November 2005

Dr. Nasser Barghouty,

Project Scientist, Space Radiation Shielding Project, NASA’s Marshall Space Flight Center,

Huntsville, AL

Some scientists believe that materials such as aluminum, which provide adequate shielding in Earth orbit or for short trips to the Moon, would be inadequate for longer-duration space missions, such as a manned trip to Mars. NASA scientists recently have invented a polyethylene-based material called RXF1 that's even stronger and lighter than aluminum -- compared to aluminum, polyethylene is 50% better at shielding solar flares and 15% better for cosmic rays. Dr. Barghouty is a co-inventor of RXF1. He has been serving as the project scientist for the Space Radiation Shielding Project (SRSP) at Marshall since May of 2002.

NASA Tech Briefs: What is the mission of NASA’s Space Radiation Shielding Project?

Dr. Nasser Barghouty: NASA needs the capability to gauge how effective a certain material is for shielding astronauts and systems in general against space radiation. To be able to do that, there are some basic developments of transport codes that are needed. As inputs to these transport codes, you need some basic physics information – essentially nuclear physics information because the radiation issue is related to the nuclear issue (basic nuclear interactions of radiation and matter). Matter in this case could be the skin of the spacecraft or tissue (i.e., liver) of an astronaut.
We help support measurements of these nuclear physics cross-sections. These are probabilities of these nuclear processes to take place. We also fund efforts to develop these radiation transport codes. These are the two important things we do in addition to funding work to develop materials that are lighter and stronger than aluminum that can meet the shielding requirements. So there is the analysis part and the development part of novel materials for radiation shielding.

NTB: What is deep-space radiation?

Dr. Barghouty: On Earth, the atmosphere protects us, for the most part. We also are protected to a lesser degree by the Earth’s magnetic field. In our area of work, we are dealing with very high-energy and highly charged particulate radiation, which means that they [the particulates] can actually zip through a path unaltered; you can barely do anything to them as they travel through matter. So these are deeply penetrating, high-ionizing types of radiation – the worst kind.

NTB: What is it about polyethylene that makes it an appealing alternative to shield against space radiation?

Dr. Barghouty: First, there is very little that can be done about deep-space radiation, unless you use 1-meter-thick steel, which in reality makes things worse. What we can do is alter the particulate radiation ionization damage by making them fragment as they go through a shielding material, and, as we do this, not produce secondary products as a result of this interaction. In other words, minimize the waste. Aluminum does not do this. Aluminum, to a certain degree, can actually make things worse because while it fragments the particulate radiation, it produces a lot of secondary products. These are bad and essentially add to the dose of radiation.
What you want is something that is effective at fragmenting but does not produce a lot of secondary products. It turns out materials that are rich in hydrogen and carbon – like polymers – can accomplish this. Polymer-based composites have theoretically been thought of as good candidates because they not only meet the requirements for shielding, but also because they are composites and therefore can be designed as strong or as flexible as desired (designer materials). RXF1 typifies this polymer-based composite; it is certainly not the only one, but so far it is the only one that has been tested and proven.

NTB: What is RXF1? What are its benefits?

Dr. Barghouty: Theoretically, these polymer composites were already known to be good shielding materials. RXF1 is different in the sense that it actually was the first to prove this as a real material. We lab tested RXF1 at three different accelerators and exposed the material. As we predicted, the results of those test were very promising. The mechanical tests that the materials have passed with flying colors. We are currently conducting environmental tests and are addressing issues of flammability, toxicity, and other structural matters that are required by structural engineers and we are near to closing these gaps. This is where we believe that some work is needed.
A patent on the material is pending, so the specifics of how RXF1 is made are secret. However, generally speaking, RXF1 begins with a polyethylene matrix, the backbone of which is graphite fiber giving it strength. These are the types of materials that are good for radiation shielding and are the basic building blocks of the material.
What’s novel is that the fibers could be designed any way you want. You also could design the shape as well as how to put the fiber into that shape. There is a beautiful synergy between what you want and what you can do, a synergy between function and property. This is what makes composites attractive. What we have added is the idea of radiation, which is new to the composites industry. It turned out to be easy because polymers are naturally rich in hydrogen and that’s all we needed. Additionally,for the nuclear industry, it turns out that materials with a high concentration of hydrogen also address their issues, which are neutrons. Nuclear reactors produces a lot of neutrons and materials rich in hydrogen tend to slow down or “thermalize” these neutrons.

NTB: How will NASA utilize RXF1?

Dr. Barghouty: NASA has plans, for example, to go to the Moon first before we go to Mars. These missions [to the Moon] typically do not last more than a few days to two to three weeks. For these types of missions, the issue of deep-space radiation is not that critical. The issue of radiation shielding becomes critical the longer you stay, because the risk of exposure is too high. Keep in mind that NASA does not yet have limits on what is considered dangerous and what is not. We have a lot of limits and information from the International Space Station and the shuttle, but nothing from staying on the Moon for a month. For anyone to actually put the limits down on paper you need some real data on Earth, which is also very limited – how many people have actually gone out into space and come back with radiation sickness? We really do not know what is safe and what isn’t, so these limits are going to have to be put forward first. We do, however, have a rough idea of what the limits should be.
For these long-duration missions, on the Moon for example, one of the most immediate dangers is that while there a solar flair erupts. For some of these solar particle events, the doses received on the surface of the Moon or in transit to the Moon can be quite high. Protection from solar activities is an immediate need for mission designers. With a material like RXF1, a storm shelter can be built either on the surface of the Moon or in the vehicle that can serve as a good shield against solar particle events. Some of these events can actually be quite nasty, they are very unpredictable, and from our early warning system we have only about two hours to do something about them.

NTB: Do commercial applications exist for RXF1?

Dr. Barghouty: A good portion (60-70%) of the newest Airbus A380 is made of composites – not necessarily polymer composites, but they are carbon composites. Wherever carbon composites are applicable, polymer composites can actually perform similar functions. If carbon composites are being used in a specific application, one may want to look at polymer composites once the issues of flammability and toxicity are resolved. Europeans are making a lot of progress on this front. In terms of the Airbus, there was a need for them to make it lighter, and aluminum obviously would not work. So they utilized carbon, and I believe some polymer composites.
We know it can be strong enough for structural applications and the chemical tests are very promising. When it comes to properties needed by structural engineers and designers, if the gaps are closed then you have a material that can be designed according to your specific application. It is light and it is strong, so it could replace almost anything. Carbon composites have been made really strong, but they are still much to heavy for what we need at NASA.
They also could be used in the radiation shielding business for terrestrial applications, such as the nuclear industry, nuclear medicine, and nuclear power generation. The issues of radiation are somewhat different in these industries, but the superior properties of these composite materials for your own radiation issues can be custom tailored.
These are just two general examples of structural and radiation uses of the material; between the two, I believe the commercialization opportunities are very promising.
For more information, contact Dr. Nasser Barghouty at Abdulnasser.F.Barghouty@nasa.gov.


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