ing free carriers intrinsically, rj, should be independent of the absorption coefficient k of the light used. This expectation was realized in studies by Dr. Pope and Dr. Geacintov. They found that rf was independent of the angle of polarization of the absorbed light, 0. By contrast, k was a strong function of $. Dr. Pope and his coworkers also found that impurities in the surface region play an important role in determining important surface reactions. For example, they found that if crystals of a compound such as anthracene are exposed to air, even in the dark, oxygen diffuses into the surface region to a distance of 100 to 200 A. This impurity concentration reaches a mole fraction approaching 10 - 4 and quenches excitons that arrive in this region. Thus, the lifetime of a singlet exciton in the surface region of anthracene that has been exposed to air is about 5 X 10 - 9 second compared with a lifetime of about 2 X 10" 8 second in the interior of the crystal, Dr. Pope says. The NYU group came to this conclusion by adding a known exciton quencher, tetracene, to the anthracene crystal. They found that the surface concentration of excitons was not affected by tetracene until the tetracene concentration exceeded a certain amount. Adding tetracene molecules represented a type of titration. The end point represented the point where the tetracene concentration was equal to the impurity concentration in the surface region. Observations of earlier workers seemed to indicate that the energy required to produce free holes and electrons in the bulk of an anthracene crystal, Egy was less than that required to excite the first singlet state in this material, Es. But the NYU group has shown that Eg is greater than E8 and that the apparent ability of the singlet excitons to produce a free carrier— which would imply that Es is greater than Eg—is due to the dissociation of the exciton by the electrode or by an impurity near the surface. The recombination of a hole with an electron would be expected to be kinetically second order, Dr. Pope says. However, there is good evidence that at least a sizable portion of the recombination process is first order (and not pseudo first order). In anthracene this results from two properties. One is a large Coulomb capture radius of an electron by a hole in this low-dielectric-constant medium. The other is the small mean free path of an electron in its normal conduction band. Thus, an electron that has received enough energy to leave its parent molecule will often fail to escape from the capture radius unless it
receives additional energy from an external electric field. The electron will therefore fall back into the same molecule that it started from, producing the observed kinetic behavior for the recombination process. This is also called geminate recombination. From the AI mechanism, Dr. Pope predicts that by decreasing the vibrational frequencies in the molecule, he can increase the ionization efficiency of the molecule in the crystal state. One recent experiment performed at Stanford University by Dr. W. E. Spicer and Dr. Barry Schechtman indicates that chlorinated phthalocyanine has a higher normalized efficiency of ionization than normal hydrogenic phthalocyanine. This seems to bear out Dr. Pope's prediction. The prevalence of autoionization and geminate recombination processes with low ionizing efficiencies also helps explain why x-rays produce such low yields of free electrons and holes.
