1. The problem statement, all variables and given/known data How much energy does an alpha particle need to penetrate the minimal protective epidermal layer of skin (Thickness ~7 mg cm-2 2. Relevant equations Possibly R=0.56E, E,4 OR R=1.24E-2.62, 4<E<8 3. The attempt at a solution I used the equation R=0.56E so, I said that: 7 mg/cm^2=0.56E then divided by 0.56 to get E=12.mEV I guess I'm jsut not sure if I used to correct equation and am confused if the units match up. Thanks for your help!