This is very similar to how inflation works in reality.
The central bank has what you call the "source coin" -- the authority to issue new money. They can choose the rate at which they issue money, including issuing no money or even taking some out of circulation (effectively issuing negative money).
Although real-world money does not experience radioactive decay, its value diminishes over time due to inflation. If you stuffed 100 U.S. Dollars under your mattress in 1975, you would still have 100 U.S. Dollars today -- but you'd be able to buy between four and seven times less stuff with it, because things are four to seven times more expensive now. (Very rough estimate for the purposes of illustration.) Even though the physical bill hasn't changed, the purchasing power of the dollar has diminished. All dollars, everywhere -- how egalitarian! So this is very similar to your proposal, as if the dollars were radioactive and some fraction of them had decayed.
Printing money is an important part of setting monetary policy and managing the rate of inflation, but it cannot be the only part. The central bank can control how much money is in circulation, but they don't directly control what it's worth to people. This can be a problem if the government needs a lot of money, say for some infrastructure project. (Your libertarian colleagues would say that this shouldn't happen because the government shouldn't need a lot of money.) Assuming they decide to get this money by issuing new money instead of increasing taxes or borrowing it, then that will naturally devalue the existing money as the supply increases. In particular, it devalues the money they just printed, so if the amount printed was too great then they can no longer afford the project they issued it to pay for! So they issue more money to fix the problem... and I think you can see where this is going. Compounding the effect, people will see that their money is devaluing fast, and will sell it to buy a more stable currency or commodity (e.g. gold). So demand for this currency will drop as the supply massively increases. The process is called hyperinflation, and it effectively wipes out the value of the currency entirely.
So that's the most likely consequence.
Another point: If the government doesn't collect taxes at all, there's little to no reason for people to keep using this radioactive money. They would be able to sell it and do all their transactions in gold or Euros or bitcoins or goats or what-have-you, in some form that's better because it won't randomly decay. In reality, even if you get paid in goats you pay your taxes in U.S. dollars, so as an American citizen you legally have to interact with the dollar at some point (or go to prison I guess) and the grocery shop has an incentive to accept the dollars you're buying groceries with.
One of the comments asks why this would cause inflation, since the total amount of money in existence remains constant.
Consider redenomination: changing the face value of a currency in circulation. For example, in 1922 Germany was going through a period of hyperinflation, and redenominated their currency from the Papiermark to the Rentenmark. One Rentenmark was worth 1000000000000 Papiermarks. During the changeover period, you could use both interchangeably at this rate, but no new Papiermarks were printed and the existing ones could be exchanged at the bank.
Redenomination doesn't change the money supply, it doesn't create new value or destroy any, nor make anyone rich or poor who wasn't already. It can be of great symbolic value, but the practical effect is that people won't lose count of the number of zeroes in the price of bread.
Radioactive money is basically a continuous process of redenomination. A wad of a thousand radiodollars today will be redenominated in a half-life into a wad of five hundred radiodollars. The actual decay process is purely psychological. If the decayed coins were destroyed entirely, this would be a pure redenomination and wouldn't have any monetary impact. (I don't know what would be the psychological impact of knowing that for every radiodollar in your bank account now there would have been 1024 radiodollars ten half-lives ago.)
But the decayed coins aren't being destroyed, they're going to the central bank.
So that's why I think this is exactly equivalent to printing money. If the central bank decided to have 10% of all coins decay, this would be like redenominating the currency to have 10% less value (basically meaningless)... and then printing a large sum of money, which is where the real monetary impact comes from.