Although our sources aren't very active, it is best to develop good safety habits concerning radioactivity. Use tongs when handling the sources. Make sure that the souces are kept in a shielded location. Also, lead is very bad for your health, wash your hands after touching the lead.

When dealing with radiation, it is best to keep your exposure low. In fact, one of the buzzwords in radiation safety training is "ALARA" which means "as low as reasonably achievable." This philosophy advocates three methods in minimizing your risk due to radiation: time, distance, and shielding.

Time is fairly clear, the less time you spend near a radioactive source, the less exposure you will have.

To see the effects of distance and shielding we'll have to use a Geiger counter to measure radiation. How does a Geiger counter work? We can start with a similar instrument, the ion chamber. I assembled a few of these in graduate school when I worked with high-intensity x-ray beams. Ion chambers consist of two metal plates separated by an air gap, with a high voltage between them. In other words, they are just like the simple capacitors studied in Physics 4B. Under normal conditions, no current flows between the plates. However, if an x-ray beam is aimed at the gap, the beam has enough energy to ionize the air molecules that are between the plates, and the electric field between the plates will attract the ions. Since the field accelerates the ions, they pick up energy, and when they collide with other air molecules they create even more ions. This leads to a measurable current that is proportional to the intensity of the x-rays. If the voltage between the plates is too low, not enough ions are created, if it is too high, the opposite occurs (the signal won't go back to zero when the x-rays are turned off). Geiger counters use the same physics, but instead of a pair of parallel plates with an air gap, they are usually sealed cylinders with an inert gas acting as a ion source. There is a "window" at one end so that particles can enter the detector. The window is very thin, and is usually very brittle, so care needs to be taken with the instrument. It's best if you simply leave the tube in its stand. You instructor will go over the workings of the counting device with you (one of the key points is that you need to set your high voltage to 900V). Once you are ready to go, take data for five minutes interval so you have some idea of the background radiation. Remember to subtract the background radiation from your later measurements. Note that uncertainity on these types of measurements is the square root of the total counts. In addition, remember that when you subtract the background from your measurement, the result has error bars related to both the uncertainty in the background and the uncertainty in the measurement. When looking at your graphs, make sure to take this into account (in other words, due to the large error bars that are possible this week, make sure all of your data points are plotted with error bars.)

The second factor in ALARA is distance. Place a source on the top tray (distance = 4cm) and and take data for one minute. Move the source away from the detector and observe how the counts drop as a function of distance. If your counts drop below a statistically reasonable number, measure for more than a minute. Plot the counts vs. 1/(distance)² in your lab report and comment on the relationship. Do this for a beta source and a gamma source. You should also do an alpha source. The results for alpha are very different. Why?

Today's Sources

The combination of a 5.3 MeV alpha particle and a very short half-life means that this is a powerful energy source, 140 watts/gm. Due to its high power density, Po-210 is sometimes used in space probes as a power source. Its major drawback is that the half-life is not long enough for spacecraft with longer lifetimes.

Since Po-210 is an alpha source, your skin will protect you from its radation. However, if Po-210 is inhaled or ingested, then all of the energy of decay is deposited into your body. In a famous incident, Po-210 was used to poison Alexander Litvinenko, a former KGB agent. It is believed he ate 50mCi of P0-210. Our lab sources 0.1μCi sources, a half-million times less radioactivity.

For the polonium, the sources need to be label side down. All other sources are label side up.

Strontium-90
β Decay, 28.8 Year Half-life
Strontium-90 beta decays to yttrium-90, which also beta decays to zirconium-90, which is stable. It is a daughter product of uranium fission, so it is found in spent nuclear fuel rods (six out of every 100 U-235 fissions result in Sr-90). In the United States it is common practice to separate out the Sr-90 from other nuclear waste, as it is a significant heat source. Most of the Sr-90 found in the environment is the legacy of above-ground nuclear weapons testing.

Like Po-210, Sr-90 has been used as a compact power source. The Soviets used it to bring power to remote locations. In 2001 three men described as wood-cutters "came across" a pair of cylinders containing Sr-90, which they used for warmth. They were all hospitalized for radiation sickness. That being said, Sr-90 is widely used in the medical field for radiation therapy, and as a tracer.

