beta or gamma, or combinations of these. the uk national physical laboratory publishes a good practice guide through its ionising radiation metrology forum concerning the provision of such equipment and the methodology of calculating the alarm levels to be used. portable instruments hand-held ion chamber survey meter in use for surface dose rate on one of three radioisotope thermoelectric generators (rtgs) for the cassini spacecraft. portable instruments are hand-held or transportable. the hand-held instrument is generally used as a survey meter to check an object or person in detail, or assess an area where no installed instrumentation exists. they can also be used for personnel exit monitoring or personnel contamination checks in the field. these generally measure alpha, beta or gamma, or combinations of these. transportable instruments are generally instruments that would have been permanently installed, but are temporarily placed in an area to provide continuous monitoring where it is likely there will be a hazard. such instruments are often installed on trolleys to allow easy deployment, and are associated with temporary operational situations. in the united kingdom the hse has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned. this covers all radiation instrument technologies, and is a useful comparative guide. instrument types a number of commonly used detection instrument types are listed below, and are used for both fixed and survey monitoring. ionization chambers proportional counters geiger counters semiconductor detectors scintillation detectors airborne particulate radioactivity monitoring the links should be followed for a fuller description of each. radiation related quantities the following table shows the main radiation-related quantities and units. ionizing radiation related quantities quantity unit symbol derivation year si equivalent activity (a) becquerel bq s−1 1974 si unit curie ci 3.7 × 1010 s−1 1953 3.7×1010 bq rutherford rd 106 s−1 1946 1,000,000 bq exposure (x) coulomb per kilogram c/kg c⋅kg−1 of air 1974 si unit röntgen r esu / 0.001293 g of air 1928 2.58 × 10−4 c/kg absorbed dose (d) gray gy j⋅kg−1 1974 si unit erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 gy rad rad 100 erg⋅g−1 1953 0.010 gy equivalent dose (h) sievert sv j⋅kg−1 × wr 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr 1971 0.010 sv effective dose (e) sievert sv j⋅kg−1 × wr × wt 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr × wt 1971 0.010 sv spacecraft radiation challenges spacecraft, both robotic and crewed, must cope with the high radiation environment of outer space. radiation emitted by the sun and other galactic sources, and trapped in radiation "belts" is more dangerous and hundreds of times more intense than radiation sources such as medical x-rays or normal cosmic radiation usually experienced on earth. when the intensely ionizing particles found in space strike human tissue, it can result in cell damage and may eventually lead to cancer. the usual method for radiation protection is material shielding by spacecraft and equipment structures (usually aluminium), possibly augmented by polyethylene in human spaceflight where the main concern is high-energy protons and cosmic ray ions. on uncrewed spacecraft in high-electron-dose environments such as jupiter missions, or medium earth orbit (meo), additional shielding with materials of a high atomic number can be effective. on long-duration crewed missions, advantage can be taken of the good shielding characteristics of liquid hydrogen fuel and water. the nasa space radiation laboratory makes use of a particle accelerator that produces beams of protons or heavy ions. these ions are typical of those accelerated in cosmic sources and by the sun. the beams of ions move through a 100 m (328-foot) transport tunnel to the 37 m2 (400-square-foot) shielded target hall. there, they hit the target, which may be a biological sample or shielding material. in a 2002 nasa study, it was determined that materials that have high hydrogen contents, such as polyethylene, can reduce primary and secondary radiation to a greater extent than metals, such as aluminum. the problem with this "passive shielding" method is that radiation interactions in the material generate secondary radiation. active shielding, that is, using magnets, high voltages, or artificial magnetospheres to slow down or deflect radiation, has been considered to potentially combat radiation in a feasible way. so far, the cost of equipment, power and weight of active shielding equipment outweigh their benefits. for example, active radiation equipment would need a habitable volume size to house it, and magnetic and electrostatic configurations often are not homogeneous in intensity, allowing high-energy particles to penetrate the magnetic and electric fields from low-intensity parts, like cusps in dipolar magnetic field of earth. as of 2012, nasa is undergoing research in superconducting magnetic architecture for potential active shielding applications. early radiation dangers using early crookes tube x-ray apparatus in 1896. one man is viewing his hand with a fluoroscope to optimise tube emissions, the other has his head close to the tube. no precautions are being taken. monument to the x-ray and radium martyrs of all nations erected 1936 at st. georg hospital in hamburg, commemorating 359 early radiology workers. the dangers of radioactivity and radiation were not immediately recognized. the discovery of x‑rays in 1895 led to widespread experimentation by scientists, physicians, and inventors. many people began recounting stories of burns, hair loss and worse in technical journals as early as 1896. in february of that year, professor daniel and dr. dudley of vanderbilt university performed an experiment involving x-raying dudley's head that resulted in his hair loss. a report by dr. h.d. hawks, a graduate of columbia college, of his severe hand and chest burns in an x-ray demonstration, was the first of many other reports in electrical review. many experimenters including elihu thomson at thomas 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 experienced pain, swelling, and blistering. other effects, including ultraviolet rays and ozone were sometimes blamed for the damage. many physicists claimed that there were no effects from x-ray exposure at all. as early as 1902 william herbert rollins wrote almost despairingly that his warnings about the dangers involved in careless use of x-rays was not being heeded, either by industry or by his colleagues. by this time rollins had proved that x-rays could kill experimental animals, could cause a pregnant guinea pig to abort, and that they could kill a fetus. he also stressed that "animals vary in susceptibility to the external action of x-light" and warned that these differences be considered when patients were treated by means of x-rays. before the biological effects of radiation were known, many physicists and corporations began marketing radioactive substances as patent medicine in the form of glow-in-the-dark pigments. examples were radium enema treatments, and radium-containing waters to be drunk as tonics. marie curie protested against this sort of treatment, warning that the effects of radiation on the human body were not well understood. curie later died from aplastic anaemia, likely caused by exposure to ionizing radiation. by the 1930s, after a number of cases of bone necrosis and death of radium treatment enthusiasts, radium-containing medicinal products had been largely removed from the market (radioactive quackery).