the past year, a special mass spectrometer was developed for analyzing uranium hexafluoride; it is being used to evaluate standards having low concentrations of uranium 235. This instrument has also been used to compare the natural abundances of uranium in samples from different geographical areas.

In the past several years, measurements of very low levels of radioactivity have become more numerous and exacting in such fields as archeological dating, biological and medical studies, and health physics. Because of this increased activity, a thorough investigation into the radioactive contamination of materials used in radiation detection has become necessary, and a demand for radioactivity standards at very low concentrations has arisen. To meet this need, a new laboratory facility for the measurement of very low levels of radioactivity-down to a millionth of a curie—was constructed. This facility is being used to study methods of measuring the amounts of radionuclides present at very low concentrations and in making international comparisons of radioactive samples at these concentrations. It will also be used to prepare accurate radionuclide standards for a number of scientific and industrial applications.

Other work on radioactivity standards resulted in the development of a manganese-54 point source standard, a scandium-46 gamma-ray standard, an iron-55 electron-capturing nuclide standard, and a promethium-147 betaray standard. In addition, a more accurate value for the half-life of carbon 14-important in geological and archeological dating—was obtained. The new value is 5,760 years with an overall probable error of 1 percent, and is about 4 percent greater than the previously accepted value of 5,568 years.

Measurements of the acidity or basicity of solutions, expressed on the pH scale, are of critical importance not only in chemical analysis and medical research but in the control of many industrial processes. Some years ago the Bureau took the lead in establishing a standard pH scale that would meet the needs of both science and industry. Standards for the adjustment of pHmeasuring equipment to conform to this scale have been issued by NBS for more than 15 years. However, fundamental difficulties in the calculation of a standard pH have made it necessary to limit the accuracy in the assignment of pH values to ±0.01 unit. Within the past year a mutually satisfactory convention was developed in cooperation with the pH committee of the British Standards Institution, and the third decimal place is now being assigned to pH standard values.

In response to many requests, a new standard was established especially for the measurement of the pH of blood and physiological media. Accurate measurements of the pH of blood are of great importance both in medical research and in the diagnosis of pathological conditions. However, the changes in pH that must be detected are very small. To increase the accuracy with which these measurements can be made, the new standard was required to have, at body temperature, about the same pH as blood. It was prepared from pH standard materials already issued by the Bureau.

Studies of Matter and Materials

Water, because of its abundance, its importance to the physical sciences, and its role as a life-supporting liquid, has been the subject of intense study for many years. Recently, by applying an electrophoretic ion-exclusion technique, the Bureau succeeded in preparing water of extremely low ion content. This water has an electrical conductivity of 0.039 × 10-6 ohm-1 at 18 °C, indicating a residual ion content which is equivalent to a sodium chloride concentration of one part per billion. Containing less than onethird of the ionic impurities of the water prepared by Kohlrausch and Heydweiller in their historic purification experiments, this water approaches the theoretical conductivity--and ideal purity-more closely than any previously reported.

In 1960 Bureau scientists found that ethane molecules lose molecular hydrogen when subjected to ultraviolet light of very short wavelength. During the past year additional studies were made of the effect of radiation on other simple molecules. Ethylene was found to decompose by a similar process, and further experiments with ethylene showed that molecular detachment of hydrogen also occurs under the action of gamma rays. Such experiments give valuable insight into the detailed processes induced by high-energy radiation and provide information on the origin of radiation damage to materials. The formation of molecular hydrogen by action of ultraviolet radiation on water vapor was also observed; this process may account for the presence of hydrogen molecules in the upper atmosphere. Detailed investigations of the structures of several important molecules were carried out by spectroscopic studies in the ultraviolet, visible, infrared, and microwave regions of the spectrum. Through the use of microwave techniques, it was possible to measure interatomic distances with very high accuracy in a variety of hydrocarbons and their simple derivatives. Small variations were detected in the lengths of the bonds between carbon atoms in these molecules, and these changes shed some light on the nature of the chemical bonds. The microwave studies also provided other molecular information, such as electric dipole moments and quadrupole coupling constants, which can be correlated with the geometric structure of the molecules. High magnetic fields have important uses as deflectors of charged particles in the particle accelerators and detection devices of nuclear physics, in nuclear power converters, and for plasma containment in fusion reactors. If the magnet is cooled to low temperatures so as to greatly reduce its electrical resistance, a considerable amount of power that would otherwise be lost as heat becomes available for producing a higher magnetic field. To take advantage of this principle, a high-purity aluminum foil magnet with liquid hydrogen cooling for low-temperature operation was recently designed and is nearing completion. Using only 4 kilowatts of power, it is designed to produce a magnetic field of 100,000 gauss in a cylindrical volume 3 inches in diameter by 8 inches long.

At very low temperatures some metals such as lead and tin become superconductors, that is, they completely lose their electrical resistance. Obviously a superconducting electromagnet would provide a very effective means of obtaining extremely high magnetic fields. Until recently, however, such a superconducting magnet was not regarded as practical because most superconductors are driven into the normal, conducting state by rather small magnetic fields. Within the past year several alloys or compounds have been discovered that remain superconducting in the presence of high magnetic fields and while carrying large currents. One of these, a niobium-tin compound (Nb,Sn) clad in niobium, has been investigated by the Bureau, with the support of the Atomic Energy Commission, in fields up to 190,000 gauss. The results indicate that this material can be used to make solenoidal magnets that will produce magnetic fields of well over 100,000 gauss if operated from 1 to 4 degrees above absolute zero.

