Main source :
https://www.cloudylabs.fr/wp/wp-content/uploads/2020/08/Delta-ray-Production-in-MCNP.pdf
other alternatives :
http://www.cloudylabs.fr/wp/wp-content/uploads/2019/09/about-deltaray-curtis.pdf
http://www.cloudylabs.fr/wp/wp-content/uploads/2014/03/Simulation-of-charge-deposition-IPHE.pdf
« number of delta rays » is the same thing as « probability of collision » φcol(T, T’)
http://www.cloudylabs.fr/wp/wp-content/uploads/2015/07/katz-delta.pdf