Here's a breakdown:

* 9039Y represents Yttrium-90, an isotope of Yttrium with 39 protons and 51 neutrons.

* 9040Zr represents Zirconium-90, an isotope of Zirconium with 40 protons and 50 neutrons.

* 0-1e represents an electron (beta particle) with a charge of -1 and negligible mass.

The reaction:

Yttrium-90 decays into Zirconium-90 by emitting a beta particle (electron). This happens because a neutron within the Yttrium-90 nucleus transforms into a proton, an electron (the beta particle), and an antineutrino. The electron is ejected from the nucleus, while the proton remains in the nucleus, increasing the atomic number by 1.

In summary:

* A neutron in Yttrium-90 decays into a proton, an electron, and an antineutrino.

* The electron is emitted as a beta particle.

* The proton remains in the nucleus, changing the element from Yttrium (atomic number 39) to Zirconium (atomic number 40).

This is a common example of a beta minus decay, which is a type of radioactive decay where a neutron is converted into a proton, releasing an electron and an antineutrino.