SP-401 Skylab, Classroom in Space

[112] Part II - Student Experiments

Chapter 10: Particle Physics in Space.

picture of an  astronaut  aboard Skylab

 

[113] Man has been probing the nature of matter since the ancient Greeks hypothesized that everything is made of four elements: fire, air, water, and earth. The medieval alchemist believed that all kinds of matter had a common origin, that they possessed one permanent "soul" housed in a variety of temporary bodies, and that these bodies could be transmuted, i.e., converted from one element to another.

The sciences of chemistry and physics, evolving over the years, have established a remarkable similarity to these primitive concepts. Some of the earliest systematic investigations into the nature of matter were made by Robert Boyle in 1661. He proposed the idea of elements, which he described as being "certain primitive and simple or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies are immediately compounded, and into which they are ultimately resolved."

By 1789, the French chemist Lavoisier had listed 23 known elements. It was the Englishman John Dalton, however, who put the concept of an element on a firm foundation. In 1808, he theorized that elements are composed of atoms. All atoms of a given element have the same mass but differ in mass from atoms of other elements. An atom's mass, therefore, is a characteristic of an element. In 1868, Mendeleev, a Russian, prepared a chart of the then known elements, classifying them in the order of increasing atomic masses. It is now called the periodic chart or table.

Early in the 20th century, it was shown that the nucleus of an atom contains the vast majority of its mass. The nucleus consists of protons and neutrons and carries a positive electrical charge balancing the negative charge of the electrons surrounding the nucleus. Thus, atoms differ from one another in their mass according to the number of protons and neutrons in their nuclei.

Matter is fundamentally molecular rather than atomic, but the molecules are made up of atoms. It is the manner in which atoms stick together that determines the structure of the molecules. The force which binds atoms together into molecules is electrical, as is the force that holds the atom's electrons in orbit about the nucleus. The question then arises, "What is the force that holds the particles of the nucleus together?" This force has been the subject of investigation by modern physicists since Carl Anderson discovered the positron in 1932. He also revealed the existence of the meson in 1936. Since then some 200 "particles" have been found to originate in the nucleus!

Some subatomic particles exist in a free form in outer space. Among these are electrons, protons, neutrons, alphas, and higher mass particles resulting from the processes that take place in the Sun and stars. Some subatomic particles result from natural radioactivity when atoms with relatively unstable nuclei spontaneously radiate energy or matter.

One of the more subtle of the nuclear particles, and one of the more difficult to detect, is the neutron. Space radiation in the form of galactic....

 


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A beam of neutrons is attenuated while passing through material because the particles are either scattered or absorbed. [Link to a larger picture]

A beam of neutrons is attenuated while passing through material because the particles are either scattered or absorbed.

 

....and solar cosmic rays, X-rays, and gamma rays as well as the alpha particles, protons, and electrons in the Van Allen belts are relatively well known. However, neutron radiation is not nearly so well understood, largely due to the fact that neutrons have no electrical charge. They are classified according to their energies as cold, thermal, slow, intermediate, and fast neutrons.

Since neutrons have no charge, they do not interact with atomic electrons. They lose no energy through ionization or atomic excitation and are able to penetrate matter much more easily than charged particles with the same energy. The detection of energetic neutrons is quite difficult because of the small likelihood of their interaction with most all substances except those rich in hydrogen. As a result, most neutron detection methods rely on detecting a "secondary" effect caused by the passage of a neutron-as in the recoil of protons from hydrogen nuclei in emulsions, induced fission, or induced radioactivity in foils. The effective target area presented by a nucleus to an incident neutron, expressing the probability that an interaction of a given kind will take place, is known as the neutron cross section of the material.

 

Neutron Analysis

The rate of neutron flow is commonly referred to as a "flux." The measurement of neutron fluxes in Skylab was the subject of a proposal by Terry Quist of Thomas Jefferson High School, San Antonio, Tex. These measurements were considered important not only by NASA but also by....

 


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Terry Quist's experiment aboard Skylab consisted of measuring neutron fluxes. He is shown, top, with his science adviser Harry Coons.

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picture of Harry Coons

Terry Quist's experiment aboard Skylab consisted of measuring neutron fluxes. He is shown, top, with his science adviser Harry Coons.

