 Calculation of volume change (Linear Elasticity)

Hello
I want to find the volume change in a linear elasticity problem. Simply I am using FEniCS demo for linear elasticity. I am trying to obtain:
det (F) = V_f / V_0 where F is deformation gradient tensor, V_f is the final volume and V_0 is the initial volume. Here is the minimal work example:

from fenics import *

# Scaled variables
L = 1; W = 0.2
mu = 1
rho = 1
delta = W/L
gamma = 0.4*delta**2
beta = 1.25
lambda_ = beta
g = gamma

# Create mesh and define function space
mesh = BoxMesh(Point(0, 0, 0), Point(L, W, W), 10, 3, 3)
V = VectorFunctionSpace(mesh, 'P', 2)

# Define boundary condition
tol = 1E-14

def clamped_boundary(x, on_boundary):
return on_boundary and x < tol

bc = DirichletBC(V, Constant((0, 0, 0)), clamped_boundary)

# Define strain and stress

def epsilon(u):

def sigma(u):
return lambda_*div(u)*Identity(d) + 2*mu*epsilon(u)

# Define variational problem
u = TrialFunction(V)
d = u.geometric_dimension()  # space dimension
v = TestFunction(V)
f = Constant((0, 0, -rho*g))
T = Constant((0, 0, 0))
a = inner(sigma(u), epsilon(v))*dx
L = dot(f, v)*dx + dot(T, v)*ds

#############################
# Kinematics
I = Identity(3)    # Identity tensor
C = F.T*F          # Right Cauchy-Green tensor

# Jocobian : Determinant of F
J  = det(F)
##############################
# Compute solution
u = Function(V)
solve(a == L, u, bc)
##############################

space = FunctionSpace(mesh, 'P', 1)
result = interpolate(J,space)
print (result)

It return this error:

AttributeError: 'Determinant' object has no attribute '_cpp_object'

In general my goal is to find the volume change due to applying a load. Any idea how it can be done?
Thanks

Use a projection, as explained by @kamensky in: Project(x) works, interpolate(x) does not - #2 by kamensky