Nuclear Decay (unit 3) Flashcards
What is alpha radiation?
-fast moving particles, helium nuclei
- 4
Helium
2
What is beta radiation?
Electrons with speeds just less than speed of light ejected from radioactive nuclei
- 0
B
-1
What is gamma radiation?
- photons of high energy
- 0
Y
0
A
Z
x
-a is the mass number (number of protons and neutrons)
- x is the atomic number (number of protons)
What is half life?
-the time taken for the number of radioactive nuclei, to reduce to one half of it’s initial value
- unit - s
What is activity?
-the rate of decay of a sample of radioactive nuclei
-unit - Bq or s^-1
Alpha radiation properties?
-positive charge, +2e.
-strongly ionising.
-low penetrating power
-stopped by skin or paper.
-deflected by electric and magnetic fields.
Beta radiation properties?
-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.
Gamma radiation properties?
-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.
How do alpha move in magnetic and electric fields?
- magnetic - deflected slightly
- electric - deflected slightly towards negative plate
How do beta move in magnetic and electric fields?
- 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.
How do you make allowances for background radiation in experimentsl measurements?
-subtract background count from count from material
Activity equation?
Initial activity/ 2^half life
What is one Becquerel equal to?
- 1 Bq = 1 disintegration per second
Why is the actvity equation the way it is?
-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
How to derive time constant equation?
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
Investigation of radioactive decay using dice practical?
- 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.
What is radioactive decay based on?
- assumed that disintegrations are completely random
Decay constant units?
- s^-1