G.e.v. protons induce m.e.v. product trend
154 TH
ACS NATIONAL MEETING Nuclear Chemistry and Technology
Brookhaven National Laboratory scientists have found that the bombardment of uranium with very-high-energy protons results in fission processes characteristic of the deposition of only tens of millions of electron volts of
energy in the uranium nucleus. This behavior, according to BNL's Dr. Gerhart Friedlander, was indicated when they found that the yields of neutron-excess products in the rareearth region—masses from 143 to 161 —from bombarding uranium with 28G.e.v. protons, fall off sharply with increasing mass the same as did yields with low-energy interactions. Dr. Friedlander explains that the knowledge of the distribution of reaction yields by mass and atomic number is a prerequisite for any detailed understanding of nuclear reactions induced in uranium and other highatomic-number elements by protons in the G.e.v. energy range. He points out that this new research confirms an earlier BNL finding that neutron excess products of multi-G.e.v. fission result from processes involving only relatively small amounts of deposition energy (about 50 m.e.v.) in a uranium nucleus. In earlier work the BNL group had found that at proton energies of 1 to 28 G.e.v., the distribution of product yields along isobaric chains in the mass region 130 to 140 shows a doublehumped structure with peaks both on the neutron-excess and neutron-deficient side of beta stability. In the present work, Dr. Friedlander says, the double-humped structure in the charge dispersion curve—first noted for products with masses under 140—not only persists but is more pronounced with products where the mass number (A) is greater than 140. The peak-to-valley ratio of the
RARE-EARTH SEPARATION. Elna-Mai Franz of Brookhaven National Laboratory loads a rare-earth sample onto an ion exchange column. The method is used to separate rare earths formed by irradiating uranium-238 with high-energy protons SEPT. 25, 1967 C&EN 45
former (A < 140) was found to be about 2, whereas that of the latter (A > 140) is approximately 10. From this information it should become pos sible to delineate the mechanisms giv ing rise to the neutron-excess and neutron-deficient products much more sharply in this slightly higher mass re gion, the BNL scientist adds. Of late, Dr. Friedlander and his co workers, Dr. Y. Y. Chu, Ε. Μ. Franz, Dr. Ε. Hechtl, and Dr. P. Karol, have irradiated uranium-238 targets with 28-G.e.v. protons in the circulating beam of an alternating gradient syn chrotron for 30 minutes to three hours. With such irradiations and with mod ern separation techniques, it was pos sible for them to measure yields of products with half-lives ranging from about one hour to about 100 years. To study the cross sections for the formation of rare-earth nuclides, they use both chemical and mass separa tion techniques. After irradiation of the target, the gross rare-earth fraction is first isolated chemically from the uranium target by removing the bulk uranium by anion exchange chroma tography. The mixed-rare-earth frac tion is then either introduced in the form of oxides into the ion source of the isotope separator, or subjected to cation exchange separation. Both chemical and mass separations show less than 1% contamination of any nuclide with neighboring isobars or isotopes, Dr. Friedlander says. For each element of interest, either 0.4 to 2 mg. of carrier or a "spike" of a radioactive tracer isotope is added at the beginning of the chemical ma nipulation when the target is dissolved. The chemical yields are determined by neutron activation of a suitable stable isotope of the element in question or by measuring the radioactive tracer used as a spike. In the isotope separator about 25 neighboring mass numbers in the rareearth region can be simultaneously col lected. The overall efficiency for rare earths (which are converted from oxides to chlorides by a stream of car bon tetrachloride in a heated tube and then ionized by an electron-emitting filament) is 0.1 to 0.5%. The separated nuclides are collected on a strip of aluminum foil. The dis tance between neighboring mass num bers is V 2 to 1 inch. The strip is cut into squares for radioactivity measure ments. From this point, Dr. Fried lander says, the yield of each nuclide is measured by determining the radiation it emits. End-window proportional counters are used for beta particles, lithium-drifted germanium detectors for gamma rays, thin sodium iodide scintillation counters for x-rays, and surface-barrier silicon detectors for alpha particles. 46 C&EN SEPT. 25, 1967
From these radioactivity measure ments, fission product cross sections are calculated for the interaction of 28-G.e.v. protons with uranium 238. Then with the cross section data, charge dispersion and yield-us.-mass curves are plotted. The double-humped charge disper sion curves which clearly indicate two different reaction mechanisms are likely to disappear at still higher mass numbers (A > 160). This is because of the rapid drop in the yields of the neutron-excess products with increas ing mass, Dr. Friedlander points out. Moreover, he adds, only the neutrondefficient products which are presum ably due to a high deposition of energy process—other than ordinary fissionwill be observable above A=160.
