Vol.5, No.2, pp.lli—12O, 1983 0273—1177/85 $0.00 + .30Printed in Great Britain. All rights reserved. Copyright ~ COSPAR

METHODSOF REMOTESURFACECHEMICAL ANALYSIS FORASTEROIDMISSIONS

R. Z. Sagdeev,*G. G. Managadze.*1. Yu. Shutyaev,*K. SzegO** and P. P. Timofeev** Institutefor SpaceResearch,Moscow, U.S.S.R.**Cenlral ResearchInstitutefor Physics,H-1525 Budapest,P.O.B. 49. Hun gary

ABSTRACT

In this paper different active remote sensing methods are discussed which canbe applied to investigate the composition of’ minor solar bodies. Methods us-ing ion, laser and electron beams are treated in detail.

INTRODUCTION

The most common method of the investigation of mass and isotope compositionof the soil on the atmosphered planets of the solar system is to land a cos-mic probe on the planet’s surface, take a soil sample and analyse it onboard. This method has been used by the “Viking” space station for the studyof Mars /1/. For smaller planetary bodies that have no atmosphere, like theMoon, the composition of the soil is determined by detecting the X—rayemission of the surface induced by cosmic radiation. This case also involvedlanding on the planet’s surface /2/.

In the study of the mass and isotopic composition of the small planetoids ofthe solar system, e.g. those of the Asteroid Belt, the remote probing methodhas significant advantages over the above techniques. This method allows forthe investigation of the soil without previous contamination of the targetarea from the engines of the landing module. It is also possible to performa repeated study of the same object at various spots, including those whichare not easily accessible, as well as to move from one asteroid to anotherwith little energy consumption.

The active remote methods usually require the presence of some radiationsource (ion, electron or laser beam) on board of the cosmic probe that inter-acts with the surface. Laser methods differ considerably from those involvingelectromagnetic radiation in the range of radio frequencies.

When the cosmic probe irradiates the surface of the asteroid by an ion beam;the current of secondary (i.e. sputtered) ions detected by an on board devicecarry information about the mass composition of the asteroid surface. Inlaboratory practice this method is widely used for the analysis of solidsamples /3/ and has been adjusted to space conditions.

In space conditions the method of secondary ion mass spectroscopy (SISISmakes possible the remote determination of the mass and isotope compositionof the surface layer of the soil in a depth of max. 10 A. The characteristicsize of the target area is rather large in this case; for a flight height of50—lOOm— few m. The method requires a relatively long exposition (1—10 see’and charge neutralization of the cosmic probe during ion emission.

A very promising original idea is to use solar wind in the cosmic SIMS methodas primary ion beam. This technique will be discussed later on.

The remote laser method of the investigation of the mass and isotope composi-tion of planets /4/ is based on the time of flight (TOF) principle. Thefocused laser radiation produces vaporization and ionization of the soil. Theshort (i -10 ns ) laser pulse serves as a zero signal for time counting aswell. The ions accelerated by the laser plasma are detected after reflection

ill

112 R.Z. Sagdeev at al.

by a reflector field which ensures space and time focusing of the ionbursts. This technique also has its laboratory analogue, where ions producedwithout previous acceleration in laser sparks are analysed by a reflectordevice /6/. The absence of accelerating device distinguishes this methodfrom the well known LAMMA system /7/ where the ions of the substance underconsideration are accelerated to sizeable energies.

Of the methods discussed above the most perfect and suitable one is thelaser technique. It provides a high mass resolution and sensitivity. How-ever, contrary to other methods, the laser technique needs a precise deter-mination of the distance between probe and planet. At present such measure-ments are performed by semiconductor laser telemetry.

A remote sensing method based on the bombardment of the surface of thecosmic body by a beam of energetic electrons and the determination of itscomposition by detecting the characteristic X—rays was proposed in ref. /8/.This method is able to perform an analysis at remarkable distances (>10 km),though it requires neutralization of the charge on the space device and hasa limited mass resolution. It is an analogue of the method of X—ray spec—t ro 5 Copy.

For the study of thin surface layers of small planetoids and asteroids onemay use the so—called low energy secondary electron spectroscopic methods/9/. These methods use a low energy (>2000 eV) electron source and the masscomposition of the object under study can be roughly estimated on the basisof the secondary electrons (including also Auger electrons).

