Introduction
Report Format: • General text, Figures, Tables: Times New Roman, 12 pt • Section headings: Times New Roman, 13 or 14 pt
Results
Part-1: Detector operating voltage
Setup: ST-360 GM-Tube setup with counter and computer (software STX)
Objective : Plot and Understand the GM Tube Voltage plateau
Source used: 5 μCi 137Cs source at second position in the detector
Run
Voltage (V)
Counts
Uncertainty
1
50
0
0.00
2
100
0
0.00
3
150
0
0.00
4
200
0
0.00
5
250
0
0.00
6
300
254
15.94
7
350
314
17.72
8
400
316
17.78
9
450
323
17.97
10
500
296
17.20
11
550
294
17.15
12
600
332
18.22
13
650
304
17.44
14
700
321
17.92
15
750
318
17.83
16
800
301
17.35
17
850
305
17.46
18
900
374
19.34
19
950
534
23.11
Slope of plateau region
V2 (V)
850
V1 (V)
300
R2 (Count)
305
R1(Count)
254
S (% PER 100 V)
3.318
Optimal operating voltage
V
575
Part-2: Vary the Counting Duration and Understand its Effects
Number
Counts in 60s
Uncertainity (Sigma Count)
1
20
4.47
2
14
3.74
3
30
5.48
4
20
4.47
5
19
4.36
6
26
5.10
Mean count
21.5
Mean count per sec
0.358
Sigma avg
1.89
Sigma avg per sec
0.03
Average background counting rate (cps)
0.358 ± 0.03
Count duration (s)
Gamma Count
Uncertainity
Estimated Background
Uncertainity background
Real gamma count
Uncertanity of real gamma count
5
38
6
2
0.16
36
6.17
10
84
9
4
0.32
80
9.17
20
154
12
7
0.63
147
12.43
40
305
17
14
1.26
291
17.51
50
387
20
18
1.58
369
19.74
60
459
21
21
1.89
438
21.51
Part-3: Vary the Detector-Source distance & Understand its Effects
BACKGROUND
Number
Time (s)
Voltage
Counts
Uncertainty
1
60
600
25
5.000
2
60
600
14
3.742
3
60
600
12
3.464
AVERAGE BACKGROUND COUNTS
17
AVERAGE count per 60 sec
0.28
Uncertanity
2.38
Uncertainty rate
0.04
Average background counting rate (cps)
0.28 ± 0.04
Distance cm
Number
Time
Counts
UNCERTANITY
AVERAGE BACKGROUND COUNTS
Real counts
Uncertanity of real count
2.5
1
60
1494
38.65
17
1477
38.73
3.5
2
60
851
29.17
834
29.27
4.5
3
60
678
26.04
661
26.15
5.5
4
60
466
21.59
449
21.72
6.5
5
60
368
19.18
351
19.33
Part-4: Vary the Counting Duration and Understand its Effects
Thickness (mg/cm2)
Counts
Uncertanity
Real count
Real count uncertanity
Background
0
38
6.16
Lead
7367
148
12.17
110
13.64
3448
230
15.17
192
16.37
2066
291
17.06
253
18.14
1120
300
17.32
262
18.38
Thickness (mg/cm2)
Counts
Uncertanity
Real count
Real count uncertanity
Background
0
27
5.20
Al
840
278
16.67
251
17.46
655
286
16.91
259
17.69
645
306
17.49
279
18.25
522
328
18.11
301
18.84
425
319
17.86
292
18.60
Discussion
Part-1: Detector operating voltage
Q8: Is it a good or a bad GM tube? A slope of plateau below 2% to 3% is considered good, while a bad tube has slope of about 10% or more.
The slope of of the platuea of the GM tube was calculated to be 3.3%. Therefore, it is considered a good GM tube as the slope is below 10%.
Q9: Can you anticipate what will happen if the applied voltage is above V3?
Above V3 is the ischarge region where rapid rise in count rate occurs. Going above the voltage of V3 damages the detector.
Q10: Determind the optimal operating voltrage for this tube, given that it should lie around the middle of the voltage plateau (often, about 500V).
The optimal voltage is 575 V, calculated using the equation below where V2 is 300 and V3 is 850:
Q11: Will your above observations be any different if you plotted counting rate instead of counts?
