 (in an excited state). In its ground state, Iodine-128 is metastable (with a lifetime of about 25 minutes), and decays into either the first excited state of xenon-128 (via $\beta^-$ decay) or the ground state of tellurium-128 (via $\beta^+$ decay).
   • A range of possible gammas are emitted as the excited iodine-128 relaxes to the ground state.
   • A 443 keV gamma is emitted as the excited xenon-128 relaxes to ground.
   • A 511 keV gamma is produced when the positron emitted in the decay to tellurium-128 annihilates with an electron.

Energy level diagrams and decays schemes are shown below

Many of the possible emitted gammas appear in the neutron spectra (if you know where to look). Note that peaks due to neutrons interacting with iodine are more prominent when the port is open than when it is closed. Also (though not shown), the intensity of these peaks does not decrease significantly with increasing paraffin blocking material as the gammas are produced inside the detector (which is after any possible shielding); Any decrease in amplitude is due to a decrease in neutron flux.

The 511 keV peak which remains prominent when the port is closed is the portion of that peak due to pair-production from the 2.2 MeV gamma (not pair-production from iodine-128 decay).