Goi goc giua hat He va proton la $\alpha$ thi
Theo dinh ly ham sin co
$\frac{m_{p}.v_{p}}{sin(180-2\alpha )}=\frac{m_{He}.v_{He}}{sin\alpha }$
$<=>\frac{sin^{2}\alpha }{sin^{2}2\alpha }=\frac{m_{He}.K_{He}}{m_{p}.K_{p}}$
Lai theo bao toan nang luong co
$K_{p}=2K_{He}$
Theo do $ \frac{sin^{2}\alpha }{sin^{2}2\alpha }=\frac{1}{2}.4=2$
Tu do tinh dk $\alpha =69,29^{o}$
Vay goc can tim la $2\alpha =138,59^{o}$