Cloud Chamber Activities for the High School Classroom John Timothy Perry and Mary Ann Sankey Mt. Hebron High School, 9440 Route 99, Ellicott City, MD 21042 Teachers who use cloud chambers with their students most often use them for the sole purpose of observing radiation trails. The phenomenon is quite impressive, berausr the mwnations go on at a ronsisnt rat; with no apparent dccrcasc in art8virv. despite lrngthy observations of the rhnmber. However. we would like to suceest that the cloud chamber also can be used by students a s a n experimental tool. allowine them to ~ e r f o r mtheir own investieations of radiation. ~ I h i sactivekxperimentation on the s&dents' part will help them to understand some aspects of the behavior of radiation through first-hand experience and is much more interesting for them than the traditional simulations of nuclear experimentation. For anyone unfamiliar with the apparatus, the cloud chamber is used to "see" radioactivity. A student model cloud chamber is usually a short cylindrical box made of transparent plastic. See Figure 1. It has a removable lid, a black bottom surface and a n absorbent paper wrapped around the sides. The paper has a few windows cut into it so that a bright light may be shined into the chamber. A small amount of ethanol is used to soak the absorbent paper. The lid is set firmly in place and the chamber is placed on dry ice. A source of alpha or beta particles is placed inside the chamber by insertion through a hole i n the side of the chamher. As the radioactive emissions from the source streak around the chamber, they leave ionized gas particles in their wake. Suoer-cooled alcohol vaDor then condenses on these charged nucleation points. The path of the radiation is thus seen a s a fog trail of condensed alcohol. Cloud chambers may be purchased from many science supply companies. A typical cloud chamber, with a radiation source, costs about 20 dollars. The series of experiments that we present here works best when the students have access to a number of cloud chambers. As such, we uu
have found i t much more economical for the students to construct their own chambers from less expensive materials. I n addition, because the entire chamber is constructed bv the students. alterations in desien to accommodate diffirent experiments are more easily Lade. One added benefit for the students is the o.~.~ o r t u n i to t v construct their own experimental equipment. Design and construction of instrumentation is often a n integral of a research - Dart . chemist's work, though student experiments rarely include such opportunities. The cloud chamber activities we suggest i n this article allow the teacher to include these aspects of scientific research in the students'experience. Construction of an Inexpensive Cloud Chamber We have found that disposable polystyrene Petri dishes make excellent cloud chambers with a minimum of alterations. A sleeve containing 20 dishes will make 20 cloud chambers a n d costs about five dollars. We have used dishes of normal depth (15mm, which produces a chamber 30 mm deep) as well as the very shallow dishes (10 mm, the bottoms of which produce a chamber 20-mm deep and . constructhe tous ~ r o d u c ea chamber 10-mm d e e ~ )Other tion materials required include black paper and clear adhesive tape. (The black paper not onlv ~ r o v i d e sthe dark background so that the Gapor trails aie'more easily seen, but also serves a s the alcohol absorbent in the self-constructed chambers. Some papers contain dyes that are extracted readily bv the alcohol and stain anvthing they contact. If you chiose to use t h e l a n t e r n manties ih the chamber and you want to use the same mantles year after year, select a paper that is minimally affected b; the alcohol. "Fadeless" black paper for bulletin boards works well.) e can he com~letedin The construction orocess is s i m ~ l and a matter of minutes. Each chamber is constructed from two top halves of a Petri dish, or two bottom halves. 1. Cut a piece of hlack paper to a diameter slightly smaller
than the inside of one of the dish halves. Place the round of paper into one of the dishes. It is very important that the paper lie perfectly flat against the bottom, with no bulges or buckles. If necessary, remove the paper and trim off any excess that is preventing it from lying flat against the plastic. 2. Take the two tops (or two bottoms) of the Petri dishes and put them together, open-side to open-side. Run a piece of 112-in. wide, transparent tape around the dishes at their joint. (The tape has to be clear so that you can shine a bright light into the chamber from the side.) 3. Wearing gaggles and leather gloves, heat the bottom of a small heat-proof test tuhe (about 13 mm OD) in a hot flame. When the tuhe is hot, remove it from the flame and carefully push it onto the side of the joined dishes so that the tuhe melts a small round hole. It is desirable to have this hole somewhat small, about the size of the small end of a #3 rubber stopper, so don't push the tube all the way through. This melting process creates same fumes so you may want to do this in a fume hood or outside.
