Measurement and Detection of Radiation

Nicholas Tsoulfanidis, Sheldon Landsberger

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000417821
• Edition: 5
• Topic: Physical Sciences
• Subtopic: Nuclear Physics

As useful to students and nuclear professionals as its popular predecessors, this fifth edition provides the most up-to-date and accessible introduction to radiation detector materials, systems, and applications. There have been many advances in the field of radiation detection, most notably in practical applications. Incorporating these important developments, Measurement and Detection of Radiation, Fifth Edition provides the most up-to-date and accessible introduction to radiation detector materials, systems, and applications. It also includes more problems and updated references and bibliographies, and step-by-step derivations and numerous examples illustrate key concepts.

New to the Fifth Edition: • Expanded chapters on semiconductor detectors, data analysis methods, health physics fundamentals, and nuclear forensics.
• Updated references and bibliographies.
• New and expanded problems.

1
Introduction to Radiation Measurements

1.1 WHAT IS MEANT BY RADIATION?

The word radiation was used until about 1900 to describe electromagnetic waves. Around that time electrons, x-rays, and natural radioactivity were discovered and were also included under the umbrella of the term radiation. The newly discovered radiation showed characteristics of particles, in contrast to the electromagnetic radiation, which was treated as a wave. In the 1920s, de Broglie developed his theory of the duality of matter, which was soon afterwards proved correct by electron diffraction experiments, and the distinction between particles and waves ceased to be important. Today, radiation refers to the whole electromagnetic spectrum as well as to all the atomic and subatomic particles that have been discovered.

One of the many ways in which different types of radiation are grouped together is in terms of ionizing and nonionizing radiation. The word ionizing refers to the ability of the radiation to ionize an atom or a molecule of the medium it traverses.

Nonionizing radiation is electromagnetic radiation with wavelength λ of about 10 nm or longer. That part of the electromagnetic spectrum includes radiowaves, microwaves, visible light (λ = 770 − 390 nm), and ultraviolet light (λ = 390 − 10 nm).

Ionizing radiation includes the rest of the electromagnetic spectrum (x-rays, λ ≈ 0.01 − 10 nm) and γ-rays with wavelength shorter than that of x-rays. It also includes all the atomic and subatomic particles, such as electrons, positrons, protons, alphas, neutrons, heavy ions, etc.

The material in this text refers only to ionizing radiation. Specifically, it deals with detection instruments and methods, experimental techniques, and analysis of results for radiation in the energy range shown in Table 1.1. Particles with energies listed in Table 1.1 are encountered around nuclear reactors and low-energy accelerators; around installations involving production or use of natural or manufactured radioisotopes; in medical research and nuclear medicine installations; in biological, biochemical, geological, and environmental research involving radioactive tracers; and in naturally occurring radioactive materials (NORM).

TABLE 1.1 Maximum Energy Considered

Particle	Energy (MeV)
α	20
β	10
γ	20
n	20
Heavy ions	100

1.2 STATISTICAL NATURE OF RADIATION EMISSION

Radiation emission is nothing more than release of energy by a system as it moves from one energy state to another. According to classical physics, exchange or release of energy takes place on a continuous basis; that is, any amount of energy, no matter how small, may be exchanged as long as the exchange is consistent with conservation laws. The fate of a system is exactly determined if initial conditions and forces acting upon it are given. One may say that classical physics prescribed a “deterministic” view of the world.

Quantum theory changed all that. According to quantum theory, at the molecular, atomic, and subatomic levels, energy can be exchanged only in discrete amounts when a system moves from one energy state to another. The fact that conservation laws are satisfied is a necessary but not a sufficient condition for energy exchange and the change of a system. The fate of the system is not determined exactly if initial conditions and forces are known. One can only talk about the probability that the system will follow this development or that; or it will do something or nothing. Thus, with the introduction of quantum theory, the study of the physical world changed from “deterministic” to “probabilistic.”

The emission of atomic and nuclear radiation obeys the rules of quantum theory. Thus, one can only deliberate about the probability that a reaction will take place or that a particle will be emitted.

Consider a radioactive source emitting electrons and assume that one attempts to measure the number of electrons per unit time emitted by the source. For every atom of the source, there is a probability, not a certainty, that an electron will be emitted during the next unit of time. One can never measure the “exact” number of electrons emitted. The number of particles emitted per unit time is different for successive units of time. Therefore, one can only determine the average number of particles emitted. That average, like any average, carries with it a statistical uncertainty. The determination of this uncertainty is an integral part of any radiation measurement.

1.3 UNCERTAINTY^*, ACCURACY AND PRECISION OF MEASUREMENTS

* The word ‘error’ is still also used instead of ‘uncertainty’.

A measurement is an attempt to determine the value of a certain parameter or quantity. Anyone attempting a measurement should keep in mind the following two axioms regarding the result of the measurement:

• Axiom 1: No measurement yields a result without an uncertainty.
• Axiom 2: The result of a measurement is almost worthless unless the uncertainty with that result is also reported.

The term uncertainty is used to define the following concept:

1. (Measured or computed value of quantity Q) − (True value of Q)

or

1. Estimated uncertainty of the measured or computed value of Q.

Related to the uncertainty of a measurement are the terms accuracy and precision. The dictionary gives essentially the same meaning for both accuracy and precision, but in experimental work, they have different meanings.

The accuracy of an experiment tells us how close the result of the measurement is to the true value of the measured quantity. The precision of an experiment is a measure of the exactness of the result. As an example, consider the measurement of the speed of light, which is known, from measurements, to be equal to 2.997930 × 10⁸ m/s.

Assume that a measurement gave the result 2.9998 × 10⁸ m/s. The difference between these two numbers is an estimate of the accuracy of the measurement. On the other hand, the precision of the measurement is related to the number of significant figures^* representing the result. The number 2.9998 × 10⁸ indicates that the result has been determined to be between 2.9997 and 2.9999 or, equivalently, that it is known to 1 part in 30,000 (1/29998).

* As an example of the number of significant figures, each of the following numbers has five significant figures: 2.9998, 29998, 20009, .0029998, 2.9880 × 10⁸.

If the measurement is repeated and the new result is 2.9999 × 10⁸ m/s, the accuracy has changed but not the precision. If, on the other hand, the result of the measurement is 2.99985 × 10⁸ m/s, both precision and accuracy have changed.

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