Radiation HW on Alpha Particles

[cconradi]

Homework Statement

How much energy does an alpha particle need to penetrate the minimal protective epidermal layer of skin (Thickness ~7 mg cm^-2

Homework Equations

Possibly R=0.56E, E,4
OR R=1.24E-2.62, 4<E<8

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 just not sure if I used to correct equation and am confused if the units match up.

Thanks for your help!

 

[cconradi]

I think it may be correct if I use to other formula actually. 7 mg/cm^2=1.24E-2.62
Do the math and it equals 7.8 mEv. Sound like a better answer?
Thanks!