i wouldnt worry too much about americium 241 (im assuming that's what you'll be using becuase of its availibility) because if you got enough of it to make a critical mass, putting you hand within the range of the alpha radiation would instantly give you a radiation burn. If you some how do acuire enough radioactive material ("hello, chemical supply store? i need about 50 kg of plutonium, preferably of reagent grade") to pose a neutron hazard, i recommend alternating peices of you material with peices of cadmium.
Below (or perhaps above, i dont know where this attachment is ending up) is my plan for a nuclear battery. First, the Americium emits alpha radiation, which then hits the copper-activated Zinc sulphide, emitting a glow, just like in radium watches. Because the Americium salt and the Zinc salt are in a mixture, almost all of the alpha radiation emitted by the Am hits the ZnS, making visible light. The visible light then hits a sillycone (i like spelling it that way) solar panel, making energy. The quarts is there so that visible light from the Am and ZnS hits the sillycone, but not alpha. This is a design (but they used a more sane alpha emitter) that is used by some for small electronics that barely use energy, such as a watch, or a pacemaker, or something like that. And also, they didnt use silicone solar panels, they simply used a sheet of silicone with a transparant conductor sandwiched between it and the quartz.
By the way, on the subject of solar panels, a company is now developing a new solar panel that not only absorbs infrared, red, yellow, orange, and green, as silicone panels do (did you notice that they are blue? its becuase they reflect blue, these ones are black), but also some ultraviolet, making lots and lots of electricity. It is also cheap and easy to make, and can even be painted on to a surface.
EDIT: Turns out it's below.