Science Teacher Workshop Meter Exercises - Greg's Answers. Hands on demonstration with Geiger Counters and experiments for the classroom.

Transcription

1 Science Teacher Workshop Meter Exercises - Greg's Answers Hands on demonstration with Geiger Counters and experiments for the classroom.

2 1 Survey Bingo Needed: Several Lantern mantles (or suitable radiation sources) File folders Survey meter 1. Turn the survey meter on and select x1 scale. 2. Perform a response check by placing the probe near a known source (side of meter) 3. Now hold the probe a few feet above the table and observe the background count rate. 4. While watching (or listening) to the meter response, begin lowering the probe toward the table top. 5. Try to locate the position that has the highest response. 6. You may need to change the knob to the x10 or x100 position if the count rates go off scale. 7. When you have located the area of the highest reading. Note the nearest letter that corresponds to that location. Letters A T O M Mat #1 Mat #2 Mat #3 Mat #4 Optional: Place shielding over some of the sources. Student can both locate source and measure count rate.

3 2 Background Needed: Survey Meter Stopwatch or Room clock with second sweep hand. 1. Turn the survey meter on and select the x1 setting. 2. Make sure the window of the probe is open. 3. Observe the needle on the instrument for sudden deflections (events). 4. If audio enabled, the speaker may produce a click 5. Count the number of events in a 1-minute interval. 6. Repeat the count 5 times. 7. Compute the average {Optional: compute the standard deviation} #1 9 #2 9 #3 12 #4 15 #5 11 Average: 11.2 counts (obviously answers will vary... Std dev: 2.5 counts but these values are fairly typical) Questions: 1. What is causing the events? Where do they come from? Background radiation. Most common sources are cosmic (ie our sun) and terrestrial (rocks etc). Some building materials (concrete, etc) may contain terrestrial sources. 2. Why does the number of counts change? Radioactive decay occurs randomly in a sample (can not predict which atom in a sample will decay next). Counting for longer periods or averaging more than 5 samples will yield better statistics. For 15 min, I counted 176 => cpm.

4 3 Distance Needed: Survey Meter Lantern Mantle (or other source) Ruler, meter stick or tape measure. 1. Turn the meter on and set to x1 scale 2. Position a source near the detector. 3. Move the detector/probe away from the source until the count rate can be reasonably counted (ie. ~1 count per second) 4. Measure the distance with a ruler and record below as the starting distance. (be sure to include units of inches or cm) 5. For 30 seconds, count the number of events from the meter. Record this value beside the starting distance. 6. Now move the probe twice as far from the source (starting distance x 2). 7. Repeat the count again for 30 seconds. 8. Continue to increase the distance by a factor of 2 until the count rate approaches background rate determined in exercise #2. Hint: when there are very few counts, (less than 5-10), try counting for a full minute and divide the counts by 2. I did this exercise twice. The first using a lantern mantle and again using a fiestaware plate. Both values are summarized and plotted in a separate spreadsheet. Starting distance: Counts 2 x Starting distance: Counts 4 x Starting distance: Counts 8 x Starting distance: Counts Questions: When you double the distance, does the count rate drop in half? Ideally it should drop by a factor of 4 when distance is doubled. I actually found slight variations...experimental error and beta attenuation in air. What is the relationship between distance and count rate? Inverse square 1/(dist) 2

5 4 Shielding Needed: Survey Meter Lantern Mantle (or other source) 1 deck of playing cards. Ruler 1. Position the detector and source approximately 1 inch (2.5 cm) apart. 2. Turn meter on and turn the knob to a setting (i.e. x1, x10, x100) such that the needle is midrange (not touching the left or right side of the scale) 3. Note the count rate here 200 counts per minute (cpm). 4. Now begin inserting playing cards between the source and the meter probe. 5. Watch the meter response and stop inserting cards when the count rate is half of the initial count rate. Measure the thickness of the cards needed. _~5 cards = 1/8" 6. Now try substituting the cards with other objects. (try items like a CD or jewel case, floppy disks, pieces of cardboard, plastic or metal rulers, foam cup etc,). 7. List the objects which are good at blocking the radiation. CD platter (also ~1/8" thick drops counts by factor of 4) Jewel Case drops (~double thick CD; counts drop by a factor of ~8 Aluminum ruler (also ~1/8" thick drops counts by factor ~10) My hand (~1/2" thick) drops count rate by factor ~ List the objects that are poor at blocking the radiation. Styrofoam cup (need at least a 1/2" to drop by only factor of 2) Questions: If I double the amount of cards determined in step #5, what will happen to the count rate? If just photons were present, it would drop by factor of 2. Due to betas, I observed about a factor of 3. What physical properties (shape, color, size, weight) are important in selecting good objects to block or shield the radiation? density (mass/thickness), items with a high atomic number (high Z ie. metals).

6 5 M & M Half-life Needed: one small bag of plain M&Ms. # of "M"s Empty the bag onto a table. Count and separate the number with an M showing. 2. Pickup the remainder and drop them back on the table. 3. Again record and remove the number with an M showing. 4. Repeat until all M&M s are gone. Attempt # of Attempt M s # of M s Number of "M" vs. Number of attempts # of Attempts