Radioactivity (Unit 5)

Question 1

Definition of an isotope

Answer 1

are atoms of the same element with the same number of protons but a different number of neutrons

Question 2

Properties of alpha, beta and gamma radiation

Answer 2

See table on sheet

Question 3

Identifying alpha, beta and gamma radiation

Answer 3

Put the detector within a few cm of the source and put some paper between the source and the detector. If the count rate drops significantly, then the source is alpha.
If the count rate doesn’t change, remove the paper and replace it with a few mm of Aluminium. If the count rate drops significantly, then the source is beta. If the count rate doesn’t change, then the source is gamma.

Question 4

Applications of alpha, beta and gamma radiation

Answer 4

alpha – smoke detectors
beta – thickness measurement of cardboard (in a paper mill)
gamma – detecting leaking pipes / medical tracer

Question 5

Safe handling of radioactive sources in a laboratory

Answer 5

• handle with (long) (30 cm) tweezers because the radiation intensity decreases with distance
• store in a lead box (immediately) when not in use to avoid unnecessary exposure to radiation

Question 6

Examples of background radiation

Answer 6

• Cosmic rays
• Ground, rocks and buildings
• Radon (in atmosphere)
• Nuclear fallout (from weapons testing/nuclear accidents)
• Discharge/waste from Nuclear power

Question 7

Experimental verification of inverse square law for gamma rays

Answer 7

• Count rate measured by GM tube from a gamma source (gamma rays not stopped by air)
• Measured count rate equals counts from source PLUS background counts
• Measure background count rate and subtract this from the measured rate with the source present. This gives the corrected count rate (counts just from the source).
• Vary the distance of the GM tube from the source and plot a graph of corrected count rate against 1/(distance)2 to establish an inverse square relationship between intensity and distance.

Question 8

What is meant by random nature of radioactive decay

Answer 8

• there is equal probability of any nucleus decaying,
it cannot be known which particular nucleus will decay next.
• it cannot be known at what time a particular nucleus will decay.
• the rate of decay is unaffected by the surrounding conditions.

Question 9

Definition of decay constant

Answer 9

the probability of (a nucleus) decay per unit time (usually per second).

Question 10

Definition of activity

Answer 10

The number of nuclei of an isotope that decay each second.

remember each decay produces one radioactive particle that can then be detected with a Geiger Muller tube

Question 11

Units of activity

Answer 11

Bq (Becquerel) – number of decays per second.

Question 12

Definition half life

Answer 12

Time taken for half the nuclei of a particular isotope present to decay OR time taken for the activity of a particular isotope to half

Question 13

Decay curves

Answer 13

See sheet

Question 14

Decay constant from a log graph

Answer 14

decay constant = - gradient

Question 15

Plot sketch of graph of N against Z for stable nuclei

Answer 15

See sheet

Question 16

Decay equations for alpha, beta+, beta- and electron capture

Answer 16

See sheet

Question 17

Decay modes for alpha, beta+, beta- and electron capture

Answer 17

See sheet

Question 18

Existence of nuclear excited states within nucleus

Answer 18

• Nuclei can be in excited states (eg following radioactive decay).
• Nuclei can de-excite (and lose energy) by emitting a photon.
• As energy levels differences are really big in nuclei, the photons have very large energies.
• And hence high frequencies and short wavelengths (gamma part of spectrum).
• Gamma emission is therefore often associated with alpha and beta decay.

Question 19

Why is beta emission associated with gamma rays of discrete frequencies

Answer 19

• Following beta decay the nucleus is in an excited state
• Which are at discrete energies
• And emit gamma rays when they de-excite/fall down to lower states
• Reference to E=hf and stating gamma rays (or drop in energy level) have discrete energies.

Question 20

Why is the gamma source technicium-99 used in medical diagnosis.

Answer 20

• It only emits gamma rays
• gamma rays can be detected outside the body/are weakly ionising and cause little damage
• It has a short enough half-life and will not remain active in the body after use
• It has a long enough half-life to remain active during diagnosis
• The substance has a toxicity that can be tolerated by the body
• It may be prepared on site (at hospital)

Question 21

Features of Rutherford scattering

Answer 21

Experimental set-up
• Air must be removed (vacuum) because alpha particles only travel short distances in air due to collisions with air molecules.
• Gold foil must be thin so that alpha particles are not absorbed by the foil (have more than one collision).
Observations
• Majority of alpha particles pass straight through (without any deflection).
• A small number of alpha particles are scattered through very large angles, some even come back towards the alpha source (scattered through 180 degrees).
• Conclusions about structure of atom
• The atom is mostly empty space because most alpha particles do not pass close enough to the nucleus to be deflected.
• The nucleus is very small and positively charged to provide necessary electric field strength to repel alpha particle.
• Nucleus contains most of the mass (of the atom) because the alpha particles are scattered through very large angles
• Nucleus is much smaller compared to the separation between nuclei (and hence to the size of the atom itself).

Question 22

Maximum size of nuclear radius from estimate of closest approach

Answer 22

conversion of initial ke (of alpha particle) to electrical potential energy at point of closest approach.
Ek=[1/2mv2]=1/(4pi(epsilon 0)) x QaQN/r
Where Qa is charge on alpha particle, QN is charge on nucleus and r is effectively the distance of closest approach to nucleus.

Question 23

Why are other methods for measuring the nuclear radius other than alpha scattering used?

Answer 23

• strong force acting between alpha particle and nucleus complicates results
• scattering is produced by the distribution of protons, not the whole nucleon distribution
• alpha particles are relatively massive, causing recoil of nucleus which complicates results

Question 24

Advantage of using electrons to measure nuclear radius

Answer 24

• electrons are not subject to the strong force so, electron scattering patterns are easier to interpret.
• electrons give greater resolution (or are more accurate) because they get closer to the nucleus and alpha particles cannot get so close to the nucleus due to electrostatic repulsion so only provide information on the closest distance of approach, not radius.
• electrons produce less recoil in nucleus because electrons are much less massive (than nucleus).
• high energy electrons are easier to produce because electrons have a lower specific charge so are easier to accelerate.

Question 25

Experimental method for determining size of nucleus

Answer 25

Electron scattering

Question 26

Typical size of nuclear radius

Answer 26

1x10-15 m (1 fm)

Question 27

Information that can be gained about the nucleus using alpha particles

Answer 27

• maximum diameter of the nucleus
• proton number and nuclear charge
• that the mass of the nucleus is most of the mass of the atom

Question 28

Information that can be gained about the nucleus using high energy electrons

Answer 28

• Nuclear radius (diameter)

* Nuclear density

Question 29

Determination of nuclear radius from electron scattering

Answer 29

See sheet

Question 30

Derivation of radius from experimental data

Answer 30

See sheet

Question 31

Calculation of nuclear density

Answer 31

See sheet