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# Three Types of Nuclear Decay Processes for AP Physics B

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By McGraw-Hill Professional
Updated on Feb 12, 2011

Practice problems for these concepts can be found at:

Atomic and Nuclear Physics Practice Problems for AP Physics B

When physicists first investigated nuclear decay in the early twentieth century, they didn't quite know what they were seeing. The subatomic particles (protons, neutrons, electrons, and so forth) had not been definitively discovered and named. But, physicists did notice that certain kinds of particles emerged repeatedly from their experiments. They called these particles alpha, beta, and gamma particles.

Years later, physicists found out what these particles actually are:

• Alpha particle, α: two protons and two neutrons stuck together
• Beta particle,β: an electron or a positron4
• Gamma particle, γ: a photon

Be sure you recall the properties of these particles; if you have to, look in the glossary for a reminder. A couple of observations: The alpha particle is by far the most massive; the gamma particle is both massless and chargeless.

### Nuclear Notation

The two properties of a nucleus that are most important are its atomic number and its mass number. The atomic number, Z, tells how many protons are in the nucleus; this number determines what element we're dealing with because each element has a unique atomic number. The mass number, A, tells the total number of nuclear particles a nucleus contains. It is equal to the atomic number plus the number of neutrons in the nucleus. Isotopes of an element have the same atomic number, but different mass numbers.

A nucleus is usually represented using the following notation:

where "symbol" is the symbol for the element we're dealing with. For example, He represents helium with two protons and two neutrons. A different isotope of helium might be He, which contains three neutrons.

Now, you don't have to memorize the periodic table, nor do you have to remember what element is number 74.5 But you do have to recognize what's wrong with this nucleus: Since we just told you in the footnote that tungsten is element 74, then an element with atomic number 76 can't be tungsten!

### Alpha Decay

Alpha decay happens when a nucleus emits an alpha particle. Since an alpha particle has two neutrons and two protons, then the daughter nucleus (the nucleus left over after the decay) must have two fewer protons and two fewer neutrons than it had initially.

The answer is found by simple arithmetic. The atomic number decreases by two, to 90. The mass number decreases by four, to 234. (The element formed is thorium, but you don't have to know that.) This alpha decay can be represented by the equation below:

### Beta Decay

In beta decay, a nucleus emits either a positron (β+ decay) or an electron (β decay). Because an electron is not a normal nuclear particle, the total mass number of the nucleus must stay the same after beta decay. But the total charge present must not change—charge is a conserved quantity. So, consider an example of neon (Ne) undergoing β+ decay:

Here e+ indicates the positron. The mass number stayed the same. But look at the total charge present. Before the decay, the neon nucleus has a charge of +10. After the decay, the total charge must still be +10, and it is: +9 for the protons in the Fluorine (F), and +1 for the positron. Effectively, then, in β+ decay, a proton turns into a neutron and emits a positron.

For β decay, a neutron turns into a proton, as in the decay process important for carbon dating:

The mass number of the daughter nucleus didn't change. But the total charge of the carbon nucleus was initially +6. Thus, a total charge of +6 has to exist after the decay, as well. This is accounted for by the electron (charge –1) and the nitrogen (charge +7).

In beta decay, a neutrino or an antineutrino also must be emitted to carry off some extra energy. But that doesn't affect the products of the decay, just the kinetic energy of the products.

### Gamma Decay

A gamma particle is a photon, a particle of light. It has no mass and no charge. So a nucleus undergoing gamma decay does not change its outward appearance:

However, the photon carries away some energy and momentum, so the nucleus recoils.

Practice problems for these concepts can be found at:

Atomic and Nuclear Physics Practice Problems for AP Physics B

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