Solid-State Radiation Detectors

Technology and Applications

Salah Awadalla

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351830904
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Integrating aspects of engineering, application physics, and medical science, Solid-State Radiation Detectors: Technology and Applications offers a comprehensive review of new and emerging solid-state materials-based technologies for radiation detection. Each chapter is structured to address the current advantages and challenges of each material and technology presented, as well as to discuss novel research and applications.

Featuring contributions from leading experts in industry and academia, this authoritative text:

• Covers modern semiconductors used for radiation monitoring
• Examines CdZnTe and CdTe technology for imaging applications including three-dimensional capability detectors
• Highlights interconnect technology for current pixel detectors
• Describes hybrid pixel detectors and their characterizations
• Tackles the integrated analog signal processing read-out front ends for particle detectors
• Considers new organic materials with direct bandgap for direct energy detection
• Summarizes recent developments involving lanthanum halide and cerium bromide scintillators
• Analyzes the potential of recent progress in the field of crystallogenesis, quantum dots, and photonics crystals toward a new concept of x- and gamma-ray detectors based on metamaterials
• Explores position-sensitivity photomultipliers and silicon photomultipliers for scintillation crystals

Solid-State Radiation Detectors: Technology and Applications provides a valuable reference for engineers and scientists looking to enhance the performance of radiation detector technology for medical imaging and other applications.

1	Modern Semiconductor Pixel Detectors Used as Radiation Monitors Martin Kroupa, Nicholas N. Stoffle, Son Hoang, J. Idarraga-Munoz, and Lawrence S. Pinsky

CONTENTS

1.1 Introduction

1.2 Radiation in Space

1.2.1 Radiation near Earth

1.3 Radiation Monitors

1.4 New Radiation Environment Monitor Based on Pixel-Detector Technology

1.4.1 Charge-Integrating Devices

1.4.2 Single-Quantum Counting Devices

1.4.3 Timepix Detector

1.4.3.1 Energy Calibration

1.4.3.2 Timepix on ISS

1.5 Advantages of Pixel Detectors

1.6 Future of Radiation Monitors Based on Pixel Detectors

Acknowledgment

References

1.1 INTRODUCTION

In our daily lives, radiation cannot be seen,^* smelled, or heard; that might be why it retains an aura of mystery. The truth is, radiation is all around us, and we are exposed to it on a daily basis. Indeed, the evolution of life on Earth may have actually required radiation [2]. However, when radiation levels exceed values to which the human body is adapted, it can present significant health hazards [3]. Generally speaking, radiation measurement and monitoring on Earth is required only if some special event occurs, or in places where we deal with increased radioactivity on a daily basis (e.g., nuclear power plants). There is, however, a place where radiation monitoring is a necessity—space [4]. Normally, humankind is protected from the radiation found throughout space by Earth’s atmosphere and magnetic field. What would happen to us if we did not have this protection? Also, can we establish and minimize the risks associated with exposure to space radiation? These questions have to be answered if humankind is to expand its influence to other planets or to the stars beyond.

Radiation dosimetry is the science dealing with the accurate determination of the energy deposited in a material, such as living tissue, by radiation. Properly establishing the threat that radiation, ionizing or otherwise, poses to humans is a daunting task due to the complexity of the interaction between radiation and matter. Not only are there many different types of radiation, but the radiation in space has a continuous energy spectrum, ranging from energies that can be shielded by a single piece of paper to incredible energies that represent orders of magnitude higher than those we can create in the largest accelerators on Earth [5,6]. How radiation interacts with matter also depends on the material composition, and thus a detailed calculation is needed when information from a silicon-based detector is used to assess effects on human tissue.

Such complexities make dosimetry a complex, interdisciplinary field, wherein there is always a demand for new radiation monitors. Developments in microelectronics open new possibilities in this area, and development of leading-edge radiation monitors based on pixel-detector technologies is currently under way. These new monitors will be used for future crewed space missions, as well as for portable radiation monitors, which may be used both within the crewed space program and back on Earth.