The general drawbacks of the electron beam injection methods are: limitedmass resolution, the need of neutralizing the space probe as well as thesevere restrictions on the strength of the magnetic field.

In the following the various realizations of remote probing methods will be

discussed.

SECONDARY—ION~‘ETH0DS

In this case an ion source and the necessary detecting apparatus are in-stalled aboard a space probe which can manoeuvre and approach the planetoidto 50 — 100 meters. For such distances the ion source should provide acurrent of about Ii = I — 10 mA at an energy of Ei — 2—3 keV. The ions maybe O~, Ar’

4’, Xet It is recommended that a quasi—pulsed regime be applied,with 1 s — injection and 5—10 s — pause.

It is known /10/ that at a given parameter set of the ion beam, for theemission of one secondary ion on the average ~ primary ions should hit thesurface. The secondary ion current is reduced by 10° to 107 timgs in the

2flight from the surface to the probe which receives a flux — 10 ions/cm /s;the energy of the secondary ions is in the region of 0 to 50 eV with amaximum of about 10 to 20 eV.

For the detection of the secondary ions various analyzers can be used, e.g.,radiofrequency, time—of—flight, or quadrupole systems, depending on thegiven problem and the required mass resolution.

The recently developed foil reflectron (FORTRON) and foil induced—polariza-tion analyzer (SIMS) , according to preliminary calculations, should providea mass resolution up to 200 in the mass regi8n of 1—150 a.m.u. These deviceshave an effective collection surface — 10 cm— at a sufficiently large aper-ture and can provide the spectrum for the elemental and isotope analysis ofthe studied surface at the expected current of secondary ions.

The small planetoids in the Solar System which have no atmosphere and possesspresumably very weak magnetic fields ( 10 -r) are exposed to a constantbombardment by the ions of the solar wind. Let us consider the possibilityto analyze asteroid surfaces by using the solar wind as primary ion beam.The mean ion energy in the solar wind consisting mainly of hydrogen (,SO to95 %) and helium ( 5 to 20 %) is about I keV and the flux is _1Om/cm~/s/11/.

Let us assume that on the space probe approaching the asteroid from its sun-lit side0there is device for detecting secondary ions. Let us consider a

20 cm window on the detector with a total aperture _400. Let the set—up

Rere:e Surface Chemical Analysis 113

provide a mass resolution not less than 150 and a detection time — l~s. The“FORTRON” apparatus, for example, has such specifications.

Let us estimate the secondary ion current hitting th~ detector at the dis-tance of —50 m which means that it covers •.l07 cm’ of the asteroid surface.Each ion of the solar wind knocks Out a secondary ion with a probability of10—3. Assuming that the secondary ions are isotropically emitted the totalreturp current of secondary ions under 2n 5E solid angle will be— 1014 ions/s from ~~hich, however, Only —lOs’ ions/s hit the detector locatedOn 50 in (as the current is reduced by

a—s /s — _____

— direct R=5O m — ________

At a 10 ~ detection efficiency and 1 s exposition time the detector counts— 106 ions which is sufficient to determine the mass spectrum of the sub-stance studied.

In the case of hydrogen bombardment of the asteroid surface relatively heavyelements as, e.g., calcium or iron, can be knocked out only with a probabili-ty of lO—~/12/. Increasing the exposition time by an order of magnitude

from 1 5 to 10 s ) one can compensate for this loss.

With increasing distance the ion current decreases with the square of thedistance but also increases in the same way due to the increasing effectivesurface area. Consequently while the asteroid completely covers its solidangle the detector will receive a constant secondary—ion current.

REMOTELASER MASS-ANALYZER

The method is based on the acceleration effect of laser plasma on ions. Itwas shown /13/ that when a sample Is irradiated by focused laser radiationthe emitted ions do not need acceleration to higher energies (as e.g., in“LAMMA”, up to I to 3 key) hut one can perform the analysis at the energiesobtained by the ions during the so—called “free flight” /6/. Depending on thediameter of the laser—inflicted crater from 15 lam to 1 mm the maximum of theion energy distribution is shifted from —90 to 600 eV at a radia-tion power flux 109 k/cm

2 /i~+/.

Using the free flight method /6/, on the basis of the first version of the“PUMA” set—up, the “LIMA” laser mass—analyser has been developed at I5R,Moscow. The device /13/ has a high (— 300) mass resolution at a focusing dia-meter of the laser radiation on the spot —150urn. Let us note that in anacceleration regime set—up ‘LANMA” at a crater diameter of l0~m or morethe mass resolution of the device is sharply reduced, presumably due to theformation of a dense plasma and its interaction with the accelerating elec-tric field.

The idea of developing a remote mass analyser /4/ was conceived when underlaboratory conditions it was confirmed that it is possible to construct adevice working in the regime of free flight and it was experimentally proventhat the laser radiation can he focused at 50 to 100 m distance onto a spotof I to 2 mm diameter.

For obtaining the necessary high mass resolution the reflectron /5/ was chosenas basic device. In the remote probing experiments the flyby base ( 50 to100 in) considerably increases the characteristic length of the reflector.Therefore, the time—of—flight of the ions from the planetary surface to thedetector, with no special measures, significantly exceeds the time spent bythe ions in the reflector excluding thereby the possibility of space—timefocusing,

For the removal of these time differences it was decided to slow down theions before entering the reflector and to ensure In this way the closenessof those times.

The time—of—flight of the particles from the formation site to the detectorconsists of the time of its drift along a zerofield of length L and the time—of—flight in the reflector:

114 R.Z. Sagdeev ~t ~i.

t(~) = ~114 + ~ 4 (/~7T’’/E~—eV~

1) (X.~1—X.)2q ~ 1=1 2q (eV,÷1—eV,)

+ 4 p’Eev~1 (Xk2_x)(eVk+2_eVk+l)

where m, c, and q are the mass, energy, and charge of particle, respectively;X1 — the location coordinates of the reflector wires, V1 — their potentials;X1 = 0, V1 = 0, k is determined from the condition

eVk+l < t < eVJç4~2

For a given set of particles with a given energy dispersion one can calculatethe bunch width at and the average arrival time t ; the H resolution can beestimated by the equation a

R

2~t

By varying the parameters X~and V~, the maximum of R can be determined. Thefocusing effect of the reflector is based on the fact that the time—of—flightof the particles depends on their energy in the opposite way as on theirdrift length.

The reflector has been designed numericafly by a computer. The results haveshown that for a resolution > 200 with a + 5 ‘~ dispersion in particle energyone has to use a reflector of three apertures, since in the case of a simplereflector with a homogeneous field it is impossible to compensate the disper-sion of the drift times after a 50 m flight more precisely, in this case areflector of more than 10 in length would be needed) . According to the cal-culations the first aperture should be narrow — 3 mm, equal to the step of

— and the particles should lose about 90 of their energy in it i.e. atthe entry to the reflector the potential distribution will have a step. As aresult, the particles cross the reflector with a velocity 3—10 times lessthan the drift value and the time spent in the 30 cm long reflector becomesclose to their drift time along L = 50 m which allows to focus the bunchesin time.

The concrete values of V1 and X~depend on L. The X~grid distances are opti-mized for an average drift length and are kept fixed; the reflector is tunedby varying V1 only. According to the calculations for obtaining a resolutionof —200 the V~values should be adjusted at every 3 m change of L, i.e. thedrift length is to be measured with a precision of + I m.

In order to increase the sensitivity a reflector of toroidal form was design-ed. In this configuration the reflector directs the particles entering thewindow—ring of 300 cm

2 area to the detector with 7 cm~area located on theaxis. For ensuring the necessary radial velocity for the particles the gridsare bent inwards under an angle of 10. The transparency of the torus reflec-tor was calculated to be 40 with no account for the transparency of thegrids. (This means that 40 ‘~ of the particles entering the window with thenecessary energy will hit the detector.)

Let us estimate the number of particles entering the reflector. Assume thatthe laser evaporated a surface layer of diameter 1 mm ~S — 1O2 cm2) and withdepth h =~O.l ~m = 10—5 cm containing ~h = Sh = 3xlO~ atoms wherea = 5xlO~ cm~ i~ the density of solids (p..) . From the emerging plasma_Nj = TI = 5x1O~- ions will be emitted without recombination (rI — lO’~ isthe Ion yield efficiency /15/). Considering that the ions are isotropicallyemitted in a hemisphere, the number of i~ns entering the reflector window ofS

1~ = 300 cm2 area at an L = 50 m = 3xl0~ cm distance is equal to

S~ 5.1012.3.102—

N. . = N. — = ____—— - 1O~ ions1,lfl ~ 2~L2 2~.25.1O6

Remote Surface Chemical Analysis 115

At a 10 t/ total reflector transparency (including all factors) one expects

N = 106 ionsi, del

to be detected.

The power flux 2.lO~ W/cm2 needed for the vaporization and one—electron ioni-

zation of the matter in a crater of diameter 1 mmcan be obtained by using alaser of variable Q—factor with — 0.3 J energy per — 10 ns long pulse. Theuse of a YAG or glass laser activated with Nd ions is recommended(A = l.O6um.Such a laser has a divergence of -3~ lO~ rad when working in a singlemoderegime. The beam can be focused to a spot of 1 mm diameter using an objectiveof — 20 cm diameter. Allowing a flux variation of + 30 the maximal fluxwill be twice the minimal one which corresponds to a change in the spot dia-meter by a factor of 1,4. Such a change would be caused by a shift of thefocus plane to 10 cm at L = 50 in that is the focusing system needs a telemet-ry precision of < + 10 cm.

Such a telemeter can be constructed by using the measurement of time—of—flightof light, with a semiconductor laser as light source. The frequency of themeasurements is determined by the requirement that the dynamical precision ofthe telemetr~ should also be less than + 10 cm. For a space probe of velocity2 m/s and 45 mean declination the frequency of distance measurements shouldbe at least 5 Hz.

The last condition imposes a limit on the refocusing speed of the objective.In the case of a mirror objective with a piezo—ceramic small mirror of vari-able curvature, however, the refocusing speed of the objective will be mainlydetermined by the transient processes in the high voltage transformer of thepiezo—ceramic mirror and can easily be increased to 100 Hz.

The details listed above clearly show that the construction of a remote lasermass analyzer for space research is a problem which can be solved with presentday techniques.

For measuring the mass composition of the ions of the laser plasma variouskinds of fast mass analysers can be used. For example, the above mentionedF’ORTRON or SIMS set—ups have a time resolution which allows for the detec-tion of —IO~ions during the existence of the laser plasma (— 1 ins ) whichmay be sufficient for obtaining a mass spectrum of the investigated sub-stances (chondrul, basalt).

Thus, within the actual conditions of remote mass composition analysis ofcosmic bodies a simultaneous application of secondary—ion and laser methodsis recommended in order to achieve higher reliability.

ELECTRONBEAM INDUCED X—RAYEMISSION

Information on the concentration of elements in the surface layer can he ob-tained by exciting their X—ray spectra. The remote sensing X—ray fluores-cence method was already used to investigate the geochemical composition ofthe lunar surface during the Apollo—l5,l6 flights /20/. The Al/Si and Mg/Siconcentration ratios were determined in the vicinity of Mare Tranquillitatis.In that case the characteristic lines were excited by the primary X—rayemission from the solar corona.

Going away from the Sun to Mars distance or farther the intensity of the Sunis not enough to do similar experiments. Therefore an active experiment ofsimilar type was elaborated by Hrehuss et al. /5/. They considered a situa-tion when the distance of the probe from the target is 1—10 kin, the measuringtime at least 100 see; there is no significant atmosphere and magnetic field.To resolve the K

0 characteristic line of the most important elements, theenergy resolution of the detector should not be worse than about 0.2 keV.

Specially it was proposed to mount 15—30 keV electron gun of 1—10 mA inten-sity on a space probe. There is no requirement to ~ocus the beam. The detec-tor is a planar Si(Li) semiconductor with a few cm surface.

It is an important question whether the measured intensities are statistical-ly significant. Considering a given element of the surface, during a measur-ing period t[sec] the number of quanta within the K0—line in question isgiven by

116 R.Z. Sagdeev et Ci.

N = 6.24.1018 ‘~ Itf/R2

where I[A] is the current emitted from the gun, f[m] is the effective sur-face of the detector system and R[m) is the distance of the probe from thesurface, ,~ [.sterad’* is the probability that one electron generates oneLi—quantum in unit solid angle.

TABLE 1 The yield parameter n in units of l0~ photons/electronsteradian for different elements in the case of meteorites asindicated

Element dust—like grain iron meteorite“Saratov” “Kunashak’ “Sihote AIm’

Mg 1.43 0.69 —

Al 0.13 0.21 —

51 3.88 3.37 —

p 0.089 - 0.20S 0.22 0,41 0.24K — 0.066 —

Ca 0.54 0.34 —

Cr 0.091 0.11 —

Mn 0.054 0,071 —

Fe 1.94 2.09 12.62Ni 0.054 0.047 0.39

To find out n in the appropriate energy range experiments were performed onthree different meteorite samples: on a dust—like sample “Kunashak”, on agrain sample “Saratov” and on small rough pieces from the iron meteorite“Sihote AIm”. The result are summarized in Table 1.

Using the measured n values, assuming lO_~ abundance of the element examin-ed in the surface layer, I = 1 mA, f = i0~ in2, R = I ~m, t = 100 sec oneobtains N — 6000 with a relative statistical error l/Y’N= 1,3

The same characteristic line is also excited by the radiation of the Sun aswell, whereas the measured spectrum are superimposed on a continuous back-ground the major part of which is hremsstrahluns. The detected photon inten-sities are summarized in Table 2.

TABLE 2 Detected photon

Characteristic radiation Continuous backgroundradiation

Detected .. DetectedSource sourcephoton/sec photon/sec

electron excitation 600 bremsstrahlung 150

fluorescence excita— photons scatteredtion* for quiet Sun back from solarcondition 20 radiationt 40

for flare activity 100 cosmic radiation 10

Total max. 700 Total 200

*at Mars’ distance

The region in which the proposed method can be applied is

Remote Surface Chemical Analysis 117

io-13~s ‘f’,R2 � 10_li A

as estimated by the authors. It is also necessary that the electron beamShould travel without any essential change of direction, i.e. the Larmorradius of the electron path should exceed the approach distance R. Thisyields -

HR << gauss cm.

It is also required that no local magnetic field be present on the surface.

T}{F~ PHYSICAL LIMITATION OF THE METHODS

Magnetic field

A magnetic field of unknown characteristics in dependenceof its value andorientation around a planetoid may become a factor in performing a distantprobing. In the case of having information on the main characteristics of thefield the geometry of the experiment can be correspondingly changed.

In general, the magnetic field does not disturb the distant ionic measurementsindependently of its orientation if the Larmor radius p of the secondaryhydrogen ion Is much larger ( 10 times~ than the distanc~ of the satellite fromthe object. At a distance of L = SO in should be at least 500 m.

The energy of the ions formed in the ion and laser plasma is different: inthe former case the energy is below 10 to SO eV and thus the maximal allowedmagnetic field is

143 JEH = —-—--—~ io2 oe.oe

In the case of laser plasma the ion energy goes up to 1 keV that is the limitimposed by the magnetic field is much weaker.

The magnetic field of a single asteroid should not exceed IO~ /16/. Con-sidering the condition PH>> L this means that the ionic and laser measure-ments can be started from 5 and 50 km respectively, if the necessary fluxesof the primary beams and sensitivity of the measuring apparatus are available.

For the beam of energetic electrons :io to 30 keV) the magnetic field does notcause problems; for the low—energy Auger electrons, however, the condition

>> L not be feasible.

Solar wind background and its effect on the laser studies

If solar wind ions can be used in the secondary—ionremote probing method asprimary particles then for the laser time—of—flight method these ions generatea parasitic signal perturbing the measurement.

Let us estimate the maximal parasitic effect caused by the elastically orquasi—elastically scattered ions of the solar wind. The flux of these par-ticles is higher by two orders of magnitude than the secondary ions knockedout of the surface. We should take into account that the energy of thescattered solar wind ions corresponds to the energy window of the reflector.

As mentioned earlier the effective surface of the reflector is 300 cm2 at anaperture =20. If th~ asteroid—satellite distance is 30 m then the aper-ture covers —lO~cm~surf~ce area (i.e. a circle of —l m diameter). Theprimary beam is reduced 10° times due to the distance to the detector and byanother order QI magnitude due to reflection. Thus, the total flux reductionis at least 10’.

In the laser technique each mass spectr~m is coll~cte~ for —l0~ s; as theinitial flux of the solar wind is ,—lO~ ions cm~’ s during the spectrumcollection the detector collects 10- background ions as a maximum:

1bgr be cm’~ s~ lO’ cm2 1O~ S b0~ = 102

118 R.Z. Sagdeev et al,

Considering that the expected flux of laser—induced,ions is about 106 tO 10’particles and the dynamic range of the device is 10* one can conclude thatthe 102 background particles, uniformly distributed in time, do not distortremarkably the measured spectra.

The solar wind electrons scattered from the planetary surface with energies— 10 to 20 eV may, however, parturb the detection of Auger electrons ofsimilar energies.

Electrostatic potential of the space apparatus

In the case of injecting a charged particle beam from the board of a cosmicprobe /17,18,19/ the charge neutralization of the probe is a serious problemin the environment of the low—density plasma.

If the cosmic probe has an electric capacity _lO_~0 F (a sphere with a I mradius) then the injection of a bO’-~ A current from the board in the absenceof neutralizing particles leads to a 10 kV potential in ~ I ins time.

There are two problems: how can one measure the potential of the satelliteand how can one neutralize it. The measurement of the potential of a cosmicprobe in an environment of low—density plasma is a difficult and so far un-solved problem. The neutralization can be accomplished by emitting particlesof opposite charge as well /19/. In this case, however, the two beams shouldhave the same current to a high precision. Such a solution, together with thetechnical difficulties, involves e.g. in the electron experiments an onboard ion injector as well, which is considerably heavier and more complexthan the electron gun.

In the case of ion beam injection the problem has a much simpler solution:thermal electrons are emitted simultaneously with the ion beam. These elec-trons raised by the ions due to the space charge have a somewhat smallercurrent than the ions, thus create a not too high (10 to 100 U) potential onthe probe. One can measure this potential under laboratory conditions and— neglecting the influence of the low—density plasma in the space — shift theenergy window of the detector, i.e. detect the ions accelerated in the poten-tial of the probe.

There is another way of detecting the secondary ions knocked out by tile pri-mary beam with 10 to 50 eV energies and accelerated by the 100 V potential ofthe probe to energies 15O eV. In the detector the ions can he acceleratedto 1—2 key and the 10 Ci energy window ~l5O to 200 eV) of the system ensuresthe detection and analysis.

Solar IN radiation

The detectors generally used for detecting ions (e.g. micro—channel plates)detect the UV radiation as yell with a high (10 ‘~) efficiency. This UV back-ground, which gives a =101 flux/cm

2/s when scattered from the surface understudy should be absorbed using special techniques.

LABORATORYTESTS OF THE REMOTELASER METHOI)

In order to demonstrate the capability of the distant laser TOF mass—analyzera i/s part of the torus reflector was tested in the large vacuum chamber atthe ISR, Moscow. The apparatus installed in it consisted of two systems: theion source and the IOF’ mass analyser.

The first system included a small—size multi—mode laser working in a variableQ—factor regime with a — 20 uJ pulse energy and pulse length T

1 10 uS,

a focusing system, and a target with interchangeable Ag and S~samples. Asthe single—mode laser of small divergence angle C— 3 to ‘~xlO

9 rad) was notcompleted until the beginning of the test we had to make changes in the ex-perimental se$—up. As it was difficult to focus the available laser beamof u -5xl0’ rad divergence onto the 150 tim spot needed for the 1O9 W/cmpower flux at the 10.5 in distance we placed it together with the focusingoptics near (-30 cm) the target. This system was mounted on a platform move—able in 3 orthogonal directions inside the vacuum chamber. The platform couldmove 6 in along the length of the chamber and 0.3 m from the centre in hori-zontal and vertical directions to each side. The maximal distance between thesamples and the window of the reflector was Lmax = 10.5 m. The mass analyzerconsisting of the reflector and the detector was fixed ~.n the prechambe:’. Theexperiment was performed at a pressure of p ~ 1.3 x lO~ torr. The reverse

Remote Surface Chemical Analysis 119

field of the reflector was previously calculated. A ‘j—slit reflector waschosen, the grid locations in it were optimized for an S in drift distance ofthe ions, and for all the other distances the grid voltages were optimized.

After the preliminary tests of the reflector the grid voltages were found toproduce maximum resolution. The reproducibility of the spectra was — SO %.This can be increased by including the single—mode laser expected to be usedin the full reflector maquette.

As a result of the work with the mass analyzer it was confirmed that thismethod provides the necessary resolution and sensitivity. The average resolu-tion during the series of experiments was found to be —200 for the silvertarget and in some cases it reached 500. The total ion flux to be detected bythe TOF mass analyzer in the real experiment can be estimated by relativecalculations using the experimental data obtained in the ISR large vacuumchamber. Indeed, the signal amplitude of the detector is sufficient for a re-liable determination of the spectrum. However, when the distance is increasedto 50 in the particle flux at the detector decreases by a factor of 25. In-creasing the signal by increasing the reflector surface, additional innerfocusing when using the full torus configuration and also by increasing thelaser beam spot )S times) compensates for the loss due to higher distance anda simultaneous usage of the torus reflector and the high—power laser leads toan increase in the flux by an order of magnitude.

REFERENCES

I. D.M. Anderson, Mass Spectrometric Analysis of Organic Compounds Water andVolatile Constituents in the Atmosphere and Surface of Mars, The VikingMars Lander., Icarus 16, Ill 1972

2. Yu.A. Surkov, Eksperimental’noe modelirovanie gamma—izlucheniya, obuslov—lennogo yadernymi vzaimodeistviyami chastits vysokih energii 5

veschestvom Luny I planet, Kosm. issled., 15, 1 1977

3. R.C. Bradley, .Journ. Appi. PIns. 30, 1 1939

4. I.D. Kovalev, G.G. Managad7r, A.M. Natanzon, I.Vu. Chutyaev, R.Z. Sagdeev,Distantsionnyi lazernyi mas-~—analizator, in press in: II. Vsesoyuznomsoveschanii p0 lazernol niass—spektrometrii, Gorki, 1—S June, 1983.

5. B.A. Mamyrin, Vremyaproletnyi mass—speh’trometr, Bulletin izobretenii,13, 2(1967)

6. I.D. Kovale~’, N.y. Larin, ~T.Ya. Motovlchev, A.I. Suchkov, \Tremyaproletnyi

mass—spektrometr, Patent, prioritet No. 5~*4l5S5, Aug. 1954

7. R. Vehsung, Analizator mass s lazernym mikrozondom L~~NA,in: Tehnologiyatohkih plenok dlya elektroniki, tech, rep. Leybold—Heraens, Company,PEG, 1978.

8. G. Hrehuss, T.I. Gombosi, I. Náday, L. Pogilny, K. Szeg~, Proposal forElectron Beam Induced Remote Sensing X—Ray Fluorescence Investigation ofMinor Bodies in the Solar System ~Experiment Ind—X/. KFKI—1963—l09,Central Research Institute for Physics, Budapest. Hungary 19S3)

9. I.G. Kozlov, Sovremennye problemy elektronnoi spektroskopii, Atomizdat,Moscow, 1978.

10. M. Kaminski, Atomnye i ionnye stolknoveniya na poverhnosti metalla,Izd. ‘lir, Moscow, 1967.

11. A. Hundhausen, Solnechnyi veter, Izd. ‘lir, Moscow, 1975.

12. B.V. Panin, Vtorichnaya ionnaya emissiya metallov pod deistviem ionov senergiyami 10—100 key, ZhETF, 41, 313 1961)

13. G.G. Managadze, I.Yu. Shutyaev, A.L. Bondarenko, La7ernyi mass—analizatorso svobodnym razletom ionov, in press in: II. Vsesoyuznom soveschanii p0lazernoi mass—spektrometrii, Gorki, 1—S June 1983.

14. G.&. Devytyh. S,V, Gaponov, 0 vozmozhnosti mikrobokal’—nogo analizatorana lazernom mass—spektrometre, Pis’ma v ZhETF, 2, 90o 1976)

120 R.Z. Sagdeev et al.

IS. A. Kornyu, Analitjscheskoe primenenie mass—spektrometra s odinarnoifokusjrovkoi 1 lazernym istochnikom, in: Kineticheskaya mass—spektro

—

metriya i ee analiticheskie primene~4~, Nauka, Moscow, 1979, p. 209.

16. Sh.Sh. Dolginov, Magnetizm i elektroprovodnost’ Luny po dannyni“Lunohoda—2”, preprint (1974)

17. A,A, Galeev, Ye,V, Mishin, R.Z. Sagdeev, v.D. Shapiro, V.1, Shevchenko,Razryad v okoloraketnoi oblasti pri inzhektsii elektronnyh puchkov, DAN231, 71 (1976)

18. G.G. Managadze, W. Riedler, Plasma processes in the region of electronbeam injection from high—altitude payload, in: Active experiments inspace, ESA SP—195, ESTEC, Noordwijk, The Netherlands, 19S3, p. 161.

19. H.A. Cohen, C. Sherman, E.G. Mullen, Spacecraft charging due to positiveion emission. An experimental study. Geophys. Res. Lett. 6, 515 ( 1979,

20. C.G. André, Science 197, 986 (1977)