Frg~re1 C oud chambers-The top two cnamoers are commerc a y ava ab e cnambers Tne bohom chamoer was rnaoe accoro ng to tne directions in this paper.
Setting Up the Cloud Chamber 1. Squirt a small amount of alcohol into the claud chamber, such that the black paper is thoroughly saturated. Two or three milliliters usually suffices. 2. Set the cloud chamher an tap of same dry ice. You want to maximize the contact between the bottom of the dish and Volume 72 Number 4 April 1995
339
the surface of the dry ice. The more contact, the quicker and more effectively the chamber will respond to the radioactive particles, producing the fog trails. Our experience indicates that slabs of solid dry ice are the least desirable farm to use. We purchase pelletized dry ice that we then grind to the consistency of coarse sand with a mortar and pestle. We pour this into a small dish and settle the cloud chamber into the dry ice. (Here, again, the plastic Petri dishes work very well.) We have had success with dry ice snaw, produced from a carbon dioxide gas cylinder. However, the snaw tends to stick to itself, making it difficult to conform to the flat surface of the cloud chamber. The problem may be solved by pushing an the tap of the snow with a flat, sturdy board. You may want to place an insulating material between the dish of dry ice and yaur table or desk surface to prevent the extreme cold from doing any damage. 3. Insert the radioactive source into the opening in the side of the cloud chamber. Turn off the overhead lights. Using a flashlight to illuminate the chamber through the sides of the dish, observe the vapor trails caused by the passage of radioactive particles through the air in the chamber. (Dependingon the amount of contact hetween the bottom of your chamber and the dry ice, these trails could appear immediately, or they could take several minutes.)
Radioactive Sources Most commercial cloud chambers are sold with a lead210 radiation source. Some science supply companies list the lead-210 as a n economical source of alpha particles. Do not mistake such descriptions a s a claim to be solely alpha particle emitters. The decay series for lead-210 inclides
Figure 2. Alead.-210 radiation source age tube.
?",withits protective stor-
beta as well as alpha decay reactions. (See Table 1.) The radioactive material is in the eye of a sewing needle, the needle firmly inserted into a rubber stopper for handling. See Figure 2. When not in use, the needle is kept in a thickwalled plastic tube to protect the source. The sources also can he bought separately. The half-life of lead-210 is 22.3 years, so the sources should last for quite some time, as long as they are handled properly. I t still is possible currently to purchase lantern mantles that contain thorium, a naturally radioactive element. The lantern mantle contains substances which incandesce a t the temperature of a propane or methane flame, providing a bright white light ( I ) . We have been able to purchase these mantles a t stores t h a t sell camping supplies. The cost is minimal, about one dollar per mantle. See Figure 3. When buying the lantern mantles, check the contents list to make sure they contain thorium. (Thorium exists naturally a s the single isotope, Th-232.) In some cases, the contents may only mention that the mantles contain a radioactive substance. These are acceptable for our purposes. (Coleman has begun to market lantern mantles t h a t
340
Journal of Chemical Education
Table 1. Decay Series for Lead-210 Parent Nuclide
Decay Mode
Daughter Half Life of Nuciide Parent Nuclide
+ 4 ~ e + '06pb 138days (3) '06pb + stable isotope; no decay Information from 'Table of the Isotopes", CRC Handbook of Chemistry and Physics 71st ed.; David R. Lide, Ed. "OPO
DO NOT contain thorium and, thus, would produce no trails in the cloud chamber.)
