Activation

Material Activation: A Concern for Health Physicists

Material activation is a significant concern in radiation protection and a key focus for health physicists. It occurs when a stable material is exposed to radiation, causing some of its atoms to undergo nuclear transitions (typically through the absorption of a neutron, proton, among many others) and become a radioactive isotopes. These activated materials then emit their own radiation, posing potential hazards to workers and the environment.  

Why is this a concern for health physicists?

• Unexpected Radiation Sources: Activated materials can become sources of radiation in areas where it's not anticipated, leading to unexpected exposures if not properly monitored and controlled.
• Variety of Radiation Types: Activated materials can emit various types of radiation, including gamma rays, beta particles, and alpha particles, each with different penetration abilities and health effects.
• Waste Management Challenges: Activated materials become radioactive waste that needs to be carefully handled, stored, and disposed of according to regulations to minimize environmental and public health risks.
• Decommissioning Complexities: Activation products can significantly complicate the decommissioning of nuclear facilities, requiring specialized techniques and extending the time required for safe dismantling.
• Understanding the Nuclear Physics Behind Activation

Activation Equation

The process of activation can be understood through the following simplified nuclear equation:

$A=\sigma *\phi *N*(1-e^{(-\lambda *t_1)})*e^{(-\lambda *t_2)}$

Particle Energy: The energy of the incident radiation influences the probability of activation. Certain isotopes have higher probabilities of activation at specific energies. This energy directly affects the cross-section.
Activation Cross-Section ( $\sigma [barn]$ ): An inherent property of the target material that describes the probability of a nucleus interacting with a neutron. The unit of $1barn$ is equal to $1E-24c{m}^2$.

Particle Fluence ( $\phi [particles\bullet se{c}^{-1}$ ): The intensity of the incident radiation field. Higher flux leads to faster activation.

Target Atoms ( $N[atoms]$ ): The number of atoms onto which the particle flux is incident.

Decay Constant ( $\lambda [se{c}^{-1}]$ ) : This is the decay constant of the activation product which will determin how long it will take to approach saturation activity and how long the activation product will remain radioactive. $\lambda =ln(2)/T_{1/2}$

Irradiation Time ( $t_1[sec]$ ) : The time during which the material is irradiated by the particle fluence above. The longer the material is exposed to fluence, the more activation will occur. The activation will approach a saturated activity after a $t_1$ equal to 5x the half life.

Time since Irradiation ( $t_2[sec]$ ) : The value for $t_2$ is the time since the end of irradiation. This term is simply a radioactive decay term which accounts for the natural decay of the activation product.

Assumptions

• The particle flux occurs over an area equal to the cross sectional area of the target N.
• The target is sufficiently thin to not cause attenuation, reduction of the energy of the incident particles, which would affect the cross section.
• There is a sufficient number of target atoms such that the reaction rate does not decrease with their depletion (transmutation into another isotope)

Deeper Dive

While this topic is typicall called neutron absorption or neutron activation, there is a long list of nuclear reactions which can result in the activation of the target isotope. Linked here is a list of many possible nuclear reactions for which there may be Evaluated Nuclear Data Files (ENDF)s which will provide cross section values as a function of the nuclear reaction, target, and incident radiation energy.

There is a limit to the maximum amount of product material which may be produced, called the saturation activity. The saturation activity for the general equation occurs when time approaches infinity or half life approaches zero. This elimitates the exponential term and reduces to $A=\sigma *\phi *N$. A general rule of thumb is that saturation occurs after 5 half lives.

The following presentation from the NRC provides a good overview of neutron activation and activation analysis.

• Neutron Activation and Activation Analysis

Below are some applicable and possibly useful Wikipedia pages

• Wikipedia page on Neutron Activation
• Wikipedia page on Neutron Capture
• Wikipedia page on Proton Capture

Neutron Activation Example

To exercise the knowledge from this section we can evaluate the production of${}^{16}N$ from the neutron absorption of${}^{16}O$. Assume that there is a fluence of 2.7E15 neutrons/sec and they are all thermalized.

${}^{16}O+{}^{1}n⟶{}^{16}N$

Replicating the equation above we can step through the variables $A=\sigma *\phi *N*(1-e^{(-\lambda *t_1)})*e^{(-\lambda *t_2)}$

Particle Energy: The neutrons are thermalized, which at STP means 0.025 eV, or 2.5E-8 MeV.
Activation Cross-Section ( $\sigma [barn]$ ): Using the IAEA ENDF database

An inherent property of the target material that describes the probability of a nucleus interacting with a neutron. The unit of $1barn$ is equal to $1E-24c{m}^2$.

Particle Fluence ( $\phi [particles\bullet se{c}^{-1}$ ): The intensity of the incident radiation field. Higher flux leads to faster activation.

Target Atoms ( $N[atoms]$ ): The number of atoms onto which the particle flux is incident.

Decay Constant ( $\lambda [se{c}^{-1}]$ ) : This is the decay constant of the activation product which will determin how long it will take to approach saturation activity and how long the activation product will remain radioactive. $\lambda =ln(2)/T_{1/2}$

Irradiation Time ( $t_1[sec]$ ) : The time during which the material is irradiated by the particle fluence above. The longer the material is exposed to fluence, the more activation will occur. The activation will approach a saturated activity after a $t_1$ equal to 5x the half life.

Time since Irradiation ( $t_2[sec]$ ) : The value for $t_2$ is the time since the end of irradiation. This term is simply a radioactive decay term which accounts for the natural decay of the activation product.

Future Additions

This section provides a basic introduction to material activation. Further exploration could include:

• Specific examples of activation products in different industries (e.g., medical, industrial, research)
• Detailed discussion of activation cross-sections and their importance
• Methods for measuring and monitoring activation levels
• Strategies for minimizing and managing activation in various settings
• Properly format the nuclear equations in the examples with simultaneous sup/sub scripts