Gamma decay
Gamma decay is the simplest form of nuclear decay—it is the emission of energetic photons by nuclei left in an excited state by some earlier process. Protons and neutrons in an excited nucleus are in higher orbitals, and they fall to lower levels by photon emission (analogous to electrons in excited atoms). Nuclear excited states have lifetimes typically of only about
s, an indication of the great strength of the forces pulling the nucleons to lower states. The
decay equation is simply
where the asterisk indicates the nucleus is in an excited state. There may be one or more
s emitted, depending on how the nuclide de-excites. In radioactive decay,
emission is common and is preceded by
or
decay. For example, when
decays, it most often leaves the daughter nucleus in an excited state, written
. Then the nickel nucleus quickly
decays by the emission of two penetrating
s:
These are called cobalt
rays, although they come from nickel—they are used for cancer therapy, for example. It is again constructive to verify the conservation laws for gamma decay. Finally, since
decay does not change the nuclide to another species, it is not prominently featured in charts of decay series, such as that in
[link] .
There are other types of nuclear decay, but they occur less commonly than
,
, and
decay. Spontaneous fission is the most important of the other forms of nuclear decay because of its applications in nuclear power and weapons. It is covered in the next chapter.
Test prep for ap courses
A nucleus in an excited state undergoes
decay, losing 1.33 MeV when emitting a
ray. In order to conserve energy in the reaction, what frequency must the
ray have?
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is commonly used in smoke detectors because its α decay process provides a useful tool for detecting the presence of smoke particles. When
undergoes α decay, what is the resulting nucleus? If
were to undergo β decay, what would be the resulting nucleus? Explain each answer.
When
undergoes α decay, it loses 2 neutrons and 2 protons. The resulting nucleus is therefore
.
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For β decay, the nucleus releases a negative charge. In order for charge to be conserved overall, the nucleus must gain a positive charge, increasing its atomic number by 1, resulting in
A
nucleus undergoes a decay process, and the resulting nucleus is
. What is the value of the charge released by the original nucleus?
- +1
- 0
- -1
- -2
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Explain why the overall charge of the nucleus is increased by +1 during the β decay process.
During this process, the nucleus emits a particle with -1 charge. In order for the overall charge of the system to remain constant, the charge of the nucleus must therefore increase by +1.
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Identify the missing particle based upon conservation principles:
-
-
-
-
-
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Are the following reactions possible? For each, explain why or why not.
-
-
-
-
- No. Nucleon number is conserved (238 = 234 + 4), but the atomic number or charge is NOT conserved (92 ≠ 88+2).
- Yes. Nucleon number is conserved (223 = 209 + 14), and atomic number is conserved (88 = 82 + 6).
- Yes. Nucleon number is conserved (14 = 14), and charge is conserved if the electron’s charge is properly counted (6 = 7 + (-1)).
- No. Nucleon number is not conserved (24 ≠ 23). The positron released counts as a charge to conserve charge, but it doesn’t count as a nucleon.
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