Radioactivity©98
EXPERIMENT 11
Objective: To observe the radioactive decay of certain isotopes, to measure the
penetrating power of their radiations, and to calculate their half-lives and decay
constants.
DISCUSSION:
The nuclei of certain isotopes are unstable, eventually breaking up spontaneously
into stable particles. The most frequent products of this decay are alpha particles
(helium-4 nuclei), beta particles (electrons), gamma rays (very high energy photons)
and whatever is left of the nucleus after decay has occurred.
It is not possible to predict when a given nucleus will decay. However, given a
very large number of a given nucleus, it is possible to predict that a certain fraction of
them will remain unchanged after a certain lapse of time. Specifically, if initially there
are n0 nuclei, then after a lapse of time t, the number n remaining is given by
n  n0 e t ,
(1)
whereis a constant that depends upon the isotope undergoing the nuclear
transformation. This kind of relationship is known as exponential decay. It is also
encountered, among other places, in the expression that gives the current delivered by a
capacitor while discharging through a resistor.
A characteristic of exponential decay is that the time in which any given fraction
of the nuclei disintegrates is always the same. For example, if after 50 years, 15% of the
nuclei have decayed, then in another 50 years, 15% of the remaining nuclei will decay.
An indirect way of expressing the value of the decay constantis to give the
timein which half of the sample decays. This time is called the half-life. In this case n
= n0 /2, and so e = 2. The value offor which eis 2 is the natural logarithm of 2,
0.69315… That is, the half-life is given by

0.69315

.
(2)
The value ofcan be determined experimentally by observing the decay of the
radioactive substance. Suppose, for example, that the sample is polonium-210, which
decays by the emission of a helium nucleus to become lead-206. In a certain time
interval, say one minute, just a small part of the polonium nuclei disintegrate, but this
number is proportional to the total number of nuclei in the sample. As this total number
gets smaller, so also does the number which decay. That is, the number of disintegrating
nuclei is directly proportional to the total number in the sample. Thus Eq. (1) can also be
written in terms of the rates of decay. Letting ro be the initial number of disintegrations
per unit time, and r the number at time t, we have
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r

