Principles of Nuclear Radiation Detection

Geoffrey G. Eichholz

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351091978
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Nuclear Physics

This book is intended for senior undergraduate and beginning graduate students in physics, nuclear engineering, health physics and nuclear medicine, and for specialized training courses for radiation protection personnel and environmental safety engineers.To keep the size of the book manageable, material has been selected to stress those detectors that are in widespread use. Attempts have also been made to emphasize alternatives available in approaching various measurement problems and to present the criteria by which a choice among these alternatives may be made.

CHAPTER 1

INTRODUCTION

Nuclear and atomic radiations find many applications in our society, from chest X-ray radiography to the control of the thickness of aluminum foil, from tracer measurements of stream flow or natural gas reservoirs to measurements of cardiac output from the heart, from determination of cosmic ray intensities on mountain tops to the monitoring of radiation levels around nuclear power reactors or supervoltage particle accelerators. In each case the measurement of radiation intensity or radiation exposure requires a thorough understanding of the characteristics of alternative detection systems available, an appreciation of the physical problem to be solved, and an informed selection of the detector type that is most suited to the particular application. Since nuclear radiations are not sensed by the common human senses, a detector is required to provide a conversion to a readily measurable phenomenon. The properties of these detectors and any refinements that can be supplied to improve their performance must form the core of any discussion of radiation detection systems. The performance of a detector system depends on the characteristics of all of its components, that is, the detector proper which responds to the incident radiation and converts it into a more tractable form, usually electric charges or voltages, the amplifier system, and the data display or evaluation system.

Nuclear radiations comprise several forms of energy transmission that are characterized by their ability to excite, ionize or otherwise interact with most atoms or molecules that they encounter. They derive their name from the fact that they are emitted as the result of the de-excitation processes that a nucleus undergoes following bombardment or capture reactions; the type and energy spectrum of the emitted radiation reflect the characteristic energies associated with these processes. The radiations of particular importance are called alpha (α), beta (β) and gamma (γ) radiations and neutrons, though a whole host of other charged and uncharged particles fit the same general description and may be encountered in specific research situations. X-Rays, whose characteristics are closely allied to gamma radiation, are usually included in considerations of nuclear radiation detectors as a matter of convenience, though strictly they are not of nuclear origin but arise from transitions between orbital electron energy levels.

Radiation detection may consist of different types of operations; these are variously referred to as detection, monitoring, analysis or dosimetry with each type requiring slightly different equipment and emphasizing a different aspect of the measurement process. In detection one tends to concentrate on verifying the presence or absence of a given kind of radiation above a certain level of intensity. In monitoring operations this process is typically related to a time-dependent observation or to established safety standards of exposure. Analysis of radiation requires further identification of various radiation sources, if present, and a determination of energy distributions or spectra, and, in some cases, discrimination in time sequence of events. Finally, dosimetry relates the type and energy distribution of the radiation field to the absorption of this energy in body tissue and to any related health effects that may arise from this exposure.

Consequently, one may divide detector systems into those that are intended and most suited for the measurement of radiation intensity, those dedicated to the measurement of radiation dose, and those capable of determining energy distributions or spectra. In general, each detector system will be capable of responding principally to only one type of radiation, though high levels of other radiation types may cause interference; hence, a wide variety of detector types and detector systems have been developed for different purposes.

All radiations represent a transfer of energy from the emitting source to the surrounding media where they may interact to an extent determined by the characteristics of the radiation and the nature and structure of the intervening medium. If the interaction between the radiation and the target medium is strong, energy is transferred at a rapid rate and the radiation is strongly attenuated. If radiation attenuation is weak, the radiation may have an appreciable range and may be detected at great distances from the source. Since all detection methods depend on finding an efficient interaction mechanism to transfer some of this energy so that it can be converted into readable form, these interaction processes are central to any discussion of radiation detection.

The energy of radiations emitted by nuclei is of the order of nuclear excitation energies or binding energies; these are of the order of kiloelectron-volts (keV) and megaelectron-volts (MeV), where 1 MeV = 0.16 picojoule (pJ).

The radiations will interact with materials, that is, they will lose kinetic energy to any solid, liquid or gas through which they pass by a variety of interaction mechanisms that depend on the energy of the incident radiation, the density and atomic number of the absorbing medium, and the relative probability for one or another process to take place. These probabilities are called cross sections and are usually expressed in barns (1 barn = 10⁻²⁴ cm² = 10⁻²⁸ m²). The result of these interaction processes is a gradual slowing down of any incident particle until it is brought to rest or “stopped” at the end of its range. Although the terminology varies among the nuclear physicist, the health physicist, the radiobiologist and the radiation chemist, they all are interested in the rate of energy loss or transfer (“dE/dx, LET”) as the incident particle is slowed down, the total energy lost or transferred by various secondary processes and the consequent effect per unit mass or unit volume on the target material.

Since charged particles lose energy at a fairly steady rate when transversing material substances, it is possible to relate their initial energy to the distance traveled in that medium. This is called the range of the particle and represents the average distance traversed in a given medium by a particle of specified energy.

The emission of radiation from nuclear processes occurs randomly in time; consequently all radiation detection processes deal with statistically random events and the data evaluation and interpretation of detector response must take into account the statistical nature of all events. This has considerable bearing on the sensitivity and accuracy of radiation detection systems and this aspect will be treated in some detail in Chapter 3.

RADIATION INTENSITY

The intensity of a radiation field can be described in several ways. The simplest is to describe it in terms of a particle flux or flux density, i.e., the number of particles passing unit area per unit time, with all particle paths projected normally onto the reference plane. This is a convenient way of expressing field intensities for parallel beams of particles or for fields created by a point source. It is the customary way of describing neutron fields around a reactor where one also needs to distinguish between neutrons in different energy ranges. Flux density is usually expressed in number of particles/cm²-sec.

For X-rays and gamma-rays, a more experimentally oriented approach is customary in which the field intensity is expressed in terms of the number of ions produced in a known mass or volume of detec...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• 1. INTRODUCTION
• 2. INTERACTION OF RADIATION WITH MATTER
• 3. COUNTING STATISTICS AND ERROR DETERMINATIONS
• 4. GAS-FILLED DETECTORS
• 5. SCINTILLATION DETECTION SYSTEMS
• 6. SEMICONDUCTOR DETECTORS
• 7. TRACK DEVICES
• 8. MISCELLANEOUS DETECTORS
• 9. NEUTRON DETECTION
• 10. CALIBRATION AND STANDARDS
• 11. ELECTRONIC SYSTEMS
• Appendix A—Notes on Detector Geometry
• Appendix B—List of Elements
• INDEX