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# Detection of Radioactivity

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Invest in uranium!

Uranium is going to be worth more on the open market after a steep decline in prices several years ago. So people are out there looking for more uranium. But they are not waving Geiger counters around to find this material. All the surface uranium has been found. Instead, they must study geology, such as the rocks in the picture above. Deep holes are drilled to explore the underground geology of a site and the rocks obtained are chemically analyzed. No more clicking – just modern-day geology and chemistry.

### Detection of Radioactivity

Radioactivity is determined by measuring the number of decay processes per unit time. Perhaps the easiest way is simply to determine the number of counts/minute, with each count measuring a single decay process, such as the emission of an α-particle. A particular isotope may have an activity of 5,000 counts/minute (cpm) while another isotope might only have 250 cpm. The amount of activity gives a rough indication of the amount of the radioisotope present – the higher the activity, the more radioactive isotope in the sample.

#### Units of measurement

The curie (Ci) is one measure of the rate of decay (named after Pierre and Marie Curie). One curie is equivalent to 3.7 × 10 10 disintegrations per second. Since this is obviously a large and unwieldy number, radiation is often expressed in millicuries or microcuries (still very large numbers). Another measure is the becquerel (Bq) , named after Henri Becquerel. The becquerel is defined as an activity of one disintegration/second. Both of these units are concerned with the disintegration rate of the radioactive isotope and give no indication of dosage to the target material.

Exposure gives an indication of the amount of radiation that travels through the air. Two factors influence the amount of exposure a person may receive – time and intensity. Acute exposure indicates a large amount of radiation received over a short period of time. Chronic exposure deals with lower levels of exposure over a longer period of time. Dose equivalence combines the amount of radiation received and the medical effect of that radiation. Calculations of exposure and dose equivalence are complicated and will not be pursued at this time.

#### Personal Dosimeters

Measurement of exposure to radioactivity is important for anyone who deals with radioactive materials on a regular basis. Perhaps the simplest device is a personal dosimeter – a film badge that will fog up when exposed to radiation. The amount of fogging is proportional to the amount of radiation present. These devices are not very sensitive to low levels of radiation. More sensitive systems use crystals that respond in some way to radioactivity by registering the number of emissions in a given time. These systems tend to be more sensitive and more reliable than film badges.

Dosimeter

#### Geiger Counters

A Geiger counter provides a sensitive means of detecting radioactivity. A tube is filled with an inert gas, which will conduct electricity when radiation enters it. When a charged particle comes into the tube, it changes the electrical potential between the anode and the cathode. This change in potential in the tube produces a change in voltage in the electrical circuit and registers as a count. Geiger counters are fairly inexpensive and reliable, so they are useful in a wide range of applications. More complicated types of counters are also available, but are generally used in sophisticated experiments.

Geiger counter.

#### Summary

• Units for the measurement of radioactivity are described.
• Instruments for the detection of radioactivity are described.

#### Practice

Read the material at the link below and answer the following questions:

1. Who invented the Geiger counter?
2. What happens when a radioactive particle collides with gas molecules inside the tube?
3. What causes current to flow and register a count?

#### Review

1. What is the most common unit of radioactivity?
2. How does a dosimeter detect radioactivity?
3. What detects the presence of a radioactive emission in a Geiger counter?

# Reviews

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