1.2 RADIATION IN SPACE

Over the last several decades, since humans first ventured into space, our understanding of the ionizing radiation environment near Earth and within the solar system has expanded, but this understanding is far from complete. Several models for the radiation spectra at Earth have been developed, based on data gathered both from terrestrial measurements and from satellites in orbit around our planet as well as from interplanetary probes [7,8,9]. One of the biggest obstacles for all future crewed missions traveling beyond the protection of Earth’s magnetic field will be the radiation environment [10]. Without the protection of Earth’s magnetic field and atmosphere, radiation represents a very significant threat to astronauts’ health. Moreover, the radiation in space is not constant in time. For example, solar particle events (SPEs) that erupt from the sun can significantly increase the radiation in the interplanetary environment and have the potential to seriously impact, or possibly cause the death of, insufficiently protected astronauts on interplanetary or deep-space missions. Thus, fast and precise measurement of the radiation field is one of the priorities for all future missions.

1.2.1 RADIATION NEAR EARTH

Earth’s magnetic field and atmosphere provide terrestrial life with a robust shield from cosmic and solar radiation, though relatively small amounts of such radiation do reach sea level. For example, muons and neutron cascades are measurable, along with a range of other particle types, at ground-level observation stations. As altitude increases, the amount of matter for particles to interact with decreases. Hence, the dose from ionizing radiation increases with altitude, and the background radiation dose at sea level is lower than the background measured at higher elevations [11]. The surface radiation levels also decrease with latitude, the equatorial regions having lower surface radiation levels.

Earth’s magnetic field can be thought of as a dipole field, which is tilted and offset relative to Earth’s spin axis. Charged particles become trapped in the geomagnetic field within regions known as the Van Allen Belts. The offset and tilt of the geomagnetic field give rise to the South Atlantic Anomaly, a region of increased radiation in low Earth orbit (LEO) where the inner Van Allen Belt, containing primarily trapped protons, is nearest the surface of Earth [12,13].

At LEO altitudes (altitudes between 160 and 2000 km), the radiation field has a distinct separation in components based on geographic location. Trapped electrons populate high-latitude regions over North America and above the Southern Indian Ocean, as well as in regions near the magnetic poles. These are regions where the trapped electrons can reach LEO altitudes as they bounce between north–south magnetic mirror points. These regions are also populated by those galactic cosmic rays with sufficient energy to penetrate the magnetic field.

Closer to the planet’s equator, the magnetic field is more effective at similar altitudes, resulting in only the higher-energy cosmic rays being able to penetrate to LEO altitudes. In addition, geomagnetic cusp regions allow access of solar particles to LEO altitudes. Geomagnetic cusp regions are regions at higher latitudes where geomagnetic field lines have been opened to the interplanetary magnetic field through interaction with the solar wind [14,15].

Solar phenomena, such as coronal mass ejections (CMEs) and proton events, also have an impact on radiation components in LEO. CMEs are shock fronts in the interplanetary medium composed of plasma swept up following a solar eruption. When such a shock passes Earth, it can cause disturbances in the geomagnetic field, which have the ability to cause variations in the radiation belt location and composition. The result is a widening of the areas of effect associated with the magnetic field and, in some cases, the formation of temporary belts of trapped particles [12].

SPEs are also a concern at LEO. High-energy protons and other solar products are accelerated toward Earth as a result of disturbances or eruptions in the Sun’s corona [16,17]. The high-energy protons arrive at Earth within minutes to hours and can cause a dramatic increase in both the energy spectrum and in the overall proton flux [18]. If a vehicle is not well shielded by the geomagnetic field, SPEs can result in greatly increased radiation exposure relative to quiescent periods [16,19]. For deep-space crewed missions outside Earth’s magnetic field, SPEs can represent potentially fatal radiation risks in the absence of sufficient shielding to protect the crew.

