MODULE 4

Section 1: 

Prompt neutrons

Delayed neutrons

Section 3: 

Photoneutrons

Section 4: 

(n,2n) and (n,3n) reactions

Section 5: 

Exercises

When fission ceases, the emission of prompt neutrons also automatically ceases, but (some) fission fragments continue to emit neutrons via radioactive decay.

Let's look at the following reaction:

Br⁸⁷ decays by means of b- to ₃₆Kr⁸⁷, sufficiently excited to emit a neutron and become ₃₆Kr⁸⁶. 

(4-3)

 Figure 1

The neutron emitted is a prompt neutron, but it occurs some time after the original fission, since the Br⁸⁷ must decay first. The Br⁸⁷ is called the precursor.

There are more than a score of precursors and they are divided into 6 groups in accordance with their half-life T½.

Let's look at the total proportion of delayed and prompt neutrons:

As we will see, the proportion of delayed neutrons has a very important effect on the control of the reactor. (b = 0.0065  U-235).

Table-2 displays the characteristics of the delayed neutrons in thermal fission.

Example on production of prompt neutrons and the effective delayed neutron fraction.

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