EXPERIMENT #13: RADIOACTIVITY

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EXPERIMENT #13: RADIOACTIVITY
OBJECT: To study radioactivity, measuring background radiation, determining the half-life of
Co-60, and looking at radiation shielding.
APPARATUS: We detect radiation with a Geiger tube. In the
center of the tube is a wire charged to a high electric potential.
When an alpha, beta, or gamma ray enters the tube it can break apart
atoms of the gas inside the tube, or atoms in the walls of the tube,
into free electrons and ions. The electrons are electrically attracted
to the wire in the center of the tube and rush toward it; they ionize
more atoms by knocking against them on the way, so that an
avalanche of electrons crashes into the wire. The electrical wire
pulse so produced is counted by a box of electronics.
The number of pulses registered in some time interval measures the amount of radiation entering
the Geiger tube.
CAUTION: The radiation source we use in parts 2 and 3 is in a green or orange plastic disc. It
is weak enough to be handled without special precautions, but avoid unnecessary contact: Don't
play with it. Don't put it in your pocket.
OPERATION. You have one of two different kinds of counters:
If you have a digital (off - white) counter:
1. Press power to turn it on.
2. Check that it's working correctly:
a. Press test, then stop, then reset.
b. Set count interval for one minute. Now, pressing count, and waiting one minute,
should give a reading of 3600. (When the test switch is depressed, the counter is looking
at the signal from the 60 Hz electric line, rather than the signal from the Geiger tube.)
c. If the result is incorrect, tell the instructor.
3. To take readings:
a. Check that the high voltage control is set for 400 V and press test again (The light on
the switch should go out).
b. Pressing reset then count will now count pulses from the tube for the amount of time
you've set on count interval.
If you have a non-digital (blue) counter:
1. Turn it on with the volume knob.
2. Press and hold in the red button and set the high voltage at 900 V, reading the red scale on the
meter. Watch it for a moment until it's fairly stable, and then do not disturb the voltage again.
3. The response knob should be at "fast". Set the "range" knob for the most sensitive scale
which does not pin the needle.
4. The center part of the tube turns; open it up to take readings.
- 2 PART ONE. Background radiation.
You do not use a radioactive source for part one. If you have already gotten one, place it at least
one meter away.
Measure radiation from the earth, the building, and outer space as follows: Lay the tube
horizontally (to present the same cross section to cosmic rays from above as to natural
radioactivity from the side.) Count the number of pulses in each minute for five minutes. (A
pulse makes the digital counter increase by one, or makes the light flash, the loudspeaker click,
and the needle jump.)
Multiply the average number of counts per minute by 100 to compensate for the fact that Geiger
tubes are usually under 1% efficient. Also multiply by the ratio of your cross sectional area to
the detector's. (Inside its protective case, the dimensions of the actual tube are 1.5 cm by 8.3 cm.
Similarly, find your cross section from your height times width.)
You now have the approximate number of times per minute that natural radiation interacts with
your body. Note that this does not include the many muons, neutrinos, etc, which pass right
through you; just rays which are actually causing ionization. Kinda scary, isn't it?
PART TWO. Penetration and shielding.
1. Obtain a beta ray source (Sr-90 in a green disk). If one is not available, go on to step 5 or to part
three, and someone will be done with one soon.
2. Place it, writing side down, about 3/4 of an inch below your detector's window. (Blue: Clip the
tube in the holder on the front of the counter, with its window on the bottom. White: Use the
second slot from the top.)
3. Record the average counting rate. (Blue: Watch the needle wave around for a couple of minutes,
and record the highest and lowest readings you see. Then, average those two numbers. White:
Make a two minute run, and then divide by two to get counts per minute.)
4. Repeat step 3 with three pieces of lead stacked between the source and detector. Repeat once
more with the same thickness of aluminum, and then cardboard.
5. Replace the beta source with a gamma source (Cobalt-60, orange disk). Don't use one that says
"old"; that's for part three. Repeat steps 2- 4.
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.
- 3 PART THREE. Half life.
You will determine the half life of Co-60 by comparing the activity of a newer sample to an
older one's.
Put the cobalt-60 gamma ray source, writing on the bottom, right next to the tube. (Blue
counters: Lay the disk on the table with the detector's window right on top of it, no gap in
between. White counters: Use the top slot of the tube stand.) Find the average counting rate as
before.
To determine the half-life, it is necessary to observe the decay rate again at a later time to see
how rapidly it is decreasing. In the case of cobalt-60, several years would be necessary to see a
significant decrease. For those of you who don't want to wait that long, we have some older
samples. Subtract the date on the old one from the date on the new one, and assume that the new
one will be down to the activity of the old one that many years from now. Measure the decay
rate of the old sample, with the same method as before.
From the two decay rate measurements and the time between them, calculate the half-life of Co60. Do not expect an especially accurate value, because the original activity of the old sample
probably wasn't quite the same as the new sample's. However, your results should be within
20% of the accepted value printed on the sample. Compare these values in your conclusion.
- 4 Phy 133
Report on Experiment 13: Radioactivity
PART ONE:
Number of counts in one minute:
____________
____________
____________
____________
____________
Ave: ____________
Find your exposure:
PART TWO:
Beta: Nothing between: __________________________ counts/min
Lead:
__________________________ counts/min
Aluminum:
__________________________ counts/min
Cardboard:
__________________________ counts/min
Gamma: Nothing between: __________________________ counts/min
Lead:
__________________________ counts/min
Aluminum:
__________________________ counts/min
Cardboard:
__________________________ counts/min
PART THREE:
New sample, Ave = ______________
Calculate T1/2:
Old sample, Ave = ______________
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