http://www.ck12.org Chapter 24. Nuclear Chemistry
to their larger mass, ejectedαparticles also move much slower than other types of nuclear decay products. A stream
ofαparticles can be blocked by a sheet of paper or a human hand.
Beta particles (electrons) have a higher penetrating power than alpha particles, but they can still be stopped by a
thin sheet of aluminum. However, the materials being bombarded with a high-energy stream of electrons can also
become excited, potentially leading to additional reactivity.
Of the three basic types of emissions, gamma radiation has the highest penetrating power. Thick, high density
materials (such as lead) are required to stop gamma emissions. The thickness of the shielding will determine the
effectiveness of the protection offered by the lead.
FIGURE 24.4
Detection of Radioactivity
Units of Measurement
Radioactivity is quantified by measuring the number of decay processes per unit time. For example, we can measure
radioactivity in terms of counts per minute (cpm), where each "count" is a single decay process, such as the emission
of anα-particle. A sample of one particular isotope may have an activity of 5,000 cpm, while an equal amount of
another isotope might result in a radiation level of only 250 cpm. For a given nucleus, the amount of radioactivity
gives a rough indication of the amount of the radioisotope present –the higher the activity, the more of the radioactive
isotope in the sample.
The curie (Ci), named after Marie and Pierre Curie, is another unit that is commonly used to measure the rate of
decay. One curie is equivalent to 3.7× 1010 decay processes per second. Since this is a fairly large value, radiation
is often expressed in millicuries or microcuries. Another common unit is the becquerel (Bq), named after Henri
Becquerel, which is simply defined as one count per second.
Exposure to Radiation
The units discussed above give us information about the rate of decay for a radioactive sample, but they do not tell
us anything about the level of exposure experienced by nearby targets, such as people. The amount of exposure and
the resulting medical consequences depend on various factors, including the type of radiation, its intensity, and the
length of time that the person is exposed to the radioactive material. A large amount of radiation received over a
short period of time is referred to as acute exposure, whereas low levels of exposure over a long period of time are
referred to as chronic exposure.