A sample of emits an electron as it transforms into

PHYSICS · Subjective · A Level · Physics · Particles and radiation · Particles and radiation

Problem

A sample of

^{90}_{38}\text{Sr}

emits an electron as it transforms into

^{90}_{39}\text{Y}

. The incomplete reaction is

^{90}_{38}\text{Sr} \;\to\; ^{90}_{39}\text{Y} + e^- + X.

Identify particle

X

and, in one or two sentences, explain how its presence ensures conservation of both electric charge and lepton number.