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: