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Home , Autoradiograph, Fermium, Fritz Strassmann, Neutron capture, Nuclear chemistry, Nuclear fission

, These data would provide infor- mation on the change in amount of dense absorber between the source and the detec- tors and thus on the change in configuration of (he pit caused by the implosion, The key to this diagnostic scheme was to find such a What Are source. The fission products -140 and lanthanum-140 (the same that led to the discovery, of fission) and provided the solution. Barium-140 has a half-life of 12.8 days and decays to lantha- num 140. which has a 40-hour half life and Nuclear ? emits an energetic gamma ray with its decay. As the operation was finally worked out. the lanthanum-140 was extracted from a large he definition of the terms “radiochemistry” and “,” and batch of its barium 140 “parent” (obtained T the delineation of the presumed difference between them, is a problem that from Oak Ridge) and put into a compact bedevils the toilers in this vineyard whenever they try to tell what they do. receivcr in the center of the pit to be tested, Historically, radiochemistry appears to have been the first in line, When Chemical separation and further purification Madame was carrying through her laborious chemical procedures to of the lanthanum-140 were carried out by a isolate and identify the radioactive elements and to establish their transforma- headed by Gerhart Friedlander at a tions, she was doing radiochemistry. It was not until some time later that the tcmporary building a mile or two from the of the nucleus and its reactions became clear, In most of the early firing site in Bayo Canyon. There were then researches, chemical manipulations were essential, and by the late a no elegant “hot cells”’ nor sophisticated re- recognizable body of techniques and strategies had evolved to deal with the mote-handling apparatus. To minimize rapidly growing list of radioactive species. And it was by such chemical exposure. The radiochemists rigged manipulations that and first demonstrated the up the chemical processing equipment so occurrence of . Examining the products resulting from that the essential operations could be irradiation of , with an atomic weight around 238, they found performed and monitored from a distance unmistakable evidence for radioactive barium with an atomic weight around with a system of cables. mirrors. and 140; they were forced to conclude that the barium came from the uranium by a telescopes. The final lanthanum 140. concern process that could only have been some kind of a splitting of the uranium tratcd into a of 0.1 milliliter. was nucleus. Their employment of chemical techniques to study nuclear phenomena then heavily shielded and trucked to the may be considered nuclear chemistry. firing site, where another simple remote- Unfortunately for those who would compartmentalize science into neat bins, control rig transferred it into the implosion our subject has broadened greatly in the span of time since the discovery of test assembly. The source. implosion as nuclear fission, Nowadays, its practitioners range from the “pure” radio- sembly, and gamma-ray detectors were , whose primary concern is with the chemistry involved, to some destroyed in the test explosion. but the “nuclear chemists” who are concerned only with nuclear problems and barium-140 supply back at the processing who rarely get their hands on any radioactivity—or vice versa. In our article we site was available to serve another day: occupy something of a middle ground. Those pursuits in which the chemical because of its relatively long half-life it could operations are clearly of first importance we call radiochemistry, and those in “grow in” one or more new lanthanum-140 which the problems of and reactions are of first importance we sources for use a few days later. The RaLa call nuclear chemistry. If in telling our story we turn out to have been implosion tests became a regular diagnostic inconsistent in employing this criterion, we plead guilty and ask our radio- practice: with steady improvements in tech /nuclear chemist critic to cast the first stone. ■ nique and with increasingly strong sources. they were continued until 1962, when they

4 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

sample of the debris from the explosion and performing radiochemical separations to isolate. individually. the and at least one radioactive fission product. By measuring the plutonium sample in an alpha- particle counter, he obtains the number of remaining plutonium nuclei—those that did not fission-which we shall call p. By measuring the fission-product sample in a beta- or gamma-ray counter, he obtains the number of fission-product nuclei. He then employs an empirical constant to convert this number to the number of fissions that had occurred in the debris sample, a quantity we shall designate by f. (The empirical constant is derived by measuring the same fission product in a calibration standard of plutonium-239 in which a known number of fissions has been induced.) The efficiency of

fissions — fissions + remaining Pu nuclei

Wartime photo showing trasfer of a heavily shielded gamma-ray source at the RaLa — f chemical processing site in Bayo Canyon. Motive is being supplied by group f+ p leader Gerhart Friedlander and anchorperson Norma Gross, a WAC assigned to Los Alamos. Such sources were used to study the process of implosion, a method for Thus, we see that we are able to determine achieving a critical configuration of the fissionable in a nuclear . the explosion efficiency by measuring the fission products and the unburned fuel in a small fraction of the explosion debris without were supplanted by other diagnostic The efficiency of a . like having to know explicitly the fraction of the schemes. that of an industrial process such as the total debris represented in that sample. In conversion of into electrical , is actual practice we can perform this measure- The Test measured by the fraction of the energy ment on only ten trillionths of the total debris content of the valuable starting material, the created by a 10-kiloton test, if the efficiency The task that was to become the dominant fuel, that appears as usable output energy. is the only performance parameter desired. occupation and responsibility of the nuclear Thus, in the case of an explosion produced Also, in actual practice there are a number of and radiochemists up to the present day was by nuclear fission of a fuel such as pluto- nontrivial complications that we have swept one that began with the first nuclear ex- nium-239, the efficiency is given by the under the rug: some of the plutonium-239 is plosion, the Trinity test of July 16. 1945 at fraction of the plutonium-239 originally in consumed by the nonfission reaction 239Pu the Jornada del Muerto site in central New the device that actually underwent fission, or (n,y)240Pu (that is, plutonium-239 + neutron Mexico. That task was the measurement of “burned.” The nuclear chemist proceeds to the efficiency of the explosion. derive this fraction by gathering a small additional plutonium-239 may have been

LOS ALAMOS SCIENCE Summer 1983 5 created by neutron reactions with any uranium-238 present in the device followed

caused our sample to become enriched or depleted in the fission product chosen for analysis, so that f and p may not be found in the ratio in which they originally occurred. There are methods for dealing with these and other complications. By multiplying the efficiency by the num- ber of plutonium-239 nuclei known to have been put in the device, we can calculate the total number of fissions that occurred in the explosion. This is the number that the nu- clear chemist actually determines, and it can easily be converted into a yield, or total energy released. For practical reasons nu- clear explosion yields have traditionally been expressed in terms of tons or kilotons of TNT equivalent. Conversion of the chemist’s number to kilotons involves multiplying total fissions by a nominal energy release per The energy generated by both reactors and so-called atomic fission and inclusion of minor additional arises from the neutron-induced fission of certain isotopes such as uranium-235 and energy contributions from sources such as plutonium-239. Shown here is a typical fission event: absorption of a neutron by radiative . There are other uranium-235 to an excited state of uranium-236, which splits into two primary measures of the energy release in a test fission products, in this event -93 and cesium-141, within about 10-21 second. explosion (fireball expansion rate, atmo- (Not shown are the additional emitted as the excited nucleus splits; these spheric pressure wave, underground seismic neutrons make the fission reaction a self-sustaining, or chain, reaction.) Each of the signal, and so on) but the direct nuclear chemistry measurements provide the stan- neutron-rich primary fission products undergoes a series of beta decays leading dard energy release to which all the other eventually to a stable . As the residues of a test, the fission measurement techniques are calibrated. products contain information about the weapon’s performance. For example, the The determination of the energy released number of fissions that occurred can be determined by assaying a sample of the in the Trinity explosion employed just these explosion debris for one or more of the fission products. A radiochemist setting out to principles and measurements, albeit with do this by assaying for one of the fission products in the decay chains shown here equipment that would be considered prim- would most probably pick -93 or -141. The preceding chain members itive by modern standards. The job was decay too rapidly, and the subsequent members decay too slowly (or not at all). The performed by a team headed by Herbert excited uranium-236 nucleus can also split initially into any one of a large set of Anderson and Nathan Sugarman, the latter possible primary fission-product pairs. The resulting mixture of primary fission on temporary assignment from the Manhat- products and their decay products, which contains various isotopes of about three tan Project at the University of Chicago. dozen elements, is the almost universal diet of Los Alamos radiochemists. Although a Their final report of the measurement, dated September 25, 1945 and still bearing a secret nuisance when a particular isotope must be isolated, the fission-product mixture is a classification, contains some interesting rich lode of material for basic research in nuclear chemistry, including investigations notes on the planning and execution of this of the fission process and studies of individual on the neutron-rich side of unprecedented operation. Since it was recog- stability. Such studies at Los Alamos have resulted in the discovery of at least fifteen nized that, if the explosion went as it was new nuclides with half-lives ranging from less than a second to millions of years.

6 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

I supposed to, the best samples would probably be found in the ground beneath the blast. means had to be devised to get at this material. Two Sherman tanks were made available for the purpose. One of them was fitted with -shielded compartments for the driver and the observer/sampler, who gathered samples through a hole in the floor of his compartment with a vacuum cleaner or by driving a hollow pipe into the ground. The second tank was equipped to tire into the center of the crater from a distance of 500 yards; the rockets were fitted with sampling noses and with cables for retrieval. Both methods worked, and a good set of samples was brought back to Los Alamos for processing and radiochemical analysis. One’s first reaction on reading these ac- counts today is astonishment that such measurements could be made with useful accuracy at all, given the primitive instru- ments available at the time. Apparatus now considered indispensable—high-resolution gamma-ray detectors, multichannel pulse- height analyzers. and stable transistorized electronics, to name just a few—did not exist. In point of fact, the efficiency and yield numbers generated by this were

respectable error even today. The nontrivial lesson to be learned is that ingenuity and scientific craftsmanship can achieve im- pressive results even with rudimentary equip- ment.

More Testing and a Radiochemistry Group

The experience gained in the Trinity test was soon put to use in the summer of 1946. when similar diagnostic measurements were made on the two test of at in the Marshall The crew that collected samples of radioactive debris from the Trinity crater rode to Islands. There was still no specifically desig- its destination in a Sherman tank outfitted with lead shielding and bottles of breathing nated nuclear and radiochemistry group at air. The samples were gathered through a trapdoor in the bottom of the tank. The Los Alamos, most of the experienced person- lower photo is an aerial view of the crater and the tracks left by the tank. nel having left in the great diaspora following

LOS ALAMOS SCIENCE Summer 1983 7 the end of World War II, Thus, when the call came for a working group to carry out the task at Crossroads, a team of experienced individuals was gathered from Los Alamos, Argonne, Oak Ridge. and various univer- sities under the leadership of William “Buck” Rubinson. This time, two kinds of postexplosion de- bris samples were gathered: from the atmo- sphere, by drone B-17 and F-6F aircraft, and from the sea, by drone boats. To provide early information in the field, a temporary radiochemical laboratory with appropriate Samples of airborne debris from the Operation Crossroads tests of 1946 were radiation counting equipment was set up gathered by drone aircrqft. Shown here is a sampling unit being installed behind the nearby at Kwajalein Atoll. The shorter lived cockpit of a drone B-17. The filter paper in the sampling unit was specially treated so products from the explosion (such as that the debris would stick to its surface. -97 and -99 from fis- sion and -239 from neutron cap- ture on uranium-238) were analyzed at Kwa- that fission product relative to others and to establishment and to the fledgling Atomic jalein, and the longer lived products (stron- neptunium and plutonium. No way has ever Energy Commission the importance of con- tium-89, zirconium-95, barium-140, and been found to exploit this fact for study of tinuing Los Alamos as an integral part of the cerium-144 from fission) were analyzed in -cloud phenomenology; its only effect national defense research and development samples taken back to Los Alamos. The has been to restrict the choice of fission- effort. From a more technical standpoint total plutonium content of the individual product nuclides suitable for weapons test these tests showed that radiochemical analy- samples was measured at both laboratories, diagnostics. ses of atmospheric samples alone could but measuring the ratio of plutonium-239 to The beginning in late 1946 and provide essential efficiency information and total plutonium, which involved fission early 1947 saw the formation at Los Alamos in addition detailed information on the per- counting at a reactor, was of course done at of a permanent nuclear and radiochemistry formance of individual components of the Los Alamos. [This ratio provides informa- group led by R, W. Spence and soon to test device. The demonstrated success of air tion about the fraction of a weapon’s original become a part of J Division, the test or- sampling followed by radiochemical analysis plutonium-239 content that is “wasted” in ganization. This change had two important was important not only to the nation’s nonfission neutron reactions such as (n,2n) effects. First, it provided the organizational domestic program and (n,y). ] focal point for dealing in a more coherent but also to the development of its atmo- The fission-product measurements for way with the spectrum of contributions that spheric surveillance system for detecting and Operation Crossroads showed for the first nuclear and radiochemists could make to a evaluating possible foreign weapons tests. It time in a practical way the consequences of weapons development laboratory. More im- was such a system that picked up evidence of the occurrence of noble-gas elements in portant from a scientific standpoint. it set the a nuclear event in the Soviet Union in fission-product decay chains. Fission stage for the transition of the nuclear August 1949, and radiochemical analysis of products such as -89 and chemists from their status as primarily con- the evidence was able to show that the event barium-140, which derive in large part from sumers of basic science information to pro- was indeed the test of a nuclear the noble-gas ancestors 3. 15-minute kryp- ducers as well. device. ton-89 and 13.7-second -140, did not Operation Sandstone in 1948, in which “track” well with other fission products three new experimental fission-weapon de- Continued Growth and the Mike Test derived from chains with very short-lived or signs were tested at Eniwetak Atoll, was an insignificant noble-gas ancestry. It was clear important landmark for the new group and Following the Operation Sandstone tests, that even less than a minute spent as a gas for the entire Laboratory. The successful and with them the assurance of a continuing during decay of a predecessor was enough to conduct of these tests and the information contribution to the Laboratory’s future. cause significant chemical fractionation of gained from them confirmed to the defense members of the newly organized nuclear and