Caution: Radioactive Materials Teachers who use radioactive substances with students should be prepared to discuss the hazards involved with such use. Caution your students NOT to touch the eye of the needle, nor to disturb it in any way that might loosen any of the radioactive material from inside the eye. Similarly, devise appropriate procedures for the handling of the lantern mantles. Because the mantle must be handled directly, you might possibly require the use of gloves. (Students may question such precautions, because there are no such instructions on the mantle bag. Point out to them that the camper is expected to handle the mantle far the short time required to install it in the lamp. In the lab, they may be required to handle the mantle several times and, therefore, should take proper precautions.) One other concern with the lantern mantles would be the fact that the thorium produces a n isotope of radon in the fifth step of its decay series. See Table 2. Radon could be inhaled and its alpha emanations could endanger the lung tissues. The radon produced i n the decay series of Th-232 is Rn-220. Radon-220 has a half-life of less than one minute. Rn-222, with a half-life of more than three days, has been the concern for home environmental safety reported by the media. I t is the much longer half-life of the Rn-222 which provides the time necessarv for the radon to find its way into the lungs (2,3).~ c c o r d i n gto the Personal Radiation Dose Chart published bv the American Nuclear Society (4),a person who uses lantern mantles containing tho-
F gdre 3 -amern n a v e s As ndcaleo n me lexl Coleman now mardels a antern manlle lnal ooes no1 conla n rao oacl ve thornom
,
Table 2. Decay Series for Thorium-232
Daughter Half Life of Nuclide Parent Nuclide
Decay Mode
Parent Nuclide 232~h
+
"%a
+ + +
"'AC
228~h
'"Ra 220~n 2'6~o '"pb
+
21ZBi
+
+ + +
4 ~ e + 2 2 8 ~ a 1.4 x 10lOy 'e + 2 2 8 ~ ~ 5.75 y 6.13h 'e + "'~h 4 ~ e + 2 2 4 ~ a I .9l y 'Rn 3.66 da 4 ~ e + 55.6 S 4 ~ e + 2'%0 0.15 s 4 ~ e + '12pb + "'~i 10.64 h Oe (can undergo alpha OR beta decay)
alpha decay
21zBi
+
208~1
+
'08pb
4 ~ e
+ +
2 0 8 ~ ~
'e 208Pb + stable isotope; no decay
1h
3 min
Oe
+
'"PO
I h
'O'P~
0.3
+
"'PO
+
'08Pb
+ stable isotooe: no decav
4 ~ e+
Student Assignment 1 Insert a lead-210 radiation source "needle" in your chamher. Describe the appearance of the fog trails i t produces. Notice that all the trails originate from the eye of the needle. Remove the needle. What do you observe now? Notes to the Teacher The fog trails are mostly straight lines, though a few appear to curve slightly. See Figure 4. All the fog trails originate from the eye of the needle. When the student removes the needle from the cloud chamber, no more trails will be visible. All the decay products of lead410 are solids a t room temperature and, thus, remain in the eye of the needle. (See Table 1for the lead-210 decay series.) This is setting the stage for what the student will observe with the lantern mantle. Student Assignment 2
beta decay 21zBi
perimental challenges and should be available to the students in the lab.
Information from Table of the Isotopes'. CRC Handbook of Chernislw and Physics, 71st ed.: David R. Lide. Ed.
