Your common all garden improvised terrorist thermonuclear device
Sorry, do what?
I’ve never heard of an improvised terrorist before.
What is it and why would they need the whole of a garden? Are they really common?
Serious answer,
A small, as in smallest possible, nuke, there was a survivor of Hiroshima somewhat incredibly just 300m from the centre of the blast, protected by the massive concrete bank building, but mostly anything within a couple of km is dead, at 20 miles you’d almost certainly survive but would want to leave the area for a while.
The supersize Russian efforts, you’d be instant charcoal.
Fortunately, they’ve been dismantled, I believe, mainly because it’s easy to fly a small one into “the right place” now that guidance systems are what they are. And small ones are cheaper.
Also, nobody improvises a nuclear weapon. It’s not like you just knock one up from stuff lying about the place like MacGyver.
https://en.wikipedia.org/wiki/Effects_of_nuclear_explosions