NUCLEAR FISSION AND FUSION

19.5. NUCLEAR FISSION AND FUSION

Nuclear fission refers to splitting a (large) nucleus into two smaller ones, plus one or more tiny particles listed in Table 19-3. Nuclear fusion refers to the combination of small nuclei to make a larger one. Both types of processes are included in the term artificial transmutation.

Table 19-3 Nuclear Projectiles and Products∗

Nuclear Rest Mass Name

Hydrogen nucleus

Heavy hydrogen nucleus

Tritium

3 H Tritium nucleus

Helium-3

3 He Light helium nucleus

Free neutron

Alpha

Helium nucleus

High-energy electron

High-energy light particle

Positron

Positive electron

∗ Larger projectiles are identified by their regular isotopic symbols, such as 12 6 C.

Transmutation means converting one element to another (by changing the nucleus). The first artificial transmutation was the bombardment of 14 7 N by alpha particles in 1919 by Lord Rutherford.

7 N+ 4 2 He −→ 17 8 O+ 1 1 H

The alpha particles could be obtained from a natural decay process. At present, a variety of particles can be used to bombard nuclei (Table 19-3), some of which are raised to high energies in “atom smashing” machines. Again, nuclear equations are written in which the net charge and the total of the mass numbers on one side must be the same as their counterparts on the other side.

EXAMPLE 19.8. What small particle(s) must be produced with the other products of the reaction of a neutron with a 235 92 U nucleus by the following reaction?

92 U+ 1 0 n −→ 90 38 Sr + 143 54 Xe + ?

Ans. In order to get the subscripts and the superscripts in the equation to balance, the reaction must produce three neutrons:

92 U+ 1 0 n −→ 90 38 Sr + 143 54 Xe + 3 1 0 n

This reaction is an example of a nuclear chain reaction, in which the products of the reaction cause more of the same reaction to proceed. The three neutrons can, if they do not escape from the sample first, cause three more such reactions. The nine neutrons produced from these reactions can cause nine more such reactions, and so forth. Soon, a huge number of nuclei are converted, and simultaneously a small amount of matter is converted to a great deal of energy. Atomic bombs and nuclear energy plants both run on this principle.

EXAMPLE 19.9. If each neutron in a certain nuclear reaction can produce three new neutrons, and each reaction takes 1 s, how many neutrons can be produced theoretically in the 15th second?

Ans. Assuming that no neutrons escaped, the number of neutrons produced during the 15th second is

The number produced in the 60th second is 4.24 × 10 28 .

CHAP. 19]

NUCLEAR REACTIONS

These nuclear reactions actually take place in much less than 1 s each, and the number of reactions can exceed 10 28 within much less than 1 min. Since the energy of each “event” is relatively great, a large amount of energy is available. Such nuclear reactions are controllable by keeping the sample size small so that most of the neutrons escape from the sample instead of causing further reactions. The smallest mass of sample that can cause a sustained nuclear reaction, called a chain reaction, is called the critical mass. Another way to control the nuclear reaction is to insert control rods into the nuclear fuel. The rods absorb some of the neutrons and prevent a runaway reaction.

When a positron is emitted from a nucleus, it can combine with an electron to produce energy. Show that the following equations, when combined, yield exactly the number of electrons required for the product nucleus.

22 Na −→ 22 Ne + + 1 β

e − + + 1 β −→ energy

One of the 11 electrons outside the Na nucleus could be annihilated in the second reaction, leaving 10 electrons for the Ne nucleus.