Radioactivity

Invisible Radiation

I purchased a small radioactive specimen at the recent New Jersey Mineral, Gem, Fossil and Jewelry Exhibition. Although the dealer assured me the radiation level was completely safe, I was curious about the actual amount. Eventually, I bought an inexpensive radiation detector to measure the radiation myself.

The detector pictured above has a small Geiger-Muller tube located inside the plastic case along the bottom. It detects beta, gamma, and x-rays. According to the user manual, it is sensitive to gamma rays with energies between 0.1 and 1.25 Mev and beta rays with energies between 0.25 and 3.5 Mev. The given x-ray energy range in the manual was probably printed incorrectly, so I don't know the x-ray values. The detector can register each particle with an audible click, and it sounds an alarm when the count rate exceeds 100 counts per minute.

Upon receiving the detector I immediately measured radiation count levels at various places around my house. I let the detector count for 10 minutes and calculated the average count rate for this amount of time. The count rate is measured in counts per minute, or CPM.

• At the dining room table the average count rate was 20 CPM.
• On the granite kitchen counter the average count rate was 20 CPM.
• At the computer in my "office" the average count rate was 16 CPM.
• On my bed the average count rate was 17 CPM.

All these values are described as "safe, no worry at all" in the detector's user manual. They apparently represent normal background radiation in my house. Any count rate below 50 CPM is classified as safe according to the user manual.

Next, I put the detector near the radioactive specimen I had purchased at the rock show. The next picture shows the pea-sized uraninite crystal enclosed in its small plastic box.

Uraninite is uranium dioxide. Over billions of years the uranium has been decaying into a number of radioactive decay products including thorium, protactinium, radium, radon, and polonium. Some of these radioactive isotopes emit beta radiation which my detector records.

When I placed the detector 18 centimeters (about 7 inches) from the encased uraninite, the detector recorded an average count rate of 38 CPM - significantly higher than background, but still safe. When I placed the detector 5 centimeters (about 2 inches) from the uraninite, however, the average count rate jumped to 726 CPM - well above the warning level! At this close position the detector was clicking like crazy and its alarm was continuously sounding!

I certainly don't want the uraninite too close to anyone for extended amounts of time. I've placed it at least 36 centimeters (about 14 inches) from any position a person might occupy, so it's in a safe place within my mineral display.

The count rates I mentioned are different than biological dose rates. Radiation dose is measured in a unit called grays. One gray is defined as one joule of radiation energy absorbed in one kilogram of matter. Different forms of radiation cause differing amounts of biological effects. For example, the biological effect of absorbing one gray of protons is about twice the effect of absorbing one gray of x-rays. The differing biological effects are incorporated into a radiation weighting factor which, when multiplied by the dose in grays, gives a biological dose unit called sieverts. Radiation dose safety limits are often stated in terms of sieverts. (Dose units are named after Louis Harold Gray (1905-1965) and Rolf Maximilian Sievert (1896-1966) who were pioneers in the measurement of radiation doses and the biological effects of those doses.)

My radiation detector, apparently, calculates biological dose rates as well as count rates. The background level around my house of approximately 20 CPM is equivalent, according to the detector, to a biological dose rate of about 0.13 microSieverts per hour. The Nuclear Regulatory Commission (NRC) recommends the general public not be exposed to rates exceeding 20 microSieverts in any one hour. My background levels are well below that threshold - less than 1/100th of the warning threshold. Even 18 centimeters (7 inches) from the uraninite the dose rate was 0.25 microSieverts per hour - still less than 1/10th the warning level for an hour of exposure. At 5 centimeters (2 inches) from the uraninite the dose rate was about 4.5 microSieverts per hour - still below the short term warning level.

For long term exposures the NRC recommends the general public not be exposed to more than one milliSievert (above background level) in a year. If my background level is 20 CPM, or 0.13 microSieverts per hour, I receive about 1.14 milliSieverts in a year just from background sources alone. I would have to double my background exposure to exceed what I understand is a very conservative safety level. 

Along with the radiation detector I also purchased a spinthariscope. It came in the colorfully labeled box shown in the next two pictures.

The principle of the spinthariscope was accidentally discovered in 1903 by William Crookes who had spilled some radioactive material containing radium on a specially prepared thin layer of zinc sulfide. When he used a magnifying glass to help pick up the spilled material, he noticed tiny flashes of light on the zinc sulfide. The light flashes were caused by impacts of individual alpha particles emitted by the radioactive material. In this way, normally invisible radiation became visible! Crookes named the device a spinthariscope by using the Greek word, spinther, meaning, spark or scintillation. This device was one of the first radiation detectors.

The next two pictures show the spinthariscope. Inside the white metal container a bit of radioactive thorium ore sits upon a moveable platform which can be raised or lowered slightly by turning the screw on the bottom of the white container. A viewing screen with zinc sulfide is located between the thorium ore and the black eyepiece at the top of the spinthariscope. The view looking down into the eyepiece is shown in the second picture below. Thorium ore emits alpha particles which hit the zinc sulfide viewing screen. The screen emits a small flash of light with each alpha particle hit.

Looking in the eyepiece.

The small light flashes are really dim. In order to see them well your eye must be completely dark adapted. This means sitting in a dark room for at least 5, 10, or 15 minutes, the longer the better. When I first looked in the eyepiece I could see flickering light that looked like small bubbles on the surface of a boiling liquid. I tried turning the eyepiece focus screw back and forth, but I couldn't get the bubbles to focus into pinpoints of light. Eventually, I tried using a magnifying glass between my eye and the eyepiece. When I did this, the result was spectacular! The viewing screen was covered with uncountable sharp speckles of light much like the magnified view of an astronomical globular star cluster filling a telescope eyepiece. Instead of constant starlight, however, the "stars" were flickering on and off. The next image of globular star cluster M5 was taken by the Hubble telescope. If you imagine all these stars rapidly winking on and off, you will get some idea of the view through the spinthariscope eyepiece.

It's amazing to see how many alpha particle bullets are continually shooting out of the radioactive ore!

Science has given us many devices that enhance and extend the senses. These radiation detectors make the invisible, visible.