The book is intended as a textbook for a radiation physics course in academic medical physics graduate programs as well as a reference book for candidates preparing for certification examinations in medical physics sub-specialties. It may also be of interest to many professionals, not only physicists, who in their daily occupations deal with various aspects of medical physics or radiation physics and have a need or desire to improve their understanding of radiation physics.
Today physicists and mathematicians throughout the world are feverishly working on one of the most ambitious theories ever proposed: superstring theory. String theory, as it is often called, is the key to the Unified Field Theory that eluded Einstein for more than thirty years. Finally, the century-old antagonism between the large and the small-General Relativity and Quantum Theory-is resolved. String theory proclaims that all of the wondrous happenings in the universe, from the frantic dancing of subatomic quarks to the majestic swirling of heavenly galaxies, are reflections of one grand physical principle and manifestations of one single entity: microscopically tiny vibrating loops of energy, a billionth of a billionth the size of an atom. In this brilliantly articulated and refreshingly clear book, Greene relates the scientific story and the human struggle behind twentieth-century physics' search for a theory of everything.
Through the masterful use of metaphor and analogy, The Elegant Universe makes some of the most sophisticated concepts ever contemplated viscerally accessible and thoroughly entertaining, bringing us closer than ever to understanding how the universe works.
This new edition is thoroughly revised and updated in line with latest clinical practice, knowledge and guidelines. The first section presents lists of differential diagnoses, supplemented by notes on useful facts and discriminating factors. These assist with the systematic assessment of radiographs and images from other modalities. The second section offers detailed summaries of the characteristic radiological appearance of a wide range of clinical conditions.Lists of differential diagnoses.Notes on radiological apperances.Ideal for preparation for radiological examinations.Contents revised and reduced to reflect current radiological practice.Existing lists modernised to reflect modern imaging practice, particularly where technological advances have been made (eg multislice CT and PET CT).Revised to take account of new imaging guidelines (eg Royal College of Radiologists: Making Best Use of the Radiology department; SIGN; NICE).Journal references updated.Increased number of diagrams.
There are many biochemistry textbooks in the market. Some of them are purely basic while others are applied, and there are very few books which cover both these aspects together. For this reason, the students learning biochemistry in their undergraduate courses have to depend on multiple books to acquire a sound knowledge of the subject.
This book, ‘Biochemistry’ is unique with a simultaneous and equal emphasis on basic and applied aspects of biochemistry. This textbook offers an integration of medical and pure sciences, comprehensively written to meet the curriculum requirements of undergraduate courses in medical, dental, pharmacy, life-sciences and other categories (agriculture, veterinary, etc.).
This book is designed to develop in students a sustained interest and enthusiasm to learn and develop the concepts in biochemistry in a logical and stepwise manner. It incorporates a variety of pedagogic aids, besides colour illustrations to help the students understand the subject quickly and to the maximum. The summary and biomedical/clinical concepts are intended for a rapid absorption and assimilation of the facts and concepts in biochemistry. The self-assessment exercises will stimulate the students to think rather than merely learn the subject. In addition, these exercises (essays, short notes, fill in the blanks, multiple choice questions) set at different difficulty levels, will cater to the needs of all the categories of learners.
New to This Edition
The book offers an integration of medical and pure sciences, and is comprehensively written, revised and updated to meet the curriculum requirements of Medical, Pharmacy, Dental, Veterinary, Biotechnology, Agricultural Sciences, Life Sciences, and others studying Biochemistry as one of the subjects.
It is the first text book on Biochemistry in English with multi-colour illustrations by an author from Asia. The use of multicolours is for a clearer understanding of the complicated biochemical reactions.
It is written in a lucid style with the subject being presented as an engaging story growing from elementary information to the most recent advances, and with theoretical discussions being supplemented with illustrations, flowcharts, and tables for easy understanding of Biochemistry.
It has each chapter beginning with a four-line verse followed by the text, biomedical concepts, a summary, and self-assessment exercises. The lively illustrations and text with appropriate headings and sub-headings in bold type faces facilitate reading path clarity and quick recall.
It provides the most recent and essential information on Molecular Biology and Biotechnology, Diabetes, Cancer, Free Radicals, Free radicals and Antioxidants, Prostaglandins, etc.
