Free Nuclear Physics MCQs with Answers

11. Carbon dating is possible because

• A. carbon 14 is stable
• B. living things take in carbon 14 while alive, and it then decays with a known half life after death
• C. all carbon is radioactive
• D. carbon 14 is produced in the body

Explanation: While an organism lives it exchanges carbon with its surroundings and holds the atmospheric ratio of carbon 14 to carbon 12, but after death intake stops and the carbon 14 decays with a half life of about 5730 years. Measuring the remaining ratio therefore gives the time since death. The method is useful up to roughly 50,000 years, after which too little carbon 14 remains.

12. Nuclear fission is the process in which

• A. two light nuclei join to form a heavier one
• B. a heavy nucleus splits into two lighter nuclei, releasing energy and neutrons
• C. a nucleus emits an alpha particle
• D. an electron is captured by a nucleus

Explanation: Splitting a uranium 235 nucleus releases about 200 MeV together with two or three neutrons, and those neutrons can trigger further fissions in a chain reaction. Joining light nuclei is fusion, which powers the Sun and releases even more energy per unit mass. Both release energy because the products are more tightly bound than the reactants.

13. In a nuclear reactor, the moderator is used to

• A. absorb excess neutrons and shut the reactor down
• B. slow fast neutrons so that they are more likely to cause further fission
• C. cool the reactor core
• D. shield the operators from radiation

Explanation: Graphite or heavy water slows the neutrons through repeated collisions, because slow neutrons are far more readily captured by uranium 235 and sustain the chain reaction. Absorbing surplus neutrons to control the rate is the job of the control rods, usually boron or cadmium. Confusing the moderator with the control rods is the standard error here.

14. The energy released in a nuclear reaction comes from

• A. the chemical bonds between atoms
• B. the small loss of mass, converted according to E equals mc squared
• C. the kinetic energy of the electrons
• D. friction inside the nucleus

Explanation: The products of fission or fusion have slightly less total mass than the reactants, and that mass defect appears as energy, which is why nuclear reactions release millions of times more energy per event than chemical ones. The binding energy per nucleon curve, peaking near iron, explains why light nuclei release energy by fusing and heavy ones by splitting. Chemical bonds involve only the outer electrons.

15. The binding energy per nucleon is greatest for nuclei near

• A. hydrogen
• B. iron
• C. uranium
• D. helium

Explanation: The curve of binding energy per nucleon rises steeply from hydrogen, peaks around iron and nickel at about 8.8 MeV, then falls slowly towards uranium. Nuclei on the left release energy by fusing towards the peak and those on the right by splitting towards it. This is why iron is the end point of fusion in a massive star.

16. Cobalt 60 is used in hospitals mainly as a source of

• A. alpha particles for imaging
• B. gamma rays for treating cancer
• C. neutrons for sterilising instruments
• D. X rays for dental work

Explanation: Its penetrating gamma radiation can be focused on a deep tumour from outside the body, and because dividing cells are more sensitive, the cancer is damaged more than the surrounding tissue. The same gamma source is also used to sterilise medical equipment. Alpha particles cannot penetrate the skin and so are useless for external imaging or therapy.

17. A radioactive tracer used for medical diagnosis should ideally have

• A. a long half life and emit alpha particles
• B. a short half life and emit gamma rays that escape the body for detection
• C. a very long half life and emit beta particles
• D. no radiation at all

Explanation: Gamma radiation escapes the body so an external camera can detect it, and a short half life means the activity falls quickly once the scan is done, limiting the patient's dose. Technetium 99m, with a six hour half life, is the workhorse of nuclear medicine for exactly these reasons. Alpha emitters would deposit all their energy inside the tissue and be detected by nothing.

18. Radiation damages living tissue mainly because it

• A. heats the tissue significantly
• B. ionises molecules, including DNA, producing reactive fragments
• C. makes the tissue radioactive
• D. changes the pH of the cells

Explanation: Ionisation breaks chemical bonds and generates free radicals that go on to damage DNA, which may kill the cell or, if it is repaired imperfectly, cause a mutation and later a cancer. The energy deposited is far too small to matter as heat. Exposure to gamma or X rays does not make the patient radioactive, which is a common public misconception.

19. The equivalent dose that accounts for the different biological effects of different radiations is measured in

• A. gray
• B. sievert
• C. becquerel
• D. joule

Explanation: The sievert multiplies the absorbed dose in grays by a quality factor, which is 1 for beta and gamma but about 20 for alpha particles, because alpha deposits its energy in a very short track and does far more local damage. Legal exposure limits for workers and the public are therefore set in millisieverts. The becquerel describes the source rather than the dose received.

20. Nuclear fusion in the Sun releases energy by converting

• A. uranium into lead
• B. hydrogen into helium
• C. helium into hydrogen
• D. carbon into oxygen

Explanation: Four hydrogen nuclei combine through the proton proton chain to give one helium nucleus, and the small mass difference appears as energy. Enormous temperature and pressure are needed to overcome the electrostatic repulsion between the positively charged nuclei, which is why sustained fusion is so hard to achieve on Earth. Fusion releases more energy per kilogram of fuel than fission and leaves far less long lived waste.