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ContentsInitial Problem Statement 2 Narrative 3-4 Notes 5 Appendix 6

Non-Destructive Weld InspectionHow does the strength of a radioactive source change with time?

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How does the strength of a radioactive source change with time?

It is common practice to manufacture items

using separate parts which are then assembled

to produce the desired item. One method of

assembly that applies to metal parts involves

the use of welding to create a strong joint. For

safety-critical joints the weld has to be inspected

to ensure that there are no defects which may

lead to a future joint failure.

One method of weld inspection uses a

radioactive source that passes gamma rays

through the metal and onto a photographic

film which is then developed and inspected for

defects, such as those shown in the image,

right.

Figure 1

This method of inspection testing does not

involve any physical change to the weld. It is

described as a non-destructive test.

In order to determine the exposure time for the

film the “strength”, or, technically, the activity

measured in GBq (giga-becquerels) of the

radioactive source is required. This changes with

time as the source decays.

Non-Destructive Weld InspectionInitial Problem Statement

See “Radiographic imaging” on page 5

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NarrativeIntroductionA typical source isotope used for testing is iridium-192 (192Ir). A typical graph of how activity changes with time for such a source is shown below.

0

10

20

30

40

50

0 40 80 120 160 200 240 280 320 360 400

Time (days)

Activity (GBq)

Figure 2

The operator will need to assess the age of the source and adjust the exposure time of a radiograph accordingly.

Discussion 1What shape is this curve?

The key constant that describes radioactive decay for a particular material is the half-life. This is the time it takes for the activity to fall to one half of its original value.

Activity 1Use the above graph in Figure 2 to measure the half-life of 192Ir in days by determining how many days it takes for the 40 GBq source to decay to 20 GBq.

Discussion 2Do you get the same answer when you find the number of days required for the source to decay from 20 GBq to 10 GBq? What about other values?

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2. A mathematical descriptionAs the half-life time is the same between any two activity values, the curve can be described mathematically by the following equation A A kt= −

0ewhere A is the activity at time t and k is a constant.

Discussion 3What is the meaning of A0 and what is its value.

Activity 2Rearrange the above to find the general formula for k in terms of A0, A and t?

Activity 3Use the half-life results previously determined from the graph to calculate k.

Discussion 4To how many significant figures do you think you quote k?

Discussion 5In what other areas of engineering are you likely to meet exponentials?

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NotesRadioactivity and imagingChemical elements can exist as different isotopes. These have the same number of protons and electrons in the atom but a different number of neutrons. This means they have the same atomic number, but different mass numbers. Some isotopes are stable, which means they do not change over time. Others are radioactive. This means the nucleus has excess energy which it can shed by changing its make up. There are several ways a nucleus can do this, including

Alpha decay: The nucleus releases two protons and two neutrons.

Beta decay: The nucleus converts a neutron into a proton and an electron.

As a result of the decay, the nucleus transmutes from one element to another. Such decays are also often accompanied by the release of gamma rays. This is a very energetic form of light, similar to an X-ray. As with X-rays, gamma rays can pass through materials, although they pass through some materials easier than others. This is used to our advantage with X-rays, which pass more easily through soft tissue than they do through bone, to provide an internal view of, for example, broken bones or tooth development.

In the context of weld inspection, the gamma rays are used to “see” the internal structure of the weld. They will pass more easily through a gap in a weld than through a solid piece of metal and so can be used as the equivalent of a medical X-ray for weld inspection.

The number of radioactive decays that happen each second in an amount of material is called the activity of the material, and the SI-derived unit is the becquerel (Bq). A substance with an activity of 1 Bq is one in which one decay per second occurs. If you are using the gamma rays from this decay to inspect a weld you wouldn’t have a very bright source. Typical sources are about one thousand million (109) times more active than this, that is the activity is of the order

109 Bq = 1 GBq (giga-bequerel).

In this example you will use activities measured in giga-bequerels.

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Appendixmathematical coverageUse algebra to solve engineering problems• Know how to write any number to a specified number of decimal places or significant figures, or

to some other level of accuracy• Be able to give numerical answers to an appropriate level of accuracy• Solve problems using the laws of logarithms• Solve problems involving exponential growth and decay