Gamma Emission and Nuclear Energy Levels

Gamma emission as nuclear de-excitation

Gamma (γ) emission happens when a nucleus is left in an excited (higher-energy) state and then releases that excess energy as a high-energy photon. The key idea is that γ emission changes the energy of the nucleus but not its composition: the mass number and atomic number stay the same.

• What changes: nuclear energy state (excited → lower energy).
• What does not change: atomic number Z and mass number A.

You can think of γ emission as the nucleus “cooling down” after being formed in an energetic configuration.

Gamma rays vs X-rays: same type of radiation, different origin

Gamma rays and X-rays are both photons (electromagnetic radiation). The practical distinction is based on where the photon comes from:

• Gamma ray: emitted by the nucleus during nuclear de-excitation.
• X-ray: emitted by the electron cloud, typically when electrons rearrange between atomic shells (for example after an inner-shell vacancy is created).

Because both are photons, their energies can overlap; origin is the reliable way to distinguish them in nuclear/atomic descriptions.

Nuclear energy levels and excited states

Nuclei have discrete energy levels, analogous in spirit to electron energy levels in atoms, but governed by nuclear structure. An excited nucleus is often written with an asterisk: ^A_ZX*. When it drops to a lower level, it can emit one or more γ photons whose energies match the differences between levels.

Continue in our app.

• Listen to the audio with the screen off.
• Earn a certificate upon completion.
• Over 5000 courses for you to explore!

Or continue reading below...

Download the app

Metastable states (nuclear isomers)

Some excited states live unusually long before emitting γ radiation. These are metastable states (also called isomers) and are written with an m, such as 99mTc. The longer lifetime occurs because the transition to a lower level is “hindered” (for example by angular momentum or parity change requirements), making γ emission less probable per unit time.

Why gamma often accompanies alpha or beta decay

After an alpha or beta decay, the daughter nucleus is frequently produced in an excited state. That happens because the decay changes the nuclear configuration suddenly, and the daughter is not guaranteed to land directly in its ground (lowest-energy) state. The daughter then de-excites by emitting γ radiation (sometimes in a cascade of multiple γ rays).

Conceptually, treat many decays as a two-step process:

• Step 1: α or β decay changes Z and/or A and creates a daughter nucleus, often excited.
• Step 2: the excited daughter emits γ to reach a lower energy level (often the ground state).

Practice: writing decay schemes with excited daughters and gamma emission

In decay schemes, it is useful to explicitly show the excited daughter and the subsequent γ emission. The core bookkeeping rules are:

• For γ emission: A and Z do not change.
• The asterisk * (or m) indicates the nucleus is not in its ground state.
• Energy released in γ emission is the level spacing between the initial and final nuclear states.

Step-by-step method

1. Write the primary decay (α or β) using correct changes in A and Z, and mark the daughter as excited if the problem states it or if you are constructing a scheme that includes γ emission.

2. Write the de-excitation as a separate line: excited daughter → same nuclide (no change in A, Z) + γ.

3. Check conservation: A and Z must balance on each line; γ carries no A or Z.

4. (Optional) Add γ energies if level spacings are given, labeling each γ photon with its energy.

Worked example 1: beta decay followed by gamma

Suppose a nucleus undergoes β⁻ decay to an excited daughter, then emits γ:

^A_ZX → ^A_{Z+1}Y* + β− + ν̄e

Then the daughter de-excites:

^A_{Z+1}Y* → ^A_{Z+1}Y + γ

Notice: the β⁻ step changes Z by +1; the γ step changes neither A nor Z.

Worked example 2: alpha decay followed by gamma

Alpha decay reduces A by 4 and Z by 2. If the daughter is excited:

^A_ZX → ^{A-4}_{Z-2}Y* + ^4_2He

Then:

^{A-4}_{Z-2}Y* → ^{A-4}_{Z-2}Y + γ

Practice set (fill in the blanks)

1) β⁻ to an excited daughter

Complete the scheme:

^137_55Cs → ________ + β− + ν̄e

________ → ________ + γ

Hint: In β⁻, A stays the same and Z increases by 1. Mark the first daughter as excited with *.

2) α to an excited daughter

Complete the scheme:

^241_95Am → ________ + ^4_2He

________ → ________ + γ

Hint: In α decay, A decreases by 4 and Z decreases by 2.

3) Isomeric transition (metastable state)

Write the γ de-excitation:

99mTc → ________ + γ

Hint: Same A and Z; only the energy state changes.

Adding gamma energies (optional extension)

If a level diagram indicates that an excited state is 1.33 MeV above the ground state, you can annotate:

Y* → Y + γ (1.33 MeV)

If there are multiple steps (a cascade), list each γ with its own energy corresponding to each drop between levels.

Gamma interaction tendencies: penetration and shielding (conceptual)

Because γ rays are photons with no charge and no rest mass, they tend to be highly penetrating compared with charged particles. They do not steadily lose energy by Coulomb interactions the way charged particles do; instead, they are more likely to travel some distance and then interact in a relatively discrete event.

Common interaction modes (qualitative)

• Photoelectric absorption: the γ photon is absorbed and an electron is ejected. More likely at lower γ energies and in higher-Z materials.
• Compton scattering: the γ photon scatters off an electron, losing some energy and changing direction. Often important at intermediate energies.
• Pair production: at sufficiently high γ energy (above 1.022 MeV), the photon can convert into an electron–positron pair near a nucleus.

These processes explain why γ rays can pass through significant thicknesses of material and why shielding design focuses on materials and thickness that increase the probability of interaction.

Shielding tendency: why dense materials help

In general terms, γ shielding is more effective with dense, high-Z materials (such as lead) because they provide more electrons and stronger nuclear electric fields per unit thickness, increasing the chance of photoelectric absorption and other interactions. Thicker layers increase the probability that a γ photon will interact before emerging.

A useful mental model is attenuation: as γ rays pass through matter, intensity decreases approximately exponentially with thickness:

I = I0 e^(−μx)

where I0 is the initial intensity, x is thickness, and μ is an attenuation coefficient that depends on photon energy and material. This is not about stopping every photon with a single collision; it is about reducing intensity by making interactions more likely.

Exposure reduction ideas: distance and shielding (conceptual, not procedural)

Two broad concepts help explain why γ exposure can be reduced without changing the source:

• Distance: for a point-like source, intensity decreases with the square of distance (inverse-square behavior). Doubling distance reduces intensity to about one quarter.
• Shielding: placing material between source and observer attenuates the γ beam by increasing interaction probability, reducing transmitted intensity.

These ideas connect directly to the physics of photon propagation and interaction: geometric spreading with distance and probabilistic attenuation in matter.