We use two facts about it. First, it damages matter it passes through. Second, because the alpha particles are fast moving, they tend to penetrate far into matter they strike. Thus, if the matter they pass through is sufficiently rigid alpha particles will leave tracks in the material. What material will we be using? A polymer called CR39. This is the most common material used in making eyeglass lenses. Will we be able to see the tracks as they are made? No, for three reasons. First, they are too small, so a microscope is required. Second, they need to be developed by soaking the CR39 in the proper solution. Third, the alpha emission is so rare in most locations, that a long period of exposure is necessary to observe a detectable number of tracks. How long is a long exposure? Four weeks. FOUR WEEKS? Yep, four weeks. So what’s the plan for today? Make the detectors according to the directions in your manual. Record what you do in your notebook. Put your detectors in a place you want to investigate. Are there especially good places to put the detectors? Basements and rooms with poor ventilation are good places. Anything I need to worry about? Yes. If a janitor finds your detector and doesn’t know what it’s for he or she might throw it out. So what do I do to prevent that?
First, put a note next to your detector. Second if you can find the janitors who are in charge of the place you’re putting your detector, let them know what you’re doing. OK, so now I have an exposed disk? What now? You’re going to soak the disk in hot concentrated NaOH. Be very careful when you handle the NaOH, because NaOH is damaging to the skin. It is essential that you work in the hood, keep your safety glasses on at all times and wear skin protection. What does soaking the CR39 in NaOH do? It makes the tracks of the alpha particles visible (at least using a microscope). What now? You’ll use a microscope to determine the number of tracks per unit area, and compare it to a standard. The standard gives the number of tracks that a sample containing 370 pCi per liter of gas would generate. The ratio of the radon concentration of your sample to 370 pCi will be the same as the ratio of the number of tracks you observe to 2370 tracks and therefore
pCi / L
tracks in your sample / cm2 / day x370 pCi / L 2370 tracks / cm2 / day
What’s a pCi, a politically correct organ for vision? It’s a unit of radioactivity. A Curie (Ci) is an amount of radioactive decays equal to that produced by a gram of radium, and is equal to 3.7 x 1010 disintegrations per second. The p stands for pico, a prefix meaning multiply by 10-12. Thus a pCi is equal to 3.7 x 10-2 disintegrations/second. This means that for a one L of a one pCi /L sample, there will be 37 decay events every 1000 seconds. How high a value is dangerous? The EPA has set a limit of 4pCi/L as being safe. Do you have any experimental tips for us? Of course.
1. Please be very careful with the NaOH. Gloves, eye protection, hood. 2. You’ll clip the CR39 to a small ring, and then hook the ring to a bent paper clip. Try hard not to immerse the paper clip, since it will dissolve in the NaOH. 3. Make sure that you clip the ring to the edge of your CR39, not the corner. To make it easier to open up your ring to clip it to the CR39, use your flat spatula. 4. You’ll boil water in a flask and then insert your test tube containing the CR39 and NaOH to heat your NaOH. Add a boiling chip to the water. 5. When etched for 40 minutes, carefully rinse the NaOH off. The remaining NaOH should be disposed of in the aqueous waste container, but only after it’s cooled down. 6. When you use the microscope to count the tracks, you’ll need to determine the area you’re looking at. Because of the optics of the microscope this will be circular, so once you’re focused on your sample, we’ll use a vernier caliper to measure the diameter of the area in mm and then divide by 10. This will be the diameter of your circle in cm. Your area will be given by area (diameter / 2) 2 7. Determine the number of tracks in 10 different parts of your CR39. How do we obtain tracks/ cm2 /day? 1. Determine the average number of tracks you observed. 2. Divide by the area you determined. This gives you tracks / cm2 3. Divide by the number of days your detector was left in place. This yields tracks / cm2 /day. Honor Stuff All calculations on page 145, the questions on page 146, and all data and results on page 147 may be done with your lab partners
Experiment 21: Measurement of Radon in Air – Prelab for assembly of Radon Detectors What is the purpose of this experiment? The purpose of this experiment is to measure the levels of Radon in a location of your choice. Why are we interested in Radon levels? Radon is a radioactive inert gas. It is a decay product from Uranium. Uranium is present in trace quantities in granite that may lie beneath the foundations of many houses. It can pass from cracks in granite through soil into houses. Radon became a serious concern in the 80’s in the first fuel crisis, when many people began insulating their households more tightly. Ironically a by product of the insulation was that Radon, which had been previously kept at low levels because of regular circulation and replacement of the air in a house, was able to build up to unacceptable levels. Why does it matter that Radon builds up? While radon is a decay produce of uranium, radon itself is subject to radioactive decay, emitting high energy alpha particles. These alpha particles represent a health hazard. What are alpha particles? Alpha particles are helium nuclei, with a mass of 4 atomic mass units (AMU) and a charge of +2. They are a class of radiation called ionizing radiation. They tend to strongly damage matter (and tissue) that they pass through. How do we detect radon? Because radon is one of the inert gases, we can’t use chemical reactivity to detect it, so we instead we’ll detect it’s alpha radiation. How do we detect alpha radiation?