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Human subjects responded on two panels. A differential-reinforcement-of-low-rate schedule with a limited-hold contingency operated on Panel A. In Condition 1, responses on Panel B produced a stimulus on the panel that signalled whether reinforcement was available on Panel A. In Condition 2, responses on Panel B briefly illuminated a digital clock. In both conditions, performance on Panel A was very efficient; with few exceptions, Panel A was pressed only when reinforcement was available. Thus, in effect, a fixed-interval schedule operated on Panel B. In Condition 1, a "break-and-run" response pattern occurred on Panel B; with increasing temporal parameters, the duration of the postreinforcement pause on Panel B increased linearly while overall response rate and running rate (calculated by excluding the postreinforcement pauses) remained approximately constant. In Condition 2, the response pattern on Panel B was scalloped; the postreinforcement pause was a negatively accelerated increasing function of schedule value, while overall response rate and running rate were negatively accelerated decreasing functions of schedule value. The performance of subjects in Condition 2, but not in Condition 1, was highly sensitive to the contingencies in operation, and resembled that of other species on the fixed-interval schedule.

This paper reviews the current knowledge of cosmic radiation and its applicability to commercial aviation. Galactic cosmic radiation emanates from outside the solar system, while occasionally a disturbance in the sun's atmosphere leads to a surge in radiation particles. Protection is provided by the sun's magnetic field, the earth's magnetic field, and the earth's atmosphere. Dose rates are dependent on the altitude, the geomagnetic latitude and the solar cycle. For occupational exposure to ionising radiation, which includes aircrew, the International Commission on Radiological Protection recommends maximum mean body effective dose limits of 20 mSv per year (averaged over 5 years, with a maximum in any one year of 50 mSv). Radiation doses can be measured during flight or may be calculated using a computermodelling program such as CARI, EPCARD, SIEVERT or PCAIRE. Mean ambient equivalent dose rates are in the region of 4 -5 µSv per hour for long-haul pilots and 1 -3 µSv per hour for short-haul, giving an annual mean effective exposure of the order 2 -3 mSv for long-haul and 1 -2 mSv for short-haul pilots. Epidemiological studies of flight crew have not shown any increase in cancer mortality or cancer incidence that could be directly attributable to ionising radiation exposure. Whilst it is accepted that there is no level of radiation exposure below which effects do not occur, all the current evidence indicates that the probability of airline crew or passengers suffering any abnormality or disease as a result of exposure to cosmic radiation is very low.Keywords: Galactic cosmic radiation, solar flare, coronal mass ejection, ground level enhancement (GLE), International Commission on Radiological Protection (ICRP), dose rate effectiveness (DREF), linear energy transfer (LET), relative biological effectiveness (RBE), quality factor (QF), chromosome aberrations, CARI, EPCARD, SIEVERT, PCAIRE. IntroductionThe planet earth is continuously bathed in high-energy ionising radiation known as galactic cosmic radiation (GCR), emanating from outside the solar system, and sporadically exposed to bursts of energetic particles from the sun referred to as solar particle events (SPEs).The main source of GCR is believed to be supernovae (exploding stars), while occasionally a disturbance in the sun's atmosphere (solar flare or coronal mass ejection) leads to a surge of radiation particles with sufficient energy to penetrate the earth's magnetic field and enter the atmosphere. Ionising RadiationIonising radiation refers to subatomic particles that, on interacting with an atom, can directly or indirectly cause the atom to lose an electron or even break apart its nucleus. It is when these events occur in body tissue that health effects may result if the human body's self-repair mechanism fails.Ionising radiation is a normal part of the environment. It is emitted in all normal human tissue cells, and emanates from the ground and some building materials. It is used as a medical diagnostic and treatment tool, and is received from GCR. 2Io...