is used to calculate this. for example, a practical shield in a fallout shelter with ten halving-thicknesses of packed dirt, which is roughly 115 cm (3 ft 9 in), reduces gamma rays to 1/1024 of their original intensity (i.e. 2−10). the effectiveness of a shielding material in general increases with its atomic number, called z, except for neutron shielding, which is more readily shielded by the likes of neutron absorbers and moderators such as compounds of boron e.g. boric acid, cadmium, carbon and hydrogen. graded-z shielding is a laminate of several materials with different z values (atomic numbers) designed to protect against ionizing radiation. compared to single-material shielding, the same mass of graded-z shielding has been shown to reduce electron penetration over 60%. it is commonly used in satellite-based particle detectors, offering several benefits: protection from radiation damage reduction of background noise for detectors lower mass compared to single-material shielding designs vary, but typically involve a gradient from high-z (usually tantalum) through successively lower-z elements such as tin, steel, and copper, usually ending with aluminium. sometimes even lighter materials such as polypropylene or boron carbide are used. in a typical graded-z shield, the high-z layer effectively scatters protons and electrons. it also absorbs gamma rays, which produces x-ray fluorescence. each subsequent layer absorbs the x-ray fluorescence of the previous material, eventually reducing the energy to a suitable level. each decrease in energy produces bremsstrahlung and auger electrons, which are below the detector's energy threshold. some designs also include an outer layer of aluminium, which may simply be the skin of the satellite. the effectiveness of a material as a biological shield is related to its cross-section for scattering and absorption, and to a first approximation is proportional to the total mass of material per unit area interposed along the line of sight between the radiation source and the region to be protected. hence, shielding strength or "thickness" is conventionally measured in units of g/cm2. the radiation that manages to get through falls exponentially with the thickness of the shield. in x-ray facilities, walls surrounding the room with the x-ray generator may contain lead shielding such as lead sheets, or the plaster may contain barium sulfate. operators view the target through a leaded glass screen, or if they must remain in the same room as the target, wear lead aprons. particle radiation particle radiation consists of a stream of charged or neutral particles, both charged ions and subatomic elementary particles. this includes solar wind, cosmic radiation, and neutron flux in nuclear reactors. alpha particles (helium nuclei) are the least penetrating. even very energetic alpha particles can be stopped by a single sheet of paper. beta particles (electrons) are more penetrating, but still can be absorbed by a few millimetres of aluminium. however, in cases where high-energy beta particles are emitted, shielding must be accomplished with low atomic weight materials, e.g. plastic, wood, water, or acrylic glass (plexiglas, lucite). this is to reduce generation of bremsstrahlung x-rays. in the case of beta+ radiation (positrons), the gamma radiation from the electron–positron annihilation reaction poses additional concern. neutron radiation is not as readily absorbed as charged particle radiation, which makes this type highly penetrating. in a process called neutron activation, neutrons are absorbed by nuclei of atoms in a nuclear reaction. this most often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next-heavier isotope, many of which are unstable. cosmic radiation is not a common concern on earth, as the earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a significant problem for satellites and astronauts, especially while passing through the van allen belt or while completely outside the protective regions of the earth's magnetosphere. frequent fliers may be at a slightly higher risk because of the decreased absorption from thinner atmosphere. cosmic radiation is extremely high energy, and is very penetrating. electromagnetic radiation electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength. x-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. in some special applications, depleted uranium or thorium are used, but lead is much more common; several cm are often required. barium sulfate is used in some applications too. however, when the cost is important, almost any material can be used, but it must be far thicker. most nuclear reactors use thick concrete shields to create a bioshield with a thin water-cooled layer of lead on the inside to protect the porous concrete from the coolant inside. the concrete is also made with heavy aggregates, such as baryte or magnadense (magnetite), to aid in the shielding properties of the concrete. gamma rays are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray. ultraviolet (uv) radiation is ionizing in its shortest wavelengths but is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing, and protective eyewear. protection from uv is simpler than for the other forms of radiation above, so it is often considered separately. in some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily. for example, although high atomic number materials are very effective in shielding photons, using them to shield beta particles may cause higher radiation exposure due to the production of bremsstrahlung x-rays, and hence low atomic number materials are recommended. also, using a material with a high neutron activation cross section to shield neutrons will result in the shielding material itself becoming radioactive and hence more dangerous than if it were not present. personal protective equipment (ppe)—radiation personal protection equipment (ppe) includes all clothing and accessories which can be worn to prevent severe illness and injury as a result of exposure to radioactive material. these include an sr100 (protection for 1hr), sr200 (protection for 2 hours). because radiation can affect humans through internal and external contamination, various protection strategies have been developed to protect humans from the harmful effects of radiation exposure from a spectrum of sources. a few of these strategies developed to shield from internal, external, and high energy radiation are outlined below. internal contamination protective equipment internal contamination protection equipment protects against the inhalation and ingestion of radioactive material. internal deposition of radioactive material result in direct exposure