8 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

tory employees to carry out graduate-level studies and eventually to earn advanced degrees while continuing their active commit- ment to ongoing Laboratory programs. Though the Ph.D. requirements included an academic year in residence at Albuquerque. in practice a candidate could and did return frequently to Los Alamos to take part in the activities of his group. After the year of residence. the candidate returned to Los Alamos to do his thesis research: the topic The neutron-induced fission of uranium-235 results in a wide range of products. The was chosen to be relevant to Laboratory graph above shows that the yield of these fission products has a relative minimum in interests, and the progress of the work was guided jointly by Professor Kahn and a the region corresponding to symmetric and nearly symmetric fission and that the Laboratory staff member. A total of thirteen yields in this region increase with the energy of the neutrons inducing the fission. Yield University of doctorates were curves for other fissile isotopes exhibit similar shapes and variations with neutron awarded on radiochemistry and nuclear energy. chemistry theses under this program. Doc- toral thesis work of students from the radiochemistry group (now a part of J Chemistry, are believed to be the first of their Massachusetts Institute of . the Division) began a program of research and kind. Studies were also begun on the decay University of Wyoming, and the University development along two complementary lines. properties of individual fission products and of , San Diego has also been First and perhaps the most immediately a variety of other nuclides and on nuclear performed here under similar arrangements. urgent was development of radiochemical cross sections, in particular cross sections The major expansion in the Laboratory’s procedures. instrumentation, and calibra- for reactions with 14-MeV neutrons. Some weapons design and testing effort that began tions to support both programmatic func- of this work led in 1949 to the group’s first in 1950 had a corresponding impact on the tions and research in this field. The second discovery of a new —an iso- size and scope of the group’s activity. The was the research itself. Of particular interest meric state of -97 denoted by 97mNb. first Nevada test series in early 1951, a test was the study of fission-product yields and The scheme for decay of zirconium-97 to series at Eniwetak in the spring of 1951, and decay properties. Careful measurements another Nevada test series in the fall of that were made of the mass distributions of reported in Physical Review and became the same year presented the group with the fission-product yields from the fission of first in a large number of research papers to sharp challenge not only of providing diag- uranium-235, uranium-238, and pluto- be published by Los Alamos nuclear nostic information on a large number of nium-239 induced by various reactor neu- chemists and their collaborators from other fission explosions. but also of exploring and tron spectra. Similar measurements were groups and institutions. eventually developing means for obtaining made for fission induced by monoenergetic An important part of the Laboratory’s quantitative diagnostic information on the 14-MeV neutrons. [Neutrons with this scientific history. and one that contributed phenomena associated with thermonuclear energy are a product of the reaction significantly to the development of components, The latter led to the first 3H(2H,n)4He induced by bombarding targets and radiochemistry, was the creation in 1949 use of “radiochemical detectors”: small containing (-3) with deu- of a special arrangement with the University amounts of suitable target elements in- terons (hydrogen-2 ) from the Labora- of New Mexico to provide scientific instruc- corporated into or near the device compo- tory-s Cockcroft-Walton accelerator. The tion at Los Alamos. As part of this program, nents to provide, by their level of activation, same reaction between tritium and Professor Milton Kahn of the University’s a measure of the neutron spectrum and at thermonuclear temperatures is the domi- Chemistry Department began commuting fluence at these sites. The success of these nant reaction and energy source for con- from Albuquerque to give a two-semester early detector trials has led in succeeding trolled thermonuclear power. ] The latter re- night-school course entitled “Chem. 213. years to a steady growth in the scale and sults, subsequently reported at one of the Radiochemistry.” This program helped fill sophistication of this diagnostic tool, to the Gordon Research Conferences on Nuclear an urgent need: an opportunity for Labora- point where nowadays the detector informa-

LOS ALAMOS SCIENCE Summer 1983 9 tion is the most important radiochcmical contribution to weapons testing. The first large-scale thermonuclear ex- plosion, the Mike test of 1952, handed the nuclear chemists an unexpected bonus: from its unprecedented neutron fluxes came a grand assortment of transuranium nuclides, including two new elements. Using samples of airborne debris collected by aircraft from the Mike cloud, a collaboration of nuclear chemists from thc University of California Radiation Laboratory. Argonne National Laboratory, and Los Alamos undertook a thorough analysis of the transuranium ele- exchange is a powerful technique for separating chemically similar elements. ments by means of cation-exchange resin Chemist Deva Handel here loads an ion-exchange column with a solution of mixed separations. These experiments showed a rare . The columns ( vertical tubes at right center) contain a resin similar to number of previously unknown radioactive the zeolite in softeners. Flow of a completing agent causes the rare earths to species. among them an isotope of element migrate through the resin, each at a different rate. The plastic tubing on the left 99 emitting 6.6-Me V alpha particlcs and a carries the emerging solution to test tubes mounted on an automatic sample changer. 7.1 MeV alpha emitter growing from an element-99 parent. Further work identified the first of these activities as by of californiun-253 and the A Spin-off in Space he success of radiochemical detectors for weapons diagnostics led directly to the eventually named and , T development of a device performing a similar function in quite a different respectively. Also discovered in the Mike environment—space. of spacecraft and their crews was of debris were the new nuclides plutonium-244 course a concern of the nation’s manned space-flight program, but measuring the kind (the longcst iived of the plutonium isotopes) and amount proved an awkward task. The instruments that do the job well in the and the beta emitters plutonium 246 and laboratory require power, room, and attention that are at a premium in a spacecraft. -246. Detailed examination of the Through previous collaborations on NASA space experiments, a member of the abundances of the transuranium isotopes nuclear and radiochemistry group became aware of the problem and organized the revealed what had happened. Some of the designing of a radiochemical dosimeter that offered a partial but economical solution. uranium in the device together with some of Designed to measure neutron spectra and weighing approximately one pound, the the neptunium produced by various nuclear dosimeter contained target specimens of uranium-238, yttrium-89, scandium-45, and reactions during the explosion. was exposed -46, -47, -48, -49, and -50. After a nine-day journey in space aboard the to neutron fluxes high enough to produce Apollo spacecraft that rendezvoused with the Soviet spacecraft Soyuz in July 1975, the strings of succcssive neutron capture reac- dosimeter was returned to Los Alamos for radiochemical analyses within twenty-four tions leading to uranium and neptunium hours after splashdown. The analysis of the radioactivities induced in the target isotopes with sixteen units or more isotopes, in conjunction with the body of neutron cross-section data derived from the heavier than those of the starting isotopes: group’s experience with radiochemical detectors for weapons diagnostics, gave these neutron-rich product isotopes then information on total neutron fluxes and on fluxes within various energy bins up to underwent successive beta decays culminat- about 40 MeV. The data, apart from their intrinsic interest, were of value in my in the nuclides later observed by the interpreting other experiments carried out during the space mission. One noteworthy radiochemists. Indeed, because of the new result was that the overall neutron exposures in the spacecraft had been lower by a family of nuclear phenomena made possible factor of about 2.5 than was expected on the basis of calculations from previous by the great neutron densities achieved in the flights. ■ thermonuclear burn, the Mike debris turned

