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Nuclear Decay

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Nuclear Decay
Jr. Lab II sp’01
Lise Johnson
Phillip Forkner
List of Experiments
•
•
•
•
•
Geiger Plateau
Background Radiation Measurment
Resolving Time
Shelf Ratios
Randomness of Disintegration
Geiger Tubes
•
•
•
•
Gas filled cylindrical tubes with an applied electric field
Charged particles entering the tube cause a measurable pulse
Rely on gas multiplication/ Townsend Avalanches
Not proportional Counters
Geiger Plateau
•
•
•
•
•
•
No gas multiplication at low
voltages
At minimum voltage avalanches
begin to form
On the plateau avalanche increases
are minimal
At higher voltages avalanches are
uncontrollable
Operating above the plateau fries
the tube
Optimal operating voltage is at the
knee of the plateau
Geiger Plateau (Tl-204)
200
180
160
CPM
140
120
100
80
60
40
20
0
400
500
600
700
800
Voltage (V)
900
1000
1100
Background Measurement
• No source, Voltage = 850, with and without
shielding
• Found that approx 66% of the background
was shielded
Resolving Time
• There is an amount of time after a “hit” that
the counter will not register subsequent hits
• Dead time =  
r1 пЂ« r2 пЂ­ r3
r1 пѓ— r2
Shelf Ratios
• Radiation is emitted from a
source in all directions
• Only a fraction reaches the
counter
• How does the count fall off
when the source is moved ?
• Beta sources fall off faster
because they are absorbed by
air
1800
1600
1400
1200
1000
800
600
400
200
0
data
shelf^-2
0
2
4
6
8
distance (shelf)
Tl-204
CPM
CPM
Sr-90
100
90
80
70
60
50
40
30
20
10
0
Data 2
shelf^-1
0
2
4
Distance (shelf)
6
8
Randomness of Disintegration
12
Frequency
10
8
6
4
2
0
1
2
3
4
5
6
Counts / 10sec
7
8
9
10
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