of radiation to organs and tissues inside the body. the respiratory protective equipment described below are designed to minimize the possibility of such material being inhaled or ingested as emergency workers are exposed to potentially radioactive environments. reusable air purifying respirators (apr) elastic face piece worn over the mouth and nose contains filters, cartridges, and canisters to provide increased protection and better filtration powered air-purifying respirator (papr) battery powered blower forces contamination through air purifying filters purified air delivered under positive pressure to face piece supplied-air respirator (sar) compressed air delivered from a stationary source to the face piece auxiliary escape respirator protects wearer from breathing harmful gases, vapours, fumes, and dust can be designed as an air-purifying escape respirator (aper) or a self-contained breathing apparatus (scba) type respirator scba type escape respirators have an attached source of breathing air and a hood that provides a barrier against contaminated outside air self contained breathing apparatus (scba) provides very pure, dry compressed air to full facepiece mask via a hose air is exhaled to environment worn when entering environments immediately dangerous to life and health (idlh) or when information is inadequate to rule out idlh atmosphere external contamination protective equipment external contamination protection equipment provides a barrier to shield radioactive material from being deposited externally on the body or clothes. the dermal protective equipment described below acts as a barrier to block radioactive material from physically touching the skin, but does not protect against externally penetrating high energy radiation. chemical-resistant inner suit porous overall suit—dermal protection from aerosols, dry particles, and non hazardous liquids. non-porous overall suit to provide dermal protection from: dry powders and solids blood-borne pathogens and bio-hazards chemical splashes and inorganic acid/base aerosols mild liquid chemical splashes from toxics and corrosives toxic industrial chemicals and materials level c equivalent: bunker gear firefighter protective clothing flame/water resistant helmet, gloves, foot gear, and hood level b equivalent—non-gas-tight encapsulating suit designed for environments that are immediate health risks but contain no substances that can be absorbed by skin level a equivalent—totally encapsulating chemical- and vapour-protective suit designed for environments that are immediate health risks and contain substances that can be absorbed by skin external penetrating radiation there are many solutions to shielding against low-energy radiation exposure like low-energy x-rays. lead shielding wear such as lead aprons can protect patients and clinicians from the potentially harmful radiation effects of day-to-day medical examinations. it is quite feasible to protect large surface areas of the body from radiation in the lower-energy spectrum because very little shielding material is required to provide the necessary protection. recent studies show that copper shielding is far more effective than lead and is likely to replace it as the standard material for radiation shielding. personal shielding against more energetic radiation such as gamma radiation is very difficult to achieve as the large mass of shielding material required to properly protect the entire body would make functional movement nearly impossible. for this, partial body shielding of radio-sensitive internal organs is the most viable protection strategy. the immediate danger of intense exposure to high-energy gamma radiation is acute radiation syndrome (ars), a result of irreversible bone marrow damage. the concept of selective shielding is based in the regenerative potential of the hematopoietic stem cells found in bone marrow. the regenerative quality of stem cells make it only necessary to protect enough bone marrow to repopulate the body with unaffected stem cells after the exposure: a similar concept which is applied in hematopoietic stem cell transplantation (hsct), which is a common treatment for patients with leukemia. this scientific advancement allows for the development of a new class of relatively lightweight protective equipment that shields high concentrations of bone marrow to defer the hematopoietic sub-syndrome of acute radiation syndrome to much higher dosages. one technique is to apply selective shielding to protect the high concentration of bone marrow stored in the hips and other radio-sensitive organs in the abdominal area. this allows first responders a safe way to perform necessary missions in radioactive environments. radiation protection instruments practical radiation measurement using calibrated radiation protection instruments is essential in evaluating the effectiveness of protection measures, and in assessing the radiation dose likely to be received by individuals. the measuring instruments for radiation protection are both "installed" (in a fixed position) and portable (hand-held or transportable). installed instruments installed instruments are fixed in positions which are known to be important in assessing the general radiation hazard in an area. examples are installed "area" radiation monitors, gamma interlock monitors, personnel exit monitors, and airborne particulate monitors. the area radiation monitor will measure the ambient radiation, usually x-ray, gamma or neutrons; these are radiations that can have significant radiation levels over a range in excess of tens of metres from their source, and thereby cover a wide area. gamma radiation "interlock monitors" are used in applications to prevent inadvertent exposure of workers to an excess dose by preventing personnel access to an area when a high radiation level is present. these interlock the process access directly. airborne contamination monitors measure the concentration of radioactive particles in the ambient air to guard against radioactive particles being ingested, or deposited in the lungs of personnel. these instruments will normally give a local alarm, but are often connected to an integrated safety system so that areas of plant can be evacuated and personnel are prevented from entering an air of high airborne contamination. personnel exit monitors (pem) are used to monitor workers who are exiting a "contamination controlled" or potentially contaminated area. these can be in the form of hand monitors, clothing frisk probes, or whole body monitors. these monitor the surface of the workers body and clothing to check if any radioactive contamination has been deposited. these generally measure alpha or