Limitations on Proton Decay Modes from a Passive Detection Scheme

Abstract

Passive proton decay experiments are based on the observation that a kiloton of material after one year will have experienced as many proton (or bound neutron) decays as a single gram of a billion year old piece of matter. The observations I will discuss are based on the following. First, following a proton decay event within a nucleus, such as ¹⁶0 or ⁵⁶Fe, a spallation of that nucleus is fairly probable. The size of the probability depends somewhat on the nucleon decay products, e.g. a pion will scatter or react approximately half the time in ¹⁶0. However, even if nucleon decays were into all neutrinos, the “hole” left in the daughter nucleus would leave it with an excitation energy of up to about 50 MeV. This highly excited nucleus would then emit protons, neutrons and alphas until only a stable particle fragment was left. Second, it is known that the heavy recoiling fragments from such spallation reactions are highly ionizing enough to cause damage in solids such as mica, and indeed short (l to 2µ) spallation recoil tracks have been observed.¹