This allows you to transport the Co-60 taking advantage of square-cube law scaling. In summary its a zillion times easier to keep a million Co-60 ball bearings cool and frozen than one big sphere because of surface area.
If you're really bored you can do the thermodynamics calculations for how small a pellet has to be to remain cool enough to touch safely (well, other than the radioactivity) in still room temp air, etc. Or if you're willing to clamp the pellet to a 10 C/W transistor heatsink (that heatsink is about the size of a postage stamp, is if you figure on a hot day your skin can tolerate about 10C temp rise without getting a burn, than that means your pellet has to dump less than a watt or so, of course there's no rule you couldn't use a much better larger heatsink, or a modest cooling fan, or drop it in a barrel of water, etc)
The biggest problem you run into with this kind of stuff is thermal shock from expansion. You might be well advised to find some low coeff of expansion cobalt alloy rather than using the straight up stuff. A straight up sphere, would likely shatter if big enough and the exterior is frozen and the inside is very hot.
This allows you to transport the Co-60 taking advantage of square-cube law scaling. In summary its a zillion times easier to keep a million Co-60 ball bearings cool and frozen than one big sphere because of surface area.
If you're really bored you can do the thermodynamics calculations for how small a pellet has to be to remain cool enough to touch safely (well, other than the radioactivity) in still room temp air, etc. Or if you're willing to clamp the pellet to a 10 C/W transistor heatsink (that heatsink is about the size of a postage stamp, is if you figure on a hot day your skin can tolerate about 10C temp rise without getting a burn, than that means your pellet has to dump less than a watt or so, of course there's no rule you couldn't use a much better larger heatsink, or a modest cooling fan, or drop it in a barrel of water, etc)
The biggest problem you run into with this kind of stuff is thermal shock from expansion. You might be well advised to find some low coeff of expansion cobalt alloy rather than using the straight up stuff. A straight up sphere, would likely shatter if big enough and the exterior is frozen and the inside is very hot.