CP 15: Absorption of Gamma ray through lead

Question 1

CP 15 Equipment list

Answer 1

• Radioactive source
• Ruler
• Mount for radioactive sources
• GM tube
• Tongs
• Different thickness of lead
• Lead lined container to store source when not in use

Question 2

CP 15 Method

Answer 2

1. Connect the Geiger-Müller tube to the counter and, without any sources present, measure background radiation over a five-minute period
2. Record this value
3. Calculate the average background rate per minute
4. Measure the thickness of the lead absorbers using Vernier callipers at three points on each sheet.
5. For each sheet record the average thickness
6. Place the radioactive source a fixed distance of 10 cm away from the tube
7. Record the count rate over one minute
   8.Repeat steps 3 and 4 a further two times, recording the count rate each time
8. Place the thinnest absorber directly in front of the gamma ray source
9. Repeat steps 3-5
10. Replace the sheet with another thickness and continue taking three readings per thickness

Question 3

Safety precautions in CP15?

Answer 3

Keep source stored in lead lined box when not in use
Do not point source to anyone
Keep distance (min 1m) from source

Question 4

How to increase accuracy of CP15?

Answer 4

Ensure repeats are taken and mean is calculated as decay is random; reduces effect of random error
Ensure experiment is done in same location to avoid changes in background radiation

Question 5

Reasons for inaccurate count rate

Answer 5

• Counting too short of time
• No record of background radiation
• Mean not taken
• GM tube too far away from source
• Radiation spreads out from source so not all emitted radiation will reach GM tube

Question 6

Why i there an exponential decrease in rate of decay for a sample containing large number of unstable nuclei

Answer 6

• Fixed probability of individual nucleus undergoing decay
• For sample with more unstable nuclei there is a predictable pattern
• Fraction of nuclei decaying in the next second is equal to decay constant
• Hence number of nuclei decaying depends on number of unstable nuclei A=lambda x N
• Number of unstable nuclei decreases exponentially as N=Ne
• So rate of decay and activity decreases exponentially as A=Ae