10 Summer 1983 LOS ALAMOS SCIENCE out to be a proverbial mine for basic formance by radiochemical techniques. and above, more than half of the delayed- nuclear science as well as for the science and The program to investigate the use of neutron species were retained; and some of technology of thermonuclear explosions. We nuclear reactors for propulsion, the the fission-product elements, such as silver return to this exciting subject in a later Rover program, represented a major new and , were found to be very mobile. section. initiative for the Laboratory. The basic con- In one experiment the sharp variation in The remaining years of the ’50s up to the cept was to a low-molecular-weight mobility was exploited to measure the half- moratorium on testing of November 1, 1958 gas—hydrogen was the obvious choice—by life of -115, a newly discovered were a period of hectic activity and vigorous passing it through a compact reactor and to fission-product isotope. Uranium-loaded scientific growth for the group. During this direct the now high-temperature gas stream slugs were irradiated with neutrons, period there were four weapons test series at through a nozzle as a high-specific-impulse allowed to decay at room temperature for Nevada and three in the Pacific, for all of propellant. Practical exploration and devel- varying lengths of time, and then heated to which the group provided essential diag- opment of this concept involved diagnostic expel the more volatile products, among nostic information. In addition, research on needs for which radiochemical techniques them being the cadmium-115 granddaughter radiochemical procedures and on nuclear were well suited: measurement and calibra- of palladium-115. (This procedure is some- properties proceeded along several lines, tion of fission-energy release, analysis of fis- what analogous to a chemical “milking” some of them prompted by the needs of sion distribution in the reactor fuel elements, experiment.) Analyses for the cadmium- 115 weapons diagnostics, others by ideas, special assays of the amount of fuel and fission showed the half-life of palladium- 115 to be sources, and facilities deriving from the test products escaping from the reactor into the activities. An especially important factor in propellant, and a variety of problems as- The studies of the variations in diffusion this work was the availability of state-of-the- sociated with creation and migration of rates with element and temperature led also art radiation measuring equipment, including radioactivity under high-temperature condi- to the development of a means for indirect tions. determination of temperature distribution in instruments for counting, multi- For a reactor with uranium-impregnated the reactor. Small cartridges of graphite channel pulse-height analyzers for alpha and graphite fuel, the design type provisionally loaded with radioactive tracers selected to gamma scintillation spectrum measurements, selected for Rover, there were three concerns cover a range of diffusion rates were em- and a magnetic beta-ray spectrometer, of special importance to high-temperature placed at various locations in the reactor operation: the loss of uranium fuel by dif- during assembly. These “radiochemical ther- Diagnostics for Nuclear Rocketry fusion out of the fuel elements, the loss of mometers” were recovered after a reactor delayed-neutron-emitting fission products test run and analyzed for tracer content. The middle ’50s saw another significant that provide the margin of reactor control, Comparison of the tracer losses with losses change for the nuclear and radiochemistry and the escape of fission products in general from identical units put through calibration group: R. W. Spence, who had created the to the propellant stream. With these con- runs in ovens back at the laboratory gave the group in late 1946 and led it through nine cerns foremost in mind, the group began approximate temperatures at the reactor years of expansion. was appointed second in immediately an experimental study, in the locations. Our records show that in a typical command of a new division set up to study temperature range expected for the reactor, full-scale reactor test we employed sixty such nuclear rocket propulsion. and his place as of the rates of diffusion and volatilization thermometers, each one loaded with the group leader was taken by G. A. Cowan. An into a gas stream of a large number of tracers barium-133, americium-241, and plu- early member of the at elements from fission of uranium in a tonium-239, listed in order of decreasing and the University of graphite matrix. Although some data were diffusivity. Chicago and with Los Alamos for Operation available from previous studies conducted By far the largest component of the nu- Crossroads, Cowan had been with Spence’s elsewhere, it was necessary to have a wider clear and radiochemistry group’s participa- group since 1949 until he was brought to the and self-consistent data base suited to the tion in the Rover program was the detailed J Division office in early 1955. Shortly after Rover components. From the large amount post-mortem of the fuel component of the assuming the group leadership in November of these data three important results various reactors after full-scale tests. The 1955 (the group was then known as J- 11), he emerged: none of the uranium diffused out; reactors were disassembled at the Nevada initiated what was to become a second major although substantial amounts of the delayed- Rocket Development Site, and the fuel ele- program for the group over the next fifteen neutron emitters bromine and were ments were returned to Los Alamos for years: the diagnosis of nuclear rocket per- lost at temperatures of 2000 degrees Celsius determination of fission distribution, total

12 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

fission, and fission-product loss to the graph- ite moderator and to the atmosphere. This large-scale effort. extending from 1958 to 1967, had important consequences for future undertakings of the group. It brought to the radiochemistry site a versatile chemical hot- cell facility and a complement of up-to-date radiation-measuring apparatus with com- puterized data-processing equipment (the lat- ter included a PDP-8 computer acquired in 1966). Even more important than the facili- ties, it brought the group valuable experience and a broadened outlook in state-of-the-art radiochemical operations on the 100-curie scale.

Scientific Applications of Nuclear Explosions

In the middle and late 1950s the nuclear chemistry group undertook another activity that was to make a significant contribution to basic nuclear science: the specific applica- tion of nuclear explosions to the measure- ment of quantities and the production of materials that lay outside the reach of con- ventional laboratory facilities. This research direction, initiated and actively promoted by Cowan. had its first noteworthy success with From 1955 to 1970 Los Alamos was investigating the principles of nuclear rockets, the “wheel experiment” carried out during a rockets propelled by a low-molecular-weight gas heated to high temperature by nuclear weapon test in 1961. This experi- passage through a . The upper figure shows schematically the design of ment was aimed at measuring the variation one of the rocket engines built and tested by the Laboratory. [From R. W. Spence, in mass symmetry of uranium-235’s fission International Science and Technology, 58-65 (July 1965).] The hydrogen not only at its neutron resonances. [A little expla- provided the rocket’s thrust as it exited through the exhaust nozzle but also cooled the nation may be in order here: for the neutron- nozzle as it entered the system in liquid form through the coiled pipe. The induced fission of, say, uranium-235, the Laboratory’s nuclear and radiochemistry group was called upon to study various term “mass symmetry” describes the extent aspects of rocket reactor behavior, including the diffusion fission products from the to which the compound nucleus ura- graphite fuel matrix at the expected operating temperatures. The lower figure shows nium-236 (the intermediate product formed data from one such study in which members of the group measured the amounts of by absorption of a neutron) splits nearly in half, and the term “neutron resonance” de- various fission-product elements remaining in a graphite sample after it was held at scribes the enhanced absorption of neutrons 2400 degrees Celsius for 4 minutes. The variation of diffusivity with element and with with corresponding to the dif- temperature led to the development of “radiochemical thermometers ’’for determining ferences between that of uranium-235 and temperatures within the reactor core during a test run. The thermometers contained those of semistable states of the compound radioactive isotopes of elements covering a range of diffusivities. The losses of the nucleus.] A disk faced with uranium-235 was isotopes measured after the test run gave the temperatures at various reactor set spinning in front of a slit at the upper end locations. of a vacuum pipe leading to an underground

LOS ALAMOS SCIENCE Summer 1983 13 235 U-Faced O* Disk

Shown schematically at left is the setup for the “wheel experiment” to determine the variation in mass symmetry of the fission induced in uranium-235 by neutrons with energies corresponding to its neutron resonances. The was an underground nuclear explosion, which produces, essentially in a single burst, neutrons with speeds ranging from one-fifth to ten millionths the speed of light. By the time the neutrons reach the target disk, they have been separated in energy by time of flight; that is, neutrons with different energies reach the target at different times—the fast first, the laggards later. Because the disk is rotating, the neutrons arriving at different times strike the target at different locations. Those neutrons with resonant energies induce many fissions and create bands of fission products in the uranium-235. Shown above is an of the fission products on the disk’s surface. The energy

large number of fast-neutron -induced fissions) and a few prominent resonances.

nuclear explosion. The pulse of neutrons lower and the higher symmetric-fission from the explosion, separated in energy by yields, respectively, could be identified with time of flight, created in the uranium-235 a states in uranium-236 with total angular series of fission-product bands correspond- momentum./ of 4 and 3. Similar results were ing to the resonances. Radiochemical analy- obtained for the resonances of pluto- sis of the individual bands for total fissions nium-239, but in this case the higher and and for silver-111 and cadmium-115, prod- lower symmetries are associated with the J= ucts of nearly symmetric fission, gave the 1 and the J = 0 states of plutonium-240. symmetric-fission yields at the resonances. Another scientific application of nuclear This and subsequent experiments with im- explosions was one that had been brought to proved techniques showed that the sym- light in startling fashion with the Mike test: metric-fission yields fall into two groups the production of very heavy nuclides. The averaging 0.59 and 1.11 times the sym- results of that test had given rise to a wave of metric-fission yield for fission induced by hope amongst some of the heavy-element thermal neutrons. The resonances with the research fraternity. They reasoned that since

14 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

the rapidly increasing probability of spon- taneous fission with increasing isotopic mass set an effective limit on the masses of the products attainable. The heaviest product recovered was fermium-257, with a half-life of 100 days. [The heavier isotope fermi- um-259, discovered in 1976 by a Los Ala- 3 mos/Livermore team employing the ( H,P) reaction on a fermi urn-257 target, decays by with a half-life of only 1.5 seconds.] There has been some specula- tion on the possibility of reaching the predicted “’” near 298114 by irradiating suitable heavy-element targets in a very high-flux nuclear explosion (or per- haps one bootstrapped on another) to make an “end run” around the zone of nuclides prone to spontaneous fission. but the prospects do not at present appear suffi- ciently promising to warrant a test. Mean- while, this heavy-element work has indicated that a large area on the neutron-rich side of the isotope chart represents nuclei known to Attempts to reach a region of predicted semistable nuclides in the vicinity of Z = 114 have been produced (or to be producible) in and N = 184, the Wand of stability, ” have occupied nuclear since the ‘50s. nuclear explosions but remaining un- According to current thinking, the best prospect of reaching it lies in single-step described and currently unavailable by any collisions, with minimum excitation, between neutron-rich projectiles and neutron-rich other known means. Here the limitation lies targets—in effect a quick, shallow underwater trip to the island. (Adapted from not in the technology of thermonuclear ex- computergraphics by Ray Nix.) plosives but in the technology of sample recovery. Development of the means to retrieve and partially process samples from the Mike device was designed to achieve doubly closed-shell nuclides are -40 an underground explosion within five or ten other technical goals—the production of and lead-208. ) Moreover, by the ’60s there minutes of the event could open up a large heavy elements was, after all, only inciden- was reason to believe that the high neutron new field of nuclear studies. tal—it did not seem unreasonable that other fluxes could be achieved in explosions devices of this kind could be modified to producing much less radioactivity and releas- Tools Old and New achieve even higher instantaneous neutron ing much less energy. say of the order of a fluxes and thus provide a path to synthesis of few kilotons. Between 1962 and 1969 several In science it is axiomatic that new ad- still heavier elements and possibly even the sophisticated devices modified for produc- vances follow the introduction of new tools super heavies, the predicted semistable tion of heavy elements were actually fired in and new techniques. Pressed by insistent nuclides in the vicinity of the doubly closed- underground tests in Nevada, some by Los demands for accurate and timely diagnostic 298 Shell 114. (The stability of nuclides Alamos and some by Livermore. Although data, the group and its instrumentation czar with approximately 114 and 184 radiochemical examination of the products J. P. Balagna had early on recognized the neutrons is predicted by the shell model of of these tests turned up valuable nuclear necessity for first-rate equipment. New tools the nucleus. This model explains the ob- information, it also clearly brought to light a of the trade were tested and incorporated served stability of nuclides with “closed” fundamental limitation on heavy-element into the group’s instrumentation complex as shells containing certain “magic” numbers of synthesis—spontaneous fission. The fluxes soon as they appeared on the scene. A protons or neutrons. Other examples of achieved in these later tests were high, but particularly useful addition was an isotope

LOS ALAMOS SCIENCE Summer 1983 15 separator purchased in 1964. With it we became available for other studies. One of now not only supply essential isotope data could separate interfering isotopes of various the first studies involved flights through the on fissile elements and on radiochemical elements and thus perform for the first time discharge plumes of the Rover reactor tests detector elements but also have made accurate analyses for second- and third- to provide information on the radioactivity possible some new initiatives in basic re- order (and in some cases even higher order) carried out by the propellant gas. Then, in search. products of neutron reactions. This capabil- the middle to the late ‘60s, the rapidly One quite recent initiative that demands ity significantly broadened the range and increasing national concern over environ- the capabilities of the utility of the radiochemical detector tech- mental pollution steered the sampling efforts facility is the use of “heavy” methane nique for weapons diagnostics, The isotope toward investigation of airborne pollutants. as tracers to study atmospheric separator has been used as well for experi- The aircraft sampling facilities were used to circulation and mixing patterns. (“Normal” ments in basic nuclear science, including a survey and measure forest-tire clouds, indus- methane contains the common isotopes tour de involving tightly scheduled trial smokestack discharges, and regional air - 12 and hydrogen-1; in the heavy operations at Oak Ridge, Los Alamos, and quality. By microscopic examination, neu- methanes these isotopes are replaced by the the in which the fission tron-activation analysis, and other special- rare but stable isotopes carbon-1 3 and/or cross section of 6.75-day uranium-237 was ized techniques group members were able in hydrogen-2.) In a 1980 trial experiment measured at neutron energies from 43 eV to many cases to establish telltale “fingerprints” carried out in collaboration with the Na- 1.83 MeV. of individual sources by which their con- tional Oceanic and Atmospheric Administra- Semiconductor radiation detectors, silicon tributions to atmospheric pollution could be tion, heavy methanes were released at Nor- units for charged particles and soft x rays tracked far from the point of origin. man, Oklahoma, and their concentrations and -drifted germanium units for The mid ’70s brought to the nuclear and were measured in samples collected at a gamma rays, also became available to the radiochemistry group a state-of-the-art mass network of stations and by the U.S. Air group in the ‘60s. With their fifty-fold in- spectrometry facility that made possible Force. Evidence of the heavy methanes was crease in energy resolution over the then quantitative measurement of isotopes by found even at East Coast locations. Since the standard scintillation detec- direct counting of . Its development samples contained much larger concentra- tors, semiconductor detectors made it represented the convergence of two trends, tions of normal methane, state-of-the-art possible to identify and precisely assay indi- one in supply and one in demand. The mass spectrometry was essential to the experi- vidual in mixtures and thereby demand lay in the pressure to extract more ment. The mass spectrometers currently in permitted us to redirect a great deal of detailed and sophisticated diagnostic infor- use at Los Alamos can measure about one radiochemical processing and counting time mation from the debris of nuclear weapons part of methane-20 in a billion parts of to new measurements. At Los Alamos as tests. On the supply side, steady improve- normal methane and one part of methane-21 elsewhere, the new detectors also brought ment in the apparatus and techniques avail- in 200 billion parts of normal methane. about a jump in the detail and range of able to mass spectrometry was bringing it (These discrimination capabilities may be information to be obtained from basic nu- within reach as a cost-effective adjunct to the increased by a factor of a thousand with new clear research. diagnostics program. The most common techniques involving sophisticated particle- There has been another component of the diagnostic task requiring mass spectrometry identification apparatus at the Laboratory’s group’s facilities and capabilities that has is determining the isotopic compositions of vertical electrostatic accelerator.) The suc- expanded its interests to areas not commonly the uranium and plutonium remaining cess of the trial experiments has led to associated with nuclear and radiochemistry. in the debris. From such data is obtained the preparations for further experiments on a Ever since the first atmospheric tests in fraction of the fissile fuel consumed by continental scale. Nevada in 1951, explosion-cloud sampling nonfission reactions. But, as in other pur- Members of the group also, of course, had been performed by the U.S. Air Force suits, with every boon comes a burden. Mass have access to other Laboratory facilities of with manned aircraft; the actual cloud spectrometers are notoriously finicky, and great value for their programmatic and re- sampling was directed at close range by a every sample must be meticulously prepared search interests. Among these facilities, until from the group serving as a crew to free it of all other elements, real or it was shut down in 1974, was the old member in the sampling squadron. With the suspected, that come with it in the raw brought here during World War II, cessation of atmospheric testing in 1963, the debris. Still, the effort spent in setting up the which provided beams of low-energy sampling director, some of the aircraft, and, facility and getting it to work as it should has charged particles. This cyclotron was the most important, the accumulated experience begun to pay oK. The mass spectrometrists world’s first practical source of “lightweight

16 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

alpha particles,” or -3 ions, and nu- impossible to obtain directly. The cross sec- backs and advantages for radiochemical clear chemists had measured excitation func- tion for, say, the (n,p) reaction can be diagnostics. Access to the debris was tions (cross section dependence on energy) of determined indirectly, however, by creating slower—days compared to hours—and the the reactions between helium-3 and target excited states of yttrium-87 [the intermediate material of interest in the debris had to be nuclei ranging from carbon to uranium. product of the (n,p) reaction] and measuring extracted from large amounts of fused earth, Another valuable facility, the vertical elec- the fraction of these excited nuclei that decay usually containing unwanted products from trostatic accelerator/FN Tandem Van de by emission. The excited yttrium-87 of trace elements in the Graaff complex, came on line in 1964; it was nuclei are produced through the reaction earth. But for most diagnostic purposes the 86 3 2 87 the first to offer accelerated beams of tritons Sr( He, H) Y by employing helium-3 ions benefits more than compensated for the with energies up to more than 20 MeV. With from the Van de Graaff and a target of drawbacks. The debris stayed where it was these tritons nuclear researchers could study strontium-86 (an uncommon but stable iso- made, available for recovery in large nuclei by reactions such as (3H,p), (3H,a), tope available from Oak Ridge). This exam- amounts at any time and with simpler con- and (3H,ap). Also available were high-energy ple illustrates that by using well-known nu- trol of personnel radiation exposure. neutrons [from the 2H(3H,n)4He reaction] for clear physics strategies we can circumvent The experience and expertise gained in measuring excitation functions of (n,xn) re- the difficulties sometimes arrayed against us dealing with the products associated with actions at neutron energies well above 14 when we try to study a as it underground nuclear explosions contributed MeV, a regime of considerable interest to the actually occurs. to some noteworthy accomplishments and weapons program. This accelerator complex The Laboratory’s reactor complex, in new initiatives. For example, starting with 85 has served as the focus for a nuclear particular the series of high-neutron-flux in- kilograms of bastnasite ore from a rare-earth chemistry/ (the boundary line stallations (of which the mine near Mountain Pass, California, D. C. is sometimes indistinct) collaboration on re- and the Pajarito Site critical assemblies are Hoffman and her colleagues succeeded in searches of frontline interest: the fission the current representatives), has also been an isolating a plutonium fraction that, when process in a wide range of nuclei and excita- essential and versatile resource for the nu- subjected to mass-spectrometric analysis at tion, the properties of the heaviest nuclei, and clear and radiochemistry group. Nuclear the General Electric Research Laboratory, the processes occurring when complex nuclei explosive devices are essentially one-shot showed the presence of plutonium-244. are bombarded with other complex nuclei. neutron reactors specially designed to (Bastnasite is a rare-earth mineral whose In addition, the charged particles available achieve very rapid power buildup and energy formation should have concentrated also any from the Van de Graaff provide a way— release, and the great variety of radio- primordial plutonium.) Reported in 1971, often the only way—to study neutron reac- chemical procedures needed to diagnose this discovery of plutonium-244 in nature tions of isotopes important to radiochemical their performance has to be developed and had significant consequences for our ideas weapons diagnostics. As an example, con- calibrated with more conventional neutron about and the distribution of sider the isotope yttrium-86, which is among sources. elements in the solar system. In other high- the neutron-deficient yttrium isotopes Mentioned last only because of its more sensitivity measurements another staff mem- formed by (n,xn) reactions when yttrium-89, recent vintage, the Laboratory’s meson phys- ber and his postdoctoral associate, in col- the common stable isotope of yttrium, is ics facility (LAMPF) has considerably laboration with associates at Livermore and employed as a radiochemical detector in a broadened the scope of the group’s nuclear Hanford, obtained evidence for niobium-92 nuclear device. Analyses of debris samples chemistry research. We elaborate on this and -94 in natural niobium. These investiga- for the various yttrium isotopes, together topic in a later section. tions were the precursors of what is today a with the known (n,xn) cross sections, are large and far-ranging research effort in iso- expected to provide values for the flux of From Underground Testing tope . neutrons from the of to The group’s capabilities in applying radio- deuterium and tritium. But the flux de- chemical techniques to measuring very mi- termination is complicated by the fact that The Limited Test Ban Treaty of 1963, nute quantities of elements in earth samples, some of the yttrium-86 is consumed by which restricted all nuclear test explosions to coupled with the interests of group member further neutron reactions, such as (n,y), (n,p), underground, had a far-reaching effect on C. J. Orth, led recently to the discovery of and (n,np). To correct for this complication, the course of nuclear and radiochemistry at evidence in support of a controversial expla- information about the cross sections of these Los Alamos. Compared with atmospheric nation for the extinction of many plant and parasitic reactions is needed but is difficult or testing, underground testing had both draw- animal species, including the dinosaurs, that

LOS ALAMOS SCIENCE Summer 1983 17 Device

Data Analysis and Interpretation

Radiochemical Analysis Element Separation Mass Spectrometry Alpha, Beta, and Gamma Measurements

I

Radiochemical diagnostics provide essential feedback to nu- called upon to produce the necessary range of information clear weapon design. A variety of analytical techniques are about a weapon’s performance during a test explosion.

18 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

marks the boundary between the Cretaceus search was initiated on nuclear structure and physicists study nuclear reactions by measur- and Tertiary geologic periods. A Berkeley reactions (at the electrostatic accelerator ing particles emitted from a target during team headed by Luis and Walter Alvarez complex and at LAMPF), on the application bombardment.) proposed that an interruption of the food of new radioisotopes to and of An interesting example is the triplet of chain was responsible, an interruption stable isotopes to atmospheric studies, in pion-induced reactions that produce caused by a blotting out of the by debris isotope geochemistry and , carbon-11 in a natural carbon (predomi- from the impact of an extraterrestrial object. and on the migration and containment of nantly carbon- 12) target: They based this hypothesis on unusually nuclear wastes. Discussion of research in the high abundances of iridium found in marine latter area has been reserved for another sediments from Italy at a stratum coincident article in this issue of Los AIamos Science. with the Cretaceous-Tertiary boundary. Linked with the broadening range of re- (Iridium is a platinum-family element exceed- search was a sequence of leadership changes ingly scarce in the earth’s crust and some- following the appointment in 1970 of Harold what less scarce in extraterrestrial matter.) M. Agnew as the third Director of the Similar iridium “anomalies” were subse- Laboratory. Group Leader Cowan was quently discovered at other locations, but all made Leader of the new Chemistry-Nuclear Once the cross sections for these reactions were found in marine sediments and could Chemistry Division, which was created to are known at various incident pion energies, possibly be attributed to concentration by incorporate research in inorganic, physical, they become good intensity monitors for ocean processes. Orth and his radio- and nuclear chemistry. The nuclear and negative and positive pion beams. One places chemistry team members, in collaboration radiochemists under newly appointed Group a carbon-containing target in the pion beam with experts in Cretaceus stratigraphy and Leader James E. Sattizahn became CNC-11. for a few minutes, withdraws it, and plant life from the U.S. Geological Survey, measures the carbon- 11 produced; a simple selected a likely drill site in northern New Nuclear Chemistry at LAMPF calculation gives the number of pions that Mexico and succeeded in discovering, in struck the target. These reactions contain a freshwater sediment, a strong iridium con- With the start-up of the Clinton P. basic physics interest as well. Examination of centration spike that correlated exactly with Anderson Meson Physics Facility in the reactions 1 and 2 suggests that the incoming the Cretaceous-Tertiary boundary as defined early 1970s, the chemists found new pion, whether negative or positive, simply by changes in abundances of pollen species. horizons opened to them in both applied bits a single neutron in the carbon-12 Since the peak concentration of iridium in radiochemistry and basic nuclear science. nucleus and knocks it out; reaction 3 is the normal earth samples is only parts per This 800-MeV linear proton accelerator and same except that the positive pion passes its trillion, very sensitive analytical techniques its array of auxiliary equipment, built to positive charge to a neutron during the involving neutron activation and radio- produce beams of protons and pi mesons collision and a proton emerges. Now it is chemical separation were required to detect (pions) at intensities several hundredfold considered as firmly established in nuclear the anomaly. This research area is fast greater than had been available anywhere physics that, at pion kinetic energies around moving, and we expect future developments else before, brought within reach” a wide 180 MeV (the so-called 3/2,3/2 resonance), to broaden the scope of our contributions range of phenomena previously almost inac- the probabilities for the elementary reactions still further. cessible to practical measurement. One of these phenomena is the interaction of pions The Winds of Change with nuclei, and small teams of nuclear a carbon-12 nucleus is merely a collection of chemists began to study pion-induced nu- neutrons and protons, the ratio of the cross The period from the mid ’60s to the early clear reactions on targets ranging from section for reaction 1 to that for reactions 2 ’70s was a time of transition for the Labora- carbon to uranium by measuring the radio- and 3 together should be 3. (When car- tory, a transition from concentration of active products. (We note here a commonly bon-l 1 is measured, reactions 2 and 3 are effort on nuclear weapons to participation used distinction made a little less inaccurate indistinguishable.) When carefully measured also in a broad spectrum of other high- by use of the qualifier “many”: many nu- by the nuclear chemists, however, the ratio technology topics of national concern. Nu- clear chemists study nuclear reactions by came out to be not 3, but 1.6. This seemingly clear and radiochemistry entered a cor- measuring radioactive products remaining in simple discrepancy presented the theoreti- responding transition. Significant new re- a target after bombardment; many nuclear cians with an interesting problem, and they

LOS ALAMOS SCIENCE Summer 1983 19 have devised a model that solves it, The key to their solution lies in taking into account that, although reactions 1, 2. and 3 as we have written them may represent what hap- pens at the site of a collision inside a nucleus, the emerging neutron (proton) is likely to hit and trade places with a proton (neutron) while on its way out of the nucleus and produce another mass-11 nuclide, - 11 (-1 1). Thus, the carbon-1 1 nuclei that the chemist sees may be fewer than the total that were initially produced in the collision. They represent the cases where “nothing went wrong. ” Another research interest of the nuclear chemists at LAMPF concerns the “exotic” nuclei produced when 800-MeV protons strike nuclei, especially heavy ones. This interaction yields a wide assortment of parti- cles and nuclear fragments ranging from ativistic effects is small); and measuring its the one described above. At the upper end of pions and neutrons to nuclei only a little rate of energy loss in the detector assembly the mass scale they found evidence for lighter than the target. Among these frag- gives its nuclear charge. Thus, by electronic - I 1, -12 (both already known), and ments are nuclei, many of them still uniden- means one establishes the identity of the -14, but not for beryllium-13, -15, or -16. tified, that represent the extremes of what isotope without chemically separating it or Thus, it appears that the neutron drip line can exist at all. Some of them have a great measuring its decay. In a series of experi- occurs beyond beryllium-14 and that beryl- excess of neutrons, so great that their last ments using thin targets of uranium and lium-16 is probably neutron-unbound. On few neutrons are bare!y held; these are , the nuclear chemists have identified the other hand, beryllium- 11 is probably the described as almost neutron-unbound or five new examples of neutron-rich exotic heaviest odd-mass beryllium isotope that is near the “neutron drip line.” There exist isotopes: -27, -3 1, mag- neutron-bound. The difference between the other nuclei at the opposite extreme: severely nesium-32, aluminum-34, and phos- odd- and even-mass isotopes is not surpris- deficient in neutrons and rich in protons, that phorus-39. (The heaviest stable isotopes of ing to nuclear scientists: it reflects the well- is, almost proton-unbound, or near the these elements are neon-22, magnesium-26, known preference of nuclear particles to “proton drip line.” These exotic nuclei are aluminum-27, and phosphorus-3 1.) Others occur in pairs. hard to find, Not only are they produced are currently under study. So far we have talked in terms of merely very rarely in nuclear reactions, but they Present nuclear theory cannot predict ac- establishing the existence of exotic nuclei. have half-lives so short (seconds or less) that curately the boundaries of nuclear existence. The data would be much more valuable if they cannot be isolated and identified by and there is now evidence that even the they told in addition how stable such nuclei conventional radiochemical techniques. A exotic nuclei named above may be short of are. The Los Alamos team is now at work on team of nuclear chemists devised a method the neutron drip line, To illustrate this point, a new apparatus that should provide this to identify some of these exotic products in we pick an example from low-atomic- num- kind of information. It will still measure time flight, that is, as they emerge from the ber nuclei measured elsewhere. The element of flight, but with sufficient precision to give proton/heavy nucleus reaction. Measuring a beryllium, with an of 4, has a a spectroscopically useful isotope mass, product's time of flight from the production single stable isotope. beryllium-9. The ques- which is a direct measure of nuclear stability. site to a detector gives its velocity: measur- tion has been asked, “What is the heaviest When operational, the instrument should ing its as it is stopped in the possible isotope of beryllium?” A team at the yield masses on at least thirty exotic nuclei detector then gives its mass from the ex- University of California Bevatron made a with accuracies of 200 keV or better, a figure search, employing an experiment similar to that should lead to a new kind of thinking

20 Summer [983 LOS ALAMOS SCIENCE Tracking the Isotopes

about these nuclei. that got under way in the mid ‘70s, nuclear same charge orientation. After the proton beam at LAMPF has chemists have studied the intensity effects on passed through the last of the particle- a variety of target materials chosen to elicit Fission Comes to the Group generating targets, it is considered to have chemical structure factors. Along the way done its work. Attenuated to between 50 and they have turned up a number of curious The reader who has followed our account 75 percent of its starting intensity and too results, some of which are beginning to make this far is likely to have acquired the im- diffuse for normal use, it is refocused slightly sense in terms of the . For pression that the nuclear and radiochemistry and sent to the “beam dump.” But even in its example, the relative probability of capture group was becoming rather large and di- death throes it can produce large amounts of of a negative muon by the two elements in a verse, with a range of programs and interests radioactivity in the materials in its path. binary compound had been expected to vary beyond that usually associated with a single Inserting targets of their own into the beam- approximately as the atomic numbers of the discipline. As seen through the eyes of the dump zone by means of a remotely con- two elements weighted by their stoichio- participants this transformation has been trolled facility specially engineered for the metric abundances; thus, in boron nitride undramatic but noticeable in many ways. It purpose, radiochemists obtain a variety of (BN) the ratio of the capture probabilities has reflected changes both within the com- radioisotopes that they isolate and prepare c(B) and c (N) would be 5/7, or 0.714. munity of members—in interests and for experiments in . A Actual measurement, however, gave a ratio capabilities—and outside—in the spectrum separate radiochemistry group has been built of national programmatic needs and the up to explore and develop this family of have captured about 2.8 times its “fair Laboratory’s response to them. The trans- applications. share. ” Although no realistic theoretical formation, like most transformations on the Another offering on the LAMPF menu, model has yet been developed to calculate science and technology scene, had an impor- negative muons, has provided the chemists this ratio, some European workers have tant organizational facet: the question of with a new probe of matter whose utility and come up with an assumption and a recipe how people and resources were to be or- applications they are still exploring. (A that do reasonably well for BN and other ganized and allocated for the strongest con- negative muon, loosely speaking, is a semi- molecules composed of low-atomic-weight tribution to the Laboratory’s goals. Consid- stable, heavy version of an . These elements. The heart of the assumption is that eration of the overall nuclear and radio- particles are available at LAMPF from the muons are captured by an in propor- chemistry effort from this standpoint re- decay of negative pions.) Experimentally, tion to the number of loosely bound elec- ceived a great deal of attention and resulted, one places a specimen of interest in a beam trons around the atom, a quantity that can in late 1980, in the first major structural of negative muons and measures the in- be computed in a simple way from the change since the group was formed in 1947. tensities of the muonic x rays emitted as the ionicity of the B-N bond. We conclude with The nuclear and radiochemistry group, muons captured by individual atoms de- a case in which this concept has provided the which had reached a membership of 100, excite through their levels in the atoms. Each first experimental evidence for the direction was split into three groups, the first under element is identifiable by its set of muonic x of the dipole in the diatomic nitric Group Leader H. A. O’Brien and centered rays, just as it is by its set of ordinary elec- oxide (NO). Our muon team measured the on research in medical radioisotopes, the tronic x rays. The aspect of this pheno- capture ratio c (N)/c (0) for two gas targets, second under Group Leader E. A. Bryant menology that attracts the attention of one consisting of 5 atmospheres of nitrogen and centered on isotope geochemistry, and the chemists is one first observed over two (NJ and 5 atmospheres of (O2) and the third under Group Leader J. E. Sattizahn decades ago: the muonic x-ray intensity the other consisting of 10 atmospheres of and centered on weapons diagnostics and patterns of the elements in a compound and NO. The capture ratios came out to be 0.834 other topics in nuclear and radiochemistry. the relative number of muons captured by and 0.959, respectively, and the ratio of the In practice and as intended, the boundary the individual elements in a compound vary ratios was 0.870; that is, the nitrogen in the lines are fuzzy. The groups continue to share with the compound’s structure and state. NO captured a small excess of muons. By the same building complex and facilities. Thus, if one” could learn something about the our assumption, then, the loose are Most important, they continue to enjoy the nature of this variation, one might use it as a slightly displaced toward the nitrogen end of contacts and cross-fertilization of ideas that tool to investigate chemical structure. the molecule. A detailed theoretical calcula- have contributed so significantly to past Beginning with an experimental program tion of the molecular structure has led to the developments. ■

LOS ALAMOS SCIENCE Summer 1983 21 Further Reading

David Hawkins (Part I) and Edith C. Truslow and Ralph Carlisle Smith (Part II), : The Los Alamos Project (Tomash Publishers, Los Angeles, 1983).

Gerhart Friedlander, Joseph W. Kennedy, Edward S. Macias, and Julian Malcom Miller, Nuclear and Radiochemistry, Third Edition (John Wiley & Sons, New York, 198 1).

A. Ghiorso, S. G. Thompson, G. H. Higgins, G. T, Seaborg, M. H. Studier, P. R. Fields, S. M. Fried, H. Diamond, J. F. Mech, G. L. Pyle, J. R. Huizenga, A. Hirsch, W. M. Manning, C. I. Browne, H. L. Smith, and R. W. Spence, “New Elements Einsteinium and Fermium, Atomic Numbers 99 and 100,” Physical Review 99, 1048-1049 (1955).

R. W. Spence, “Nuclear Rockets,” International Science and Technology, 58-65 (July 1965).

Edward Teller, Wilson K. Talley, Gary H. Higgins, and Gerald W. Johnson, “Scientific Applications of Nuclear Explosions,“ in The Constructive Uses of Nuclear (McGraw-Hill Book Company, New York, 1968).

G. A. Cowan, B. P. Bayhurst, R. J. Prestwood, J. S. Gilmore, and G. W. Knobeloch, “Symmetry of Neutron-Induced U Fission at Individual Resonances. III,” Physical Review C 2, 615-620 (1970).

Glenn T. Seaborg, editor, Transuranium Elements: Products of Modern (Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania, 1978).

Glenn T. Seaborg, “The New Elements,” American Scientist 68, 279-289 (1980).

S. G. Thompson and C. F. Tsang, “Superheavy Elements,” Science 178, 1047-1055 (1972).

D. C. Hoffman, F. O. Lawrence, J. L. Mewherter, and F. M. Rourke, “Detection of Plutonium-244 in Nature,” Nature 234, 132-134 (November 19, 1971).

B. J. Dropesky, G. W. Butler, C. J. Orth, R. A. Williams, M. A. Yates-Williams, G. Friedlander, and S. B. Kaufman, “Absolute Cross Sections for the Physical Review C 20, 1844-1856 (1979).

Morton M. Sternheim and Richard R. Silbar, Puzzle,” Physical Review Letters 34, 824-827 (1975).

J. D. Knight, C. J. Orth, M. E. Schillaci, R. A. Naumann, H. Daniel, K. Springer, and H. B. Knowles, “Chemical Effects in Negative Muon Capture in Some Ionic and Covalent Solids and Ionic Aqueous Solutions,” Physical Review A 13, 43-53 (1976).

Charles J. Orth, James S. Gilmore, Jere D. Knight, Charles L. Pillmore, Robert H. Tschudy, and James E. Fassett, “An Iridium Abundance Anomaly at the Palynological Cretaceous-Tertiary Boundary in Northern New Mexico,” Science 214, 1341-1343 (1981).

22 Summer 1983 LOS ALAMOS SCIENCE Tracking the Isotopes

AUTHORS

Jere D. Knight joined the Manhattan Project at the University of Chicago in 1942 after receiving a Bachelor of Science in chemistry from St. John’s University (at Collegeville, Minnesota) and completing two years of graduate work at the University of Minnesota. In 1943 he transferred with the nuclear chemistry group under C. D. Coryell to Oak Ridge, where he worked on fission products, reactor neutron activations, and the first macro production of tritium. Following a return to the University of Minnesota and a Ph.D. in in 1948, he spent the next year and a half as an instructor in chemistry and research associate in physics at the University of Illinois. In 1949 he joined the nuclear and radiochemistry group at Los Alamos, where he has been involved in many of the developments described in this article. He was appointed a Laboratory Fellow in 1981. Since his retirement in 1982 he has served as a consultant to his former group.

James E. Sattizahn received his B.S. in chemistry from Lawrence College in Appleton, Wisconsin in 1942 and joined the Manhattan Project at Oak Ridge in March 1944. (He and the co-author of this article worked only 100 yards apart at Oak Ridge, but they never met until both had moved to Los Alamos.) The Bikini tests brought Sattizahn to Los Alamos on June 5, 1946 as part of the team assembled to carry out radiochemical diagnostics. He was an original member of the Laboratory’s permanent nuclear and radiochemistry group formed in early 1947 and has served as its Leader since January 1, 1971. He received a Ph.D. in radiochemistry from the University of New Mexico in 1957 under the Laboratory’s graduate study program.

LOS ALAMOS SCIENCE Summer 1983 23