P{{o}^{210}}{{\to }_{82}}{{X}^{206}}{{+}_{2}}H{{E}^{4}}\] Using conservation of linear moments \[206\,{v}'+4v=0\RIGHTARROW {v}'=-\FRAC{4v}{(206)}\]\[\Rightarrow \,|{v}'|\,=\frac{4v}{206}\]