Fig. 2.4

block for an unstable nuclide from the Chart of the Nuclides is shown in Figure 2.4.

2.2. Fill in the gaps (1-6) with the words from the box below the text. There is one extra word which you don’t need to use.

Figure 2.5 shows the distribution of the stable nuclides plotted on the same 1__________ as the Chart of the Nuclides. As the 2__________ become higher, the ratio of neutrons to protons in the nucleus becomes larger. For helium-4 (2 protons and 2 neutrons) and oxygen-16 (8 protons and 8 neutrons) this ratio is unity. For indium-115 (49 protons and 66 neutrons) the ratio of neutrons to protons has increased to 1.35, and for uranium-238 (92 protons and 146 neutrons) the neutron-to-proton ratio is 1.59.

Fig. 2.5

If a heavy nucleus were to split into two fragments, each fragment would form a nucleus that would have approximately the same 3__________ ratio as the heavy nucleus. This high neutron-to-proton ratio places the fragments below and to the right of the 4__________ displayed by Figure 6. The instability caused by this 5__________ of neutrons is generally rectified by successive beta emissions, each of which converts a neutron to a 6__________ and moves the nucleus toward a more stable neutron-to-proton ratio.

Give the definitions of the notions below.

1. chart is …

2. axis is …

3. average thermal neutron absorption cross section is …

4. thermal neutron activation cross section is …

5. neutron-to-proton ratio is …

TEXT 3

Before reading the text below complete the sentences 1-3.

1. Natural uranium …

2. Enriched uranium …

3. Depleted uranium …

For spaces 1-5, read the text below and decide which answer A, B or C best fits each space.