to the same extent. crosslinks strengthen the polymer by preventing chain sliding, effectively leading to thermoset behavior. crosslinks and branching lead to higher molecular weight and polydispersity. thus, these polymers will generally have increased stiffness, tensile strength, and yield strength, and decreased solubility. polyethylene is well known to experience improved mechanical properties as a result of crosslinking, including increased tensile strength and decreased elongation at break. thus, it has “several advantageous applications in areas as diverse as rock bolts for mining, reinforcement of concrete, manufacture of light weight high strength ropes and high performance fabrics.” in contrast, chain scission reactions will weaken the material by decreasing the average molecular weight of the chains, such that tensile and flexural strength decrease and solubility increases. chain scission occurs primarily in the amorphous regions of the polymer. it can increase crystallinity in these regions by making it easier for the short chains to reassemble. thus, it has been observed that crystallinity increases with dose, leading to a more brittle material on the macroscale. in addition, “gaseous products, such as co2, may be trapped in the polymer, and this can lead to subsequent crazing and cracking due to accumulated local stresses." an example of this phenomenon is 3d printed materials, which are often porous as a result of their printing configuration. oxygen can diffuse into the pores and react with the surviving free radicals, leading to embrittlement. some materials continue to weaken through aging, as the remaining free radicals react. the resistance of these polymers to radiation damage can be improved by grafting or copolymerizing aromatic groups, which enhance stability and decrease reactivity, and by adding antioxidants and nanomaterials, which act as free radical scavengers. in addition, higher molecular weight polymers will be more resistant to radiation. effects on gases exposure to radiation causes chemical changes in gases. the least susceptible to damage are noble gases, where the major concern is the nuclear transmutation with follow-up chemical reactions of the nuclear reaction products. high-intensity ionizing radiation in air can produce a visible ionized air glow of telltale bluish-purplish color. the glow can be observed e.g. during criticality accidents, around mushroom clouds shortly after a nuclear explosion, or inside of a damaged nuclear reactor like during the chernobyl disaster. significant amounts of ozone can be produced. even small amounts of ozone can cause ozone cracking in many polymers over time, in addition to the damage by the radiation itself. gas-filled radiation detectors in some gaseous ionisation detectors, radiation damage to gases plays an important role in the device's ageing, especially in devices exposed for long periods to high intensity radiation, e.g. detectors for the large hadron collider or the geiger–müller tube ionization processes require energy above 10 ev, while splitting covalent bonds in molecules and generating free radicals requires only 3-4 ev. the electrical discharges initiated by the ionization events by the particles result in plasma populated by large amount of free radicals. the highly reactive free radicals can recombine back to original molecules, or initiate a chain of free-radical polymerization reactions with other molecules, yielding compounds with increasing molecular weight. these high molecular weight compounds then precipitate from gaseous phase, forming conductive or non-conductive deposits on the electrodes and insulating surfaces of the detector and distorting its response. gases containing hydrocarbon quenchers, e.g. argon–methane, are typically sensitive to aging by polymerization; addition of oxygen tends to lower the aging rates. trace amounts of silicone oils, present from outgassing of silicone elastomers and especially from traces of silicone lubricants, tend to decompose and form deposits of silicon crystals on the surfaces. gaseous mixtures of argon (or xenon) with carbon dioxide and optionally also with 2-3% of oxygen are highly tolerant to high radiation fluxes. the oxygen is added as noble gas with carbon dioxide has too high transparency for high-energy photons; ozone formed from the oxygen is a strong absorber of ultraviolet photons. carbon tetrafluoride can be used as a component of the gas for high-rate detectors; the fluorine radicals produced during the operation however limit the choice of materials for the chambers and electrodes (e.g. gold electrodes are required, as the fluorine radicals attack metals, forming fluorides). addition of carbon tetrafluoride can however eliminate the silicon deposits. presence of hydrocarbons with carbon tetrafluoride leads to polymerization. a mixture of argon, carbon tetrafluoride, and carbon dioxide shows low aging in high hadron flux. effects on liquids like gases, liquids lack fixed internal structure; the effects of radiation is therefore mainly limited to radiolysis, altering the chemical composition of the liquids. as with gases, one of the primary mechanisms is formation of free radicals. all liquids are subject to radiation damage, with few exotic exceptions; e.g. molten sodium, where there are no chemical bonds to be disrupted, and liquid hydrogen fluoride, which produces gaseous hydrogen and fluorine, which spontaneously react back to hydrogen