High-energy charged particles of extra-galactic, galactic and solar origin collide with spacecraft structures in Earth orbit outside the atmosphere and in interplanetary travel beyond the Earth's magnetosphere. These primaries create a number of secondary particles inside the structures that can produce a significant ionizing radiation environment. This radiation is a threat to long term inhabitants or travelers for space missions and produces an increased risk of cancer and DNA damage. The primary high energy cosmic rays and trapped protons collide with common spacecraft materials such as aluminum and silicon and create secondary particles inside structures that are mostly protons and neutrons. Indeed, the effect of tens of grams per square centimeter of structure or atmosphere is to convert and multiply the primary proton "beam" into a secondary environment dominated by neutrons between several MeV and several tens of MeV. Charged protons are readily detected and instruments are already in existence for this task. Neutrons are electrically neutral and therefore much more difficult to measure and detect. These neutrons are reported to contribute 30-60 percent of the dose inside space structures and cannot be ignored. Currently there is no compact, portable and real time neutron detector instrumentation available for use inside spacecraft or on planetary surfaces where astronauts will live and work.
Present neutron detection systems use gas tube proportional counters for the monitoring of low energy (0.025 eV to 1 MeV) neutrons. However for higher energies the detector systems are quite large and massive and often employ passive detection methods that must be recycled and read out after the fact. Physically large neutron diffraction tables are used for accelerator experiments. Emulsions are flown on the Space Shuttle and returned to Earth for analysis. The NASA Ames aircraft uses an instrument built with Bonner spheres which are large spheres of polyethylene moderator some tens of centimeters in diameter with a photodiode in the center and weighing 1500 pounds.
In 1997 we proposed to design and build a portable, low power and robust neutron spectrometer that measures the neutron spectrum from 10 KeV to 500 MeV with at least 10 percent energy resolution in the various energy intervals. This instrument will monitor the existing neutron environment both inside spacecraft structures and on planetary surfaces to determine the safest living areas, warn of high fluxes associated with solar storms and assist the NSBRI Radiation Effects Team in making an accurate assessment of increased cancer risk, DNA and central nervous system (CNS) damage to astronauts. The instrument uses a highly efficient proportional counter Helium 3 tube at the lowest energy intervals where equivalent damage factors for tissue are the highest (10 KeV-2 MeV). The Helium 3 tube is shielded with a cadmium absorber to eliminate the much less damaging and, hence, uninteresting, but more prevalent, thermal and epithermal neutrons and to make the structure of the spectrum more accurate in the 20 KeV-2 MeV range. A second option is to use a pair of tubes, one shielded and one unshielded, combining the difference in their counts to yield the thermal neutron contribution. The spectrometer also uses a 5mm lithium drifted bulk silicon solid state detector in the neutron energy range of 2-500 MeV due to its demonstrated and modeled detection efficiency of 3-5 percent in the 5-150 MeV energy range. In high energy regions equivalent damage factors for dose equivalent are lower but hits from one or a small number of neutrons may prove to be important in sensitive localized volumes. The silicon detector system for high energy neutrons will discriminate against charged particles by using a plastic cesium iodide scintillator of an appropriate geometry (a small cup and plug configuration for a Mars Lander; a surrounding rectangular liner for a Space Station Express Rack) monitored by a silicon PIN photodiodes.
The first round of experiments with monoenergetic neutron beams on the Helium 3 tube and 5mm silicon detector systems were performed in February and June 1999. Both detector systems have previously been evaluated with Californium (mean energy ~ 1 MeV) and Americium/Beryllium (mean energy ~5 MeV) radioactive neutron sources at NIST. The Helium 3 tube exhibited energy resolution of at least 1 KeV over the energy range from 10 KeV to 3 MeV. The efficiency of detection of the tube decreased from 80 percent at energies of tens to hundreds of KeV to 0.1 percent as 1 MeV was approached as would be expected from the behavior of the neutron capture cross section for the neutron-Helium 3 reaction over this energy range. The best performing high-energy silicon detector from the set of FY 1999 tests proved to be the 5mm thick lithium drifted silicon detector. It demonstrated surprisingly good efficiencies of 1-5 percent at the Columbia RARAF (Radiological Research Accelerator Facility) for neutron beam energies of 2.46, 5.89 and 14, 16.25 and 18.5 MeV.
The 5 millimeter thickness of the silicon detector is a reasonable fraction of the neutrons' mean free paths in silicon in the 2-150 MeV energy range thereby significantly increasing the probability of a neutron-silicon nucleus interaction. We plan to overlap the detection of neutrons with the Helium 3 tube in the 2-5 MeV energy interval for cross calibration purposes. In the energy region above 20 MeV the substantial progress that we have made with modeling and experiments in FY 1999 and 2000 has yielded a conceptual design of one or more thick bulk lithium drifted silicon detectors (5-7 mm in an array or ganged together to increase count rate or roughly double the detection efficiency respectively) surrounded by an appropriately configured charged particle anti-coincidence scintillator detection system. Results from simulations of any high-energy stack or proton recoil telescope configuration in January and March 1999 showed that only detection efficiencies of 5X10 -4 would be achieved while maintaining 10 percent energy resolution for neutrons of energy greater than 50 MeV. Results from the RARAF experiments in February and June 1999 showed that the 5mm bulk silicon detector had as much as 5 percent (5X10 -2 ) efficiency between 10 and 20 MeV. Study of the Evaluated Nuclear Data Files (ENDF) archived by the Department of Energy showed that the total cross section for the neutron-silicon reaction remains fairly constant up to 150 MeV; thus, the bulk silicon detector efficiency is expected to do the same. The two order of magnitude advantage in detection efficiency for the bulk detector versus the recoil telescope disqualified the latter.
