Most of the danger is from alpha decay, the gamma are lower energy but travel further.
Your question is really two parts. The danger of gamma rays comes from their intensity - how much radioactive stuff there is and how much gamma it emits - therefore how many gamma photons you are going to receive. Gamma radiation at any energy is harmful so there isn't a huge health difference with energy.
How easy it is to stop does depend on the energy
see http://www.evs.anl.gov/pub/doc/Radium.pdf [Broken]
The danger from any alpha particle emitter is only if you get it inside the body. They are harmless external to the body. Gamma's are best attenuated by high Z materials, say depleted U or Pb. The amount of gamma's given off by Ra depends on the amount of Ra and it's half life (1600 yrs).
Calculate number of atoms you have, plug into equation on this page. You'd need to figure out whether the gamma's come along with the alpha's, or some competing decay path. Also consider the gamma's, etc. of any daughter products with short half lives....