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. 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