将下列段落翻译成中文 The history of nuclear physics as a discipline distinct from atomic physics starts with the discovery of radioactivity by Henri Becquerel in 1 896 investigating ph osphorescence in uranium salts. The discovery of the electron by J. J. Thomson a year later was an indication that the atom had internal structure. At the beginning of the 20th century the accepted model of the atom was J. J.Thomson ’s plum pudding model in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it.