Scientists may shift research interests and span multiple research areas in their careers,reflecting the research diversification of scientists.Quantifying the scientists\' research diversity can help to understand the research patterns of scientists.In this paper,we study the research diversification of scientists in Physics based on the Physics and Astronomy Classification Scheme (PACS) which can well reflect the research topics of physics papers.For each scientist,we first build a PACS codes co-occurrence network and reveal the research diversity by analyzing the connectivity and community structure of this network.Then we use diversity indicators to measure the research diversification of scientists and analyze the distribution of each indicator.Finally,we investigate the relationship between scientists\' diversity indicators and their scientific impact using multiple regression analysis.The results show that the numbers of connected components of most PACS codes co-occurrence networks are less than 5,and some networks have significant community structures.The diversity indicators show the heterogeneity of the research diversity of physicists.We also find that some diversity indicators are weakly correlated with scientific impact indicators.Based on our findings,we suggest that physicists should focus on their main research fields and span multiple research fields over their entire careers which could promote their scientific impact.