It describes a wide variety of case studies and biochemical correlations and several newer biomedical aspects- Metabolic syndrome, Therapeutic diets, Atkins diet, Trans fatty acids, Epigenetics, Nutrigenomics, Recombinant ribozymes, Membrane transport disorders, Pleural fluid etc.
It contains the basics (Bioorganic and Biophysical Chemistry, Tools of Biochemistry, Immunology, and Genetics) for beginners to learn easily Biochemistry, origins of biochemical words, confusables in Biochemistry, principles of Practical Biochemistry, and Clinical Biochemistry Laboratory.
Radiation: What could go wrong? In short, plenty. From Marie Curie carrying around a vial of radium salt because she liked the pretty blue glow to the large-scale disasters at Chernobyl and Fukushima, dating back to the late nineteenth century, nuclear science has had a rich history of innovative exploration and discovery, coupled with mistakes, accidents, and downright disasters.
In this lively book, long-time advocate of continued nuclear research and nuclear energy James Mahaffey looks at each incident in turn and analyzes what happened and why, often discovering where scientists went wrong when analyzing past meltdowns. Every incident, while taking its toll, has led to new understanding of the mighty atom—and the fascinating frontier of science that still holds both incredible risk and great promise.
"What time is it?" That simple question is probably asked more often in contemporary society than ever before. In our clock-studded world, the answer is never more than a glance away, and so we can blissfully partition our days into ever smaller increments for ever more tightly scheduled tasks. Modern scientific revelations about time, however, make the question endlessly frustrating. If we seek a precise knowledge of the time, the infinitesimal flash of now dissolves into a scattering flock of nanoseconds. Because we are bound by the speed of light and the velocity of nerve impulses, our perception of the "present" reflects the world as it occurred an instant ago – for all that human consciousness pretends otherwise, we can never catch up. Even in principle, perfect synchronicity escapes us. Relativity dictates that, like a strange syrup, time flows slower on moving trains than in the stations and faster in the mountains than in the valleys. The time for our wristwatch is not exactly the same as the time for our head. This eBook, A Question of Time, summarizes what science has discovered about how time permeates and guides both our physical world and our inner selves. That knowledge should enrich the imagination and provide practical advantages to anyone hoping to beat the clock, or at least to stay in step with it. Synchronize your watches...
Imaging Informatics Professionals (IIPs) have come to play an indispensable role in modern medicine, and the scope of this profession has grown far beyond the boundaries of the PACS. A successful IIP must not only understand the PACS itself, but also have knowledge of clinical workflow, a base in several medical specialties, and a solid IT capability regarding software interactions and networking. With the introduction of a certification test for the IIP position, a single source was needed to explain the fundamentals of imaging informatics and to demonstrate how those fundamentals are applied in everyday practice. Practical Imaging Informatics describes the foundations of information technology and clinical image management, details typical daily operations, and discusses rarer complications and issues.
A long life in a healthy, vigorous, youthful body has always been one of humanity's greatest dreams. Recent progress in genetic manipulations and calorie-restricted diets in laboratory animals hold forth the promise that someday science will enable us to exert total control over our own biological aging.
Nearly all scientists who study the biology of aging agree that we will someday be able to substantially slow down the aging process, extending our productive, youthful lives. Dr. Aubrey de Grey is perhaps the most bullish of all such researchers. As has been reported in media outlets ranging from 60 Minutes to The New York Times, Dr. de Grey believes that the key biomedical technology required to eliminate aging-derived debilitation and death entirely—technology that would not only slow but periodically reverse age-related physiological decay, leaving us biologically young into an indefinite future—is now within reach.
In Ending Aging, Dr. de Grey and his research assistant Michael Rae describe the details of this biotechnology. They explain that the aging of the human body, just like the aging of man-made machines, results from an accumulation of various types of damage. As with man-made machines, this damage can periodically be repaired, leading to indefinite extension of the machine's fully functional lifetime, just as is routinely done with classic cars. We already know what types of damage accumulate in the human body, and we are moving rapidly toward the comprehensive development of technologies to remove that damage. By demystifying aging and its postponement for the nonspecialist reader, de Grey and Rae systematically dismantle the fatalist presumption that aging will forever defeat the efforts of medical science.
Each chapter ends with clinical cases and the related questions, which evokes yet another method of active learning rather than didactic methods of imparting knowledge.