Inverse law links ion energy, fusion rate
154TH
ACS NATIONAL MEETING Nuclear Chemistry and Technology
Columbia University scientists work ing with the Yale University heavyion accelerator have found that in nuclear reactions induced by heavy ions, the probability of total fusion oc curring between the colliding nuclei
is inversely related to the energy of the bombarding particles. Dr. Ludwik Kowalski, Dr. Julian M. Miller, and Dr. Jean Claude Jodogne (on leave from the University of Louvain, Belgium) have shown that when a thin aluminum target is bombarded with neon-20 ions, the formation of compound nuclei (total fusion) occurs only in a limited number of interac tions, and that the probability de creases with the increasing energy of the projectiles. Dr. Jodogne says that a possible explanation for this relationship might be that when high-energy ions are absorbed, the angular momentum of the compound nuclei increases. With this increase, total fusion is less likely to occur. The fusion cross section in the aluminum-27-neon-20 system was found to be 380 millibarns at a beam en ergy of 206 m.e.v.; 500 millibarns at 150 m.e.v.; and more than 800 milli barns at 90 m.e.v. The theoretical total reaction cross section for this system is more than 1000 millibarns. A millibarn is 10~27 cm. 2 Previous observations on the cross sections of various direct reactions in dicated that total fusion might have reduced probability in some cases. However, this is the first time that the relationship between the cross sec tion of total fusion and the energy of the projectile has been established
Total fusion of 27AI and 20 Ne gives several isotopes
Based on theoretical predictions, Columbia University scientists say that the total fusion of aluminum-27 and neon-20 at 150 m.e.v. should result in the dis tribution of recoiling products shown above. Products outside the shadowed area should be detectable in mica with 1 0 0 % efficiency. Cross section data were de rived by counting the number of tracks registered in mica which had been exposed to irradiation with heavy ions while in contact with aluminum foil
by direct measurements, the Columbia chemist points out. This achievement was made possible by the use of a mica track technique developed at General Electric research and development center, Schenectady, N.Y., by Dr. R. L. Fleischer, Dr. P. B. Price, Dr. R. M. Walker (now at Washington University), and Dr. E. L. Hubbard (now at Lawrence Radiation Laboratory). The technique consists of using mica crystals in a way similar to the use of nuclear emulsions. According to Dr. Jodogne, the advantages of using mica in this study are: • It is almost totally insensitive to the incidence of neon-20 ions, as well as the recoiling products from nonfusion reactions. •Recoiling products from the total fusion of neon-20 and aluminum-27 are expected to be registered with 100% efficiency. These advantages stem from observations made by the GE group that for each solid, there exists a critical rate of ionization. Particles more heavily ionizing than this value produce continuous tracks with unit efficiency, while less heavily ionizing particles produce no tracks [Phys. Rev., 156, 353 (1967)]. The cross section data were derived by counting the numbers of tracks registered in mica which was previously exposed to the irradiation with heavy ions while in contact with a thin aluminum foil. The tracks—average length 10 microns—were revealed by etching the mica with hydrofluoric acid. The actual counting was done with the aid of an optical microscope. Theoretically, in the aluminum-27neon-20 system, total fusion should lead to the formation of vanadium-47 compound nuclei, which decay to residual products. These products would be isotopes of sulfur, chlorine, argon, potassium, calcium, scandium, and titanium, Dr. Jodogne says. Using the mica technique, he adds, it is not necessary to separate these isotopes and count the activity to determine the cross section data. Such separation and counting would be a difficult task because many of these isotopes are either stable to radioactive decay or have a short half-life. Although the results of this study have been enlightening, the Columbia scientist points out, it is possible that total fusion events followed by rare fission or multiple alpha particle emission could escape observation with mica detectors. Bearing this in mind, the Columbia group is using more sensitive detectors and other systems to collect more definitive data on the total fusion phenomenon.
Discharge lamp aids arsenic determination 1554TH
ACS NATIONAL MEETING Analytical Chemistry
An electrodeless discharge lamp in an Evenson cavity has been used by Dr. Oscar Menis and T. C. Rains at National Bureau of Standards (Washington, D.C.) as a primary light source in the determination of arsenic by atomic absorption spectrophotometry. The lamp provides high-intensity, far-ultraviolet, arsenic line radiation. In addition, Dr. Menis and Mr. Rains developed a new separation and concentration technique and a new fuel system, making possible a 0.1-p.p.m. detection limit for arsenic. Elements such as arsenic and selenium, whose ground-state resonance lines are in the far UV (below 2000 A.), have been difficult to determine by atomic absorption spectrophotometry because the far-UV radiation is absorbed appreciably in air. Dr. Menis and Mr. Rains found that an electrodeless discharge lamp, developed 17 years ago but seldom used for atomic absorption, provides radiation intense enough to overcome this difficulty. The lamp is powered by a radiofrequency generator which supplies 100 watts at 2450 MHz. The cavity which transfers this power from the microwave source to the lamp is of Evenson design. It eliminates objectionable and hazardous radiation to the body and to the electronic components of the instrument. This radiation was caused by the output from the hemispheric antenna first used to transmit power to the lamp. Using a new fuel system, argon-hydrogen, and a Zeiss burner, the NBS workers obtained detection limits for arsenic of 0.1 p.p.m. for the 1890-A. and 1937-A. lines and 0.2 p.p.m. for the 1972-A. line. The argon-hydrogen fuel gave lower background absorption and better detection limits for arsenic than the oxyhydrogen flame. The separation of arsenic from the selenium begins with the precipitation of selenium from a 2N hydrochloric acid solution with sulfurous acid. Since arsenic (III) can be lost from hydrochloric acid as arsenic trichloride or arsine during evaporation, the arsenic was concentrated by extraction with diethylammonium diethyldithiocarbamate (DEDC) in chloroform. To make the arsenic extraction quantitative, the arsenic was reduced to arsenic (III) with potassium iodide and sodium metabisulfite. Arsenic is then stripped from the
D E D C by an exchange reaction with cupric ion. Dr. Menis and Mr. Rains found that the separation of arsenic from DEDC and chloroform is necessary because organic materials interfere with the final determination. Carbon radicals in the flame produce a high absorption blank at all three resonance lines of arsenic. To separate and preconcentrate the arsenic in cast iron, the NBS workers dissolve the sample in nitric acid and evaporate it to near dryness. After the sample is taken up in water, the iron and copper are extracted with 2thenoyltrifluoroacetone in carbon tetrachloride, with the pH controlled at 2.5 by the addition of pH 4.6 acetate buffer. To further concentrate the arsenic and to remove acetate ion, the solution is made 3N in hydrochloric acid and the arsenic extracted with DEDC as with the selenium samples. The NBS workers verified the recovery of arsenic from the samples by following the dissolution, separation, and preconcentration steps with arsenic-77 and by using the method of standard additions. The tracer work showed that many previous analyses gave low results because of the loss of arsenic (III) from hydrochloric acid solutions.
Pyrolysis unit ties gas chromatographs 154 TH
ACS NATIONAL MEETING Analytical Chemistry
A tandem gas chromatograph—two gas chromatographs in series with a hightemperature pyrolysis unit between them—is a relatively inexpensive, rugged analog of the gas chromatograph-mass spectrometer combination. In the tandem system, developed for extraterrestrial research, as explained by Dr. Clarence J. Wolf and John Q. Walker of McDonnell-Douglas Corp., St. Louis, Mo., compounds are separated by the first chromatograph, the separation chromatograph. They then pass into the pyrolysis unit, where the temperature is between 500° and 850° C , and are degraded into simpler molecules. The decomposition products are analyzed by the second chromatograph, the analysis chromatograph, yielding pyrograms of the compounds in the sample. The apparatus consists of an F&M Model 5750 as separation chromatograph, a coiled gold tube 36 inches long and 0.04 inch in inner diameter as pyrolyzer, and a Perkin-Elmer Model 11 as analysis chromatograph. The first chromatograph is temperature-programed. The pyrolyzer SEPT. 25, 1967 C&EN
47