rium adds 0.003 mrem to their yearly exposure. The average background exposure for each person living i n the United States is approximately 350 mrems. (Package instructions suggest that the user not keep mantles near the skin for prolonged periods of time and always wash hands after handling.) Student Experiments with the Cloud Chamber
Once the students are familiar with the operation of the cloud chamber, we find it possible to assign investigation problems that require the students to design their own exoeriments. This i s m i t e a challenee for most students. though working i n &ups helps to &eve some of the anxi: etv. As lone as each student handles the radioactive materials in the manner that we have demonstrated in class, we allow them to experiment on their own. The following experiments require cloud chambers, lead210 radiation sources and lantern mantles containing thorium-232. Other simple materials are implied i n the ex-
-
F gdre 4. A.cad-210 rao~atonso.fcc 'wcu r ' r l I?@ :o..o crsrnoer Not ce that a lne Ira Is can oe l'aceu eas , 3 , m 10 tle nee0 e 5 eye
With t h e lead-210 radiation source "needle" i n your chamber, devise and carry out a procedure for determining the number of disintegrations per minute (dpm) of the source. Describe your procedure, list your data and conclusions, and discuss any errors involved in the process. Notes to the Teacher If a cloud chamber is properly operating and the source is active, the number of trails visible should be impossible to count directly by eye. Our intention is that students realize that direct counting methods are impractical and that some averaging technique should be employed. Common strategies are to divide the cloud chamber into sections and have several students count the trails in different sections and then add together the results, or to count the trails in one small section of the chamber and then multiply the result by the number of sections needed to make up the entire chamber. Most students count for a time period shorter than one minute and then determine the dpm by calculation. Student results may he in the range of several hundred per minute. Student Assignment 3 With the lead-210 radiation source "needle"in the chamber, devise and carry out an experiment to investigate the ability
.~.
Figure 5. A lead410 radiation source "needle"in the cloud chamber with a paper barrier in place. Volume 72 Number 4 Aoril 1995
341
Figure 6. A lantern mantle (thorium containing) in the cloud chamber. Notice that most of the trails can be traced back to the mantle fabric. Ho~ever,at !no lop of me pnotograph s n Ira tnal runs approxmale y en lor gnt, Inat cannot oe traced bacd lo me man1 e TI s Ira s ~cly cxplalnca oy !he aecay of a raoon-220 gas parilc e. ~~~
~
~~~
~
~~
of various materials to shield the radiation. Describe your procedure, list your data, and explain your conclusions. Notes to the Teacher The cloud chamber could be altered (or a new chamber constructed) to include a n internal barrier of paper, aluminum foil, plastic, etc., near which the radioac&e source is positioned. See Figure 5. The simplest test we have done is to place the source on top of the cloud chamber. I n such a position faint vapor trails are visible in the chamber that are not seen when the source is removed. In general, it is easier to prove that the radiation can pass through a material because the trails will be visible. To prove that a material is an effective shield for the radiation, the students must not only show that no trails exist on the side of the barrier opposite the source, hut also that trails would be visible if radiation were on that side of the barrier. Student Assignment 4 Using the list of contents on the lantern mantle hag and a Handbook of Chemistry a n d Physics (or some other appropriate resource) determine the element(s1 responsible for the lantern mantle's radioactivity. Determine the decay series for the isotopeb) involved. Notes to the Teacher The lantern mantles we have used list "aluminum, calcium, cerium, magnesium, and thorium" compounds present in the mantles, as well as other miscellaneous materials. Using t h e "Table of t h e Isotopes" in t h e CRC Handbook of Chemistry and Physics, the student should find each element in the mantle contents list, determine which of its isotopes occur naturally and then see if any of those isotopes have a half-life listed in the table, which would indicate that the isotope is radioactive. Thorium232 is the only naturally radioactive element in the lantern mantles. Using the mode of decay listed for the isotope, the Decay Series for thorium-232 may be produced. (See Table 2.) Caution: In some earlier editions of the Handbook of Chemistry and Physics, it cannot be determined that Th-232 is the naturally occurring radioactive isotope of thorium because no percent natural abundance for this isotope is listed. The 71st Edition (1990-1991) does list thorium-232 a s making up 100% of the naturally occurring thorium on Earth. 342
Journal of Chemical Education
Figure 7. A lantern mantle (thorium containing) in the cloud chamber. The V-shaped trail may be the result of a radon-220 decay followed immediately by its daughter Po216 decay. Student Assignment 5 Insen 3 lantern mantle partially through the hole in thts side mt'vour rhnmber and observe the ibntralls 11 produces for several minutes. Make a list of theVdifferen
