Core Unit IV: Nuclear Physics
A. Natural Radioactivity

Key Concepts

Becquerel accidentally discovered that uranium compounds caused a photographic plate to become fogged. (He was investigating the relationship between x-rays and fluorescence using crystals of uranium potassium sulphate.)

Radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the emission of particles or electromagnetic radiation.

Pierre and Marie Curie investigated uranium ores using chemical separation. They discovered that pitchblende and chalcocite, naturally occurring ores, were highly radioactive due to the presence of plutonium and radium.

All naturally occurring elements with atomic numbers greater than 83, as well as some isotopes of lighter elements, are radioactive.

Based on later work by Rutherford, Soddy, Villard, and others, three different types of radiation were identified.

Alpha particles are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.

Beta particles are electrons capable of travelling at speeds approaching the speed of light. Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. (Some texts refer to beta particles as "beta negative particles", to distinguish them from beta positive particles -- positrons.)

Alpha particle emissions and beta particle emissions change the composition of the nucleus.

Gamma rays are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead.

All radioactive nuclides have the following common characteristics:

• Their radiations affect the emulsion of photographic film, ionize surrounding air molecules, make certain compounds fluoresce, and have certain special biological effects.

• They undergo radioactive decay.

People are constantly being exposed to radiation from a variety of natural and human-created sources. Exposure should be minimized, but it can never be reduced to zero.

Some commonly used symbols for subatomic particles are:

neutron

proton

electron (beta particle)

positron

(A positron is a particle much the same as an electron, but with a positive charge. It is an example of "antimatter".)

    (alpha particle)

gamma ray (photon)

Radioactivity can not be detected with our senses. Special detectors are needed. Because it can not be detected by human senses it is particularly dangerous; one may unknowingly be exposed to it for prolonged periods of time. Radiation has an effect on tissue and on genetic material.

Several devices have been developed to detect radioactivity, with the earliest being an unexposed photographic plate placed in the vicinity of a source being detected. Other devices include a Wilson cloud chamber, electroscopes, ionizing chambers, the Geiger-Muller tube, liquid and electronic bubble chambers, scintillation detectors (spinthariscope), and solid state semiconductor devices.

Dosimetry is the measurement of radiation and the study of its effects on living organisms.

There are several different units used to measure radiation.

The absorbed dose describes the amount of energy deposited per kilogram of exposure time, measured in the gray (Gy).

1 Gy = 1 J/kg = 100 Rads

(Rads are non-SI, but in general use.)

The biological damage produced on a given organism is called the dose equivalent, measured in sieverts (Sv).

1 Sv = 100 rem = 10⁵ mrem
(rem -- rad equivalent man)

dose equivalent(Sv) = absorbed dose(Gy) x a quality factor(Q)

The quality factor is a number assigned to each type of radiation to describe its biological effects.

The effect that absorbed radiation has on different types of tissues varies. Furthermore, there is disagreement by scientists about the cumulative effects of low dosage exposure to radiation. For these reasons, no exposure to radioactive emissions, for any period of time, should be regarded as being "safe" to humans or other living organisms. Much research is still needed into the long-term biological effects of radiation.

The becquerel (Bq) is the activity of a source produced when one disintegration per second occurs from a radioactive source.

1 Bq = 1 disintegration per second

kBq and MBq are often used to express the radioactivity of a source. This unit does not make any distinctions between the effects of different types of radiation.

1 curie (Ci) = 3.7 x 1010 Bq

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: radioactivity, isotopes, alpha particles, beta particles, gamma rays, dosimetry, absorbed dose, dose equivalent, quality factor.

2. State how radioactivity discovered .

3. Identify some naturally occurring radioactive ores.

4. Realize that radioactivity is found in both natural and artificial sources.

5. Recognize that people are constantly being exposed to radiation from a variety of sources.

6. Recognize that, although exposure to radioactivity is inevitable, it should be minimized.

7. Develop a generalization based on atomic number regarding some radioactive elements.

8. State the number of different types of radiation found in nature.

9. Identify the composition of alpha particles, beta particles, and gamma rays.

10. Compare the penetrating power, speed, potential danger, and other important characteristics of alpha particles, beta particles, and gamma rays.

11. Identify common characteristics of all radioactive nuclides.

12. Recognize and interpret some commonly used symbols for subatomic particles.

13. Demonstrate the correct use of some commonly used symbols for subatomic particles.

14. Recognize that radioactivity can not be detected by human senses.

15. Suggest some important implications arising from the fact that radioactivity can not be detected by human senses.

16. Identify one device that can be used to detect radioactivity.

17. Identify some of the units that are used to measure radiation.

18. Demonstrate an understanding of the units that are used to measure radiation.

19. Recognize that absorbed radiation has different effects on different kinds of tissue.

20. Recognize that there is disagreement among scientists on the cumulative effects of low dosage exposure to radiation.

21. Understand that no exposure to radioactive emissions, for any period of time, should be regarded as being "safe" to humans or other living organisms.

22. Use SI fundamental and derived units and prefixes correctly.

23. Recognize that non-SI units are sometimes used.

Teaching Suggestions, Activities and Demonstrations

1. Students could research one type of radiation detection device in further detail. Many other Key Concepts in this section lend themselves well to independent learning strategies.

2. The expression used to represent an electron can be confusing

   In most cases, the subscript refers to the number of protons (e.g., ). For the electron the subscript refers to the charge. Some students might make the inference that an electron is a "negative proton". Clarify this usage of symbols.