Roentgens:
The roentgen (R) is a measure of radiation intensity of X-rays
or gamma rays.

Rads:
The rad is a unit of absorbed radiation dose in terms of the energy
actually deposited in the tissue. The rad is defined as an absorbed
dose of 0.01 joules of energy per kilogram of tissue. The more
recent SI unit is the gray.

Rems:
The biologically effective dose in rems is the radiation dose
in rads multiplied by a "quality factor" which is an
assessment of the effectiveness of that particular type and energy
of radiation.

Newton's
Inverse Square Law

Any point source which spreads its influence equally
in all directions without a limit to its range will obey the inverse
square law. This comes from strictly geometrical considerations.
The intensity of the influence at any given radius (r) is the
source strength divided by the area of the sphere. Being strictly
geometric in its origin, the inverse square law applies to diverse
phenomena. Point sources of gravitational force, electric field,
light, sound, and radiation obey the inverse square law.

As one of the fields which obey the general inverse square law,
a point radiation source can be characterized by the diagram above
whether you are talking about Roentgens,
rads,
or rems.
All measures of exposure will drop off by the inverse square law.
For example, if the radiation exposure is 100 mR/hr at 1 inch
from a source, the exposure will be 0.01 mR/hr at 100 inches.

The applet below shows a radioactive source. The distance to
the green source is shown below. You can also drag the little
person and his Geiger counter around to a distance of your choice.
When the mouse button is released, a point is plotted on the graph.
The dosage the person receives at the particular distance is shown
numerically and graphically. The graph allows you to confirm Newton's
Inverse Square Law.

If the distance is too small, the dosage will be too high and
our brave technician will face severe medical effects. To clear
the graph, select a new material, or the same one again. Moving
the mouse from the white area to the gray will turn off the sound!

What dosage in mR/hr is considered safe?
Better find out!
The red dosage lines represent 2, 5, and 100 mR/hr levels.

Exercise: Assume you are standing three feet from a a 15 Curie
cobalt-60 source. How many mR/hr dosages are you getting?