Radiological Protection and Medical Dosimetry for the Skylab

RADIOLOGICAL PROTECTION PLANNING for the Skylab missions encompassed two major areas; those radiation exposures that were "expected" whose components were known with relative certainty and those radiation exposures that were "unexpected" or completely indeterminant. The expected radiation components were the trapped protons and electrons of the Van Allen Belts (figure 9-1), galactic cosmic rays, and the emissions of onboard radiation sources (table 9-I). The possibilities of unexpected exposure included energetic solar particle events, high altitude nuclear tests, and potential problems with onboard sources.

Premission analyses indicated that dose equivalents from the nominal environment of trapped (Van Allen Belt) particles and galactic cosmic radiations would be well below the limits adopted by National Aeronautics and Space Administration from the National Academy of Sciences recommendations for manned space flight (table 9-II) (ref. 1). These analyses indicated that the Skylab 2 mission (28-day duration) would be within the 30-day limit category, while Skylab 3 and 4 (59 days and 84 days, respectively) would be within the 90-day category. Because the nominal environment would result in doses well below these limits, operational radiation support was geared toward rapid identification and reaction to any enhanced radiation situation.

Spacecraft Radiation Monitoring

Mission rules establishing mandatory onboard decisions concerning a radiation enhanced environment were written only for the relatively radiation sensitive intervals of extravehicular activity. Therefore, the astronauts were provided instrumentation and training to insure that the crews aboard Skylab could act autonomously during periods of planned or unexpected communication loss.

The onboard instruments available for crew readout included a portable rate survey meter and three (plus a spare) personal radiation dosimeters which display integrated dose in 10 millirad integrals. The personal radiation dosimeters and rate survey meter provided the dual functions of extravehicular activity dosimetry and dose rate monitoring, plus vehicle area monitoring in the intervals between extravehicular activities.

Routine monitoring of dose rates at a fixed location aboard the Skylab vehicle was performed by an ionization chamber instrument, the Van Allen Belt Dosimeter. Electron and proton fluences (particles/cm-square) were monitored by an electron-proton spectrometer mounted on the exterior of the spacecraft. Rate data from these instruments were telemetered or recorded for later transmission to ground, and were not available for direct crew readout.

Passive Dosimetry

Each crewman was provided with a passive dosimeter packet to be worn continuously throughout the mission. The packet weighed approximately 14 g (one-half oz), and was designed to be worn on a soft strap on the ankle or wrist. The packet contained the following dosimetry materials for postflight analysis: densitometric film, nuclear track emulsions, polycarbonate and cellulose nitrate track detectors, lithium fluoride (TLD-700) chips, and tantalum/iridium foils.

In addition to passive dosimeters worn by the crewmen, passive dosimeters were placed within the Orbital Workshop’s film storage vault for the intervals from the beginning of Skylab 2 to the end of Skylab 2 (28 days) and from the beginning of Skylab 2 to the end of Skylab 3 (123 days). The film vault dosimeters were placed in locations with aproximate 2 pia shielding values of 13 and 23 g/cm/2 aluminum. Relative to proton range in tissue, these depths in aluminum correspond to soft tissue depths of approximately 10 and 19 cm, respectively.

Ground Radiation Monitoring

Radiation protection support was provided by specialists in communications, computational analysis, and radiological health. Spacecraft data, satellite information, and solar observatory reports were utilized in evaluating the space environment, especially relative to radiation enhancement. The crewmen reported their personal radiation dosimeter readings (as integrated dose) on a daily basis, plus additional readings before and after each extravehicular activity. These readouts confirmed a continuously nominal radiation environment throughout each of the three missions.

Although there were no radiation enhancements, the mission was not totally uneventful from a radiation standpoint. A few highlights are as follows.

Solar Activity.—The Skylab missions were flown during a period when solar activity was approaching a minimum in the Sun’s solar cycle. Nevertheless several events of scientific interest occurred during the Skylab missions, however, particle emissions from these events were of low energy and relatively low intensity. These characteristics, coupled with the shielding effect of the Earth’s magnetic field, reduced radiation doses from solar particles to below the limits of detect-ability for onboard dosimetry instrumentation (<10 millirad per event).

Nuclear Events.—A series of four nuclear devices were detonated by France at their Murora Test Site during Skylab 3. The tests produced no ionizing radiation problems for Skylab. However, the possibility of eye damage to the crew from accidental observation of a test was recognized. Therefore, visual observation of ground sites in the vicinity of the test area was completely avoided.

Onboard Radiation Source Problems.—One of the larger onboard sources (approximately 200 mCi of promethium-147) was radioluminescent markings on knobs and dials of an experimental device, the experiment S019 "Articulated Mirror System." Roughly half of the total activity was applied to digital readout belts and wheels within a readout subassembly. Two malfunctions occurred with the device in-flight. First, a number of radio-luminescent numerals (about one mCi each) became detached from one of the dial wheels, and second (perhaps because of the first), a belt of numerals became jammed and failed to indicate instrument position in the 10’s and 100’s places of rotational attitude.

The possibility of numeral detachment had been recognized late in the preflight preparations for the missions and the dial subassembly had been gasket-sealed to preclude escape of promethium-147 into the spacecraft atmosphere. The problem during the flight became one of how to obtain valid experimental results, either by fixing the jammed belt (without release of promethium-147) or by finding an alternative alignment method for the experiment. Ground based testing with a training model of the experiment equipment determined that the numeral belt could not be freed without breaking into the sealed dial unit. In the meantime, an alternative alignment method was devised and tested. The alternative method was successful and was utilized for the remainder of the mission.

Dosimetry Results

Integrated radiation doses at a tissue depth equivalent to lens of the eye were obtained daily by crew readout of personal radiation dosimeters. These dosimeters were worn the first 4 days of each mission and on all extravehicular activities. During the duration of each mission, the instruments were placed in the designated assigned positions shown in table 9-III. Mean dose rates for similar positions in consecutive missions show a trend toward increased values as use of food, water, propellants, and other expendables reduced the overall spacecraft shielding. Thermoluminescent dosimeter results of the crew-worn passive packets are shown in table 9-III for comparison with the rates found throughout the spacecraft.

An upper limit estimate of the hard galactic radiation contribution is approximately 18 millirad per day; the approximate lower limit is 12 millirad per day. Comparison of these rates with the overall mean dose rates shown in table 9-III indicates that the galactic component accounted for 30 to 50 percent of the observed film vault doses, and roughly 20 to 30 percent of the crew dose means.

The majority of the remaining dose originates from protons of the Van Allen Belts and softer secondary radiations generated by passage of the primary particles through spacecraft materials.

The evaluation of dose equivalents for mixed radiations in space is a complex subject and it is recommended that the reader consult the literature for rigorous discussion on this subject. There are, however, some notable findings which should be covered.

Primary Electrons.—Van Allen Belt electrons did not penetrate into the spacecraft, nor were they found to penetrate deeply enough (3 mm tissue equivalent) during extravehicular activities to register on either the passive dosimeters or personal radiation dosimeters. Consequently, electron doses to the skin (tissue depth: 0.1 mm below 0.2 g/cm² of space suit shielding) were calculated from electron-proton spectrometer data.

Dose Versus Shield Depth.—Doses to the blood forming organs (tissue depth: 5 cm) were found to average 0.66 of the doses observed to the skin. These dose averages were obtained by integration of outputs from the dual sensors of the Van Allen Belt Dosimeter. The value of 0.66 also is in good agreement with a value obtained by interpolation between crew-worn and film vault dosimeter results.

The sole difference between skin and eye doses (0.1 mm and 3.0 mm tissue depth, respectively) is the added dose to skin from electrons during extra-vehicular activities.

Quality Factor Versus Shield Depth.—Film vault shielding was found to be relatively ineffective from a simple dose reduction standpoint (table 9-III). Despite the small dose reduction, however, quality factor could have decreased substantially if the dose reduction was solely due to filtering of lower energy particles. On the other hand, secondary buildup processes tend to increase quality factor as a function of shield depth. These competing effects could not be calculated accurately prior to the mission. Therefore, we have relied primarily on postmission nuclear emulsion analyses of the film vault dosimeters to determine space radiation quality as a function of shielding.

Comparison of emulsion data from the dosimeters worn by the crew and film vault dosimeters indicates that the filtering mechanism (reduced quality factor) is slightly dominant at shield depths up to 23.3 g/cm² aluminum. At blood forming organ depth (5 cm tissue), quality factor is estimated equal to 1.5. In comparison, a quality factor of 1.6 is found for the crew-worn dosimeters beneath 0.3 g/cm² of tissue equivalent shielding.

Neutron Dosimetry.—Details of the iridium/ tantalum neutron dosimetry system have been published previously (ref. 2). Thermal (0.02 to 2.0 electronvolts) and intermediate (2.0 to 2 X 10³ electronvolts) neutrons were found to contribute to crew dose equivalent at a combined rate of approximately 0.1 millirem/day.

Direct measurement of fast neutron fluence by suspended track analysis of crew-worn nuclear emulsions was not possible due to the high track densities obtained on the Skylab missions. However, upper limit dose calculations have been made based on nuclear emulsion disintegration star analyses (to determine neutron production rates) and iridium/tantalum evaluation, assuming that all activation is due to tissue albedo. Both methods show excellent agreement with upper limit rates of approximately 12.5 millirem per day for fast neutrons with mean energy of approximately one megaelectronvolts.

Conclusion

Table 9-IV summarizes the dosimetry results for each crewman of the Skylab missions. As indicated in this table, there were certain variations in passive dosimeter wearing habits which required adjustments for data comparison purposes.

Dose equivalents received by the Skylab 4 crewmen were the highest received in any NASA mission to date, but remained well within the limits established for the Skylab missions. Due to the low rates involved (for example, less than 100 millirem per day to blood forming organs), dose equivalents for each crewman were well below the threshold of significant clinical effect. These dose equivalents apply specifically to long-term effects such as generalized life shortening, increased neoplasm incidence, and cataract production. To place the mission values in perspective, the NASA career limits were 400 rem blood forming organs, 1200 rem skin, and 600 rem eye lens and were established from ancillary radiation exposure constraints recommended by the National Academy of Science and based upon a reference risk of doubling the incidence of leukemia and other neoplastic disease. This reference risk was taken to be a dose equivalent of 400 rem. These career limits also entail a statistical risk of nonspecific life shortening of from 0.5 to 3.0 years (ref. 3). The Skylab 4 crewman could fly a mission comparable to one 84-day Skylab 4 mission per year for 50 years before exceeding these career limits.

References

1. Space Science Board, Radiobiological Advisory Panel, Committee on Space Medicine. Radiation protection guides and constraints for space-mission and vehicle-design studies involving nuclear systems. National Acad. Sci., p. 15. Washington, D.C., 1970.

2. ENGLISH, R.A., and E.D. LILES. Iridium and tantalum foils for spaceflight neutron dosimetry. Health Phys., 22:503-506, May 1972.

3. Space Science Board, Radiobiological Advisory Panel, Committee on Space Medicine. Radiation protection guides and constraints for space-mission and vehicle-design studies involving nuclear systems. National Acad. Sci., pp. 12-14. Washington, D.C., 1970.

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