The proton number of thorium is 90 and the proton number of radium is 88.
Which series of decays turns a thorium nucleus into a radium nucleus?
β−+α+β−+α\beta^- + \alpha + \beta^- + \alphaβ−+α+β−+α
α+α+α+β−\alpha + \alpha + \alpha + \beta^-α+α+α+β−
α+β−+β−+β−\alpha + \beta^- + \beta^- + \beta^-α+β−+β−+β−
α+α+α+β−+β−\alpha + \alpha + \alpha + \beta^- + \beta^-α+α+α+β−+β−
