Science Teacher Workshop Meter Exercises. Hands on demonstration with Geiger Counters and experiments for the classroom.

Transcription

1 Science Teacher Workshop Meter Exercises Hands on demonstration with Geiger Counters and experiments for the classroom.

2 Exercise 1 Survey Bingo Needed: Several Lantern mantles (or suitable radiation sources) File folders Survey meter Procedure: 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 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 Exercise 2 Background Needed: Survey Meter Stopwatch or Room clock with second sweep hand. Procedure: 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 #2 #3 #4 #5 Average: Questions: 1. What is causing the events? Where do they come from? 2. Why does the number of counts change?

4 Exercise 3 Distance Needed: Survey Meter Lantern Mantle (or other source) Ruler, meter stick or tape measure. Procedure: 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. 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? What is the relationship between distance and count rate?

5 Exercise 4 Shielding Needed: Survey Meter Lantern Mantle (or other source) 1 deck of playing cards. Ruler Procedure: 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 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. 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. 8. List the objects that are poor at blocking the radiation. Questions: If I double the amount of cards determined in step #5, what will happen to the count rate? What physical properties (shape, color, size, weight) are important in selecting good objects to block or shield the radiation?

6 Exercise 5 M & M Half-life Needed: one small bag of plain M&Ms. # of "M"s Procedure: 1. 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 M s Attempt # of M s Number of "M" vs. Number of attempts # of Attempts

7 Appendix A Meter Setup and Maintenance Background Civil Defense radiation instruments were issued for the purpose of measuring radiation from radioactive fallout after a nuclear attack. These meters were issued and stocked in public fallout shelters and radiological monitoring stations. The Office of Civil Defense issued these instruments to local government, state & city, Civil Defense organizations who applied for the instruments depending on their needs. The majority of these instruments that are still around today were manufactured in the early 1960's at the beginning of the community fallout shelter program. There were some instruments issued in the mid to late 1950's for Civil Defense but these weren't manufactured in the large numbers like the instruments issued later in the early 60's. How does a GM meter work? The CV-700 probe contains a hollow chamber that has inner electrodes at high voltage. A ray of radiation hitting the probe, ionizes the gas in the chamber, and discharges the electrodes. The discharge creates an electronic pulse (heard as a click,) and is called a count. The instrument scale reads counts per minute, the rate at which it detects radiation. If the detector probe has a rotating shield, turn it so the gray (or yellow) geiger tube is visible.

8 Setup Install batteries 1. Most meters use 5 D cells 2. Turn knob to off position. 3. Unbuckle both clasps and lift up by handle. 4. Carefully insert batteries without tearing the battery insulation. 5. Secure in place with clamps provided. Hint: if the battery is too short to make contact, a folded wad of aluminum foil may be inserted between positive + terminal of battery and contact strip. Maintenance If not used for long periods (> 6 months) remove batteries to avoid corrosion. Contacts may be cleaned with a nail file or sandpaper. Speaker Jack Modifications Some meter models have an earphone jack. Unless modified, the earphone jack only works with the original (non-standard) headsets. A simple modification can be made to install a 1/8 headphone jack (Radio Shack). A simple pair of speakers (commonly used with computers) can amplify clicks for classroom use. An alternate method of modifying the meter is described and illustrated at the following location: How to count/record clicks? Low rates: count by hand High rates: observe needle on analog gauge In between? Buy a different type of meter that can be attached to a computer. The Popcorn counter demonstrated may be purchased from the following supplier: Apfel Enterprises

9 Appendix B A Few Words about Radiation Sources Check source on Meter (Uranium) On the side of the Geiger counter is a check source, made of a tiny amount of uranium. It is used to check the operation of the detector. The meter dial should respond when the probe is held close to the check source. Keep the check source side of the counter pointing away from any experiments. Otherwise it may interfere with the experiment. The check source is safe to touch. However, if it is gouged or cut open, place the counter in a plastic bag and do not use. Lantern Mantles (Thorium) The best ones seem to be made outside the USA. Fiestaware (Uranium in glaze) May be found in antique stores Bright Orange color works best (formula later changed to a dull orange) Vaseline Glass (Uranium in glass) Works great with Blacklight. Safety Difficult to detect with Civil Defense meters, unless in large quantity. All of these sources are safe to use.

10 Appendix C Materials for Survey Bingo Preparation of Mats Prepare the mats in advance of the class. Print two copies of the pages in this section. Tape the first copy inside the folder. Place a source over the appropriate letter and secure with tape. Hint: To make detection less obvious, suggest using a lantern mantle or a thin source. Decoys (similar shape without a source) can also be placed in the folders. Now close the folder and tape the second copy to the outside of the folder directly above the copy inside the folder. Table Layout Be careful not to place the mats too close together since the combined strength of two adjacent sources may lead to a wrong letter being selected.

11 Mat #1 RAD FUN

12 Mat #2 T E A M W O R K

13 Mat #3 P H O N E

14 Mat #4 A M G E