Analyzing the M+2 peak in thes mass spec

76 % of Cl is Cl-35, and 24% is Cl-37. Thus, when a compound contains Cl, it will have a M+2 peak in the mass spectrum with an intensity that is 1/3 or 24/76 of the intensity of the molecular ion (M+) peak.
51% of Br is Br-79, and 49% is Br 81, so when a compound contains Br, it will have an M+2 peak that is equally intense to the M+ peak.