Physics U29

Mass defect = mass of nucleus - total mass of individual separate nucleons.

Binding energy = energy required to completely separate nucleons to infinity.

Greater binding energy gives rise to release of energy.

Total binding energy / total number of nucleons = Mass defect/ c^2

1 amu = 1 u = 1.67 x 10^-27 kg

In radioactive decay, the interval between 2 decays is random. The overall rate of decay decreases as number of undecayed nuclei decreases eventually.

We consider rate of decay of large number of nuclei and take an average to determine decay constant lambda. Lambda = Probability of decay for 1 nucleus / time interval. (Lambda = Probability of decay per unit time)

Acitivity of a sample (A) = Rate of decay = lambda x N; where N = number of undecayed nuclei.

A = N/t

A graph of N vs t is an exponential graph. No = Initial undecayed nuclei. N = Presentt undecayed nuclei.

N = No exp(-lambda x t)  *similar for A and R.

A = Activity, N = Undecayed nuclei, R = Count rate

Number of Nuclei = Total mass in kg of the sample / (Number of Nucleons x atomic mass unit).

Number of nucleons = Number of moles x Avogadro's constant. (N = n x NA)

Spontaneous: Not dependent on any external conditions.

Random: It might decay any time. (Unpredictable).

Radioactive: It is random and spontaneous emission from nucleus. Nucleus emits particles.

Decay constant: Probability of decay of a nucleus per unit time.

Q. Suggest reasons why the activity and the measured count rate may be different.

1. Radiation is emitted in all directions and as a result the activity of the source and the measured count rate are different.

2. The detector is not sensitive to all radiations.

3. Dead time of counter.

4. Window of detector may absorb same radiations.

5. Background radiation.

Q. Explain why the emitted Beta particles have a range of energies.

The energy of Beta particle is shared with anti-neutrino.

Q. State what is meant by radioactive decay.

Emission of particles or radiation from an unstable nucleus occurs through spontaneous emission.