US3916187A

US3916187A - Cosmic dust analyzer

Info

Publication number: US3916187A
Application number: US475338A
Authority: US
Inventors: James C Administrator Fletcher; Neal L Roy
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA
Priority date: 1971-10-14
Filing date: 1974-05-31
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

Methods and apparatus are provided which employ ion time-offlight techniques to determine the constituency of a high velocity particle of matter such as a micrometeorite. A charged target electrode formed of two known materials is arranged to intercept the particle, the impact of which will create a discrete clous or plasma of ions of both the known target material as well as the particle matter. A charged collector electrode is spaced a preselected distance from the target electrode to receive the ions. However, the region between the two electrodes is established as a low density field-free region, and if the ions are then accelerated into the collector electrode at a proper body, the ions in the plasma will tend to travel at a velocity which is substantially a function of only the mass of the ion. The ions in the plasma will tend to separate into groups according to mass. The fractional ionization for an arbitrary atomic species can be specified by the Saha equation if the plasma volume (V) and temperature (T) are known. T can be determined by taking the ratio of the Saha equations for two elements present in the target in known concentration. (Taking the ratio negates the requirement of knowing V.) Given T, the procedure can be reversed to yield the relative abundance of elements contained in the impacting particle.

Description

United States Patent Fletcher et a1.

COSMIC DUST ANALYZER Inventors: James C. Fletcher, Administrator of the National Aeronautics and Space Administration, with respect to an invention of Neal L. Roy, Redondo Beach, Calif.

Filed: May 31, 1974 Appl. No.: 475,338

Related US. Application Data Continuation-impart of Ser. No. 189,438, Oct. 14,

1971, abandoned.

[52] US. Cl. 250/251; 250/287; 250/423 [51] Int. Cl. H01J 39/34 [58] Field of Search 250/423, 424, 281, 282, 250/283, 287, 288, 251; 324/71 CP [56] References Cited UNITED STATES PATENTS 3,538,328 11/1970 Strausser 250 OTHER PUBLICATIONS Detection Technique for Micrometeoroids using Impact Ionization, Auer et al., Earth and Planetary Science Letters, Vol. 4, No. 2., Apr., 1968.

Primary Examiner-Craig E. Church Attorney, Agent, or Firm-Russell E. Schlorff; John R. Manning; Marvin F. Matthews [5 7 ABSTRACT Methods and apparatus are provided which employ ion time-of-flight techniques to determine the constituency of a high velocity particle of matter such as a micrometeorite. A charged target electrode formed of two known materials is arranged to intercept the particle, the impact of which will create a discrete clous or plasma of ions of both the known target material as well as the particle matter. A charged collector electrode is spaced a preselected distance from the target electrode to receive the ions. However, the region between the two electrodes is established as a low density field-free region, and if the ions are then accelerated into the collector electrode at a proper body, the ions in the plasma will tend to travel at a velocity which is substantially a function of only the mass of the ion. The ions in the plasma will tend to separate into groups according to mass. The fractional ionization for an arbitrary atomic species can be specified by the Saha equation if the plasma volume (V) and temperature (T) are known. T can be determined by taking the ratio of the Saha equations for two elements present in the target in known concentration. (Taking the ratio negates the requirement of knowing V.) Given T, the procedure can be reversed to yield the relative abundance of elements contained in the impacting particle.

11 Claims, 11 Drawing Figures l PAR 1 1 10 W3 1 /ON FLIGHT 2 1 PATH +V W 2 I 6 T g L 7 I 2 I 1 a 7 f l f 9 osc/uos COPE TO EXT TO DISPLAY TRIGGER U.S. Patent Oct. 28, 1975 Sheet 1 of9 3,916,187

FIG. I

4 L7 L- IO F3 I ION FL/GHT 2 PATH 9 +V VA A I 6 OSC/LLOSCOPE TO EXT TO OIsPLAV TRIGGER 44 HIGH VOLTAGE SUPPLW-ZO KV) j 43 51 i 48 HIGH VOLTAGE To I SUPPLY IO KV 42 h OSCILLOSCOPE i VERTICAL I 49 AMPLIFIER 40/]? 43A l '53 J 40 i 54 45/ LIGHT PHOTO J/ PIPE i i MULTlPL/ER 50 I 46 I I 43A 47 PMT HIGH VOLTAGE I 52/ SUPPLY g 43 i 48 *l T0 cOA COLLECTOR AMPLIFIER US. Patent Oct. 28, 1975 Sheet 5 of9 3,916,187

Rwmmmwww U.S. Pat ent Oct. 28, 1975 Shee't 6 of9 3,916,187

m mlk T I I l l l l l l l J qmh US. Patent Oct. 28, 1975 Sheet 7 of 9 3,916,187

Sheet 8 of 9 US. Patent Oct. 28, 1975 FIGQB COSMIC DUST ANALYZER ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 45 U.S.C. 2457) The present application is a Continuation-In-Part application of U.S. Pat. Application Ser. No.: 189,438, filed Oct. 14, 1971, entitled: COSMIC DUST ANALY- ZER (now abandoned).

