loss, and worse in technical journals of the time. in february 1896, professor john daniel and dr. william lofland dudley of vanderbilt university reported hair loss after dr. dudley was x-rayed. a child who had been shot in the head was brought to the vanderbilt laboratory in 1896. before trying to find the bullet, an experiment was attempted, for which dudley "with his characteristic devotion to science" volunteered. daniel reported that 21 days after taking a picture of dudley's skull (with an exposure time of one hour), he noticed a bald spot 5 centimeters (2 in) in diameter on the part of his head nearest the x-ray tube: "a plate holder with the plates towards the side of the skull was fastened and a coin placed between the skull and the head. the tube was fastened at the other side at a distance of one-half inch from the hair." beyond burns, hair loss, and cancer, x-rays can be linked to infertility in males based on the amount of radiation used. in august 1896, dr. hd. hawks, a graduate of columbia college, suffered severe hand and chest burns from an x-ray demonstration. it was reported in electrical review and led to many other reports of problems associated with x-rays being sent in to the publication. many experimenters including elihu thomson at edison's lab, william j. morton, and nikola tesla also reported burns. elihu thomson deliberately exposed a finger to an x-ray tube over a period of time and suffered pain, swelling, and blistering. other effects were sometimes blamed for the damage including ultraviolet rays and (according to tesla) ozone. many physicians claimed there were no effects from x-ray exposure at all. on august 3, 1905, in san francisco, california, elizabeth fleischman, an american x-ray pioneer, died from complications as a result of her work with x-rays. hall-edwards developed a cancer (then called x-ray dermatitis) sufficiently advanced by 1904 to cause him to write papers and give public addresses on the dangers of x-rays. his left arm had to be amputated at the elbow in 1908, and four fingers on his right arm soon thereafter, leaving only a thumb. he died of cancer in 1926. his left hand is kept at birmingham university. 20th century and beyond a patient being examined with a thoracic fluoroscope in 1940, which displayed continuous moving images. this image was used to argue that radiation exposure during the x-ray procedure would be negligible. the many applications of x-rays immediately generated enormous interest. workshops began making specialized versions of crookes tubes for generating x-rays and these first-generation cold cathode or crookes x-ray tubes were used until about 1920. a typical early 20th century medical x-ray system consisted of a ruhmkorff coil connected to a cold cathode crookes x-ray tube. a spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes. the spark gap allowed detecting the polarity of the sparks, measuring voltage by the length of the sparks thus determining the "hardness" of the vacuum of the tube, and it provided a load in the event the x-ray tube was disconnected. to detect the hardness of the tube, the spark gap was initially opened to the widest setting. while the coil was operating, the operator reduced the gap until sparks began to appear. a tube in which the spark gap began to spark at around 6.4 centimeters (2.5 in) was considered soft (low vacuum) and suitable for thin body parts such as hands and arms. a 13-centimeter (5 in) spark indicated the tube was suitable for shoulders and knees. an 18-to-23-centimeter (7 to 9 in) spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals. since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased in order to operate the tube for imaging. exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax. the plates may have a small addition of fluorescent salt to reduce exposure times. crookes tubes were unreliable. they had to contain a small quantity of gas (invariably air) as a current will not flow in such a tube if they are fully evacuated. however, as time passed, the x-rays caused the glass to absorb the gas, causing the tube to generate "harder" x-rays until it soon stopped operating. larger and more frequently used tubes were provided with devices for restoring the air, known as "softeners". these often took the form of a small side tube that contained a small piece of mica, a mineral that traps relatively large quantities of air within its structure. a small electrical heater heated the mica, causing it to release a small amount of air, thus restoring the tube's efficiency. however, the mica had a limited life, and the restoration process was difficult to control. in 1904, john ambrose fleming invented the thermionic diode, the first kind of vacuum tube. this used a hot cathode that caused an electric current to flow in a vacuum. this idea was quickly applied to x-ray tubes, and hence heated-cathode x-ray tubes, called "coolidge tubes", completely replaced the troublesome cold cathode tubes by about 1920. in about 1906, the physicist charles barkla discovered that x-rays could be scattered by gases, and that each element had a characteristic x-ray spectrum. he won the 1917 nobel prize in physics for this discovery. in 1912, max von laue, paul knipping, and walter friedrich first observed the diffraction of x-rays by crystals. this discovery, along with the early work of paul peter ewald, william henry bragg, and william lawrence bragg, gave birth to the field of x-ray crystallography. in 1913, henry moseley performed crystallography experiments with x-rays emanating from various metals and formulated moseley's law which relates the frequency of the x-rays to the atomic number of the metal. the coolidge x-ray tube was invented the same year by william d. coolidge. it made possible the continuous emissions of x-rays. modern x-ray tubes are based on this design, often employing the use of rotating