Re: Would several slim blocks of concrete block gamma radiation?

The two situations you propose are largely identical.  Their ability to
absorb gamma rays will depend strongly on the energy of the gamma rays.  A
12 cm thick wall will certainly strongly absorb gammas with energies of
10's of kilo electron Volts (keV), but far more weakly attenuate gamma rays
with energies of a few mega electron Volts (MeV).  A detailed simulation
will turn up minor differences based on geometry alone for various energies
of gamma radiation, but generally gamma-ray transmission through a material
is given by the exponential decay T = e(-u x).  In this equation, T is the
fraction tranmitted through a distance x in the material, e is the natural
number (2.71828...), and u is a constant that depends on the gamma energy
and material.  

To find u in the decay equation, consult the National Institute of
Standards and Technology (NIST) website 
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html
for a particular
material.  To get the actual value, use their value in (cm^2)/gram and
multiply by the density of the material (see wikipedia for a particular
material, they list many material properties) in g/cm^3, that gives you u
in units of 1/cm.  That means that when you multiply by x in centimeters
you get a pure number which can be an exponent of e.  That will help you
compute T, the transmitted fraction, which is just the number (call it N)
that pass through a particular material for a particular number (call it
N_0) that hit the material.  It turns out that by the properties of
multiplying exponents, the number transmitted through 12 individual 1 cm
thick slabs will be the same as the number transmitted through a single 12
cm thick slab.  You can try it, a typical 1 MeV gamma ray has an
attenuation coefficient of 0.06 cm^/g, and a density of 2.3 g/cm^3, giving
a coefficient of .138/cm.  Taking 1 individual slab gives you a
transmission of 87% (0.87), 2 slabs is .87*.87 = .759, 12 of them gives you
T = 0.191 or 19% transmission.  A single 12-cm slab gives e^-1.656 = 0.191
as well.