[omgJustTest]

As the article says:” The team kept neutrons in the bottle for periods of between 20 seconds and nearly half an hour, and detected sparks of light each time a neutron decayed”

The decay products of neutrons is a proton, and electron and electron antineutrino. Light emissions would most likely be in the from acceleration or impingement of charge particles in the “bottle” magnetic field.

Clearly the article claims they are detecting “sparks”.

The reason the photons are not part of the primary emission is related to the momentum / energy balance in the decay. There are other conserved quantities such as lepton number.

As you say the light is a byproduct of the acceleration/ detections scheme of the charged particles emitted.