PointSource should add values vector values, not set

[Garth Wells]

I would like to project a particle cloud represented as a sum of dirac masses on a grid using PointSource. I have tried the following code but it gives me a set of points on the mesh instead of a nice Gaussian shaped function, any ideas why?
Perhaps I'm not using the PointSource correctly?

Cheers
/J

from dolfin import *
import random

mesh = Rectangle(-10,-10, 10,10,50,50)
V = FunctionSpace(mesh, 'CG', 1)
u = TrialFunction(V)
v = TestFunction(V)
a = inner(u,v)*dx
A=assemble(a)
L= v*dx
b = assemble(L)
n = 100000
for i in range(n):
    p= Point(random.normalvariate(0,1), random.normalvariate(0,1))
    delta = PointSource(V, p, 1)
    delta.apply(b)

u = Function(V)
solve(A, u.vector(), b)