Radioactivity and Half life

Question 1

Define Radioactivity

Answer 1

Spontaneous emission of particles and energy from unstable atomic nuclei. It occurs to achieve a more stable configuration of nucleus.

Atomic nucleus undergoing radioactive decay is the parent and product arising is the daughter.

Radioactivity is measured through Geiger counter. Records all decays (a,b,g) that reach it per second.

Question 2

Alpha radiation

Answer 2

• 2 protons + 2 neutrons, no electrons, overall charge of +2
• Reduces atomic no. by 2 and mass no. by 4, parent atom becomes a new element.
• Strongly ionizing- potential for biological damage.
• Concern if alpha particles are ingested/inhaled. Large dose can cause radiation poisoning.
• Natural sources- U-238, Rd-226
• Artificial sources- Plutonium, curium radioisotopes - produced in nuclear reactor through absorption of neutrons.

Question 3

Beta radiation

Answer 3

• Charge of -1, no mass. Neutron decays into proton and e, proton stays in nucleus, e emitted at high speed.
• Can travel several cms in air. Can be stopped by 5mm of aluminum or protective clothing. Moderately ionizing.
• Sufficient intensity of B-radiation can cause burns, like severe sunburn.
• Natural sources- radioactive decay chain of uranium, thorium, e.g. lead-210, bismuth-214 etc.
• Artificial sources- radioactive byproduct of nuclear fission.

Question 4

Gamma radiation

Answer 4

• Electromagnetic radiation in form of high energy light. No mass, no charge.
• Pass straight through materials rather than collide with atoms, weakly ionizing.
• Travel long distances through air. Thick sheets of lead or layers of concrete to stop them.
• High exposure can cause direct acute effects through immediate damage to cells. Low levels of exposure increases probability of cancer induction- with increased exposure.
• Natural sources- radiation decay seriesd of uranium, thorium, actinium, K-40, C-14.
• Artifical sources- produced in fission in nuclear reactors, high energy physics experiments, nuclear explosions and among isotopes.

Question 5

Define half life

Answer 5

Time taken for a number of radioactive nuclei in the sample to halve

OR

Time taken for no. of decays (activity) in the sampleto halve.

Question 6

Formulas

Formula for radioactive decay?

Answer 6

N= N0 e^-λt

N= N0. (1/2)^ t/ t1/2

N- remaining no. of atoms
N0- initial no. of atoms.
λ- half life
t- time

Question 7

Formula

Formula to find the number of daughter atoms (D”) produced at any time t

Answer 7

D” = N0 - N
Thus D” = N0 (1 - e^-λt)

Question 8

Formula for Half life?

Answer 8

t1/2= 0.693/ λ
Or
λ= 0.693/ t1/2

Question 9

Formula to calculate time ‘t’

Answer 9

t= 2.303/λ log (N0/N)