A Kerma from neutron irradiation

[Ozymandiac]

TL;DR

Does the kinetic energy of an atom with net zero charge count towards kerma?

Hi all,

I have a question regarding kerma from a neutron source. I will be giving a lecture about this topic (and others), and was creating a figure to explain which interacties / energies contribute to kerma and which do not.

The definition of Kerma I use is taken from ICRU report 57:

In the figure (see below), a reference volume with mass dm is irradiated by neutrons. From top to bottom, there are four interactions of neutrons with nuclei. Kinetic energies are denoted with T. Secondary ionisation is shows as small black dots. The aim of the figure is to determine which energies are counted towards the calculation of kerma (and which aren't). My initial thought was that kerma would be: K = (T1+T2+T4+T6) / dm.

My questions are all very similar, because I am unsure how I have to interpret the word "charged" from the kerma definition.

• In the first interaction (counted from the top) I show an (n,p) reaction. Does the kinetic energy from the nucleus (T2) count towards kerma? Does the moving atom even ionize surrounding atoms? I suppose the atom has charge -1 for a split second (since a proton was scattered away), but that extra electron will be quickly dumped, right? After which the atom has 0 charge.
• In the second interaction I show neutron capture (n,gamma). The atom recoils with kinetic energy T4. Is T4 counted towards kerma? My initial response was yes, but since the atom is neutral it does not satisfy the above definition of kerma.
• In the third interaction I show elastic scattering (n, n'). As before: does T6 count towards kerma, since the atom has neutral charge?

I would be grateful for any help and discussion.

 

[Astronuc]

TL;DR: Does the kinetic energy of an atom with net zero charge count towards kerma?

The definition of Kerma I use is taken from ICRU report 57:

Kerma is an acronym for Kinetic Energy Released per unit MAss. It is a non-stochastic
quantity applicable to indirectly ionizing radiations, such as photons and neutrons. It quantifies
the average amount of energy transferred from the indirectly ionizing radiation to directly
ionizing radiation without concerns to what happens after this transfer. In the discussion that
follows we will limit ourselves to photons.

Ref: https://recherche-expertise.asnr.fr...s_sante/documentation/syllabus_chapitre_2.pdf

In the first interaction (counted from the top) I show an (n,p) reaction.

The nuclear recoil T₂ counts to the kerma. The kinetic energy of the incident neutron is transferred to the nucleus and proton. Now part of T₁ is impart to the material (dm) until the proton leaves the dm, so one has to account for the portion of the kinetic energy transferred from the proton. The remaining energy of the proton would be imparted to the media outside dm.

• In the second interaction I show neutron capture (n,gamma). The atom recoils with kinetic energy T4. Is T4 counted towards kerma? My initial response was yes, but since the atom is neutral it does not satisfy the above definition of kerma.
• In the third interaction I show elastic scattering (n, n'). As before: does T6 count towards kerma, since the atom has neutral charge?

T₄ and T₆ count towards kerma. Based on the diagram, the photon and neutron (n') do not interact with the media of dm, so hν₃ and T₅ are not transferred to the media in dm.

The (n,α) reaction occurs outside dm, so the nuclear recoil, T₈ would not contribute to kerma in dm, but some of the alpha particle kinetic energy of the alpha particle is transmitted in the media of dm, so one could have to consider the contribution of the appropriate fraction of T₇ lost in dm. Since, there is an interacting medium outside dm, as evidence by the (n,α) reaction, one would have to consider energy of particles energy dm due to external reactions.

One would divide a target into many dV or dm, where dm = ρ dV. The finer the incremental volume the better the resolution of gradients, but one has to count more incident particle histories (usually with a Monte Carlo code).