Finally, galactic cosmic rays (GCR) comprise a fully ionized background in the space radiation field with a component that extends well into relativistic energies and a nuclear composition that ranges from electrons and protons through iron and beyond. Thought to have been initially accelerated in supernova remnants and other interstellar media and further accelerated through various processes during propagation to our solar system, these relativistic ions are the most difficult portion of the space radiation field against which to shield [16,20]. While GCRs are a relatively small portion of the particle flux in LEO, it represents a large fraction of the biologically significant radiation exposure to spacefarers. This, combined with the difficulty in shielding against GCRs, presents a unique problem as humans transition into crewed interplanetary exploration [16,21].

In summary, any generic radiation monitor that attempts to assess the content of the radiation environment needs to be sufficiently sensitive to nuclei from protons through iron with energies from tens of megaelectronvolts to fully relativistic, as well as lower-energy electrons up to tens of megaelectronvolts. No mention has been made of neutrons, as there are essentially no primary neutrons, but there are albedo neutrons that are produced by interactions of the primary nuclei with other objects like Earth’s atmosphere and planetary surfaces, and there are also secondary neutrons produced in interactions with the spacecrafts themselves.

1.3 RADIATION MONITORS

Current radiation monitoring hardware used in space exploration falls into two categories: passive and active. Passive instrumentation, such as thermoluminescent detectors, collects energy from the radiation field to which it is exposed and does not provide a signal until the detector is processed to extract the desired latent information [22]. Active instrumentation, however, provides a data stream that can be used immediately or stored for future analysis.

Passive detectors used in space dosimetry applications are well understood and are heavily relied upon in operational radiation dosimetry, especially to provide statutory records. The methods and materials have been well developed, and the material responses to radiation are understood to a high degree [23]. This translates into a reliable set of results with which to assess radiation exposures and related health risks. The downside, however, is the significant time delay between the exposure and obtaining the result, and the nonlinearity of the response to the details of the composition of the incident radiation. This is especially true for spaceborne instrumentation, which may be deployed for months before the data are extracted from the detector.

Active instrumentation based on proportional counters, such as the tissue equivalent proportional counter (TEPC), which is currently used for operational dosimetry on the International Space Station (ISS), is built on well-developed instrumentation technology [22,24]. While these instruments give real-time (or nearly so) feedback relating to the radiation exposures in a vehicle, such instruments rely on estimation of linear (or lineal) energy transfer (LET) into dose and dose equivalent values for use in assessing the dosimetric quantities of interest in the space radiation environment. The limitation is that TEPCs have a sensitive gas volume that is typically cylindrical with no tracking information. The net output is simply the total integrated cha...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editors
• Contributors
• Chapter 1 Modern Semiconductor Pixel Detectors Used as Radiation Monitors
• Chapter 2 CdZnTe for Gamma- and X-Ray Applications
• Chapter 3 CdTe and CdZnTe Small Pixel Imaging Detectors
• Chapter 4 CdTe/CZT Spectrometers with 3-D Imaging Capabilities
• Chapter 5 CdTe Pixel Detectors for Hard X-Ray Astronomy
• Chapter 6 Technology Needs for Modular Pixel Detectors
• Chapter 7 Single Photon Processing Hybrid Pixel Detectors
• Chapter 8 Integrated Analog Signal-Processing Readout Front Ends for Particle Detectors
• Chapter 9 Spectral Molecular CT with Photon-Counting Detectors
• Chapter 10 Characterization of Photon-Counting Detectors with Synchrotron Radiation
• Chapter 11 Organic Semiconducting Single Crystals as Novel Room-Temperature, Low-Cost Solid-State Direct X-Ray Detectors
• Chapter 12 Lanthanum Halide and Cerium Bromide Scintillators
• Chapter 13 Novel X- and Gamma-Ray Detectors Based on Metamaterials
• Chapter 14 Gamma-Imaging Devices Based on Position-Sensitive Photomultipliers
• Chapter 15 Silicon Photomultipliers for High-Performance Scintillation Crystal Read-Out Applications
• Index