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## Homework Statement

A beam of particles strikes a wall containing 2 × 10^29 atoms/m^3. Each atom behaves as a hard sphere of radius 3 × 10^–15 m.

Find the thickness of the wall such that exactly half the incident particles go through without scattering.

## Homework Equations

N(sc) = N(inc) × n(tar) × σ ---> (1)

N(sc) = the number of scattered particles

N(inc) = the number of incident particles

n(tar) = the target density

σ = cross sectional area of the target = πR^2

n(tar) = ρt/m ---> (2)

ρ = density

t = thickness of the wall

m = mass

## The Attempt at a Solution

Hello everyone; this is a straightforward scattering problem but I seem to be missing something here. This is what I did so far:

From the given information I have:

N(sc) = ½ N(inc) ---> 2N(sc) = N(inc)

R = 3 × 10^–15 m ---> σ = πR^2 = 9π × 10^–30 m^2

Plugging these values in equation (1), I can get the value of n(tar) which turned out to be 1.77 × 10^28 m^–2

Having this value of n(tar), I can substitute into equation (2) and solve for t. This is where I’m stuck, as I’m supposed to find numerical values for ρ and m in order for me to find t. These values can be obtained from the given information that the wall contains 2 × 10^29 atoms/m^3, and I think that I can get my values for ρ and m from the following equation:

The number of atoms per unit volume = (Avogadro’s number × density)/atomic mass

That is: 2 × 10^29 atoms/m^3 = (6.022 × 10^23 mol^–1 × density)/atomic mass

This is where I’m not sure how to proceed, as I need to find the density and atomic mass (and convert it to kg as well) from this equation; any help would be appreciated.