In a scattering experiment, a beam of X-rays scatters off a copper target.
1000 scattered X-rays are detected and the angle that each scattered ray 
makes to the beam is recorded (in units of degrees).  The file:
scatter.dat
contains this data in one column of 1000 rows.  The file:
scatterW.dat
contains exactly the same data in ten columns of 100 rows.  The file:
scatter.pdf
contains exactly the same data in a format that prints on one page.

The data have been sorted from 180 deg (backscatter), to 0 deg
(small angle scattering), but of course, the scattered rays
were detected in a seemingly random order.

Calculate the differential cross section at 10 different angles,
and plot the results.
Note that the solid angle between angles theta1 and theta2 is:
2 pi ( cos(theta1)-cos(theta2) )

The copper target has nt=1.5 x 10^(27)/m^2 target particles.
10000 X-rays were incident in the beam to produce the 1000
scattered X-rays.

Obviously target cross sections are very small.  The usual
unit for this small area is:
1 barn= 10^(-28) m^2
Use it!

Unit for solid angle is steradian (denoted sr).