Nuclear Decay (unit 3)

Question 1

What is alpha radiation?

Answer 1

-fast moving particles, helium nuclei
- 4
Helium
2

Question 2

What is beta radiation?

Answer 2

Electrons with speeds just less than speed of light ejected from radioactive nuclei
- 0
B
-1

Question 3

What is gamma radiation?

Answer 3

• photons of high energy
• 0
  Y
  0

Question 4

A
Z
x

Answer 4

-a is the mass number (number of protons and neutrons)
- z is the atomic number (number of protons)

Question 5

What is half life?

Answer 5

-the time taken for the number of radioactive nuclei, to reduce to one half of it’s initial value
- unit - s

Question 6

What is activity?

Answer 6

-the rate of decay of a sample of radioactive nuclei
-unit - Bq or s^-1

Question 7

Alpha radiation properties?

Answer 7

-positive charge, +2e.
-strongly ionising.
-low penetrating power
-stopped by skin or paper.
-deflected by electric and magnetic fields.

Question 8

Beta radiation properties?

Answer 8

-negative charge, -e.
- weakly ionising.
-greater penetrating power than α-particles.
-stopped by a few millimetres of aluminium.
- deflected easily by electric and magnetic fields.

Question 9

Gamma radiation properties?

Answer 9

-electromagnetic radiation of very high frequency.
- no charge.
-very weakly ionising.
-very high penetrating power.
- intensity is reduced by thick lead.
-not deflected by electric and magnetic fields.

Question 10

How do alpha move in magnetic and electric fields?

Answer 10

• magnetic - deflected slightly
• electric - deflected slightly towards negative plate

Question 11

How do beta move in magnetic and electric fields?

Answer 11

• magnetic - Beta particles are deflected by a much greater amount in the opposite direction
• electric -Beta particles are deflected by a much greater amount towards the positive plate.

Question 12

How do you make allowances for background radiation in experimentsl measurements?

Answer 12

-subtract background count from count from material

Question 12

Activity equation?

Answer 12

Initial activity/ 2^half life

Question 12

What is one Becquerel equal to?

Answer 12

• 1 Bq = 1 disintegration per second

Question 12

Why is the actvity equation the way it is?

Answer 12

-The negative sign is needed here as N decreases with time and A is the rate of decay ​
-This equation shows that the activity, A, is directly proportional to the number of radioactive nuclei, N, present in the sample.
-As time passes and a radioactive substance decays, then both N and A will decrease.
-After one half-life, the values of N and A will both halve

Question 12

How to derive time constant equation?

Answer 12

The activity, A, of a sample of radioactive material decreases exponentially with time, t.
So: A=Aoe^−λt

The half-life, T ½, is the time taken for the activity to fall to half of its initial value.

Therefore: when t=T½, A/ Ao =½

We can re-write the equation above as:
A/Ao =e^−λt

Substituting gives
1/2 =e^−λT 1/2

If we find the reciprocal of each side, then the minus sign disappears and we have:

2=e^λT 1/2

Taking natural logs (ln) of both sides gives:
ln 2=λT 1/2
Therefore:
λ=ln2/T 1/2
​

Question 13

Investigation of radioactive decay using dice practical?

Answer 13

• have 80 dice (or cubes) and a cup
• Throw the dice onto the table
• Suppose all the dice with the number 1 uppermost have disintegrated.
  -Remove these dice and count the number remaining
  -Repeat this for a further 9 throws (making 10 in all) and note down the number of throws and the number of dice remaining each time.
  -Plot a graph of number of dice remaining (y-axis) against number of throws (x-axis).
  This should give an exponential curve with a half-life of about 3.8 throws.

Question 14

What is radioactive decay based on?

Answer 14

• assumed that disintegrations are completely random