Overview

Neutron Energy Spectrometer Flight Experiments

Principal Investigator:
Richard H. Maurer, Ph.D.

Organization:
Johns Hopkins University Applied Physics Laboratory

NASA Taskbook Entry

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Technical Summary

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 for space missions and produces an increased risk of cancer, CNS and DNA damage.

The primary high energy cosmic rays and trapped protons collide with spacecraft materials such as aluminum and silicon and create secondary particles inside structures that are charged particle fragments and neutrons. 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. Charged protons are readily detected and instruments are already in existence for this task.

Neutrons are electrically neutral and much more difficult detect. These neutrons are estimated to contribute 10-30% 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.

As a product of our previous NSBRI funding, we had met the original aims of the project to design and fabricate an engineering prototype neutron spectrometer that was flown on F-15 and F-18 aircraft flights from NASA Dryden Flight Center. The spectrometer consists of both low and high energy subsystems. The detection of low energy neutrons (0.025 eV-1 MeV) is accomplished using a conventional helium 3 gas tube. The detection of high energy neutrons (5-800 MeV) is achieved using a 5 mm thick lithium drifted silicon solid state device.

The neutron spectrometer was flown on two flights 13-14 August 2001 in a pod under the wing of an F-18. A third successful flight in the same pod under the fuselage of an F-15 was executed in October 2001. The main positive result from the three flights was the verification of our engineering design and not the limited data obtained due to the short duration (~2 hours) of the aircraft flights. The value for our hardware was the proven approach in handling high voltage at high altitude corona region that will be employed for balloon flights.

Efforts in FY 2002 and 2003 were directed at designing and fabricating a neutron spectrometer for high altitude balloon flights. The electronics were made more robust and compact for the balloon flight instrument. The detector suite was changed to include a Medium Energy Spectrometer (MES) for the fast neutrons in the 1-20 MeV energy range in addition to the thick silicon detector for the >20 MeV neutron energies. The helium 3 tube was not included for the low energy neutrons (10 keV-1 MeV) since this system was validated on the aircraft flights and will be readily available for flight efforts. The MES is a Bicron 454 plastic scintillator detector system that borrows from the development of a similar system for the APL unmanned MESSENGER mission to Mercury.

Sophisticated energy depostion signal time discrimination allows the observation of both scattering and capture peaks of the neutrons in the Bicron detector for individual counts and energies. Development of experience in the calibration and use of this detector system was one of the interim goals of the project. Balloon flights were executed from Fort Sumner, NM at an altitude of 85,000 feet on two occasions--October 9, 2002 and October 9, 2003. The altitude of 85,000 feet was chosen since the amount of atmosphere remaining (~20 grams per square centimeter) is the same as the amount of carbon dioxide at the surface of Mars and should yield a reasonable simulation of the downward neutron spectrum on Mars. The October 2002 flight did not yield any useful scientific data due to engineering problems with the high voltage connection to the silcon detector and ground loop issues between the electronics and the aluminum container of the instrument. The problems were corrected durng FY 03, and the October 2003 flight yielded useful scientific data.

Initial analysis showed that we obtained a highly moderated neutron energy spectrum with the majority of neutrons in the energy range between 20 and 35 MeV. Modeling of the detector shielding geometry is necessary to deduce that on the lid of the instrument relative to that measured at the detector location. This task is our starting point in FY 05.

For the evaluation of spacecraft structural and shielding materials, we built a stack detector version of the neutron spectrometer compatible with ground-based accelerator research. We verified its successful operation at Columbia Universitys RARAF in November 2001, and then proceeded with spacecraft shielding experiments using 200 MeV proton beams at the Indiana University Cyclotron Facility in November 2002 and November 2003 and 500 MeV proton beam at TRIUMF in Vancouver, Canada in September 2003. Aluminum, carbon and polyethylene block targets were used to simulate spacecraft materials. The results yielded neutron production energy spectra showing the reduced yield from carbon based materials and the moderating (scattering) effects of polyethylene when compared to aluminum. Our data validate the recent conclusion about aluminum being the least suitable spacecraft material with respect to increased neutron production and human radiation effects. This was the last year for this project.

 

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Earth Applications

The main Earth-based application for this project/instrument development is related to homeland security and the monitoring of freight/cargo for the presence of fast and high energy neutrons from clandestine radioactive material.

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This project's funding ended in 2004