Dr. Richard H. Maurer is designing and building a prototype combined ion and neutron spectrometer to monitor the radiation environment in space habitats and transport vehicles. Merging the sophistication of space flight design with the low cost of non-flight electronic components, the instrument will be used for ground-based research at accelerators to gather information on high-energy protons, heavy ions and neutrons. In addition to monitoring neutron flux and energy spectrum, the instrument will provide early warning of harmful energetic solar proton events and provide a means to study the effectiveness of shielding materials.
Overview
Combined Ion and Neutron Spectrometer for Space Applications
Principal Investigator:
Richard H. Maurer, Ph.D.
Organization:
The Johns Hopkins University Applied Physics Laboratory
Technical Summary
Original Aims
The project involved the design and fabrication of a Combined Ion and Neutron Spectrometer (CINS) for space applications. This instrument improves upon existing charged particle species and energy spectrometers (e.g., Martian Radiation Environment Experiment (MARIE) on Mars Odyssey) and incorporates the capability of the previously developed NSBRI Neutron Energy Spectrometer to yield a single and complete ionizing radiation environment monitor for application in space habitats and transport vehicles.
The project involved the design and fabrication of a Combined Ion and Neutron Spectrometer (CINS) for space applications. This instrument improves upon existing charged particle species and energy spectrometers (e.g., Martian Radiation Environment Experiment (MARIE) on Mars Odyssey) and incorporates the capability of the previously developed NSBRI Neutron Energy Spectrometer to yield a single and complete ionizing radiation environment monitor for application in space habitats and transport vehicles.
The original objectives were refined with respect to specific metrics:
- Eliminate the gain saturation of MARIE for heavy ions with LET > 35 keV/micron;
- Increase the dynamic range vs. the MARIE instrument by a factor of 100-200 to a minimum of 10,000:1 to include protons with energies above 100 MeV; and
- Increase the maximum instrument event detection rate by at least a factor of 10 to a minimum of 100 Hz.
The end product was to be a Technology Readiness Level 6 instrument.
Key Developments
The conceptual design of the CINS instrument was facilitated through the use of Monte Carlo simulations that modeled the basic detector stack, established the energy resolution and quantified the issues that would affect the instruments efficiency using Geometry ANd Tracking (GEANT4).
Our CINS design consists of a stack of seven cylindrical detectors with a radius of 20 mm. There are three different types of detectors in the CINS stack. One is a standard Silicon (Si) solid state detector. The remaining two detector types are scintillators. The first detector in the stack is a 1 mm thick BC430 detector followed by a 5 mm thick Si detector, then a second 1 mm BC430 detector, followed by three, 5 mm Si detectors. The final detector is a 38 mm thick scintillator detector of BGO.
For the CINS system to accurately resolve solar energetic particle (SEP) events, the system must cleanly separate the protons from the electrons. We simulated spectra of electrons to 20 MeV and protons to 1 GeV. While a small percentage of the electrons and protons do overlap in this simulation, the vast majority of the protons can be separated from the electrons. Protons up to ~150 MeV are stopped in the stack. Given reasonable energy resolution of the BGO scintillator, the simulation showed that protons up to ~300 MeV can be uniquely identified.
We re-drifted eight Si (Li) detectors for charged particles and neutrons from Lawrence Berkeley Laboratorys existing stock. The second step was the creation of a guard ring accomplished by routing out the detector in a circular pattern at a fixed radius. These eight detectors have been evaluated in experiments at HIMAC (a heavy-ion synchrotron) and at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory during 2006 and 2007.
For the charged particle telescope, we procured a 38 mm X 38 mm bismuth germanate scintillator detector unit and obtained 1 mm thick scintillators. We designed and fabricated detector boards for NSRL evaluations and designed charged particle telescope electronics for control, power and data acquisition.
Ground-based accelerator experiments were executed at NSRL in March 2006 by colliding a 1 GeV/amu iron beam with a 5 inch thick aluminum target simulating a large space vehcle. The progeny of these collisions, both charged particle fragments and neutrons, were monitored by the refurbished 5 mm thick silicon detectors. During May 2007, five of the seven detector elements of the charged particle telescope were operating, and a large volume of charged particle fragment data was obtained. For the 2007 experiments, iron and silicon beams and targets of 10-20 g/cm^2 were used. All seven elements of the charged particle telescope were active for the first time in fall 2007. Additional experimental runs were made in spring, summer and fall 2008.
For the Neutron Energy Spectrometer, Johns Hopkins University Applied Physics Laboratory designed, procured and fabricated a 12 cm X 12 cm Eljen plastic boron-loaded scintillator detector for medium energy 1-14 MeV neutrons that have the maximum dose equivalent weighting factors of 10-20. This fast neutron detector system was calibrated at Columbia University Radilogical Research Accelerator Facility in November 2006 and used at NSRL for the first time in May 2007. This scintillator is operated in addition to the existing 5 mm thick silicon detectors used for high energy neutron spectra above 10 MeV. The silicon detectors must be used in conjunction with standard paddle scintillators to veto the charged particle fragments.
A custom data acquisition system was also designed and fabricated for the charged particle telescope during 2007. The output of the acquisition system has a large usable dynamic range and is able to handle the relatively high event rates that occur in an accelerator beam. This system detects events, proceeds to capture waveform data from the preamplifiers, and saves the data to a hard drive. The system is built around several digital to analog converters and a field programmable gate array and is essentially a fast 16 channel digital oscilloscope. It recorded data for the NSRL experiments beginning in fall 2007.
We have succeeded in achieving all our original aims with respect to the instrument hardware. We also were able to produce data on charged particle fragment composition and energy spectra and neutron energy spectra from several collision experiments. A sample of the data has been published in the journal, Nuclear Instruments and Methods B. A second paper in the same journal describes the custom data acquisition system.
Impact of Development
As built CINS consists of a) high and medium energy neutron detectors; b) a charged particle detector telescope based on a standard silicon stack concept and c) a custom data acquisition system.
CINS meets all the defined original aims. All its subsystems have been successfully tested and calibrated with accelerator beams. The instrument has a Technology Readiness Level of 6. A space flight quality instrument package can be easily produced by reducing volume, mass and power consumption.
Earth Applications
Neutron detection for homeland security is of interest in the areas of location of radiological weapons used by terrorists, scanning and inspecting imported cargo in ship holds or on the dock side; and smart portals for the scanning of personnel or small packages at selected points of entry.
This project's funding ended in 2008