Strontium is in the same column of the periodic table as calcium, and thus when inhaled or ingested, the body tries to integrate it into bones. This fact of chemistry makes leukemia a risk of Sr-90 exposure.

Colbalt-60
β and γ Decay, 5.3 Year Half-life
This is listed as a gamma source, which is a half-truth. Cobalt-60 goes through beta decay into nickel-60. While Ni-60 is a stable isotope, when Co-60 decays into Ni-60, the nucleus is in an excited state, and it sheds the extra energy via a pair of gamma rays. So a Co-60 sample will generate gamma rays, but it is from the nickel, not the cobalt. Like Po-210, Co-60 is produced by exposing a stable element to neutrons, in this case cobalt-59.

One of the main industrial uses of Co-60 is food irradiation. Pershiable food is brought near a Co-60 source on a conveyor belt, and the radiation from the source kills all of the bacteria inside the food, so it does not spoil as quickly. This does not make the food radioactive. Like Po-210 and Sr-90, it also has medical uses in radiotherapy and as a tracer, and it is also used to scan objects in a way similar to x-rays

Becasue Co-60 produces gamma rays, it is a source of ionizing radiation, so elevated risk of cancer is a hazard.

Atomic weapons modified by cobalt were a big fear during the height of atomic weapons testing. A bomb with an outer layer of colbalt-59 would absorb neutrons from the fission/fusion reactions in the bomb, and transmute into Co-60. With a half-life of 5.3 years, the fall out would be very radioactive, and would also stay around long enough that people could not wait in shelters until the problem went away. The movie Dr. Strangelove featured the mythical Cobalt-Thorium-G Doomsday Machine (there is some non-PC language, and can you spot the bad physics?):

Cesium-137
β and γ Decay, 30.17 Year Half-life
Cesium-137 beta decays into barium-137, normally into an excited state. The Ba-137 emits a gamma ray, and is then stable. Like Sr-90, Cs-137 is a daughter product in the fission of uranium and plutonium.

Cs-137 is one of the more common industrial isotopes, used to measure thickness of materials, and also used in radiotherapy.

What makes Cs-137 a dangerous isotope is the gamma rays from its product, Ba-137. Like Co-60, this means that there is ionizing radiation that can harm you even if you don't ingest or inhale it. Most of the current radiological harm from Chernobyl fallout is from the Cs-137.

Due to its short half-life, the only Cs-137 found in the environment came from nuclear-weapons testing and accidents like Chernobyl. Hence any item with Cs-137 in it is known to have been made since 1945, which can be helpful in detective work.

HOW MUCH RADIOACTIVITY AM I GETTING IN TODAY'S LAB?!?

One of the sources available today is Sr-90. At one meter, Sr-90 will give a dose of 9.65 mSv/hr/GBq. When bought new, the sources were 0.1uC. Since the half-life is 28.8 years, we can assume that the sources are still roughly 0.1uC, which is 3.7E-6 GBq. So at one meter, in one hour you will get 3.6E-5 mSv. Between natural and medical sources, a person gets an average background dose of about 3.6 mSv a year, or 0.01 mSv a day, 4.2E-4 mSv an hour. The radiation from the source represents roughly an 8% increase over background. Note that simply living in Denver rather than at sea level represents a 40% increase (the extra atmosphere screens out cosmic rays).

At one meter for one hour, the Sr-90 is equivalent to about an extra 5 minutes worth of background radiation. You would need about 2800 hours of exposure to equal the dose from one chest x-ray, or 840 hours to equal the radiation you get flying from here to New York.

Of course, the above analysis assumes just one Sr-90 source, there will be many sources in the room. Most can be ignored because people will shield their measurement stations with Pb, and due to distance.

Note that a physicist working for the US government is allowed a maximum dose of 50mSv in one year. Assuming a 50-week work year and a 40-hour week, that's 2000 hours on the job. That calculates to 0.025 mSv an hour. In order to reach the government's limit, a physicist would have to sit one meter away from 685 of our Sr-90 sources every working minute.