BACKGROUND OF THE INVENTION This invention relatesto improved mass spectrometry methods and apparatus and, more particularly, relates to mass spectrometry methods and apparatus employing impactionization time-of-flight techniques for investigating the elemental composition constituency of cosmic dust particles of celestial matter. I

It is now generally understood that the region which is commonly referred to as outer space," and which is the area outside of the earths atmosphere, is not at:-

vtually a space in the sense of an empty void. As a distributed throughout space. This material, which is" technically identified as micrometeorites and which is more commonly referred to as cosmic dust, is fartoo small to be seen by the most'powerful telescope. Never theless, there is considerable interest in the nature and origin of cosmic dust, since there is reason to believe that a knowledge .of the origin of this material maybe the key to a knowledge of the origin of ourearth.

A particle of cosmic dust is far too small to be captured in the manner that U.S. astronauts have taken samples of the moon. As a matter of fact, many such These and other disadvantages of the prior art are overcome with the present invention, however, andnovel methods and apparatus are herewith provided which produce a detection signal which is an unambiguous identification of the signature of the detected particle.

SUMMARY OF THE INVENTION In an optimum embodiment of the present invention, mass spectrometry apparatus is preferably provided which utilizes the ionization produced by the impact of a hyper-velocity particle upon a solid target of suitable design. The impact tends to result in the creation of micro-plasma which is composed of ions of target material as well as ions of the micrometeorite material. Accordingly, means is preferably employed which utilizes ion time-of-flight techniques to separate the microplasma into its constituent parts.

In particular, the ions of micro-plasma are preferably accelerated through a predetermined voltage gradient and allowed to drift in a field-free region to a collector electrode. During such drift, however, the ions tend to separate according to mass, and, since the mass of the target ions is known, the ions of micrometeorite material can be identified according to transit time. Moreover, means is preferably included whereby the number of ions of each species may be counted to determine the relative abundance of each'element so identified in each impacting particle ofcosmic dust.

As will hereinafter be explained in detail, the transit time measurement for each ion in the micro-plasma produced by an impacting dust particle is preferably made by collecting these ions on a flat plate which generates a functionally related signal. The collector plate is preferably connected to suitable electronics whereby this signal may be appropriately amplified and displayed so that it increases in a step-wise manner after each such impact on the target. The resulting steps are thenidentifiable as to atomic mass, and the magnitude of each will determine the number of ions of a particular'spe cies. Accordingly, the characteristic shape of the collector signal will provide an unambiguous signature of each micrometeorite which impacts on the target. If

cosmic dust particles'which may be of special interest would actually be too small to be seen even close-up," so to speak. For these and other equally obvious reasons, there has heretofore been no possible way to determine the constituency of micrometeorites notwithstanding the longstanding curiosity about this ma terial which has existed in astronomical circles.

It is well known that methods are now available for orbiting packages of test gear about'the earth at locations outside the earths atmosphere. It has been suggested that since these dust particles can'never penetrate the earthsiatrnosphere without being instantly incinerated, suitable test gear be included in one or more of these satellites which are orbited about the earth in this manner. Moreover, it has been proposed that the proper kind of test gear to analyze the constituency of micrometeorite matter is a mass spectrometer. However, the conventional mass spectrometer has not been found to be suitable for this purpose, by reason that this apparatus of the prior art tends to producespurious signals on many occasions.

the impact target contains two elements in known concentration, measurement of the relative ion signals of these two elements can be reduced to plasma temperature by taking the ratio of the Saha equation. Given the temperature, the further theoretical relationships for processing the data to yield the relative atomic abundances of the elemental constituents in the impacting particle are provided.

These and other features and advantages of the prior art will become apparent from the following detailed description, wherein reference is made to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Reference to the drawings will further explain the invention wherein like numerals refer to like parts, and in which: I

FIG. 1 is a simplified functional diagram of a time-offlight mass spectrometer which is suitable for analyzing the constituency of a particle of cosmic dust or the like.

FIG. 2 is a functional representation of one form of mass spectrometer employing the concept of the present invention and especially suitable for use in an ambient life-sustaining environment.

FIG. 3 is a functional representation of another form of mass spectrometer of the type illustrated generally in FIG. 2 but employing a different collector electrode assembly.

