PHY 1020 – Chapter 4 Nuclei and Radioactivity

A comprehensive set of question-and-answer flashcards covering core definitions, processes, examples, and safety concepts related to nuclei, radioactivity, radiation measurement, half-life, and real-world applications presented in Chapter 4 of PHY 1020.

What do we perceive as heat at the molecular level?

The random movement and vibration of atoms and molecules.

At what temperature (in Kelvin) does all molecular motion theoretically stop?

0 K, known as absolute zero.

Which two metric temperature scales are commonly used in science to measure heat?

Kelvin (K) and degrees Celsius (°C).

How is an element defined in terms of sub-atomic particles?

By the number of protons in its nucleus.

What are isotopes?

Atoms of the same element that contain different numbers of neutrons.

Name the three naturally occurring isotopes of hydrogen.

¹H (protium), ²H (deuterium), and ³H (tritium).

Why are many isotopes referred to as ‘unstable’?

Because they undergo radioactive decay to achieve a more stable configuration.

What is nuclear fusion?

The process in which two lighter nuclei combine to form a heavier nucleus, releasing energy.

Where does large-scale fusion occur naturally?

In stars, such as the Sun.

Why isn’t Jupiter a natural fusion reactor?

It is not massive enough to reach the pressure and temperature needed for sustained fusion.

Define radioactive decay.

A spontaneous process in which an unstable nucleus releases radiation to become more stable.

What particle is emitted during alpha decay?

A helium nucleus consisting of 2 protons and 2 neutrons (an alpha particle).

In beta-minus (β⁻) decay, which particle is ejected?

An electron.

In beta-plus (β⁺) decay, which antiparticle is emitted?

A positron.

What high-energy form of electromagnetic radiation is often released during nuclear decay and is highly penetrating?

Gamma rays.

Why are gamma rays especially dangerous to humans?

They have no mass or charge, allowing them to penetrate deeply and damage internal organs.

What protects Earth from most cosmic radiation?

Earth’s atmosphere and magnetic field.

How long does a free neutron typically survive before beta-decay?

About 15 minutes.

Which type of radiation can be stopped by a sheet of paper?

Alpha particles.

Which common shielding material is needed to significantly attenuate neutron radiation?

Hydrogen-rich materials such as water or concrete (due to neutron moderation by hydrogen nuclei).

What unit is commonly used in the United States to measure radiation dose?

The rem (roentgen equivalent man).

Convert 1 rem to Sieverts (Sv).

1 rem = 0.01 Sv.

What is the recommended maximum occupational dose per year for radiation workers?

5,000 mrem per year (0.05 Sv or 50 mSv).

Approximately how much radiation does an average person receive annually from natural background sources?

About 300 mrem (3 mSv).

At roughly what acute dose (in rem) does radiation poisoning begin?

Around 100 rem (1 Sv).

State the linear hypothesis for radiation-induced cancer.

The probability of cancer increases linearly with radiation dose (e.g., 2.5 rem yields ~1 extra cancer per 1,000 people).

How many of the 134 responders at Chernobyl died of acute radiation sickness?

28 individuals.

What approximate world-wide collective dose (in rem) resulted from Chernobyl?

About 60 million rem.

Why did the predicted cancer increase after the Hiroshima bombing differ from the observed value?

Actual cancer cases (~2 %) exceeded the linear-hypothesis prediction (~0.8 %), illustrating complexities in radiation effects.

What paradox is observed in Denver regarding radiation and cancer rates?

Despite receiving about 0.1 rem extra radiation per person each year, Denver’s cancer rate is lower than the U.S. average.

Define radioactive half-life.

The time required for one-half of a radioactive sample to decay.

If you start with 100 kg of carbon-14, how much remains after two half-lives (11,460 years)?

25 kg.

Describe nuclear fission.

The splitting of a heavy nucleus into lighter nuclei, releasing energy and often additional neutrons.

Which isotope powers most Radioisotope Thermoelectric Generators (RTGs)?

Plutonium-238 (²³⁸Pu).

Why does an RTG on the Mars Rover produce only ~460 W of usable power from 6.6 kW of decay heat?

Thermoelectric conversion efficiency is only about 7 %.

How does a smoke detector use radioactivity to sense smoke?

An alpha emitter ionizes air to create a small current; smoke interrupts the current, triggering the alarm.

What is the half-life of carbon-14 used in radiocarbon dating?

5,730 years.

Why is radiocarbon dating limited to objects younger than about 60,000 years?

After that time, too little ¹⁴C remains to measure accurately.

Which radioactive isotope of potassium is used for geological dating?

Potassium-40 (⁴⁰K).

What are the two stable daughter products of ⁴⁰K decay and their respective branching ratios?

⁴⁰Ca (≈89 % via beta decay) and ⁴⁰Ar (≈11 % via positron emission).

What type of radiation is classified as ‘ionizing’?

Radiation with enough energy to remove tightly bound electrons from atoms, including alpha, beta, gamma, x-rays, and high-energy cosmic particles.

List four common medical imaging techniques that contribute to ionizing radiation exposure.

X-rays, CT scans, mammograms, and PET scans.

Give one practical takeaway regarding radiation safety from Chapter 4.

Radiation is unavoidable and useful in many technologies, but exposure must be kept as low as reasonably achievable (ALARA principle).