For questions 1-7, read the text below and decide which answer (A, B, C or D) best fits each gap

In 1898 Ernest Rutherford, a young research student in Cambridge University, England, 1_______ that ra­dioactive substances 2_______ two different kinds of rays which he named α rays and β rays after the first two letters of the Greek alphabet. The negatively charged β rays turned out to be beams of electrons, and the positive α rays were found to be beams of Helium 4 nuclei. Helium 4 nuclei, 3______ of 2 protons and 2 neutrons, are thus also called α particles. After Rutherford observed a third kind of radiation he called γ rays, which turned out to be 4_______ photons. We have seen thatβ rays or electrons are emitted when a neutron sheds mass by decaying into a proton in a β decay reaction. But where do the α particles come from?

A nucleus with a(n) 5______ of electric potential energycan lose energy by converting one of its protons into a neutron in an inverse β decay reaction. This, however, is a relatively 6 _____ event. More commonly, the number of protons is reduced by ejecting an α particle. Why the nucleus emits an entire α particle or Helium 4 nucleus, instead of simply kicking out a single proton, is a consequence of an anomaly in the nuclear force. It turns out that a Helium 4 nucleus, with 2 neutrons and 2 protons, is an exceptionally 7_____, tightly bound object. If protons are to be ejected, theycome out in pairs in this stable configuration rather than individually.

1 A learned B unearthed C discovered D supposed

2 A let out B emitted C ejected D uttered

3 A consisting B containing C having D including

4 A energetic B high energy C high-power D dynamic

5 A excess B surplus C exuberance D left-over

6 A extraordinary B rare C scarce D odd

7 A reliable B stable C strong D firm

For questions 1-10, read the text below. Use the word given in capitals at the end of the lines to form a word that fits in the same line.

Date: 2015-05-18; view: 786; Нарушение авторских прав