FIG. 4 is a more detailed but nevertheless functional representation of the collector electrode assembly hereinbefore referred to in connection with FIG. 3.

FIG. 5 is a functional diagram of the major components of a mass spectrometer and registration circuit which is suitable for use in outer space.

FIG. 6 is a schematic diagram of one portion of the apparatus referred to in general in FIG. 5.

FIG. 7 is a schematic diagram of another portion of the apparatus referred to in general in FIG. 5.

FIG. 8 is a schematic diagram of another portion of the apparatus referred to in general in FIG. 5.

FIG. 9 is a schematic diagram of another portion of the apparatus referred to in general in FIG. 5.

FIG. 10 is a pictorial representation of another embodiment of the present invention which is suitable for use in conventional environments.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there may be seen a simplified sketch of the basic components of a mass spectrometer suitable for use in analyzing particles of cosmic dust. In this apparatus, a suitable metallic impact plate 2 is arranged to receive the dust particle which may approach along a path indicated by the heavy black arrow 10. The impact plate is preferably spaced a preselected distance L, from an accelerating grid 4 which, in turn, is spaced a preselected distance L from an ion collector electrode 3.

The grid 4 is preferably coupled to reference voltage (hereinafter denoted as ground), whereas the impact plate 2 is preferably coupled through a resistance 7 to a positive voltage supply. The impact plate 2 will, therefore, preferably be maintained at a preselected positive potential relative to grid 4, which will hereinafter be referred to as +V The collector electrode 3 is coupled to ground by way of a second resistance 5 of preselected magnitude, whereby the collector electrode 3 will be maintained at a preselected positive voltage relative to the grid 4 but at a preselected negative voltage relative to the impact plate 2.

As further indicated in FIG. 1, an appropriate oscilloscope circuit 9 is preferably included for the purpose of providing an observable indication of both the arrival of a particle of cosmic dust and the time-of-flight of the various constituent parts of the plasma which is created by the impact of the particle on the impact plate 2. More particularly, however, the trigger side of the oscilloscope 9 is coupled to the output of an amplifier 8 having its input side coupled to the impact plate 2. The display side of the oscilloscope 9 is, therefore, coupled to the output of another amplifier 6 having its inputcoupled to the ion collector electrode 3.

Referring again to FIG. 1, it may be seen that dust particles to be analyzed will approach and strike the impact plate 2 preferably along the path indicated by the arrow 10. Upon impacting the surface of the plate 2, however, the high velocity particle will produce a cloud of impact plasma which is composed of ions of target material as well as ions of the impacting dust particle.

The cloud of impact plasma will immediately begin to expand upon creation, and, as the plasma approaches the collisionless state, the electrons will separate from the cloud as a result of the electric field between the grid 4 and the plate 2 and will return to the plate 2. The ions, however, will be accelerated toward the grid 4 and along arrow 11 at a velocity which may be stated as:

,u ZeV Accordingly, it will be apparent that a measurement of the transit time of an ion group will uniquely define the charge-to-mass ratio of the ions in the group. Assuming that the atoms will all be singly ionized, which is most likely because of the relatively low plasma temperatures involved, a measurement of the ion time-offlight will define the atomic mass of the ion and will thus identify the element.

As hereinbefore stated, the collection of electrons at the impact plate 2 will coincide essentially with the time of impact of the particle, and this will produce the start" pulse which is applied to the trigger of the oscilloscope 9. The electron flow through the resistor 7 develops the voltage pulse required to start the sweep of the recording oscilloscope 9.

The ion collector amplifier 6 may be either current sensitive or charge-sensitive. In the current-sensitive mode, a series of peaks of varying amplitude and time locations may be observed which correspond to ions of different elements. In the integrated or charge-sensitive mode, however, the displayed signal tends to increase in steps to its maximum value and thereafter to decay according to a particular time-constant of long duration. Thus, the leading edge of each step will be functionally related to the arrival time of an ion group, and the amplitude of each step will be a function of the total charge associated with that particular ion group. Hence, the charge-sensitive mode is preferred for test apparatus intended to be employed in outer space, because the use of such a mode will clearly reduce the complexity of the various circuits which must be provided with a cosmic dust analyzer of this character.

Referring now to FIG. 10, there may be seen a simplified pictorial representation of the overall configuration of a particle analyzer which embodies the concept of the present invention but which is useful under more conventional circumstances (i.e., in ambient atmospheres which are capable of sustaining life). Accordingly, there may be seen a time-of-flight chamber which is adapted to support an ion collector electrode (not depicted) in one end and an impact plate or target (not depicted) at the other end. The end of the chamber 90 containing the target is preferably provided with a suitable fitting 91 for connecting the interior of the chamber 90 to an appropriate vacuum pump (sug gested but not depicted). The other end of the chamber 90 is coupled to means for appropriately introducing a particle to be analyzed into the chamber 90.

More particularly, the particle introduction or acceleration assembly may be seen to be composed of a 2MV Van deGraaff generator 70, or the like, which has preferably been modified for acceleration of particles on the order of one micron in size. Also provided are suitable particle injection electronics 71, for electrically charging and introducing the particle to the Van deGraaff generator 70, and suitable control electronics- 72 for the genrator 70.

Particles exiting from the Van deGraaff generator 70 are transmitted through a column assembly 74 to the chamber 90. The first stage of the column assembly 74 is a suitable T-joint 75 for coupling the interior of the column assembly 74 to an auxiliary vacuum pump (not depicted) of suitable design. From there, however, the particle travels through an appropriate magnet assembly 76 which removes unwanted ions from the beam" which may have been created by the particle charging process in the generator 70. From there, the particle will traverse a particle position detector 77 which cates the particle beam axis in order to align the particle with the system, as will hereinafter be explained in detail.

The next two stages of the column 74, which are traversed by the particle, are two

transit time detectors

78 and 80 which are spaced a preselected distance and which generate timing signals whereby the transit time or velocity of the particle in question may be determined. Accordingly these timing signals are appropriately coupled to a time interval selector and dual proportional delay generator 79. This unit has two principal functions. First, it provides an output pulse 79A to the electronics 81 of a particle deflector circuit 82 whenever the measured time interval as indicated by the pulses from the

detectors

78 and 80 falls within the limits of a predetermined time interval. Normally, the electronics 81 generates a constant input signal to the particle deflector circuit 82, which conditions the deflector circuit 82 to deflect all incoming particles by bias voltage on a pair of deflector plates (not depicted) and to prevent such particles from continuing through the column 74 to the chamber 90. When a particle traverses the

transit time detectors

78 and 80 at a proper velocity, however, the generator 79 will generate a signal 79A which conditions the electronics 8] to remove the blocking signal from the particle deflector circuit 82, whereby the particle to be tested is permitted to proceed through the column 74 to the chamber 90.

The other purpose of the generator 79 is to produce two

trigger pulses

79B and 79C at adjustable multiples of the actual transit time as determined by the interval between the timing pulses generated by the two

transit time detectors

78 and 80. By selecting appropriate multiplication factors, these two

pulses

79B and 79C can be made to appear when the approved particle arrives at two arbitrarily selected downstream locations which are indicated in FIG. 10 as X and X These two

pulses

79B and 79C may be employed to initiate the sweep of the oscilloscope in the display and recording equipment 94, and thus the first pulse 798 will preferably be set to occur just before the particle enters the particle charge detector 84 at location X and the second pulse 79C will preferably be generated when the particle subsequentlyfarrives at a location just in front of the target in the chamber 90 and indicated in FIG. 10 as X Referring again to FIG. 10, it will be noted that the column 83 is positioned between the deflector 82 and the particle position indicator 84, and which is suitable for connectingthe interior of the column 74 to an auxiliary vacuum pump (suggested but not depicted). In addition a camera 95 of appropriate design may be coupled to the display equipment 94 to obtain a permanent record of the images which appear on the oscilloscope in the display equipment 94.

Referring again to FIG. 10, it will be seen that a particle which exits the position detector 84 must first cross position X whereupon the generator 79' generates an appropriate actuating signal 79B to cycle the display equipment 94, then must traverse the sensitive particle detector 85 before entering the chamber 90, whereupon a signal is generated whose amplitude and transit time is presented to the camera 95 from which particle 1 parameters may be calculated. Upon entering the chamber 90, however, the particle will first pass the ion collector plate or electrode before impacting on the target, as hereinbefore explained. Accordingly, the chamber is preferably provided with suitable electronics 93 for registering the mass and number of ions which are generated by such impact. Other control electronics 96 will preferably be included, as will hereinafter be explained.

Referring now to FIG. 2, there may be seen a functional representation of a mass spectrometer of the type depicted generally in FIG. 10, wherein the .Van de- Graaff generator 70 and the column 74 depicted in FIG. 10 is summarized as the particle accelerator circuitry 26, and wherein the accelerator circuitry 26 is coupled to the vacuum or time-of-flight chamber 20 by the sensitive particle detector 27. As may be seen in FIG. 2, therefore, the chamber 20 contains an impact plate or target 22 disposed between the grid 23 and the evacuation port 18, and spaced a suitable distance from the collector electrode assembly 25 which is located in the other end of the chamber 20. The grid 23 is grounded, as hereinbefore stated, and the target 22 is accordingly coupled to a suitable high voltage supply 21, whereby impacting particles will produce a suitable indicating signal which is preferably enhanced by the target amplifier 24 and forwarded to the control circuitry 96 depictedin FIG. 10.

As suggested in FIG. 2, the collector assembly 25 is preferably composed of a flat ion collector plate or electrode 25A, which is disposed so that a particle issuing from the accelerator circuitry 26 will pass through a small port 258 as it travels to the target 22 by way of a preselected path 20A. The electrode 25A is preferably surrounded by a grounded shield 25B, and the electrode 25A is preferably coupled to a collector amplifier 29 which also transmits a signal to the control circuitry 96 in FIG. 10.

Referring again to FIG. 1, it will be seen that the sequence begins when a particle exits from the accelerator 26 which is found by the time interval selector and generator 79 (see FIG. 10) to have a transit time which age on the particle deflector 82 is removed, and the particle to be analyzed is permitted to enter the chamber 20.

Just before the particle reaches the sensitive particle detector 27, however, the proportional delay generator 79 produces trigger pulse 798 to initiate the sweep of the oscilloscope in the display circuit 94, whereby signal amplitude and transit time will be presented to the camera 95 as the particle traverses the detector 27. This information provides the basis for calculating the various particle parameters (velocity, mass and radius).

The particle then passes through the pinhole aperture or port 258 in the collector electrode 25A as it continues along the preselected path 20A to the target 22. Just before the particle impacts on the target 22, however (and presumably just as it reaches location X in the chamber 90 depicted in FIG. 10), the delay generator 79 initiates the second oscilloscope sweep which will present the target impact signal (and the ion charge and transit time information) to the camera 95. As hereinbefore stated, the target signal which is provided by the target amplifier 24 is essentially a step function for particle impacts of greater than a certain magnitude (i.e., about 10 km/s), whereas the signal provided by the collector amplifier 29 tends to rise in staircase fashion. Either or both waveforms may be differentiated, however, to produce sharp pulses (spikes") either at impact on the target 22 or upon arrival of each ion group at the collector electrode 25A.

Referring again to FIG. 2, it will be noted that the impacting particle will not depart substantially from the transit path 20A, and thus the target 22 may be relatively small in diameter. On the other hand, the thermal energy of the resulting ions tends to cause the ions to diverge as represented in FIG. 2 by the diverging arrows emanating from the target 22. Accordingly, a relatively larger collector electrode 25A is required to collect an adequate number of these ions. In order to further provide that an adequate number of ions be collected, the side of the shield 25C which confronts the active or receiving side of the collector electrode 25A is preferably made from screening cloth or the like which has a reasonably high transmittance.

It may be noted by those having particular skill and discernment in this art that the apparatus depicted in FIG. 2 may, on occasion, tend to have a relatively low signal-to-noise ratio. Contrary to expectation, this disadvantage is not due to any inherent defect in the apparatus, but is caused by the small size of the particles which are delivered to the chamber 20 by the accelerator assembly 26, or which are encountered by the target 2 (see FIG. 1), when the basic components of the system are employed in outer space to analyze the constituency of cosmic dust or the like.

Referring now to FIG. 3, there may be seen a functional representation of apparatus which is substantially the same in concept as that depicted in FIGS. 2 and 10, buy which has been modified to reduce or eliminate the problem of the low signal-to-noise ratio. In particular, it will be noted that an ion detector assembly has been incorporated in conjunction with the ion collector assembly 35 and that this has required that the particle be introduced into the chamber 30 on an off-axis trajectory. Thus, the particle is required to impact on the target 32 at an angle (perhaps 9 degrees) relative to the axis of the chamber 30, and this in turn requires that a smaller collecter plate or electrode 35A be employed, although the trajectories of the ions emanating from the target 32 will preferably remain the same.

Referring again to FIG. 3, it will be seen that the accelerator 36 and sensitive particle detector 37 are cou pled to the chamber 30 at an off-axis manner. as hereinbefore stated, since the collector assembly 35 and ion detector assembly 15 are necessarily positioned in the chamber 30 at a preselected spacing in the chamber 30 from the target 32 and grid 33. The target 32 is positioned adjacent the port 19 which is coupled to an auxiliary vacuum pump (not depicted) and is further cou' pled between a suitable voltage supply 31 and the input of the target amplifier 34. The collector electrode 35A is also disposed in a grounded grid 35C, as hereinbefore explained, and is coupled to a collector amplifier 39.

