Positron Emission: Videos & Practice Problems

Positron emission occurs when an unstable nucleus emits a positron, which is the antiparticle of the electron, possessing a positive charge. This process is a type of radioactive decay. For example, when the isotope Einsteinium-253 (atomic number 99) undergoes positron emission, it transforms into Californium-253 (atomic number 98), as the atomic mass remains unchanged while the atomic number decreases by one. Understanding positron emission is crucial in nuclear chemistry and applications like positron emission tomography (PET).

Positron Emission Concept 1

Positron emission is a fascinating nuclear process where an unstable nucleus releases a positron, which is the antiparticle of the electron. While an electron carries a negative charge, a positron has a positive charge, making it essentially a "positive electron." This concept may seem unusual, but it is a key aspect of nuclear reactions.

In the context of positron emission, the term "emission" refers to the decay of the nucleus, indicating that the positron is a product of this decay process. To illustrate this, consider the element Einsteinium, specifically its isotope Einsteinium-253 (symbol: Es), which has an atomic number of 99. When this isotope undergoes positron emission, the atomic mass remains unchanged at 253, as the positron has an atomic mass of 0. However, the atomic number decreases by 1, since emitting a positron effectively transforms the element into a different one. Therefore, the new element formed is Californium (symbol: Cf), which has an atomic number of 98.

This example highlights the transformation that occurs during positron emission, showcasing how nuclear reactions can lead to the creation of new elements through the emission of particles like positrons.

Positron Emission Example 1

In nuclear chemistry, positron emission is a type of beta decay where a positron (β⁺) is released from a nucleus. This process involves the transformation of a proton into a neutron, resulting in a decrease in the atomic number by one while the mass number remains unchanged. Here, we will write balanced nuclear equations for the positron emissions of uranium-235 and radon-222.

For uranium-235 (U), which has an atomic number of 92, the balanced nuclear equation for its positron emission can be represented as follows:

\[^{235}_{92}\text{U} \rightarrow ^{235}_{91}\text{Pa} + \beta^{+}\]

In this equation, uranium-235 emits a positron, transforming into protactinium (Pa), which has an atomic number of 91.

Next, for radon-222 (Rn), with an atomic number of 86, the balanced nuclear equation for its positron emission is:

\[^{222}_{86}\text{Rn} \rightarrow ^{222}_{85}\text{Ac} + \beta^{+}\]

Here, radon-222 emits a positron and transforms into actinium (Ac), which has an atomic number of 85. In both cases, the mass number remains 222 and 235, respectively, while the atomic numbers decrease by one due to the emission of the positron.

Positron Emission Example 2

In this scenario, we analyze the decay process of Thorium-225, which undergoes a series of transformations through alpha and beta decays, as well as a gamma emission. Thorium has an atomic number of 90, and we will track the changes in both atomic mass and atomic number as it decays.

First, we note that alpha decay involves the emission of an alpha particle, which is equivalent to a helium nucleus (He) with a mass number of 4 and an atomic number of 2. Each alpha decay reduces the atomic mass by 4 and the atomic number by 2. Since Thorium-225 undergoes 3 alpha decays, we calculate the total changes:

• Mass change: \(3 \times 4 = 12\)
• Atomic number change: \(3 \times 2 = 6\)

Next, we consider the beta decay, which involves the emission of a beta particle (an electron) with an atomic number of -1 and no change in mass. With 4 beta decays, the changes are:

• Mass change: \(4 \times 0 = 0\)
• Atomic number change: \(4 \times -1 = -4\)

Now, we can summarize the total changes to the original Thorium-225:

• Initial mass number: 225
• Final mass number: \(225 - 12 = 213\)
• Initial atomic number: 90
• Final atomic number: \(90 - 6 - 4 = 80\)

Thus, the new element formed after these decays is Radium (Ra), which has an atomic number of 88. However, since we calculated the atomic number to be 80, we need to correct this. The final atomic number should be 88, indicating that the product is Radium-213 (Ra-213). The gamma emission does not change the atomic number or mass but indicates that the resulting nucleus is in an excited state, which can be denoted with an asterisk (Ra-213*).

In summary, after 3 alpha decays, 4 beta decays, and a gamma emission, the product is Radium-213 in an excited state, represented as Ra-213*.

Here’s what students ask on this topic:

What is positron emission and how does it occur?

Positron emission is a type of radioactive decay where an unstable nucleus emits a positron. A positron is the antiparticle of the electron, possessing a positive charge. This process occurs when a proton in the nucleus is converted into a neutron, releasing a positron and a neutrino. The emitted positron is represented as $e^{+}$. For example, when Einsteinium-253 (atomic number 99) undergoes positron emission, it transforms into Californium-253 (atomic number 98), as the atomic mass remains unchanged while the atomic number decreases by one.

What is the difference between a positron and an electron?

A positron is the antiparticle of the electron. While an electron has a negative charge, represented as $e^{-}$, a positron has a positive charge, represented as $e^{+}$. Both particles have the same mass but opposite charges. Positrons are involved in processes like positron emission, a type of radioactive decay, whereas electrons are commonly found in atomic orbitals.

What is an example of positron emission in nuclear reactions?

An example of positron emission in nuclear reactions is the decay of Einsteinium-253. When Einsteinium-253 (atomic number 99) undergoes positron emission, it emits a positron and transforms into Californium-253 (atomic number 98). The atomic mass remains the same at 253, but the atomic number decreases by one due to the conversion of a proton into a neutron. The reaction can be represented as: ${\mathrm{^\{253\}}}^{\mathrm{Es}}\to {\mathrm{^\{253\}}}^{\mathrm{Cf}}+e^{+}$.

How is positron emission used in medical applications?

Positron emission is used in medical applications such as Positron Emission Tomography (PET). PET scans are imaging tests that help doctors observe metabolic processes in the body. A radioactive tracer that emits positrons is introduced into the body. When these positrons encounter electrons, they annihilate, producing gamma rays. These gamma rays are detected by the PET scanner, creating detailed images of organs and tissues. PET scans are particularly useful in detecting cancer, monitoring heart conditions, and studying brain function.

What changes occur in the nucleus during positron emission?

During positron emission, a proton in the nucleus is converted into a neutron. This process releases a positron and a neutrino. The emitted positron is represented as $e^{+}$. As a result, the atomic number of the element decreases by one, while the atomic mass remains unchanged. For example, when Einsteinium-253 (atomic number 99) undergoes positron emission, it transforms into Californium-253 (atomic number 98).