what is mean by Na 24 isotops ,where it is found?

There are thirteen recognized isotopes of sodium. Na23 is the only stable isotope. As such, it is considered a monoisotopic element and it has a standard atomic mass: 22.98976928(2) u. Sodium has two radioactive cosmogenic isotopes (Na22, half-life = 2.605 years; and Na24, half-life ≈ 15 hours).

Acute neutron radiation exposure (e.g., from a nuclear criticality accident) converts some of the stable Na23 in human blood plasma to Na24. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed.