Deuteron Photodisintegration

Interaction Codes
Code	Description
t*1000000 + 310000	Photodisintegration of target t

A photon of energy greater than $2.226$MeV may photodisintegrate a deuteron when travelling through heavy water. Some code has been added to the SNOMAN program to account for this physical interaction. Because photons are propagated through the detector by the EGS4 code, this has meant a modification has been made to the EGS4-SNOMAN interface. Because the cross section for photodisintegration of the deuteron is so small, the modification made is an approximation. In HOWFAR, EGS4 passes the next step length of the current particle to a user written routine that specifies the detector geometry. This step length is compared to the length $x$, defined below, and if x is shorter, a disintegration after a step of length x is triggered.

$$x = -{1 \over n\sigma}\ln(r)$$ (13.10)

where $n$ is the number density of deuterons in heavy water, $\sigma$ is the cross section for photodisintegration as a function of energy, and $r$ is a uniform random deviate between $0$ and $1$. The approximation made in this method is to leave the total cross section for a photon interaction unmodified, while adding a test for an additional interaction. In the limit that the cross section for the additional interaction is small, the approximation is valid. Since about $1$ in $500$ photons disintegrate a deuteron, we are operating in the right limit.

Once a photodisintegration is triggered, the photon is propagated the distance to the interaction point, and an interaction vertex is placed in the data structure. When this vertex is acted upon, a centre of momentum scattering angle is calculated for the neutron relative to the incoming photon, along with neutron energy, and these are transformed into the lab frame. The differential cross section is modelled on the expression;

$${d\sigma \over d\Omega } = \sum^{4}_{n=1} A_{n}.P_{n}(\cos\theta)$$ (13.11)

where $A_{n}$ is the coefficient of the $n$th Legendre polynomial $P_{n}$. The $A_{n}$ are fitted parameters based on data available in the literature, and in particular, on a phenomenological paper by Rossi et al. (Phys. Rev. 1989).