Key points have been highlighted and boxed at the end of each topic for quick revision of the core concepts.
This book comes with a free companion website which contains self-assessment exercises, detailed case discussions related to the clinical cases given inside the book, glossary and various other features for enhanced learning.
“The amazing story of an unbelievable boy . . . The world that opens up to us through his story is both fascinating and slightly terrifying . . . but in a good way. You won’t be able to walk away from this tale.” — Elizabeth Gilbert, author of Eat, Pray, Love
“Imagine if cartoon whiz kid Jimmy Neutron were real and had a brainchild with MacGyver and his adolescence got told as a rollicking bildungsroman about American prodigies and DIY nuclear reactors—well, that’s this book.” —Jack Hitt, author of Bunch of Amateurs
By the age of nine, Taylor Wilson had mastered the science of rocket propulsion. At eleven, his grandmother’s cancer diagnosis inspired him to investigate new ways to produce medical isotopes. And by fourteen, Wilson had built a 500-million-degree reactor and become the youngest person in history to achieve nuclear fusion. How could someone so young achieve so much, and what can Wilson’s story teach parents and teachers about how to support high-achieving kids?
In The Boy Who Played with Fusion, science journalist Tom Clynes narrates Taylor’s extraordinary journey—from his Arkansas home, to a unique public high school just for academic superstars, to the present, when Wilson is designing devices to prevent terrorists from shipping radioactive material and inspiring a new generation to take on the challenges of science.
“Clynes guides us on an engrossing journey to the outer realms of science and parenting. The Boy Who Played with Fusion is a fascinating exploration of ‘giftedness’ and all its consequences.” —Paul Greenberg, author of Four Fish and American Catch
“An essential contribution to our understanding of the most important underlying questions about the development of giftedness, talent, creativity, and intelligence.” —Psychology Today
This edition includes greatly expanded focus on stem cells, including adult and embryonic stem cells and progenitor populations that may soon lead to new tissue engineering therapies for heart disease, diabetes, and a wide variety of other diseases that afflict humanity. This up-to-date coverage of stem cell biology and other emerging technologies is complemented by a series of new chapters on recent clinical experience in applying tissue engineering. The result is a comprehensive textbook that we believe will be useful to students and experts alike.
New to this edition:
*Includes new chapters on biomaterial-protein interactions, nanocomposite and three-dimensional scaffolds, skin substitutes, spinal cord, vision enhancement, and heart valves
*Expanded coverage of adult and embryonic stem cells of the cardiovascular, hematopoietic, musculoskeletal, nervous, and other organ systems
In The Theory of Almost Everything, Robert Oerter shows how what were once thought to be separate forces of nature were combined into a single theory by some of the most brilliant minds of the twentieth century. Rich with accessible analogies and lucid prose, The Theory of Almost Everything celebrates a heretofore unsung achievement in human knowledge—and reveals the sublime structure that underlies the world as we know it.
Life is the most extraordinary phenomenon in the known universe; but how did it come to be? Even in an age of cloning and artificial biology, the remarkable truth remains: nobody has ever made anything living entirely out of dead material. Life remains the only way to make life. Are we still missing a vital ingredient in its creation?
Using first-hand experience at the cutting edge of science, Jim Al-Khalili and Johnjoe Macfadden reveal that missing ingredient to be quantum mechanics. Drawing on recent ground-breaking experiments around the world, each chapter in Life on the Edge illustrates one of life's puzzles: How do migrating birds know where to go? How do we really smell the scent of a rose? How do our genes copy themselves with such precision? Life on the Edge accessibly reveals how quantum mechanics can answer these probing questions of the universe.
Guiding the reader through the rapidly unfolding discoveries of the last few years, Al-Khalili and McFadden describe the explosive new field of quantum biology and its potentially revolutionary applications, while offering insights into the biggest puzzle of all: what is life? As they brilliantly demonstrate in these groundbreaking pages, life exists on the quantum edge.
– Winner, Stephen Hawking Medal for Science Communication
This inspiring guide shows you how to use Arduino to create gadgets for measuring noise, weather, electromagnetic interference (EMI), water purity, and more. You’ll also learn how to collect and share your own data, and you can experiment by creating your own variations of the gadgets covered in the book. If you’re new to DIY electronics, the first chapter offers a primer on electronic circuits and Arduino programming.Use a special microphone and amplifier to build a reliable noise monitor Create a gadget to detect energy vampires: devices that use electricity when they’re “off” Examine water purity with a water conductivity device Measure weather basics such as temperature, humidity, and dew point Build your own Geiger counter to gauge background radiation Extend Arduino with an Ethernet shield—and put your data on the Internet Share your weather and radiation data online through Pachube
In telling the story of what is perhaps the most anticipated experiment in the history of science, Amir D. Aczel takes us inside the control rooms at CERN at key moments when an international team of top researchers begins to discover whether this multibillion-euro investment will fulfill its spectacular promise. Through the eyes and words of the men and women who conceived and built CERN and the LHC—and with the same clarity and depth of knowledge he demonstrated in the bestselling Fermat’s Last Theorem—Aczel enriches all of us with a firm grounding in the scientific concepts we will need to appreciate the discoveries that will almost certainly spring forth when the full power of this great machine is finally unleashed.
Will the Higgs boson make its breathlessly awaited appearance, confirming at last the Standard Model of particles and their interactions that is among the great theoretical achievements of twentieth-century physics? Will the hidden dimensions posited by string theory be revealed? Will we at last identify the nature of the dark matter that makes up more than 90 percent of the cosmos? With Present at the Creation, written by one of today’s finest popular interpreters of basic science, we can all follow the progress of an experiment that promises to greatly satisfy the curiosity of anyone who ever concurred with Einstein when he said, “I want to know God’s thoughts—the rest is details.”
From the Hardcover edition.
Podcast — Building the H Bomb: A Personal History
Hosted by Milt Rosenberg (1590 WCGO), 25 June 2015
Building the H-Bomb: The Big Idea
APS News, June 2015 (Volume 24, Number 6)
Behind the Making of a Super Bomb
The Washington Post, 22 May 2015
Hydrogen Bomb Physicist's Book Runs Afoul of Energy Department
The New York Times, 23 March 2015
In this engaging scientific memoir, Kenneth Ford recounts the time when, in his mid-twenties, he was a member of the team that designed and built the first hydrogen bomb. He worked with — and relaxed with — scientific giants of that time such as Edward Teller, Enrico Fermi, Stan Ulam, John von Neumann, and John Wheeler, and here offers illuminating insights into the personalities, the strengths, and the quirks of these men. Well known for his ability to explain physics to nonspecialists, Ford also brings to life the physics of fission and fusion and provides a brief history of nuclear science from the discovery of radioactivity in 1896 to the ten-megaton explosion of “Mike” that obliterated a Pacific Island in 1952.
Ford worked at both Los Alamos and Princeton's Project Matterhorn, and brings out Matterhorn's major, but previously unheralded contribution to the development of the H bomb. Outside the lab, he drove a battered Chevrolet around New Mexico, a bantam motorcycle across the country, and a British roadster around New Jersey. Part of the charm of Ford's book is the way in which he leavens his well-researched descriptions of the scientific work with brief tales of his life away from weapons.Contents:The Big IdeaThe ProtagonistsThe ChoiceThe Scientists, the Officials, and the PresidentNuclear EnergySome PhysicsGoing WestA New WorldThe Classical SuperCalculating and TestingConstructing MatterhornAcademia CowersNew Mexico, New York, and New JerseyThe Garwin DesignClimbing MatterhornMore Than a Boy
Readership: A memoir for general readership in the history of science.
Key Features:It contains real physics, clearly presented for non-specialistsCombining historical scholarship and his own recollections, the author offers important insights into the people and the work that led to the first H bombPersonal anecdotes enliven the bookKeywords:Nuclear Weapons;Atomic Weapons;H Bomb;Thermonuclear Weapons;Nuclear Physics;Nuclear History;Thermonuclear History;Los Alamos;Edward Teller;Stanislav Ulam;John Wheeler;Project MatterhornReviews:
“It was a great treat to read a book that's well-written, informative, and gets the science right. It is these personal recollections and descriptions; the fact that it is a personal and first-hand account of a unique time in history and a remarkable scientific and technical achievement that made this book so enthralling. This is an engaging account of a young scientist involved in a remarkable project.”P Andrew Karam
The Ohio State University
“Ford's book is a valuable resource for anyone interested in the history of the H bomb and its role in the Cold War, and in how that work affected the life and career of an individual involved.”Physics Today
"Personal memories are the book's greatest strength. Ford doesn't glorify, or apologize for, his work on the H-bomb. He simply tells it as it was. As a result, this is an engagingly human glimpse into the world of physics in the US in the early 1950s."Physics World
Presents the images with number labeling to keep them clean and help with self-testing.Features completely revised legends and labels and over 60% new images—cross-sectional views in CT and MRI, angiography, ultrasound, fetal anatomy, plain film anatomy, nuclear medicine imaging, and more—with better resolution for the most current anatomical views.
Reflects current radiological and anatomical practice through reorganized chapters on the abdomen and pelvis, including a new chapter on cross-sectional imaging.
Covers a variety of common and up-to-date modern imaging—including a completely new section on Nuclear Medicine—for a view of living anatomical structures that enhance your artwork and dissection-based comprehension.
Includes stills of 3-D images to provide a visual understanding of moving images.
A Doody's Core Title for 2017!
THE BEST REVIEW FOR THE USMLE!
The Thirtieth Edition of Harper’s Illustrated Biochemistry combines outstanding full-colorillustrations with authoritative integrated coverage of biochemical disease and clinical information. Using brevity and numerous medically relevant examples, Harper's presents a clear, succinct review of the fundamentals of biochemistry that every student must understand in order to succeed in medical school.
All fifty-eight chapters emphasize the medical relevance of biochemistryFull-color presentation includes more than 600 illustrations Each chapter includes a section on BiomedicalImportance and a summary of the topics covered Review questions follow each of the eleven sections Case studies in every chapter emphasize the clinical relevance to biochemistry NEW coverage of toxic naturally-occurring amino acids; extraterrestrial biomolecules; computer-aided drug design; the role of complement cascade in bacterial and viral infection; secreted mediators of cell-cell signaling between leukocytes; the role of mast cells, basophils, andeosinophils; and the hazard of antioxidants that down-regulate radical signaling for apoptosis and increase risk of cancer
Applauded by medical students for its current and engaging style, Harper's Illustrated Biochemistry is an essential for USMLE review and the single best reference for learning the clinical relevance of any biochemistry topic.
It starts by introducing, in a completely self-contained way, all mathematical tools needed to use symmetry ideas in physics. Thereafter, these tools are put into action and by using symmetry constraints, the fundamental equations of Quantum Mechanics, Quantum Field Theory, Electromagnetism, and Classical Mechanics are derived.
As a result, the reader is able to understand the basic assumptions behind, and the connections between the modern theories of physics. The book concludes with first applications of the previously derived equations.
The Olympics are the world's most prestigious stage for athletic competition. Fans both casual and hardcore tune in religiously every few years to watch as men and women push themselves to the limits of human performance. But what makes a champion? Is it genetics? Hours of training? A psychological advantage? Of all the athletes who dedicate their lives - and bodies - to achieving that perfect moment of triumph, why will one person or team win out over another? Science has some compelling answers, and in this book, The Science of Sports: Winning in the Olympics, Scientific American explores this topic from various angles. Beginning with Section 1: The Psychology of Winning, the book opens with a look inside the mind of an elite athlete and tackles questions of how to face a rivalry or maintain a positive attitude in the face of defeat. Other sections discuss the sticky issues surrounding genetic advantages and physical prowess, drugs and doping, injury and recovery, and - finally - the latest scientific advice for the rest of us mere mortals to be fit and healthy. You'll find both inspiration and answers in this indispensable book from the editors of Scientific American, the leading authority on science, technology and innovation.
This new edition discusses biological systems that can be analyzed quantitatively, and how advances in the life sciences have been aided by the knowledge of physical or engineering analysis techniques. The volume is organized into 18 chapters encompassing thermodynamics, electricity, optics, sound, solid mechanics, fluid mechanics, and atomic and nuclear physics. Each chapter provides a brief review of the background physics before focusing on the applications of physics to biology and medicine. Topics range from the role of diffusion in the functioning of cells to the effect of surface tension on the growth of plants in soil and the conduction of impulses along the nervous system. Each section contains problems that explore and expand some of the concepts. The text includes many figures, examples and illustrative problems and appendices which provide convenient access to the most important concepts of mechanics, electricity, and optics in the body.