n
n0 ,
(3)
r  r e t.
0
(4)
1  r0  .
t  r 
(5)
r0
and so,
Taking the inverse of Eq. (4),we have
  ln 
We next consider the penetrating power of the three kinds of radiation. Alpha
particles consist of two protons and two neutrons, bound together in a very stable
arrangement. The alpha particle, with its charge of 2 protons, usually does not possess
much kinetic energy as it leaves the parent nucleus. Because of its charge and low speed,
it generally interacts very quickly with matter and loses what little kinetic energy it had.
It picks up two stray electrons and becomes the helium atom, which then dissipates into
the atmosphere.
Beta particles are electrons. They generally leave the decaying nucleus at high
speeds. But because of their charge and small mass, they also are quickly brought to rest
when interacting with matter. However, they are more penetrating than alpha particles.
Gamma rays are very high-energy photons. Because these photons are uncharged
and carry much energy, they do not react with matter as easily as alpha and beta particles.
Nevertheless, their penetrating power is radically limited by lead shielding.
If the half-life of a radioactive sample is very long, then during the course of a
few hours, the number of disintegrations per unit time, r, will not change appreciably. In
this case, the number of particles that are able to penetrate matter drops off exponentially
with the thickness of material. Letting 0 be the number of particles impinging on the
surface of an absorbing medium and  the number of particles emerging from the
medium after having traversed the distance x, we have that
   0 e  k ,
(6)
where k is the absorption constant. Its value depends upon the nature of the radiation and
the absorber. Eq. (6) is identical in form to that for the decrease in counting rate with
time, Eq. (4).
There is a particular thickness  such that e-k is 0.5. This thickness is called the
half-value thickness for the material for the particular radiation involved. The value of
can be determined by first finding k and setting  equal to 0.69315/k.
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APPARATUS SETUP:
1. Setup the Signal Interface as shown in Fig. 1.
a. Plug the power brick into the back of the Interface, then into the strip on your
table.
b. Connect the white cable into the back of the Interface then into your
computer’s USB port.
c. Plug the Geiger Tube into Digital Channel 1.
d. Turn on the Interface (switch is on the back).
Figure 1: PASCO Interface setup.
2. Start DataStudio. It should be located somewhere under your start menu.
3. When prompted, choose ‘Create Experiment’.
4. Set up the software to recognize the hardware:
a. In the software, add a “Geiger Counter” Digital Channel 1.
b. Notice that there is a list of Data options on the upper left (usually). Also notice
that there is a list of Display options on the bottom left (usually).
c. Click and drag the “Table” icon to “Geiger Counts, Ch1 (counts)”.
d. In the Table window, click on the Statistics button
‘Standard Deviation’.
and choose ‘Mean’ and
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e. Move and resize the table window so you can see everything, making the table
long.
EXERCISES FOR HALF-VALUE THICKNESS: Shielding of Alpha, Beta, and
Gamma Radiation: We will be setting the timer to count for five 30sec intervals.
Several counts are needed so that a mean and standard deviation of the measured quantity
can be found, thus more accurately representing the actual number of decays in a given
time period. The individual results will be listed in a table and the average will be
calculated there.
1. Set up the length of sample time and the automatic stop:
a. Set the Sample Rate interval 30 sec.
b. Under the “Sampling Options…” choose the “Automatic Stop” tab.
c. Choose ‘Time’ and set the interval to 150 sec (5*30sec).
2. Background Radiation. Background radiation is just that, radiation ever present in
the background. It can come from many sources including the sun and naturally
occurring radioactive processes. This background count is to be subtracted from all
other data taken during this experiment as it adds a systematic error to the results.
a. Adjust the Geiger detector so that it is approximately 3/4 of an inch above the
table top.
b. Remove the cap from the Geiger detector.
c. Click ‘Start’ in the DataStudio window and let the counter count for its 5, 30 sec
intervals. The counter should stop recording automatically.
Note: you will only see data show up in the table after each 30 sec interval.
d. Record the mean and standard deviation for the background radiation.
e. Save your background radiation data.
3. Alpha Radiation.
a. Under “Experiment” menu, choose “Delete ALL Data Runs”.
b. Place a source of alpha particles under the Geiger tube, hole side up.
c. Click ‘Start’ in the main window and let the counter count for the 5, 30 sec
intervals.
d. Record the mean and standard deviation for the alpha radiation without shielding.
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e. Place a piece of paper between the source and the tube.
f. Click ‘Start’ in the Science Workshop window and let the counter count again for
5 intervals.
g. Record the mean and standard deviation for the alpha radiation with shielding.
h. Save your alpha radiation data.
4. Beta Radiation. Measure the transmission of beta radiation through successively
thicker absorbers.
a. REPLACE THE CAP ON THE GEIGER TUBE!
b. Under “Experiment” menu, choose “Delete ALL Data Runs”.
c. Place a source of beta particles under the Geiger tube.
d. Click the ‘Start” button in the main window and let the counter count for its 5
intervals.
e. Record the mean for beta radiation without any absorber. This value is v0.
f. Place, in succession, each of the polyethylene absorbers between the source and
the Geiger tube and record the mean v and absorber thickness  for each absorber.
v 
g. In Excel, Graph ln  0  verses  and determine the absorption constant k from
 v 
 
the slope of the line.
h. Save your beta radiation data.
5. Gamma Radiation. Repeat Exercise 4 for gamma rays using the lead absorbers.
Save your gamma radiation data.
6. Discuss Exercises 3, 4 and 5 and what the results tell you about the nature of
alpha, beta and gamma radiation.
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EXERCISES FOR RADIOACTIVE DECAY: Lifetime of Barium Nucleus: Do not
procure your radioactive sample until you are ready to take data – read the
instructions below FIRST.
1. Get rid of old setup.
a. Under “Experiment” menu, choose “Delete ALL Data Runs”.
b. Close the table.
c. Click and drag the “Graph” icon to “Geiger Counts, Ch 1 (counts/sample)”.
2. Change the sample rate to 15 sec.
3. Set up the counter for 48, 15sec intervals (48x15 sec = 720 sec).
a. Find the “Set Sampling Options…” and choose the “Automatic Stop” tab.
b. Change the “Time” interval to 720 sec.
4. Data Acquisition
a. Place the tray with the radioactive liquid on the table with the Geiger counter 3/4
inch above the sample. The cap on the G-M tube should be removed.
b. Click the ‘Start’ button to begin taking data. Datastudio will automatically update
the Table display and the Graph as each new sample point is taken. It will
automatically stop at the end of 48 samples.
c. Go get a cup of coffee.
d. Save your half life data.
DATA ANALYSIS: Determine the half-life of the sample using Eq. (2) and the graph
generated.
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