Referring now to FIG. 4, there may be seen a more detailed representation, which is partly pictorial and partly functional, of the ion detector assembly 15 depicted in FIG. 3. In particular, the ion detector assembly l5 proper may be seen to be composed of a conventional end-window photomultiplier tube 49, which is energized in a conventional manner by a suitable high voltage supply 52, and which produces an output signal which is enhanced by an amplifier 50 before being applied to the vertical amplifier (not depicted) in the display electronics 94 represented in FIGv 10. The sensitive portion of the ion detector assembly 15 may be seen in FIG. 4 to be composed of a suitable phosphor 46 (such as a thallium-activated crystal formed of sodium iodide or the like) which has a relatively thin sheet or coating of metal 45 (preferably a 1,000 Angstrom aluminum film) on the side of the phosphor 46 which faces the collector electrode 40. The phosphor 46 is preferably optically coupled to the window end of the photomultiplier tube 49 which contains the photocathode (not depicted) by a truncated light pipe 47 or other suitable means, whereby all of the scintillations which occur in the larger diameter phosphor 46 are nevertheless visible" to the photocathode in the smaller diameter window of the photomultiplier tube 49. The phosphor 46 is preferably secured to the receiving side or face of the light pipe 47 by an annular rim bracket or clamp 48, which is preferably formed of some electrically conductive material whereby it may also be employed to couple a suitable high voltage supply to the metal film or sheet 45 on the phosphor 46.

Referring to FIG. 3, there will be noted a ring-type or annular accelerating electrode 43 which is positioned concentrically about the axis of the chamber 30 and spaced appropriately between the ion detector assembly l5 and the ion collector assembly 35. Referring again to FIG. 4, the collector assembly 35 depicted in FIG. 3 may be seen to include a slotted collector electrode 40 disposed within a shield assembly which is composed of a grounded annular grid bracket 41 which mounts a highly penetrable shield 42 on the target-side of the collector electrode 40 and which also supports a second penetrable shield 53 on the so-called rearward" side of the electrode 40.

The purpose of the ion collector assembly 35 and ion detector assembly 15 depicted in FIGv 3 is to convert the ion to be detected into one or more secondary electrons, to accelerate these electrons to increase their energy, to thereafter convert a portion of such energy into scintillations of functionally proportional magnitude,

and then to produce voltage pulses which are functionpressor grid 42 to the collector electrode 40 to activate the collector amplifier 39 as hereinbefore explained. Some of the ions will, however, pass through the slots 54 in the collector electrode 40, and, after traversing the exit grid 53, these ions will enter" the high voltage field which exists in the region between the exit screen 53 and the accelerator electrode 43.

As hereinbefore stated, the annular accelerator electrode 43 is concentrically arranged relative to the axis of the ion beam emanating from the target 32, and thus the accelerator electrode 43 will function as a secondary electron source as well as an ion accelerator electrode 43. Moreover, it will be noted that the accelerator electrode 43 is preferably provided with an ion impact surface 43A which is positioned at about 45 degrees relative to the metal film 45 on the receiving face of the phosphor 46, in order to enhance the production of secondary electrons in response to bombarding ions which pass through the slots 54 in the collector electrode 40. These secondary electrons from the accelerating electrode 43 will then be drawn into the phosphor 46 by the difference between the potential established on the electrode 43 by the high voltage supply 44, and the potential or charge established on the aluminum film 45 by the power supply 51. Since the aluminum coating or film 45 is preferably very thin, the electrons will penetrate the film 45 and be captured in the phosphor 46 without substantial loss of energy, and each electron capture will result in the production of a number of photons in the phosphor 46, the number of which is functionally related to the energy of the captured electron. The purpose of the low noise photomultiplier tube 49 is to generate a pulse for each set of scintillations or photons thus produced which is functionally related in amplitude to the number of photons present, and thus the amplitude of the pulse is proportional to the energy of the captured electron which created the photons. The output signal from the photomultiplier tube 49 is then buffered with a unity gain amplifier 50 before being presented to the vertical amplifier (not depicted) of the oscilloscope (also not depicted) in the display circuitry 94 representedin FIG. 10.

Referring now to FIG. 5, there may be seen a simplified functional overall representation of an exemplary embodiment of the present invention as adapted for use in outer space. More particularly, there is indicatedthe major components of the system in a manner so as to suggest their contribution to the operation of the system. Accordingly, when a micrometeorite strikes the target electrode 60, this will cause positively-charged ions to emanate from the target 60, whereupon a negative charge will appear on thetarget electrode 60. This negative charge is split betweentwo charge-

sensitive amplifiers

62 and 63, and their outputs are accordingly fed to a signal conditioning circuit 64, which is composed of target amplifiers and discriminators (not depicted in FIG. which operate to amplify and determine the approximate level of the input charge signal. As will hereinafter beapparent, this signal must lie withon one of five decade ranges, and thus the conditioning circuit 64 will also preferably include logic for providing a three-line output signal to identifythe particular decade range. This signal, in turn, is transmitted to the collector range switch assembly 67 which operates to set the appropriate signal gain in the collector electronics 67. It will also be noted that the three-line output signal from the target signal conditioning circuit 64 is also transmitted to the data memory circuit 69 for purposes of storage.

As will hereinafter be explained in detail, the function of the three-line output signal is to condition the switch assembly 67 so that the amplitude of the output signal will not exceed the acceptance level of the analog-to-digital converter 68. The ion charge which emanates from the target electrode 60, travels to the collector electrode 61 as hereinbefore described, whereupon the resulting signal is also split between two

chargesensitive amplifiers

65 and 66 to produce the staircaselike output signal which is presented to the amplifiers (not shown in FIG. 5) in the collector range switch assembly 67. Accordingly, it will be apparent that the two charge-

sensitive amplifiers

65 and 66 as well as the amplifiers in the switch assembly 67 are effectively underthe control of the three line output signal from the target 60.

Referring again to FIG. 5, when the most sensitive of the discriminators in the conditioning electronics 64 is tripped, a signal will be transmitted to the timing and control logic circuit 59 which is included for the purpose of establishing a time base for sampling the output from the collector electrode 61 at eight different times, to provide timing for the converter 68 of these eight sampled signals, and to provide other control functions for the entire period of signal acquisition, conversion, and transfer of acquired data to the telemetering circuit (not depicted) in the space-craft. Accordingly, experiment timing may be provided by a l0-million pulsesper-second clock circuit 58 as will hereinafter be explained.

More particularly, the staircase-like signal produced by the collector electrode 61 is preferably sampled an appropriate number of times (i.e., eight) selected arbitrarily during a preselected period (preferably 25.64:. sec.) following particle impact on the target electrode 60. These eight sampled points are then stored as analog voltages for later conversion to digital form, and after the initial 25.6;1. sec. signal acquisition period, the digital-to-analog converter 57 begins producing an output signal from a slow clock (not depicted in FIG. 5) in the converter circuit 57. Thus, the output signal from the converter 57 is a staircase ramp voltage signal which is compared to the aforementioned analog-type signals stored in the memory circuit 69.

When the clockin the converter 57 reaches its maximum count, all stored analog data will have been converted and stored in the appropriate section or stage of the memory circuit 69. In addition, attainment of the maximum count from the clock in the converter 57 will freeze the system in a holding mode and the timing and control logic 59 will simultaneously produce a data ready" signal to condition the spacecraft telemetry (not depicted). However, actual transfer of data will not occur until a "data enable signal is produced during data transfer and the application of a preselected number of telemetry pulses to the memory circuit 69. Removal of the data enable signal signifies completion of data transfer out of the memory circuit 69 to the spacecraft telemetry, whereupon the system reverts to its standby mode to await the arrival of the next impinging micrometeorite. 1

Referring again to FIG. 5, it is the function or purpose of the collector range switch assembly 67 to convert the charge appearing at the collector electrode 61 to an analog voltage output. Thus, the gain of each of the collector amplifiers is preferably set at one ofa corresponding number of ranges, depending on the charge previously sensed at the target, to accommodate the relatively wide dynamic input range. The charge is split between the two identical charge-

sensitive preamplifiers

65 and 66, and the splitting ratio for these

amplifiers

65 and 66 is preferably ten times the ratio for the collector amplifiers.

Referring now to FIG. 6, there may be seen a more detailed schematic representation of one embodiment of apparatus suitable for use as the timing and control logic circuitry 59 generally depicted in FIG. 5. In particular, it will be noted that when a start clock" pulse 101 is received, clock pulses 102 will then step the 9-bit synchronous counter which is composed of the nine flip-flop circuits 120-128 and the sixteen gates 130-145. Accordingly, as will also be noted, the states of the 9-bit counter are decoded to provide a variety of functions. The eight gates 150-157 each have eight inputs (for simplicity only one being illustrated) which are discretionary wired to the outputs of the 9-bit counter to independently set the gate companion latches (see gates 160-175) at any time from 0, which is the arrival of the start clock pulse 101, to whatever preselected time may be deemed appropriate. In this respect, it will be noted that the gate companion latches, in turn, provide hold signals 107 of the each of the eight analog converters.

When the 9-bit counter circuit has received 256 pulses 102, the last flip-flop circuit 128 will go to its true state, and the analog-to-digital converter 68 will begin its scan cycle. When the flip-flop 128 changes state, however, this causes the two

gates

118 and 119 to change state and this, in turn, causes the switching circuit composed of the gates 180-186 to change the clock pulses 102 from the 9-bit counter circuit to a divider circuit composed of the six flip-flop circuits 190-195. The 9-bit counter is now counted down to the all ones state, at which time the 9-bit counter is locked until a data enable signal 105 is received from the spacecraft data system (not depicted) which indicates that a new measurement can be taken. Accordingly, it will be seen that the 9-bit counter serves four primary purposes: (1) generate timing signals to control subsequent events, (2) provide hold command signals to the analog storage circuits, (3) provide timing signals to control the analog-to-digital converter (ADC) 68 in its processing of the data held in the storage circuits, and (4) freezing the system until all digitized data are transferred to the spacecraft data system and a data enable signal 105 is received.

Referring now to FIG. 7, it may be seen that the ADC load signal 200 and the ADC bit 1-8 signals 201-208 depicted in FIG. 6, are coupled in FIG. 7 to the analog gate circuits 210-218 respectively. These gate circuits 210-218, in turn, are coupled to an appropriate plurality of 4-bit parallel entry shift registers 220-236, and are also coupled to an 8-bit digital-to-analog converter 240.

During the analog-to-digital cycle, the states of the first eight stages of the 9-bit synchronous counter (in FIG. 6) are such that they are accordingly connected to registers 210-227, registers 229-236, and to the digital-to-analog converter 240. Accordingly, a ramp voltage 241 will be generated by the amplifier 242, which starts at zero, but which increases in a step-wise manner until it reaches the 255th step. Each step lasts a preselected time interval which is determined by the capacity of the 9-bit synchronous counter and divider circuit depicted in FIG. 6.

The inverted outputs of the analog storage circuits 250A-258A are each connected to the enable inputs of two of the 4-bit registers 220-236. The ramp, however, is simultaneously connected to all of the analog storage circuits 250-258 as will hereinafter be seen in FIG. 8.

It will be apparent that all outputs of gates 250-258 will initially be in the low state because of a high state at the enable inputs of the 4-bit shift registers 220-236. The load pulse 200 from the analog-to-digital converter 68, which occurs during each update of the 9-bit counter in FIG. 6, causes the successive counts to be transferred into each of the 4-bit registers 220-236 except for register 228. In other words, the registers will continuously track the 9-bit counter, and as the ramp voltage 241 equals the capacitor voltage in each of the analog storage circuits 250-258, its output will go high, thereby removing the enabling gate to its companion pair of 4-bit shift registers 220-236, and thereby freezing therein the previous count in the 9-bit counter. Accordingly, this count will be the digitized value of the voltage which is in the analog storage circuits 250-258.

This process will be seen to continue until the count in the 9-bit counter reaches 255, and thus all eight of the inputs to the analog storage circuits 250-258 will be scanned. However, the range bits in the circuits 261-263 will be stored in the 4-bit shift register 258 which has heretofore been skipped in the process.

As hereinbefore explained, when the 9-bit counter in FIG. 6 is full (reaches the all ones state), the data ready signal 103 will be sent to the spacecraft data system (not depicted) to indicate that the experimental data is ready for readout. Readout, of course, commences upon receipt of both the appropriate clock signal 260 from the spacecraft telemetry circuits (not depicted) and the data enable signal 105. Only sixty-eight clock pulses 260 will now be required to cause readout of all data stored in the depicted eight-channel system. Further, the data enable signal 105 will bracket the readout clock pulses 260, and will be used internally to reset the 9-bit counter and control latches. After the last bit of data has been shifted out by the spacecraft telemetry circuits, the data enable signal 105 will drop to cause the circuitry depicted in FIG. 7 to revert to its low-power standby mode.

Referring now to FIG. 8, there may be seen a simplified schematic representation of a circuit which may be employed as the analog storage circuits 251A-258A suggested in FIG. 7. For simplicity, however, it will be noted that only one analog storage circuit 251A is depicted in detail, since all eight channels are identical.

Referring again to FIG. 8, it will be seen that in the hold mode (wherein the field effect transistor coupled to input A is off, and wherein the other field effect transistor coupled to input 161A is on), the voltage across the capacitor 251AA remains within 0.1 percent of its tracked value for a preselected time interval which is the period required to perform the analog-todigital conversion on all held samples. The field effect transistor 251BB is turned on at all times except when a measurement is being made. After a start clock pulse 101 is generated by the collector circuitry 67, the comparator reset voltage 107 will go to lowthereby blocking or turning off the transistor 25188, and this will occur after the expiration of a preselected period beginning with the start of the start clock signal 101.

The two field effect transistor switches coupled to

unputs

160A and 161A are controlled by timing circuits in the logic circuitry 69 as hereinbefore explained. