News Article: Miniature Detector Measures Deep Space Radiation

July 14, 2011


[1] We report new measurements of solar minimum ionizing radiation dose at the Moon onboard the Lunar Reconnaissance Orbiter (LRO) from June 2009 through May 2010. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on LRO houses a compact and highly precise microdosimeter whose design allows measurements of dose rates below 1 micro-Rad per second in silicon achieved with minimal resources (20 g, ∼250 milliwatts, and ∼3 bits/second). We envision the use of such a small yet accurate dosimeter in many future spaceflight applications where volume, mass, and power are highly constrained. As this was the first operation of the microdosimeter in a space environment, the goal of this study is to verify its response by using simultaneous measurements of the galactic cosmic ray ionizing environment at LRO, at L1, and with other concurrent dosimeter measurements and model predictions. The microdosimeter measured the same short timescale modulations in the galactic cosmic rays as the other independent measurements, thus verifying its response to a known source of minimum-ionizing particles. The total dose for the LRO mission over the first 333 days was only 12.2 Rads behind ∼130 mils of aluminum because of the delayed rise of solar activity in solar cycle 24 and the corresponding lack of intense solar energetic particle events. The dose rate in a 50 km lunar orbit was about 30 percent lower than the interplanetary rate, as one would expect from lunar obstruction of the visible sky.


Article Excerpt: 

[2] The total ionizing dose (TID) hazard originates from the space environment and includes contributions from charged particles (electrons, ions, and secondary charged particles such as muons and pions), neutrons that undergo nuclear collisions to produce charged secondaries, and primary photons from the environment and electron bremsstrahlung. Whatever the primary origin, the effect of concern is the deposition of energy in the form of free charge within materials. The free charge in turn can affect surface chemistry, microelectronic device operation, and material properties [e.g., Stuckey and Meshishnek, 2003; Pease et al., 2009]. TID is also a concern for long-term human exploration of space [e.g., Cucinotta et al., 2005, and references therein] because there are no analogs for human exposure to TID from the space environment other than that already acquired on human space missions.

[3] One assesses the TID impacts through testing of electronic parts, testing of spacecraft materials using photon sources and particle accelerators, modeling of the space environment in the mission orbit, and ray tracing to model the distribution of mass and hence the TID reduction (or increase) at the location of interest inside the space vehicle.

[4] An independent method of addressing the TID hazard is to measure it directly on-orbit using a calibrated instrument. There are many examples of dosimetry using standard techniques of charge collection in a known detector volume. The principal benefit from a direct measurement is the reduction of uncertainty compared to that from the process of measuring the external environment, including all the particle and photon inputs, and modeling the transport of that environment and its interaction with materials. Hence, the direct measurement of TID has uncertainty that originates primarily from the dosimeter's ability to accurately capture the liberated charge in a test mass and to accurately report that result. Of course, one disadvantage is that the results are not known for a particular mission until the mission has flown.


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