N15 is not a radioactive isotope and it can be separated from N14 only base on densities. Explain

Dear student, ​ Meselson and stahl experimently demonstrated the semi-conservative nature of DNA replication. They use nitrogen isotopes to study how nitrogen incorporate in the DNA. They showed that new DNA is made by copying from the old. New DNA must be made whenever a cell divides — each daughter cell must receive a faithful copy of the parent cell's DNA. 15N (heavy) and 14N (normal) are two isotopes of nitrogen which can be distinguished based on their densities by centrifugation in cesium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15NH4Cl over many generations. As a result, 15N was integrated into the bacterial DNA. Later, they revised the 15NH4Cl medium to normal 14NH4Cl. At a regular interval of time, they took the sample and checked for the density of DNA. Based on observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively.