Experiment13: Radioactivity
OBJECT: To study radioactivity, measuring background
radiation, looking at radiation shielding and performing radioactive dating.
We detect radiation with a Geiger tube. In the center of the tube is a wire charged
to a high electric potential. An alpha,
beta, or gamma ray entering the tube can break apart atoms of gas or atoms in
the walls of the tube, into free electrons and ions. The electrons rush toward
the charged wire, ionizing more gas atoms on the way by hitting them, so that
an avalanche of electrons crashes into the wire. The electrical pulse produced is counted by
an electronic device, a computer in our case.
The number of pulses registered
in some time interval measures the amount of radiation entering the Geiger
tube.
CAUTION: The radiation sources used in parts 2 and 3 are
weak enough to handle without special precautions, but avoid unnecessary
contact. Don't play with them. Don't put them in your pocket.
Setup:
1. Mount
the Geiger-Muller
Counter on a ring stand, with the bottom of the tube about one centimeter above
the table. Do NOT remove the cap from
the end of the tube or you will expose a delicate, easily broken part.
2. Plug the
counter into channel 1 of the interface.
Connect the interface to the
computer and turn them both on. Open
PASCO Capstone. (Traditional Geiger
counters click every time they register ionizing radiation. This one makes an annoying beep instead. The only way to stop it is to pull the plug
out of channel 1 when not in use.)
3. Click
Hardware Setup at the upper left. Click
channel 1 on the picture of the interface.
Click Geiger Counter on the menu.
Click Hardware Setup again to get that out of the way.
4. Double
click “Digits” about halfway down on the right.
Click <Select measurement> at the upper left. Click Geiger Counts (counts/ sample).
5. It probably says 1.0 Hz at the bottom of the
screen. With the down arrow, change it
to 1.00 min.
Part 1. Background radiation.
With no radioactive disk nearby, click Record
near the bottom left. Nothing will
appear for a minute, then it will show the number of pulses it counted. Write it down and leave the counter running. At 2.00 minutes, it will display the number
it counted during the second minute.
Record that and also the number of pulses during the third minute. Click Stop.
Average.
The
efficiency of a Geiger tube varies for different kinds of radiation. It can be as low as 1% . Your body is thick
and dense enough to absorb a much higher percentage. Also, you are quite a bit larger than the
tube. Multiply the average number of
counts per minute by 100 to compensate for the 1% efficiency. Multiply by 100 again because your cross
sectional area is roughly 100 times the tube’s.
You now have a crude estimate of the number of times per minute that
natural radiation, from radioisotopes in the environment and cosmic rays,
interacts with your body. (This does
not include the many muons and neutrinos which pass
right through you; just rays which are actually causing ionization.) Kinda scary, isn't
it?
Part 2. Penetration
and shielding.
1. Obtain a beta
ray source (Sr-90 in a green disk) and place it below the detector. If one is
not available, go on to step 5 or to part three, and someone will be done with
one soon.
2. Temporarily
change from 1 minute to 10 seconds at the bottom of the screen. Make short data runs to see if you get a
stronger signal from the disk when the side with writing on it is up or
down. (They aren’t all the same.) Use it the way that gives a stronger signal.
3. Make four 30 s
runs. Add and divide by 2 to get average
counts per minute.
Notice that the decay rate goes up and down at
random. The shorter the time interval,
the more significant the variation is.
This is why you need to collect data for a couple of minutes each time,
to average this out. Now that you have
seen this, just do 2 minute runs and divide by 2.
4. Measure the
counting rate with two pieces of lead stacked between the source and
detector. Repeat with the same thickness
of aluminum, and then cardboard.
5. Replace the
beta source with a gamma source (Cobalt-60, orange disk). Do not use one that says "old";
that's for part three. It makes less
difference which side of the disk is up.
Let’s go with writing side on the bottom. Repeat what you did with the beta source.
6. Conclusions:
a. Compare the penetrating ability of beta to gamma
rays. That is, for which kind of rays
does a larger fraction of the rays get through a material?
b. Compare the effectiveness of these three materials in
blocking out radiation.
Part
3. Radioactive
dating.
You will
determine the age of an old Co-60 sample by comparing its activity to the newer
one you just measured.
1. Put the old
gamma ray source under the tube. Have it the
same way you had the other one: Manufacturer’s
label on the bottom, handwritten paper tag on top.
2. Determine the
counts per minute, as before.
3. The
half-life of Co-60 is printed on the disk.
Calculate the time it would take for the newer sample to decay down to
the activity of the older sample. (The
samples were not made very precisely; some were more radioactive than others
when new. So, which ones you happened to
pick up can affect your answer. What you
get may differ from other groups by a few years.)
4.
The year the newer one was made is printed on it. According to your measurements, in what year
was the old one made?
PHY 133 Report
on Experiment 13: Radioactivity
Part 1. Background radiation.
__________, __________,
__________ average =
____________ counts per minute.
Find
your exposure:
Part 2. Beta source:
Nothing between ________, ________,
________, ________, average =
________ c.p.m.
Lead: ____________ 
2
= ____________ counts per minute
Aluminum: ____________
2
= ____________ counts per minute
Cardboard: ____________
2
= ____________ counts per minute
Gamma source:
Nothing between ____________ 2
= ____________ counts per minute
Lead: ____________ 2
= ____________ counts per minute
Aluminum: ____________
2
= ____________ counts per minute
Cardboard: ____________
2
= ____________ counts per minute
Part 3. Older sample: ____________
2
= ____________ counts per minute
Calculate what year the older one was made. (Continue on back if
necessary):