Physics in Biology and Medicine will be a valuable resource for students and professors of physics, biology, and medicine, as well as for applied health workers.Provides practical techniques for applying knowledge of physics to the study of living systemsPresents material in a straight forward manner requiring very little background in physics or biologyIncludes many figures, examples and illustrative problems and appendices which provide convenient access to the most important concepts of mechanics, electricity, and optics in the body
Eve Herold's Beyond Human examines the medical technologies taking shape at the nexus of computing, microelectronics, engineering, nanotechnology, cellular and gene therapies, and robotics. These technologies will dramatically transform our lives and allow us to live for hundreds of years. Yet, with these blessings come complicated practical and ethical issues, some of which we can predict, but many we cannot.
Beyond Human taps the minds of doctors, scientists, and engineers engaged in developing a host of new technologies while telling the stories of some of the patients courageously testing the radical new treatments about to come into the market.
Beyond Human asks the difficult questions of the scientists and bioethicists who seek to ensure that as our bodies and brains become ever more artificial, we hold onto our humanity. In this new world, will everyone have access to technological miracles, or will we end up living in a world of radical disparities? How will society accommodate life spans that extend into hundreds of years? Will we and our descendants be able to bring about the dream of a future liberated by technology, or will we end up merely serving the machines and devices that keep us healthy, smart, young, and alive?
Detect harmful gases, dust particles such as smoke and smog, and upper atmospheric haze—substances and conditions that are often invisible to your senses. You’ll also discover how to use the scientific method to help you learn even more from your atmospheric tests.Get up to speed on Arduino with a quick electronics primer Build a tropospheric gas sensor to detect carbon monoxide, LPG, butane, methane, benzene, and many other gases Create an LED Photometer to measure how much of the sun’s blue, green, and red light waves are penetrating the atmosphere Build an LED sensitivity detector—and discover which light wavelengths each LED in your Photometer is receptive to Learn how measuring light wavelengths lets you determine the amount of water vapor, ozone, and other substances in the atmosphere Upload your data to Cosm and share it with others via the Internet
"The future will rely on citizen scientists collecting and analyzing their own data. The easy and fun gadgets in this book show everyone from Arduino beginners to experienced Makers how best to do that."
--Chris Anderson, Editor in Chief of Wired magazine, author of Makers: The New Industrial Revolution (Crown Business)
Interested in unraveling the physics of living things? Here's your starting point. Biophysics Demystified is the fast and easy way to understand this fascinating topic.
Written in a step-by-step format, this practical guide begins with an introduction to the science of biophysics, covering biophysical techniques and applications. Next, you'll learn the principles of physics, biology, and chemistry required to understand biophysics, including free energy, entropy, and statistical mechanics. Biomolecules and the forces that influence their structure and conformation are also covered, as are protein, nucleic acid, and membrane biophysics. Detailed examples and concise explanations make it easy to understand the material, and end-of-chapter quizzes and a final exam help reinforce key concepts.
It's a no-brainer! You'll get:Molecular, subcellular, physiological, anatomical, and environmental biophysics The laws of thermodynamics as they apply to biophysical systems Forces affecting conformation in biological molecules The composition and structure of carbohydrates, lipids, proteins, and nucleic acids The fluid mosaic model
Simple enough for a beginner, but challenging enough for an advanced student, Biophysics Demystified makes this interdisciplinary subject easy to master.
One of the most diverse of modern scientific disciplines, biophysics applies methods and technologies from physics to the study of biological systems and phenomena, from the human nervous system to soil erosion to global warming. What are the best options for satisfying the world's growing energy demands? How can we feed the world's growing population? How can we contain, or reverse, global warming? How can we vouchsafe a plentiful supply of potable water for future generations? These are among the critical questions to which biophysicists work to provide answers.Biophysics courses are increasingly taken by students of biology, physics, chemistry, biochemistry, physiology, statistics, bioengineering, neuroscience, computer science, pharmacology, agriculture,and many more Provides a friendly, unintimidating overview of the material covered in a typical college-level biophysics course A one-stop reference, course supplement and exam preparation tool for university students currently enrolled in an introductory biophysics courses An indispensable resource for those studying the natural sciences, biological sciences, and physics, as well as math, statistics,computer science, pharmacology and many other disciplines The current job market for people well versed in biophysics is very strong, and biophysics is currently listed as one of the fast-growing occupations in the North America