| 1. AIMS
Objective is to advance the state-of-the-art of solid-state microdosimeters (SSMD) to design, develop, and test a flight-qualifiable engineering model by Dec 2012. This 4 year project started in 1 Jan 2009 with this report concluding the 2nd year. Aims include:
1.1 Develop a benchtop system to advance the state-of-the-art of SSDM incorporating proven advancements into the flight engineering model
1.2 Develop a flight engineering model suitable as a personal SSDM
1.3 Develop improved SSDM sensors
1.4 Utilize computer modeling to support instrument development and compare with observations
1.5 Explore opportunities to transition to a flight program
2. KEY FINDINGS
2.1 Benchtop System - Obtained and analyzed SSMD spectra for NASA Space Radiation Laboratory (NSRL) beams (protons & heavy ions) to identify particle types, energies, and mass-to-charge ratios in the beams and produced by intervening materials. Reduced instrument noise levels near a factor of 10 during our National Space Biomedical Research Institute (NSBRI) funding period. Best noise measurements at NSRL with 200 feet of cable is ~0.3keV/micron (~0.2 keV/micron-tissue). Compared SSMD spectra from our 1st generation sensors with silicon surface barrier detectors and also obtained spectra for neutrons, the most damaging particles.
2.2 Flight Engineering Model - The instrument developed in year 1 is MIcroDosimeter iNstrument (MIDN)-II (MIDN-II). We have designed an improved version, MIDN-III, reducing size and mass with an expanded set of remote commands that should be available by the end of 2010. MIDN-II tests showed good agreement with the benchtop system observations, radiation codes, and tissue equivalent proportional counter (TEPC) results used as references. Its noise cutoffs is ~ 1keV/micron. Continued development of our unique optical calibration system and applied for a patent in Sept 2010. This provides a continuous end-to-end test and confirms the calibration or recalibrates the SSMD while in operation accomplished without a problematic radiation source.
2.3 Sensor Development - Prior observations were with the 1st generation SSMD sensors. A 2nd generation SSMD sensor was produced and tests confirmed performance using the benchtop and flight engineering instruments. A 3rd generation sensor has been designed and fabricated; preliminary tests look promising with more testing to continue in year 3.
2.4 Modeling - Several versions of the Geant4 radiation code are employed to compare to our SSMD observations. Collaboration has produced detailed SSMD distributions for each particle type and energy, critical for interpreting observations, especially since individual components can be measured with the dose equivalent (DE) coincidence system. We obtain acceptable agreement between our observations and radiation transport codes. We also employ the SolidWorks tool to develop test fixtures and housings and Electronic Workbench tool to model electrical circuits.
2.5 Flight Opportunity - Completed a conceptual design to fit a NanoRacks configuration for the International Space Station (ISS) through the auspices of the Department of Defense (DoD) Space Test Program. Our system has been approved annually for several years for inclusion on DoD space missions. We declined a flight opportunity for a potential launch in 2012 due to insufficient funds and impact to the MIDN project.
Noise measurements with the bench-top system established that SSMDs are able to be operated with noise levels as good as or better than those obtained previously by TEPCs in space. This establishes the feasibility of building space qualifiable systems with sufficiently low noise so that complete SSMD spectra for high energy protons will be able to be obtained even in the lower-lineal energy region not detected previously with space-qualified systems, a major goal of this research project. Recent measurements of SSMD spectra with high-energy neutrons (~15 MeV), considered to be the most damaging particles in space, show that SSMD can operate in high-dose radiation fields for long time periods without failures. This establishes the radiation resistance of our SSDMs, a major goal of this project. Recent measurements with SSDM systems at the NSRL facility at Brookhaven National Laboratory (BNL) establish the practicality of using our new capability of identifying particle species, energy, and charge-to-mass ratio responsible for specific individual events. These measurements provide more stringent data for establishing quality factors and the accuracy of the transport codes and theoretical calculations, a major aim of this project. Development of an end-to-end system test and calibration of a personal SSMD while operational without the need for an ionizing radiation source is a critical achievement. The development and test of the MIDN-II and the design of the MIDN-III that are early versions of a flight qualifiable personal SSMD are important accomplishments.
4.0 RESEARCH PLAN for 2011
4.1 Benchtop System - Having established the feasibility of particle identification with our SSMDs we shall:
a. build a new prototype with smaller DE detectors with lower noise characteristics to reduce the number of random events.
b. obtain data with protons and iron to investigate contributions from low-energy delta rays, typically responsible for 20-30% of the physical dose not seen by typical TEPCs.
c. compare new sensor with the previous and with silicon sensors of the same thicknesses of a few microns but with larger cross-sectional areas.
4.2 Flight Engineering Model - Complete development of our flight engineering model, MIDN-III, and carry out radiation tests at the U.S. Naval Academy (USNA) and at NSRL. The remote command capability will be expanded. Further the optical calibration technique.
4.3 Sensor Development - Carry out detailed testing of 3rd generation SSMD sensors anticipating additional improved sensors from our collaborators.
4.4 Modeling - Add to our radiation transport codes by integrating the newly available GRAS (generally recognized as safe) module into our transport code suite.
4.5 Flight Opportunity - Continue to work with the DoD Space Test Program Office to obtain a future flight opportunity.