Here's why:

* Beta emission is a type of radioactive decay where a neutron in the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino.

* Strontium-90 (⁹⁰Sr) has 38 protons and 52 neutrons.

* When it undergoes beta emission, a neutron changes into a proton, increasing the atomic number by one but keeping the mass number the same.

* This results in yttrium-90 (⁹⁰Y), which has 39 protons and 51 neutrons.

The nuclear equation for this decay is:

⁹⁰Sr → ⁹⁰Y + e⁻ + ν̅ₑ