fluoride. effects on water water subjected to ionizing radiation forms free radicals of hydrogen and hydroxyl, which can recombine to form gaseous hydrogen, oxygen, hydrogen peroxide, hydroxyl radicals, and peroxide radicals. in living organisms, which are composed mostly of water, majority of the damage is caused by the reactive oxygen species, free radicals produced from water. the free radicals attack the biomolecules forming structures within the cells, causing oxidative stress (a cumulative damage which may be significant enough to cause the cell death, or may cause dna damage possibly leading to cancer). in cooling systems of nuclear reactors, the formation of free oxygen would promote corrosion and is counteracted by addition of hydrogen to the cooling water. the hydrogen is not consumed as for each molecule reacting with oxygen one molecule is liberated by radiolysis of water; the excess hydrogen just serves to shift the reaction equilibriums by providing the initial hydrogen radicals. the reducing environment in pressurized water reactors is less prone to buildup of oxidative species. the chemistry of boiling water reactor coolant is more complex, as the environment can be oxidizing. most of the radiolytic activity occurs in the core of the reactor where the neutron flux is highest; the bulk of energy is deposited in water from fast neutrons and gamma radiation, the contribution of thermal neutrons is much lower. in air-free water, the concentration of hydrogen, oxygen, and hydrogen peroxide reaches steady state at about 200 gy of radiation. in presence of dissolved oxygen, the reactions continue until the oxygen is consumed and the equilibrium is shifted. neutron activation of water leads to buildup of low concentrations of nitrogen species; due to the oxidizing effects of the reactive oxygen species, these tend to be present in the form of nitrate anions. in reducing environments, ammonia may be formed. ammonia ions may be however also subsequently oxidized to nitrates. other species present in the coolant water are the oxidized corrosion products (e.g. chromates) and fission products (e.g. pertechnetate and periodate anions, uranyl and neptunyl cations). absorption of neutrons in hydrogen nuclei leads to buildup of deuterium and tritium in the water. behavior of supercritical water, important for the supercritical water reactors, differs from the radiochemical behavior of liquid water and steam and is currently under investigation. the magnitude of the effects of radiation on water is dependent on the type and energy of the radiation, namely its linear energy transfer. a gas-free water subjected to low-let gamma rays yields almost no radiolysis products and sustains an equilibrium with their low concentration. high-let alpha radiation produces larger amounts of radiolysis products. in presence of dissolved oxygen, radiolysis always occurs. dissolved hydrogen completely suppresses radiolysis by low-let radiation while radiolysis still occurs with the presence of reactive oxygen species has strongly disruptive effect on dissolved organic chemicals. this is exploited in groundwater remediation by electron beam treatment. countermeasures two main approaches to reduce radiation damage are reducing the amount of energy deposited in the sensitive material (e.g. by shielding, distance from the source, or spatial orientation), or modification of the material to be less sensitive to radiation damage (e.g. by adding antioxidants, stabilizers, or choosing a more suitable material). in addition to the electronic device hardening mentioned above, some degree of protection may be obtained by shielding, usually with the interposition of high density materials (particularly lead, where space is critical, or concrete where space is available) between the radiation source and areas to be protected. for biological effects of substances such as radioactive iodine the ingestion of non-radioactive isotopes may substantially reduce the biological uptake of the radioactive form, and chelation therapy may be applied to accelerate the removal of radioactive materials formed from heavy metals from the body by natural processes. for solid radiation damage solid countermeasures to radiation damage consist of three approaches. firstly, saturating the matrix with oversized solutes. this acts to trap the swelling that occurs as a result of the creep and dislocation motion. they also act to help prevent diffusion, which restricts the ability of the material to undergo radiation induced segregation. secondly, dispersing an oxide inside the matrix of the material. dispersed oxide helps to prevent creep, and to mitigate swelling and reduce radiation induced segregation as well, by preventing dislocation motion and the formation and motion of interstitials. finally, by engineering grain boundaries to be as small as possible, dislocation motion can be impeded, which prevents the embrittlement and hardening that result in material failure. effects on humans main article: radiobiology ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. its most common impact is the induction of cancer with a latent period of years or decades after exposure. high doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. controlled doses are used for medical imaging and radiotherapy. most adverse health effects of radiation exposure may be grouped in two general categories: deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells.