A Grand Canyon Radiation Lab

This is a fun lab used in our classes that you can do from a home computer.

Let’s say you have been tasked with making a direct radiation measurement of the exposure rate from a bucket of rocks for health and safety purposes. The rocks are believed to be from the Breccia Pipe formation in the South Rim of the Grand Canyon known as the Orphan Lode.

The first thing we have to do is select a survey instrument that is appropriate for this type of measurement. The meter should respond to photon radiations linearly over the range of energies that we anticipate from a natural uranium ore. Below is a high resolution gamma spectrum from a rock taken from the same ore body.

As you can see, there are many photons at different energies from the uranium daughters in the natural chain. Most of the energies, though, are from 80 keV to about 800 keV. The instrument should respond linearly and accurately over this range as a minimum.

On the shelf, we have three meters calibrated in mR/hr, the Ludlum Model 19 MiroR meter, the Ludlum 44-9 Pancake probe geiger counter, and the Ludlum Model 9 pressurized ion chamber. Now let’s look at the response of these meters and select one that is appropriate. Since this is a common task, Ludlum has made it easy for us by putting the response curves for all of their instruments on one page. See:


Now look at the curves for the different instruments that are available.

First look at the curve for the Model 19 microR meter.

Does this meter respond linearly and accurately (1.0) for the energy range we need?

If not, will it over-respond or under-respond?

Is it suitable for health and safety measurements?

Now look at the Model 44-9:

Will this meter respond correctly? If not, will it over-respond or under-respond?

Lastly, look at the Model 9 ion chamber:

Is this meter suitable for our health and safety measurement?

In the articles about the buckets of rocks in the Grand Canyon Museum, the staff used the Model 19 (apparently reading the microR scale as mR/hr), and the 44-9. They did not use the Model 9 ion chamber.

Are their measurements meaningful? If you visited the museum were you “overexposed”?

Natural uranium and thorium chain radionuclides are natural, and are a part of all soils and building materials at widely varying concentrations. To learn more about the natural decay chains see: