Remote chemical analysis during the Apollo 15 mission

Remote Chemical Analysis During the Apollo 15 Mission ISIDORE ADLER and JACOB 1. TROMBKA Goddard Space Flight Center Greenbelt, Md. 20771

PAUL GORENSTEIN American Science and Engineering Cambridge, Mass.

X-ray f Iuorescence, gamma - ray, and a Ipha- part icIe ex per iments from the exciting Apollo 15 mission yielded large amounts of data permitting interesting preliminary analysis

HE SUCCESS of the Apollo proTgraii~ for lunar exploration is by now a matter of record. A number of us have experienced the great scientific excitement surrounding the return of the lunar samples and their examination. I t is an extraordinary fact that there is now well over 300 lb of lunar material from such assorted sites as Mare Tranquillitatis, Fra Nauro, Oceanus Procellarum, and the Hadley RilleXpeiinines region. The recent Apollo 15 mission has proved in many ways to be the most exciting of all, particularly in a scientific sense. The Lunar Module ( L W touched down betn-een the Hadley Rille, a canyon nearly a mile across and about 1000 ft deep, and the base of the Apennine mountains which rise to a height of almost three miles. The astronauts were able to rove about in a wheeled vehicle (Lunar Rover), permitting them t o explore much larger areas of the landing site and to bring back an astonishing load of over 165 lb of interesting rocks, soil, and cores for analysis and study. Particularly significant was the use, for the first time, of the Command-Service Module (CSM) for

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carrying a large array of instruments to survey a number of lunar characteristics. Sector 1 of the CSM, used heretofore to carry a third oxygen tank, was packed full of scientific instruments to map the moon from orbit simultaneously iyitli the various surface activities of the Lunar Module crew. Figure 1 shows the Scientific Instrument Module ( S I N ) as seen by Scott and Irwin from the L l I . Carried in the S I N bay were eight experiments: an X-ray fluorescence spectrometer, a gamma-ray spectrometer, an alpha-particle spectrometer. a panoramic camera, a mapping camera, a laser altimeter, a mass spectrometer, and a subsatellite (carrying three geophysical experiments) which was injected into lunar orbit. Of the above experiments the X-ray spectrometer, the gammaray spectrometer, and the alphaparticle spectrometer were used to obtain information about a large portion of the moon’s surface while the mass spectrometer was employed in studying the moon’s tenuous atmosphere a t orbital heights of about 70 miles. I t is noteworthy that the X-ray and gamma-ray instruments, once through with their lunar chores,

ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972

were used during the return to earth while in trans-earth coast to perform astronomy studies as well. The X-ray, gamma-ray, and alpha-particle measurements were part of an integrated geochemistry experiment. While each experiment was designed to measure different chemical elements, the information obtained was expected to be complimentary in giving a more comprehensive answer about the moon’s chemistry. The X-ray experiment was designed to yield data about magnesium, aluminum, and silicon; the gamma-ray device to provide information about potassium, uranium, and thorium;‘ and the alpha experiment was developed to detect the radioactive decay products, radon and thoron. All of the above elements are considered important in understanding the moon’s early history. Questions About the Moon

Despite our study of the lunar samples and the moon’s geology, the origin of the moon is still obscure. The various theories, such as independent origin and subsequent capture or fission from the earth, all have their proponents. However,

REPORT FOR ANALYTICAL CHEMISTS

some variants of the fission theory are gaining fairly wide acceptance, particularly the viewpoint of Ringwood ( 1 ) of secondary accretion from a circumterrestrial sediment ring. One can also list a number of more detailed questions which still need to he solved, such a s the nature and origin of the highlands, the nature of the moon’s interior, the difference between the lunar hidden side and the side we see, and the reasons for the moon’s unusual chemistry as exemplified by the depletion of europium and such volatiles as K, Na, and TI. Our present knowledge of the moon’s chemistry comes from the samples collected a t a few chosen areas, and we have already made a number of important strides in our understanding of the moon. NASA’s Lowman (2) has pointed out, “The analysis of the returned lunar samples has begun to fill two major gaps in our knowledge: the absolute ages of the lunar geologic time scale and a definite knowledge of the composition and origin of the main lunar rock types. It is now possible to combine these early analytical results with data from earth-based studies to produce a surprisingly specific outline of the moon’s geologic evolution.” There are, however, many good reasons for desiring chemical information from other lunar sites (many of which will he inaccessible to manned landings for years to come). Certainly, a global compositional map would he of inestimable value in helping us to understand the moon. With these considerations in mind, a combined geochemical experiment was proposed as a collaborative effort involving scientists from several institutions: Goddard Space Flight Center; the University of California, San Diego; J e t Propulsion Laboratory; Figure 1. Command-Service Module in orbit. Scientific Instrument Module carrying various orbital science instruments is clearly seen

and American Science and Engineering. These experiments were implemented and flown during the Apollo 15 flight and are also scheduled for the Apollo 16 mission. They were carried in the Service Module of the CSM and employed during the orbital part of the mission. Orbital Remote Sensing

The prohlems in performing compositional analyses from an orbiting vehicle are complex. There are only a few observational phenomena able to yield unique solutions of elemental identification and concentration. An examination of electromagnetic and particulate spcctra shows only a limited number of possibilities. Furthermore, i t is apparent t h a t none yields precise results by laboratory standards. Nevertheless, such experiments (e.g., gamma-ray, or X-ray spectroscopy) can provide much nsefnl geochcmical information. The earliest American attempts a t gamma-ray spectroscopy of the moon were made in connection with the Ranger 3,4, and 5 fly-by flights. Gamma-ray experiments were also carried aboard the Russian Luna 10 and 11 which were placed in orbit around the moon. The first simplified approach t o measuring fluorescent X-rays from the lunar surface was made on the

orbiting Luna 12 by Mandel’shtam e t al. (3). Several small geiger counters having a hand-pass between 8-12 A were flown. These alternately looked at the lunar surface and out into space for comparison. Although little compositional data were obtained, there were positive indications t h a t the sun does, in fact, produce measurable fluorescent X-rays from the lunar surface. Radiation Environment at Lunar Suriace

Figure 2 summarizes our view of the radiation environment at the lunar surface. Both natural and induced sources of radiation are present. Among the principal radioactive constituents are the longlived nuclides @K, 238U,uzTh, and their decay products. These sources emit alpha, beta, and gamma radiation of various energies. Special processes such as radon diffusion (4) may also occur. Bombardment of the lunar surface by galactic cosmic rays and energetic solar protons can be expected t o produce a variety of shorter lived nuclides and, as a result, induced radioactivity. Cosmic-ray interactions with the lunar surface will also yield a prompt emission of charged particles, neutrons, and photons. Solar X-rays absorbed on t h e lunar surface will produce fluorescence X-rays characteristic of some

Report for Analytical Chemists

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of the elements in the lunar surface. The relative yields of these X-rays will depend on the intensity and shape of the solar X-ray flux, as well as on the relative abundance of the elements. Gamma Ray Experiment

Even before the hpollo program of lunar exploration, it was assumed 30A

t h a t K , U, and T h might be key elements to a n understanding of lunar processes. I n terrestrial mechanisms the decay of these elements is considered to be the source of the energy leading t o magmatic differentiations; and, in turn, the concentration of these elements becomes a measure of the extent of differentiation (e.g., K, C, and T h


tend to concentrate in the late stage crystalline rocks such as granite). Until the Apollo 15 mission, there were good determinations of K, U, and T h from the three Apollo landing sites from the various types of rocks found a t those sites. A brief summary is given in Figure 3 where potassium vs. uranium values have been plotted for various lunar materials from the three -4pollo missions. For comparison purposes, a line is drawn for terrestrial materials ranging from basalts to granite, and an achondrite field is shown. The diagonals are K/U ratios. It is obvious that the lunar materials have loiver K/U ratios than the terrestrial magmatic rocks because of the higher U and lower K values. A similar situation (not shown) also exists for the K / T h ratios. Furthermore, there is a distinct variation from one site t o the next. Because K, U,and T h are such important indicators of geochemical processes, an attempt to map these elements globally around the moon is obviou~lyof great importance. I n addition to natural gamma rays, gamma rays are also produced by cosmic-ray interactions. Strongly interacting particles constantly bombard the lunar surface. The greatest proportions of these are solar protons, although there is a galactic component from outside the solar system. The situation for the moon is unique, as compared to the earth's, because of its lack of atmosphere and extremely low magnetic field. The charged particles are not deflected a m y ; thus, strong interactions with the particles occur a t or near the lunar surface. These interactions are varied and complex and include meson production, "knock-on" phenomena, and evaporation mechanisms. Inelastic processes produce excited nucleii which then emit gamma rays, charged particles, and neutrons. These reactions are expected t o produce a spectral distribution containing a large number of lines characteristic of the elements in thc lunar surface. Figure 4 displays a calculated gamma-ray spectrum for a possible lunar anorthosite ( 5 ) . This spectrum i. the result of stochastic calculations based on nssumptions of


comic-ray fluxes, lunar composition, aiid matrix effects. I t is of 1)articulnr significance t h a t these calculations are tiased on the use of a 23/4-in. by 21/4-in. KaI gammar a y detector identical to the gamma-ray detector flown on the Apollo 15 miL:' c-1011. The actual gainnia-ray sensing : i w m b l y flon.n is composed of three niaj or sii1)assmblies : the garnmaray electroiiics, the gamma-ray zcintillation detector, and the therilia1 ohield (Figure 5 ) . K h e n the experiment is used in gathering (lata, It is deployed on a boom t o a distance of 25 ft from the SIN t o remove the detector as far as possilile from both the natural occurring radiation and the activity induced in the spacecraft by the cosmic-ray flux. Becauhe it is important t o know the extent of the vehicle backgrouiid, the boom was dcployed during the trans-earth coast a t interiiietiiate dihtaiices of 15 aiid 8 ft. Background measurements were iiiadc with the experiment stowed, as well as a t the 8- and 15-ft dist a iices . The detector (Figure 5) consists of a right-cylindrical NaI (thallium-activated) crystal, Z 3 / 4 in. by 2 V 4 in. The crystal has a thin mantle of a scintillating plastic cryotal. The niantle scintillator is optically isolated from the priinary KaI cryhtal, aiid hot11 detectors are uied in anticoincidence. The priiiiary detector h:id an approximately 8.5% resolution for the 0 661 M e V lines of laiCs. The plastic scintillator was used to eliminate the cffects of charged-particle coaniic-ray flux within the field of view of the detector. The gamma-ray electronics consisted of a 512-channel analog-todigital converter built around a 2-MHz o d l a t o r . I n the flight configuration, data were transmitted channel by channel in real time during direct earth contact and wlicn out of line of sight were ctored on tape for bubsequent telemetry.

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tral shape as well. I t is thus evident that changes in solar flux will not only produce a change in the overall emission of fluorescent X-rays but also in the relative number of photons of the various elements. T o keep track of the variable solar X-ray intensity and spectral composition during the mission, the X-ray experiment employed a small proportional counter as a solar monitor. I n addition, it was possible to obtain detailed simultaneous measurements of the solar X-ray spectrum from various OS0 (Orbiting Solar Observatories) satellites and Solrad satellites that were in operation at the time. The long-term trend of average solar intensity in the 1-8 B region, as seen in the vicinity of the earth,

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lite observations of solar X-ray emission indicated that measurable lunar X-ray fluorescence could be expected from a number of the more abundant light elements, up to at least Z = 14. Further, during the brief periods of more intense solar activity associated with solar flares, radiation from elements of higher atomic number was expected. Although the sun does provide sufficient intensity for the production of characteristic X-rays from the lunar surface, there are a number of factors which must be considered in detail t o obtain useful quantitative interpretations from the observations. These are unique problems not normally encountered in the practice of X-ray spectroscopy in the laboratory. The solar X-ray intensity is known to exhibit both short-term fluctuations on a time scale of minutes-to-hours and systematic changes associated with the 11-year solar cycle. It has been observationally established through the use of low-resolution instruments that solar spectral energy decreases with increasing energy a t such a rate that, if a thermal mechanism is assumed, thermal temperatures are calculated in the range - l o 6 to 107'K. Recent observations have also shown the presence of discrete X-ray lines of highly ionized Mg, Si, and Fe. Variations in solar flux occur not only in overall intensity but in spec32A

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is shorn in Figure 6. A curve of a smoothed sunspot number appears on the same axis. There is a distinct correlation between both, so the extrapolated smoothed sunspot number can be taken as a n indication of expected solar intensity for the next two years. It appears t h a t the peak X-ray emission occurred early in 19G3. The intensity is expected to dwline to about half t h a t value by January 1972 and reach a minimum early in 1976. Calculations of expected chara cteristic X-ray yields have been made by Gorenstein et al. (6) and Eller (7') bz sed on an assumed temperature of 4 x 10BoKfor the 1-8 A region and a solar free-free continuum. Examples of the results are shown in Figure 7 for the case of average lunar soils collected during the Apollo 12 mission. These are the intensities t h a t should appear in X-ray detectors of the type being flown. Because of the low value of the lunar X-ray brightness, high-resolution devices such as crystal spectrometers cannot be used t o isolate

the characteristic lines for measurement. T o obtain adequate counting rates and satisfactory statistics, it was necessary to employ large-area proportional counters. Further, because of the low X-ray energies, the w i n d o n were quite thin and highly transmitting. The X-ray fluoresceiice sensing assembly consists of three gas-filled (p-10) proportional counters, mechanical collimators, calibration sources for in-flight calibration, a temperature monitor, and associated electronics. The individual detectors are each approximately 30 cni2 in area and have 0.0015in. beryllium windows. Two methods for energy resolution were employed. The output of the detectors was energy-analyzed by seven discriminator channels covering in equal intervals either 0.5 to 2.75 keT- in a high-gain mode or I t o 5.5 keV in the low-gain mode. As a n additional method of energy discrimination, one detector was operated bare; two had X-ray filters. A magnesium-foil filter covered one; the other had an alu-

minum filter. The magnesium filter preferentially filters the aluminum and silicon radiation ; the aluminum filter is most selective for the silicon radiation. Figure 8 shows the X-ray assembly mounted in the same enclosure that houses the A l pha Particle Spectroineter Experiment (to be described later). The nature of the detector configuration provided a GO" field of view. At the projected niissioii altitude of 60 nautical miles, this permits an instantaneous vien- of a n area approximately GO nautical miles on edge.

The X-ray processing electronics assenibly is unique in t h a t it not only discriminated among pulses of different amplitudes but among pulse shapes as nell. Thus, the processing of the X-ray pukes from the detectors rejects non-X-ray events that produce pulses of a different shape. This mode of operation gi ves c o 11 side r a h le improvement in signal-to-noise ratios under circumstances where gamma and cosmic rays 1)roduce unwanted hackground. The output of each detec-


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tor fed an eight-channel energy analyzer. The counts were stored in registers for 8-sec intervals, and then the outputs transferred as binary coded inforination to telemetry. Backside data were stored on tape for subsequent transmission. Alpha Particle Emission Experiment

There are many possible sources for the alpha particles emanating from the lunar surface. These are: the alpha-radioactive decay of radon and thoron (and their daughter products) which have diffused out of the first few meters of lunar soil; the alpha radioactivity caused by the interaction of galactic cosmic rays with lunar surface materials; and the evaporation of protons and alpha particles during solar flares as a result of the interaction of flare-associated energetic solar protons and alphas with the nucleii of lunar surface elements. The most interesting phenomenon to investigate is the diffusion of radon and thoron. Kraner et al. ( 4 ) have proposed a mechanism for the diffusion of radon and thoron through the upper surface layer of the moon and the subsequent “painting” of the surface by the radioactive radon and thoron decay products. Some portion of the radon and thoron produced by radioactive decay escapes from the host minerals into interstitial voids in the soil and then diffuses through these voids to the surface. The rate of diffusion is related to the diffusion coefficients and soil porosity. The extent of such a process on the moon can be estimated by making assumptions about composition and porosity; it is also necessary, however. t o consider other phenomena such as surface adsorption. 34A

If thermal velocities are assumed for the emerging radon and thoron, then nearlv all the molecules will be trapped in the moon’s gravitational field. Radon n-ith its 3.8-day halflife may travel a considerable distance before undergoing decay. Thoron, on the other hand, with a 55-sec half-life, would be expected to decay near its source. Both these species would yield daughter products giving characteristic spec-

tral lines. Thus, the detection of this phenomenon could be an important indicator of active regions (such as vulcanism) of emanation from the lunar surface. The data from this experiment can also have considerable influence on the interpretations of the gamma-ray results. The necessary equipment for alpha-particle monitoring (Figure 9 1 is basically simple. The detectors are solid state of the surfacebarrier type. The flight instrument (Figure 9) consists of an alpha-detector assembly and alpha processing electronics. The alpha-detector assembly contains ten surface-barrier sensors which convert incident alpha particles into electrical pulses suitable for processing by the electronics assembly. All the sensors accept particle3 of the same energy range. The electronics processing assembly accepts the output of the detertors. analyzes the first acceptable pulse in each 100-msec telem-

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etry interval, and produccs an eight-hit hinary eodc proportional to the pulsc amplitudc on eight parallel telcmctry Inputs. An analog output idcntifies the sensor in which thc conversion occurs. A time-to-height converter indicatcs thc time elapscd bctwcen the s t a r t of the 100-mscc telemetry interval and thc arrival of thc first acccptable pulse. Scientific Instrument Module

Figurc 10 shows the S I M and its relationship to the Command-Service Module of the Apollo vehicle. M a n y of the functions involved close astronaut participation such a s boom deployment and the closing of protective covers during maneuvers involving the firing of the control jets. The most spcctacular duty of the command module pilot was the recovery of the camera by performing an EVA (extra-vchicular activity) just prior to the return from the moon.

a most exciting scientific adventure which should have great impact on future planctary exploration. References (1) A . E. Ringwood, Earth Planet. Sci. Lett., 8, 13140 (1970). (2) P. D. Lowman, Jr., “The Geologic Evolrition of the Moon,” Goddard Sone? Fheht, Center Document X-64476-31. G r h b c l t , Md.. 1970. (3) S. L. Mandel’shtam, I. P. Tindo, G . S. Chrrmemukhen, L. S . Sorokm, and A . B. Dmitriev. “Lunar X-Rmn and the Cosmic X-Rav Bnckerouid Mrasiirrd hy the L i m u Satrllitc; 1,una 12,” CJDC 523: 36:629. 192. 32 [trans. from Komieheski Issledovaniyiya, 6, 119 ,(19fi8)1. . ~.~

€1. W. Kranrr, G. L. Sehroeder, G. Dnvidson, and J. W. Carpcntcr, Seience, 152, 1235 (1W6). ( 5 ) R . Ready, J. Arnold, University of California, San Diego, Calii., private communication, 1970. ( 6 ) 1’. Gormstcin, H. Gursky, I. Adler, and J. I. Trombka, Adoon. X-Ray Anal., 13,33041 (1970). (7) E. Eller, Goddard Space Flight Centrr, Grccnbelt, Md., pnvate eommunication, 1971. (4)

Summary

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At this point in time one can report only preliminary results for the orbital measurements. We do know that, except for minor difficulties, the experiments performed exceptionally well, yielding a wealth of data t h a t will require a long time for complete analysis. For the first time in man’s history, we now have large amounts of compositional data about the moon’s hidden side. The X-ray fluorescence experiment is enabling us to compare large areas of the moon’s f a r side with the side wc see. It has already told us t h a t AI/% ratios are markedly differcnt between the highlands and thc mare areas, a fact of considcrable geologic importance in understanding thc origin of the highlands. Preliminary analysis of the gamma-ray data discloses t h a t the moon’s far side is not too diffcrent from the near side in terms of the overall concentrations of the natural radioactive elements. More detailed analysis is under way to look for more localized variations. The initial examination of thc alpha-particle data has failcd to disclosc any areas of enhanced radon emission, hut herc again onc must wait for more detailed studics. Taken as a whole. this has been

Jacob I. Trombka. an aerospace

technologist, has been at Goddard since 1966. He is project scientist and co-investigator on the Apollo X-ray spectrometer ezperiment and co-investigator on gamma-ray and alpha-particle spectrometer ezperiments. H E is also developing a neutron-gamma and X-ray fluorescence elemental analysis system. Dr. Trombka earned his PhD degree in physics from the University of Michigan in 1961. He has been a scientist at O R N L and Jet Propulsion and has been associated with program management at NASA headquarters. He is presently an adjunct professor of law at Georgetown University.

lsidore Adler, senior scientist at

Goddard Space Flight Center, joined Goddard in 1964 after working with the Geohgical Survey from 1952 as project leader in X-ray spectroscopy. H E received his PhD in physical chemistry from Brookl?jn Polytechnic Institute in 1952. I n his undergraduate work at New York City universities, he specialized in chemistry, meteorology, and physical chemistry. Dr. Adler is a member of Sigma X i , American Geophysical Union, and SAS. He was formerly associate editor for Applied Spcctroscopy and is now associate editor for Chemical Geology. A t present he is principal investigator on the X-ray fluorescence experiment on Apollo 15 and I6 as well as co-investigator with Dr. Gorenstein on the alpha-particle experiment.

Paul Gorenstein is a senior stafl scientist at American Science and Engineering in Cambridge, Mass. He earned a RS in engineering physics from Cornell University in 1957 and a PhD Cn physics from M I T in 1962. Following this, Dr. Gorenstein spent two years in Italy as a Fulbright postdoctorate Fellow. Since joining AS&E in 1965, he has been associated with a number of rocket and satellite programs of cosmic X-ray astronomy and the geochemistry experiments of Apollo 15 and 16. He is principal investigator of the alpha-particle experiment and a co-investigator with Dr. Adler of the X-ray fluorescence experiment.


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Remote chemical analysis during the Apollo 15 mission

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