targets which allow for significantly higher heat dissipation than static targets, further allowing higher quantity x-ray output for use in high powered applications such as rotational ct scanners. chandra's image of the galaxy cluster abell 2125 reveals a complex of several massive multimillion-degree-celsius gas clouds in the process of merging. the use of x-rays for medical purposes (which developed into the field of radiation therapy) was pioneered by major john hall-edwards in birmingham, england. then in 1908, he had to have his left arm amputated because of the spread of x-ray dermatitis on his arm. medical science also used the motion picture to study human physiology. in 1913, a motion picture was made in detroit showing a hard-boiled egg inside a human stomach. this early x-ray movie was recorded at a rate of one still image every four seconds. dr lewis gregory cole of new york was a pioneer of the technique, which he called "serial radiography". in 1918, x-rays were used in association with motion picture cameras to capture the human skeleton in motion. in 1920, it was used to record the movements of tongue and teeth in the study of languages by the institute of phonetics in england. in 1914, marie curie developed radiological cars to support soldiers injured in world war i. the cars would allow for rapid x-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate. from the early 1920s through to the 1950s, x-ray machines were developed to assist in the fitting of shoes and were sold to commercial shoe stores. concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s, leading to the practice's eventual end that decade. the x-ray microscope was developed during the 1950s. the chandra x-ray observatory, launched on july 23, 1999, has been allowing the exploration of the very violent processes in the universe which produce x-rays. unlike visible light, which gives a relatively stable view of the universe, the x-ray universe is unstable. it features stars being torn apart by black holes, galactic collisions, and novae, and neutron stars that build up layers of plasma that then explode into space. phase-contrast x-ray image of a spider an x-ray laser device was proposed as part of the reagan administration's strategic defense initiative in the 1980s, but the only test of the device (a sort of laser "blaster" or death ray, powered by a thermonuclear explosion) gave inconclusive results. for technical and political reasons, the overall project (including the x-ray laser) was defunded (though was later revived by the second bush administration as national missile defense using different technologies). phase-contrast x-ray imaging refers to a variety of techniques that use phase information of an x-ray beam to form the image. due to its good sensitivity to density differences, it is especially useful for imaging soft tissues. it has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. there are several technologies being used for x-ray phase-contrast imaging, all utilizing different principles to convert phase variations in the x-rays emerging from an object into intensity variations. these include propagation-based phase contrast, talbot interferometry, refraction-enhanced imaging, and x-ray interferometry. these methods provide higher contrast compared to normal absorption-based x-ray imaging, making it possible to distinguish from each other details that have almost similar density. a disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus x-ray sources, x-ray optics, and high resolution x-ray detectors. energy ranges x-rays are part of the electromagnetic spectrum, with wavelengths shorter than uv light. different applications use different parts of the x-ray spectrum. soft and hard x-rays x-rays with high photon energies above 5–10 kev (below 0.2–0.1 nm wavelength) are called hard x-rays, while those with lower energy (and longer wavelength) are called soft x-rays. the intermediate range with photon energies of several kev is often referred to as tender x-rays. due to their penetrating ability, hard x-rays are widely used to image the inside of objects (e.g. in medical radiography and airport security). the term x-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. since the wavelengths of hard x-rays are similar to the size of atoms, they are also useful for determining crystal structures by x-ray crystallography. by contrast, soft x-rays are easily absorbed in air; the attenuation length of 600 ev (~2 nm) x-rays in water is less than 1 micrometer. gamma rays there is no consensus for a definition distinguishing between x-rays and gamma rays. one common practice is to distinguish between the two types of radiation based on their source: x-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. this definition has several problems: other processes can also generate these high-energy photons, or sometimes the method of generation is not known. one common alternative is to distinguish x- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10−11 m (0.1 å), defined as gamma radiation. this criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (some measurement techniques do not distinguish between detected wavelengths.) however, these two definitions often coincide since the electromagnetic radiation emitted by x-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei. occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source. thus, gamma-rays generated for medical and industrial uses, for example radiotherapy, in the ranges of 6–20 mev, can in this context also be referred to as x-rays. properties ionizing radiation hazard symbol x-ray photons carry enough energy to ionize atoms and disrupt molecular bonds. this makes it a type of ionizing radiation, and therefore harmful to living tissue. a very high radiation dose over a short period of time causes radiation sickness, while lower doses can give an increased risk