Astrophysical and Plasma Physics Research

In recent years there has been great scientific interest in the nature and physical behavior of extremely hot gases such as occur in thermonuclear devices and in outer space. Yet this field of physics is still very poorly understood. As a result of this lack of knowledge, progress is being held up in a number of important branches of science and technology-among them space exploration and astronomy, thermonuclear power and plasma physics, ultra high temperature research, atmospheric research, and ballistic missile defense systems.

In this situation the major problem is a lack of precise measurement techniques, standards, and basic data on the fundamental properties of the hot gas or plasma. Many of the laboratories attempting to apply plasma physics to practical objectives are thus forced to rely on costly and inefficient empirical methods. To help solve this problem, the Bureau in 1960 began a special effort to unify and strengthen its work in plasma physics and astrophysics. This work is now being carefully coordinated to develop the necessary measurement standards, basic data, theoretical guidance, and interpretative techniques for determining the relevant properties of hot gases and for the solution of important problems in modern astrophysics.

The most immediate need for such knowledge and services arises in the space sciences, where satellites are used to carry equipment outside of the earth's atmosphere to study the sun and the stars. The value of the spectroscopic data thus obtained can be greatly enhanced if they can be accurately described in measurement units based on precise laboratory standards. The Bureau is making accurate measurements of atomic properties to provide the data necessary for quantitative interpretation of these astronomical obser

vations.

To study the probabilities of atomic transitions associated with hydrogen and oxygen lines observed in solar and stellar spectra, the Bureau developed a wall-stabilized high-current arc chamber operating in hydrogen at 12,000

A high-current arc chamber operating in hydrogen at 12,000 °K, a temperature twice that of the sun. The arc is used in research on the fundamental properties of extremely hot gases such as occur in thermonuclear processes and outer space. Lack of precise measurement techniques, standards, and basic data on the fundamental properties of plasmas is a major problem in the space sciences (page 6).

°K, a temperature twice that of the sun. A characteristic red light emitted by the hydrogen through slits in the arc chamber is photoelectrically recorded with a spectrometer and provides information to determine temperature and particle concentrations within the plasma.

A tabulation of the relative intensities of 39,000 spectral lines was completed during the year, providing intensity values on a uniform energy scale for 70 elements over the wavelength range from 2000 to 9000 Angstrom units. The new tables will supply much-needed quantitative intensity values for those elements most commonly encountered in spectrochemical analysis. The intensity values may be transformed into atomic transition probabilities and used to determine temperatures of laboratory light sources emitting atomic spectra and of stellar atmospheres.

In addition to the data center on atomic transition probabilities which was set up last year, a data center on atomic collision cross sections was established to gather and index all published information in this field. A complete file of scientific papers on low-energy electron cross sections has been collected, and about one-half of the papers have been coded on punched cards. Plans call for extending the data collection to other atomic cross sections as soon as is feasible.

Through the production of radio waves from plasmas in the laboratory, a major step was taken toward duplicating under controlled conditions the electromagnetic processes which occur in the upper atmosphere. Plasmas were produced in helium by a high-velocity shockwave travelling over 100 times the speed of sound. When the plasmas were studied in the presence of a transverse magnetic field, radio waves resulting from interaction between the shockwave and the magnetic field were observed. A high-speed camera, capable of operating at over 100 million frames per second, was devised to study the luminous phenomena in the shockwaves.

Radio Propagation Research

The NBS Central Radio Propagation Laboratory (CRPL) has the primary responsibility within the Federal Government for collecting and disseminating information on radio wave propagation. The results of its research program are of value to radio and television broadcasters, the military services, space scientists, and operators of many types of communication systems.

A large part of CRPL research deals with the properties of the series of electrically charged layers in the upper atmosphere known collectively as the ionosphere. Through their ability to reflect radio waves, these layers play an important part in long-distance radio communication.

By analyzing radio signals received from satellites, CRPL has been able to study the structure of the upper part of the ionosphere, measuring the density of electrons and other characteristics. Current studies are investigating the size, shape, and motion of various ionospheric irregularities as observed at a number of stations. The results obtained in this work should aid communication with space vehicles since radio signals from space are seriously affected by irregularities in the electron density of the ionosphere.

On June 24, 1961 the first rocket-borne soundings of the topside of the ionosphere were made by means of a four-stage rocket carried to an altitude of over 600 miles. Successful radio pulse reflections from the topside of the ionosphere were obtained for about 13 of the 14 minutes that the payload was above the ionosphere.

The rocket, a Javelin, was launched from the National Aeronautics and Space Administration's Wallops Island (Va.) facility. The purpose of the experiment was to test the sounding system that is to be used in a topside sounding satellite to be placed in orbit at a later date. Such a satellite will be of great value in advancing man's knowledge of the ionosphere. NBS responsibilities in this program include overall planning, design and performance of the experiment, and analysis of the resulting data. Airborne Instruments Laboratory, a division of Cutler-Hammer. Inc., is designing and building the rocket and satellite payloads and the ground data-handling equipment. Technical management and sponsorship is by the NASA Goddard Space Flight Center.