Bonner Ball Neutron Detector (BBND) measures neutron radiation (low-energy, uncharged particles) which can deeply penetrate the body and damage blood forming organs. Neutron radiation is estimated to be 20 percent of the total radiation on the International Space Station (ISS). This study characterizes the neutron radiation environment to develop safety measures to protect future ISS crews. This was a cooperative experiment with Japan Aerospace Exploration Agency.Principal Investigator(s)
Johnson Space Center, Human Research Program, Houston, TX, United States
Japan Aerospace Exploration Agency (JAXA)Sponsoring Organization
Information PendingResearch Benefits
Information PendingISS Expedition Duration:
March 2001 - December 2001Expeditions Assigned
2,3Previous ISS Missions
BBND first flew on STS-89, during which real-time, active radiation was measured as a test case for ISS missions.
The Bonner Ball Neutron Detector (BBND) developed by Japan Aerospace and Exploration Agency (JAXA) was used inside the International Space Station (ISS) to measure the neutron energy spectrum. It consisted of several neutron moderators enabling the device to discriminate neutron energies up to 15 MeV (15 mega electron volts). This BBND characterized the neutron radiation on ISS during Expeditions 2 and 3.
Due to their neutral charge, neutrons are not affected by magnetic forces and they penetrate deeper in materials and may deposit a larger fraction of their energy into human tissue than charged-particle radiations. Neutrons in the space environment are present in galactic background and solar events (primary particles), neutrons are also produced by the interaction of particles in the materials used to construct spacecraft (secondary particles). High energy secondary neutrons are produced by the interactions of high-energy charged-particles with spacecraft materials and planetary surfaces.
BBND experiment hardware consists of two assemblies: the BBND control unit, which stores radiation measurements and controls data quality; and the BBND detector unit (DU), which measures neutron radiation via a series of six stainless-steel spherical shells. Four spheres are thermal neutron detectors covered in polyethylene of different thickness, one detector is covered in gadolinium; and one detector is uncovered. The gadolinium-covered sphere acts as a control; neutrons are unable to penetrate the dense gadolinium, and the data collected by the sphere are used to determine the difference between pulses created by neutrons and protons. Data collected from the polyethylene-covered spheres will shows the amount of hydrogen surrounding the detector affects the amount of radiation penetration.
Neutrons are uncharged atomic particles that have the ability to penetrate living tissues. Because they are neutral they can penetrate farther into tissue than other types of radiation and cause severe damage, including nerve damage, cataracts and cancer. Neutron radiation can also affect the blood-forming marrow in the bones of humans and other animals. As 20 percent of the radiation that bombards the ISS is neutron radiation, it is necessary to characterize the real-time effect.
The six BBND detectors provided data indicating how much radiation was absorbed at various times, allowing a model of real-time exposure to be calculated, as opposed to earlier models of passive neutron detectors which were only capable of providing a total amount of radiation received over a span of time. Neutron radiation information obtained from the Bonner Ball Neutron Detector (BBND) can be used to develop safety measures to protect crewmembers during both long-duration missions on the ISS and during interplanetary exploration.
BBND will help determine the amount of shielding needed to protect humans from neutron radiation. This data can be extrapolated to the nuclear industry to develop more effective personal protective equipment for people working with neutron radiation.
The crew transferred and downlinked data weekly via the Human Research Facility-1 (HRF-1) workstation. They also transferred the BBND from the Shuttle to Station, and distributed the parts to the appropriate locations. After installation, BBND automatically gathered data.Operational Protocols
Following activation, BBND continuously collected data, which was stored on the BBND hard drive. The crew checked the BBND clock daily and changed-out the hard drive and downlinked data weekly. BBND was initially deployed in an empty rack in the U.S. Destiny Laboratory, and was relocated near the Laboratory's window one week prior to Expedition 3.
BBND characterized the neutron radiation on ISS during Expeditions 2 and 3 and determined that galactic cosmic rays were the major cause of secondary neutrons measured inside ISS. The neutron energy spectrum was measured from March 23, 2001 through November 14, 2001 in the U.S. Laboratory Module of the ISS. The time frame enabled neutron measurements to be made during a time of increased solar activity (solar maximum) as well as observe the results of a solar flare on November 4, 2001.
BBND results show the overall neutron environment at the ISS orbital altitude is influenced by highly energetic galactic cosmic rays, except in the South Atlantic Anomaly (SAA) region where protons trapped in the Earth's magnetic field cause a more severe neutron environment. However, the number of particles measured per second per square cm per MeV obtained by BBND is consistently lower than that of the precursor investigations. The average dose-equivalent rate observed through the investigation was 3.9 micro Sv/hour or about 10 times the rate of radiological exposure to the average US citizen. In general, radiation damage to the human body is indicated by the amount of energy deposited in living tissue, modified by the type of radiation causing the damage; this is measured in units of Sieverts (Sv). The background radiation dose received by an average person in the United States is approximately 3.5 milliSv/year. Conversely, an exposure of 1 Sv can result in radiation poisoning and a dose of five Sv will result in death in 50 percent of exposed individuals. The average dose-equivalent rate observed through the BBND investigation is 3.9 micro Sv/hour, or about ten times the average US surface rate. The highest rate, 96 microSv/hour was observed in the SAA region.
The November 4, 2001 solar flare and the associated geomagnetic activity caused the most severe radiation environment inside the ISS during the BBND experiment. The increase of neutron dose-equivalent due to those events was evaluated to be 0.19mSv, which is less than 1 percent of the measured neutron dose-equivalent measured over the entire 8-month period.
Although this experiment did not characterize the neutron radiation environment outside of Earth's magnetic field, the BBND sampling equipment provided results without return of equipment to Earth and proved that similar measurement systems could be used on missions to the moon and Mars to monitor real time radiation risks (Expedition 2 and 3 One Year Postflight Report). (Evans et al. 2009)
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