Topic 5.3: Radioactive Decay Questions

State what is meant by the activity of a radioactive source, and state its SI unit.

State what is meant by the *mass defect* of a nuclear decay.

State the two quantities that are conserved when a nuclear decay equation is balanced.

A radiation is found to travel at almost the speed of light and is deflected by a magnetic field in the direction expected for a negative charge.

Identify the type of radiation and state one further property that distinguishes it from gamma radiation.

Radium-224 (${}_{88}^{224}$Ra) decays by alpha emission to radon.

Identify the proton number and the nucleon number of the radon daughter nuclide.

State what an alpha particle is in terms of its constituent particles, and give its charge.

A radioactive isotope has a half-life of 4.0 days.

A sample starts with an activity of 2.4 × 10⁸ Bq.

Determine the activity of the sample after 16 days.

Complete the nuclear equation for the alpha decay of americium-243: ${}_{95}^{243}$Am $\to {}_Z^A$Np $+{}_2^4\alpha$.

Determine A and Z for the neptunium daughter.

Outline why the daughter nuclide in beta-minus decay is a DIFFERENT element from the parent, while having the same nucleon number.

A Geiger counter placed next to a fresh radioactive source records a count rate of 410 counts per second.

When the source is removed, the counter still records 10 counts per second from background radiation.

The source has a half-life of 15 minutes.

(a) State what is meant by the half-life of the source.

(b) Determine the count rate the counter will record 45 minutes later.

Describe what is meant by the ionising power of a radiation, and explain why alpha radiation is the most strongly ionising of the three types.

A stationary nucleus emits an alpha particle (mass 4 u) and recoils as a daughter nucleus (mass 234 u).

The total energy released is 4.80 MeV.

Estimate the kinetic energy of the alpha particle, in MeV.

A sample of magnesium-27 (${}_{12}^{27}$Mg) is a beta-minus emitter.

(a) Complete the decay equation ${}_{12}^{27}$Mg → ${}_Z^A$Y + ${}_{-1}^0$e + $\overline{<br>u}$ by finding A and Z.

(b) Explain, in terms of conservation of proton number, why the daughter Y is an element with a higher proton number than magnesium.

Outline why a background count rate must be measured and subtracted before using count-rate data to study the decay of a source.

Outline how a beam containing alpha, beta-minus and gamma radiation could be separated, and describe how each radiation behaves, when the beam is passed through a uniform magnetic field directed into the page.

A worker handling a sealed radioactive source is told that, of the three types of radiation, gamma poses the greatest hazard from outside the body but alpha poses the greatest hazard if the source material is inhaled.

Explain this apparent contradiction.

A counter near a source reads 740 counts per second.

With the source removed it reads 20 counts per second.

The source has a half-life of 25 minutes.

Calculate the count rate the same counter will read after 50 minutes.

Two sources M and N have the same half-life.

The activity of M is presently 6 times the activity of N.

Deduce the ratio of the activity of M to the activity of N after four half-lives have passed.

An unknown nuclide ${}_Z^A$X decays by alpha emission to form thallium-208 (${}_{81}^{208}$Tl).

Deduce the nucleon number A and proton number Z of the parent nuclide X.

Sketch a graph of count rate (corrected for background) against time for a radioactive source, marking the value of one half-life on the time axis, and describe two features of the curve.