PKA

PKA may refer to:

Acid dissociation constant

An acid dissociation constant, K_a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

where HA is a generic acid that dissociates into A⁻, known as the conjugate base of the acid and a hydrogen ion which combines with a water molecule to make an hydronium ion. In the example shown in the figure, HA represents acetic acid, and A⁻ represents the acetate ion, the conjugate base.

The chemical species HA, A⁻ and H₃O⁺ are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A⁻] and [H₃O⁺]

In all but the most concentrated aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply

PKA (irradiation)

A Primary Knock-on Atom or PKA is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy, or come to rest in the lattice at an interstitial site if it does not.

Most of the displaced atoms resulting from electron irradiation and some other types of irradiation are PKAs, since these are usually below the threshold displacement energy and do not have sufficient energy to displace more atoms. In other cases like fast neutron irradiation, most of the displacements result from higher energy PKAs colliding with other atoms as they slow down to rest.

Collision Models

Atoms can only be displaced if, upon bombardment, the energy they receive exceeds a threshold energy E_d. Likewise, when a moving atom collides with a stationary atom, both atoms will have energy greater than E_d after the collision only if the original moving atom had an energy exceeding 2E_d. Thus, only PKAs with an energy greater than 2E_d can continue to displace more atoms and increase the total number of displaced atoms. In cases where the PKA does have sufficient energy to displace further atoms, the same truth holds for any subsequently displaced atom.