Radioactive Paper?

I don't believe there is intelligent life on other planets. I believe they are just like us.

— A spacecraft designer whose name I don't recall.

Introduction

Figure 1: Glassware that Glows from Uranium Radioactivity (Source).

I probably shouldn't be surprised when I read that common materials are slightly radioactive. I am seeing an article on this topic every few weeks. For example, a  few weeks ago I read an article about a type of glassware that contains a small amount of uranium and is slightly radioactive. It also fluoresces when exposed to UV light.

Figure 2: MINER Neutron Imager. Using omnidirectional imaging, MINER was able to localize a radiation source in a building in Chicago (Source).

I speculate that one of the reasons that I am seeing more articles on this topic is because governments around the world are putting radiation monitors at their ports of entry (example) and these detectors are occasionally flagging shipments of kitty litter and Brazil nuts.

These radiation detectors can be quite complex. Figure 2 shows an example of a neutron detector used for detecting, classifying, and localizing fissile material.

I just read an article article that glossy paper is also slightly radioactive, a fact that I found surprising. As I read about glossy paper, it turns out that glossy paper often contains kaolin, a type of clay. So it is radioactive for the same reason that kitty litter is radioactive, which also contains clay.

Figure 3: Radiation Spectrum of a Playboy Magazine (Source).

Most of the print magazines I read today are filled with glossy paper. Much of that paper comes from the forests near my cabin in northern Minnesota, where there are large paper plants that specialize in making glossy paper.

Figure 3 shows a radiation spectrum for a 400 gram Playboy magazine – clearly the researchers are men with a sense of humor. You can see the various radiation count spikes in their data at the energy levels for:

• ²³⁴Th (93 keV)
• ²¹²Pb (238 keV)
• ²⁰⁸Tl (583 keV)
• ²¹⁴Bi (609 keV)
• ²²⁸Ac (911 keV)

My long-term goal is to understand the radiation exposure that people on Earth are exposed to everyday and how those numbers differ from a place like Mars. As part of this effort, I will duplicate the author's calculations for the radiation exposure that a human would experience holding a magazine 1 foot away from their body for an hour.

The article presents the following radiation activity information for the magazine's paper.

•   0.15 to 0.35 pCi/gm (pico-Curies per gram of magazine ) of uranium isotopes.

  I should discuss units a bit. The modern unit of radioactive decay is the Becquerel (Bq). One Bq equals one decay event per second. In the old days, radioactive decay was measured relative to radium and the unit of measure was called the Curie (Ci). One Ci equals 3.7E10 decays per second. This unit proved far too large for common usage and most folks ended up using pico-Curies (pCi).

•  0.3 to 0.6 pCi/gm of thorium isotopes.

These activity levels are not difficult to estimate. For example, I estimate the activity level from paper due to thorium in Figure 4. All the data came from a couple of web searches.

Figure4: Glossy Paper Activity Level Due to Thorium.

Let's assume that all the radiation activity is due to the paper's clay component, which contains uranium and thorium. If we assume that the magazine's mass is 400 grams, we can estimate the overall radiation level as shown in Figure 5.

Figure 5: Activity Level from a Glossy Magazine.

I then took this data and put it into the web-based radiation exposure calculator shown in Figure 6. The background behind what the calculator does is given here.

Figure 6(a) Thorium Series Calculations.	Figure 6(b) Uranium Series Calculations.

Figure 7 shows my calculation of a total radiation exposure rate of 0.0015 µREM/hour. This agrees with the results presented in the article.

Figure 7: Final Result of 0.0015 µREM/hour.