The late summer and fall of FY 1999 were occupied with two main activities: 1) our proposal to NASA in response to AO 99-HEDS-01 for a neutron spectrometer on the Mars 2003 Lander; and 2) the design and purchase of a set of detectors and associated electronics including battery packs for an engineering prototype instrument to be qualified for aircraft flight. NASA/Dryden committed to several F 15/18 flights in January 1999 for our spectrometer to monitor avionics environment neutrons and help NASA check out our design.
The proposal for MANES (MArtian Neutron Energy Spectrometer) was submitted in August 1999. It was selected for a stage of further definition in November 1999 as a potential instrument for the then-scheduled Mars 2003 Lander. The status of this instrument and its funding has evolved from potential individual Mars 2003 Lander instrument (December 1999-February 2000), to possible combination with the JSC MARIE instrument after cancellation of the 2001 Lander in April (March-May 2000), to proposal for an extended definition phase for a 2005 Lander instrument after cancellation of the large 2003 Lander and selection of the two-Athena-Rovers-in-a-bag for 2003 in July 2000 (May-August 2000), to being on hold pending a decision to fly a large Lander in 2005. That next decision on the Mars Surveyor program is expected in late September/October 2000. The changing situation has required much communication, alertness, revised statements of work and costing in addition to the original proposal with both NASA Headquarters and Johnson Space Center (JSC). A grant of 123,000 was received from JSC for accommodation and definition phase work on MANES from February to August of 2000.
The efforts on the aircraft flight hardware were interrupted by the MANES proposal. Fabrication occurred in September-October 1999. Included in this two-month period were rise time discrimination experiments with the Helium3 tube detector at RARAF at the end of October. Pictures of the instrument fabricated for aircraft flight are included in Appendix A of this report and represent the engineering prototype hardware proposed and promised to NSBRI in 1997. The instrument actually exceeds the original expectations in that it has been packaged and qualified for an uncontrolled aeronautics environment. It is more than just a laboratory instrument.
The aircraft instrument was ready for qualification in November 1999 when our selection for the Mars 2003 Lander interrupted its progress again. Analysis of its mechanical integrity for flight vibration and acoustics did continue in the November 1999-January 2000 time frame. The results of the analysis required some strengthening of the instrument and an additional cover for acoustic dampening of the Helium3 tube. The actual vibration and temperature/altitude qualification test finally took place in February 2000. The instrument readily passed the vibration test and operated successfully at low temperature; however, an isolated corona breakdown occurred in one of the high voltage supply systems at a simulated altitude of 45,000 feet. The point of voltage break down was found and fixed and the instrument was successfully qualified to the simulated 45,000 feet. The instrument was delivered to NASA Dryden on March 23, 2000 and flown the week of April 24-28, 2000 on an F-15 aircraft.
We took data up to 39,000 feet on ascent (the plane was to fly at a planned 40,000 feet cruise level) when we experienced a corona breakdown in the high voltage supply systems for both detectors. We are implementing and will implement more comprehensive and robust fixes for each high voltage supply system and plan to repeat the aircraft flights when new funding is received in FY 2001.
In January 2001 we were notified that our proposal titled "Development of a Neutron Spectrometer to Assess Biological Radiation Damage Behind Spacecraft Materials" submitted in March 1999 in response to NASA NRA 98-Heds-05 would be funded for a period of 3.5 years from May 2000 to November 2003 at a level of 90,000 per year for a total of 315,000. Our primary responsibility under this grant is to support Lawrence Berkeley Laboratory (LBL) personnel in the evaluation of spacecraft structural and shielding materials by supplying a version of the neutron spectrometer compatible with ground-based accelerator research. Due to the unavailability of accelerator facilities our first scheduled test date is now January 2001.
The major effort in detector evaluation that took place in 2000 was a series of experiments at the Los Alamos Neutron Science Center (LANSCE) to measure energy deposition in the 5mm thick lithium drifted silicon detector by neutrons with an energy range from 20-600 MeV. The experiments were performed by integrating our 5mm silicon detector with the LANSCE time-of-flight neutron spectrometer on the 90 meter beam line to give simultaneous measurements of the incident neutron energy (LANSCE fission chamber) and energy deposited in our detector. Energy depositions of up to 150 MeV were seen from the up to 600 MeV incident neutrons in our 5mm detector. Complete analysis of the data from these experiments is on hold pending the resumption of funding in FY 2001. These high-energy neutron experiments complement and complete the measurement of neutron-silicon energy depositions from known monoenergetic neutrons begun at RARAF in1999. Together the LANSCE and RARAF neutron exposure data will enable us to develop a complete response function for the 5mm detector between neutron energies of 2 and 600 MeV. Inversion of this response function will allow us to calculate a most likely incident neutron energy spectrum, previously unknown, from a measured energy deposition spectrum.
The modeling component of this research program occurred on a continuous basis in FY 2000 until funds expired. We concentrated on modeling the high-energy channel from detailed cross sections of the basic neutron-silicon interactions using state-of-the-art computer codes. There are four reasons to develop this advanced modeling capability: 1) to assess the accuracy of the codes themselves to predict energy deposition in a silicon detector (by comparison with experimental data); 2) to use the codes in understanding the experimental results; 3) to determine whether the codes can be used to calculate the shielding and scattering effects of the instrument packaging and surrounding environment (structure or atmosphere); 4) to assess the ability of the codes to supplement the determination of the instrument response function at interpolated and extrapolated energies (since it is impractical to test at intervals of 10 MeV for the whole energy range). We have found that the GEANT4 code originally developed at CERN is the easiest to use, is maintained in a timely fashion by its developers and reproduces our RARAF (2-20 MeV) energy deposition data reasonably well.