 

...the scientific community for four reasons. High-energy neutrons can be harmful to human tissue if they are present in significant quantities. Fluxes of neutrons can damage film and other sensitive experimental equipment in a manner similar to that produced by X-rays or other radiations. Furthermore, neutron fluxes can be used as a calibration source for other space-oriented particle physics experiments. Finally, neutron fluxes can affect sensitive X-ray and gamma-ray astronomy observations.

Quist had long been interested in radiation physics, and he had prepared seven science fair [116] exhibits on the subject over a 5-year period. Drawing upon his experience and knowledge of the literature, he proposed an experiment for the Skylab student program. While he had been able to find a reasonable amount of material in the literature regarding proton and cosmic ray fluxes in space, he could find little on neutron flux.

Thus, he proposed the measurement of neutron fluxes using a "solid-state track recorder." This technique utilizes a foil of metal that has a relatively large neutron cross section and is capable of nuclear fission or induced radioactivity when impacted by a neutron. When such a particle strikes the nucleus of an atom in a metallic foil or detector, fission fragments are ejected as secondary emission or induced radioactivity occurs. A second foil or film, in contact with the detector, made of a dielectric material such as muscovite mica or polycarbonate materials like lexan or cellulose triacetate, is then struck by these fission fragments or decay particles. These projectiles either disrupt the polymer chains in plastics or disturb the crystal lattice of mica in a way that the submicroscopic paths or tracks so produced can be chemically etched. Etching makes the paths visible under a microscope, so they can be counted. Calibration of such a detection scheme is carried out using a.....

 


A highly energetic particle passing through a crystalline solid such as mica knocks electrons off atoms in its path and thus ionizes them.

A highly energetic particle passing through a crystalline solid such as mica knocks electrons off atoms in its path and thus ionizes them. The path along which the particle passed is made visible by etching the material.



When an energetic particle passes through an organic polymer such as cellulose triacetate, it excites and ionizes molecules of the material.

When an energetic particle passes through an organic polymer such as cellulose triacetate, it excites and ionizes molecules of the material. Etching the material makes the path of the passing particle apparent.


 

.....known neutron flux from a nuclear reactor or particle accelerator.

Quist's objectives were to measure the neutron fluxes present in Skylab and, with the assistance of NASA and other physicists, to attempt determination of their origin as well as their energy range or spectrum. Quist's plan called for 10 identical detectors containing boron, bismuth, thorium, and uranium foils, with cellulose triacetate and muscovite mica as recording media, housed in an aluminum container. This unit enabled detection of thermal (slow), intermediate, and fast neutrons.

When the Skylab was in orbit, these detectors were deployed at various points throughout the space station by its first crew. Each was activated in orbit by removal of an aluminum shield between the metallic foil and the dielectric and deactivated for return to Earth by reinserting the shield. Four of the detectors were returned by the crew of the first mission and delivered to Quist. Under the supervision and assistance of Donald Burnett of the California Institute of Technology, Quist etched each of the 24 dielectric films and counted the tracks. Preliminary analysis indicated that the neutron flux was higher than had been expected.

 


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One of the Quist's detectors was attached to a water tank in the Skylab workshop. It consisted of metallic foils, cellulose triacetate and mica sheets, and a protective cover.

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Typical neutron tracks found in a sample of muscovite mica after being etched on return to Earth from Skylab.

One of the Quist's detectors was attached to a water tank in the Skylab workshop. It consisted of metallic foils, cellulose triacetate and mica sheets, and a protective cover.

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Typical neutron tracks found in a sample of muscovite mica after being etched on return to Earth from Skylab.


 

As a result, a request was initiated to refurbish one of the four returned detectors for launch and deployment by the third crew.

The seven detectors on board (six remaining from the first visit and one carried up by the third crew) were returned to Earth. Thus, four detectors had been exposed to the Skylab environment for 24 days, six for 251 days, and one for 81 days.

After very careful analysis of the data obtained, it was concluded that the track-density count was much higher than could possibly be expected from the identified neutron sources, i.e., the Sun, Earth's neutron albedo (escaped secondary atmospheric neutrons), and cosmic ray interactions. The high count was attributed to the impact on the materials of the space station by charged particles (mostly protons) trapped in the Van Allen belts. These, in turn, produced secondary neutrons. This is the best explanation Quist and Dr. Burnett can provide to explain their results. Their work has stimulated interest in further studies of neutron phenomena in space.


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