There will be no change if counting rate was plotted instead of count.
Part-2: Vary the Counting Duration and Understand its Effects
Q5. Explain from where this radiation is coming into your detector.
The radiation is coming into the detector due to the background radiation. The sources of background radiation are terrestrail, cosmic and internal radiation. Another source that maybe considered is electromagnetic radiation from the cables generatic electic magnetic field.
Q6. Explain why you measured different values in each of your 6 runs?
Background radiation measures the naturally occuring and different values can be measured at ifferent times and places. Moreover, systemic errors related to the detector is also a factor. Therefore, it is best to take a few measurements to minimise the error.
Q11. Explain how increasing the time duration will affect the counting statistics?
As time increased, the counts measured increased. For example, at 20 seconds, 150 counts were measured and at 60 seconds, 438 counts were measured which shows a proportianl relationship as the counts almost trippled when trippling the time.
Q12. Explain how to select an optimum counting duration in a counting experiment?
The optimum counting uran should be selected by allowing suffcient time for the experiments purpose an mimimual uncertanity.
Part-3: Vary the Detector-Source distance & Understand its Effects
Q10. Does your fit look like a 1/r2 curve, where r stands for the distance? You can try to plot 1/r2 on X-axis instead of distance to check this fact.
After plotting Count vs 1/r2 , a linear relationship is clear between count and the inverse of distance square.
Q11. Correlate your observation to the method of radioprotection using distance. In other words, describe how you would exploit this dependency to reduce your exposure.
From Q10, we can see a clear inverse relationship between count and distance. When increasing distance, the count creases, hence the radiation exposure decreases. Therefore to apply radioprotection, the further the istance kept from the radiation source, the less the exposure.
Part-4: Vary the Counting Duration and Understand its Effects
Q.5 Which material is a stronger absorber and why?
Comparing Pb to Al, Pb had a higher decrease in counts, therefore was a stronger absorber compared to Al.
Q7. From exponential curves of Al and Pb, determine the approximate μen values. Compare these values with the respective theoretical values (at energy = 661 keV), as given in the Appendix.
As shown in the table below, the mass attenuation of Lead from the plot is very close to that of the theoriatical value. However, mass attenuation of Aluminium from the plot is much higher than the theoritical value. This value is not realistic and can be due to an experimental error.
Mass attenuation of Lead
From appendix
0.1167
cm2/g
From plot
0.0001
cm2/mg
0.1
cm2/g
Mass attenuation of Aluminium
From appendix
0.07762
cm2/g
From plot
0.0004
cm2/mg
0.4
cm2/g
Conclusion
Refereneces
Run
Voltage (V)
Counts
Uncertainty
1
50
0
0.0000
2
100
0
0.0000
3
150
0
0.0000
4
200
0
0.0000
5
250
0
0.0000
6
300
254
15.9374
7
350
314
17.7200
8
400
316
17.7764
9
450
323
17.9722
10
500
296
17.2047
11
550
294
17.1464
12
600
332
18.2209
13
650
304
17.4356
14
700
321
17.9165
15
750
318
17.8326
16
800
301
17.3494
17
850
305
17.4642
18
900
374
19.3391
19
950
534
23.1084
The uncertainity in the above table was calculated using 1sigma
The slope of the plateau region identified in the above tgraph between V2 AND V3 is calculated using the below formula.
Slope of plateau region
V2 (V)
850
V1 (V)
300
R2 (Count)
374
R1(Count)
254
S (% PER 100 V)
6.948
–
Part-2: Counting duration
Objective: Vary the Counting Duration and Understand its Effects
Source used:
No source to understand background measurements
5 μCi 137Cs source at second position in the detector
BACKGROUND:
AT 600V voltage and 60s measurements
Number
Counts in 60s
Uncertainity (Sigma Count)
1
20
4
2
14
4
3
30
5
4
20
4
5
19
4
6
26
5
To calculae background counting rate in counts per second (cps)
Mean count
21.5
Mean count per sec
0.358
Sigma avg
4.60
Sigma avg per sec
0.08
Error propgation
0.88
Average background counting rate (cps)
0.358 ± 0.88
The error propgation formula used was :
5 μCi 137Cs source:
