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Laboratory #22: Absorption of
β
and
γ
Rays
Goal:
Detect nuclear radiation with a gasfilled detector; understand statistical
uncertainty. Study the absorption of
β
and
γ
rays in matter.
Equipment:
"The Nucleus" counting station with GeigerMüller tube, counter and timer.
Set of absorbers of varying thickness,
90
Sr and
60
Co source. Monitor4
Geiger counter, Vernier interface, computer.
(A)
Counting Statistics
Here, we want to convince ourselves that repeating a counting experiment N times leads
to results
n
i
(
i
=1,
N
) that are distributed like a Gaussian around a mean value
<n>.
In order
to demonstrate this well, we need many measurements. Thus we enlist the help of a
computer to do this for us.
Place a
60
Co source in the source holder in the black wooden bench, turn on the computer
and the Monitor4 (its little red light should now start flashing with each count). Click on
SW137 and then doubleclick on the &Exp04 Statistics± icon. Click &experiment±, then
&sampling± and set the experiment length to 300s and the count interval to 1s. Click
&collect±. You now should be making 1s measurements, acquiring a histogram as you go.
Adjust the distance between the counter and the source until the count per s is between 25
and 30. Click &stop± and &collect± again to restart the experiment. Wait till the experiment is
finished.
Copy the values
f
n
(number of times, the result
n
occurred) of the measured distribution
into your logbook. Calculate the average
<n>
for the
N
=300 measurements from
∑
∑
>=
<
n
n
f
f
n
n
/
,
(
)
where the sum extends over the
bins
of the histogram.
Plot the measured distribution
f
n
. On the same graph also plot the theoretically expected
distribution. This is a Gaussian with a mean
=
<n>
and a standard deviation
σ
2
=
<n>
:
>
<
−
>
<
−
=
n
n
n
o
n
e
m
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 Spring '11
 Urheim

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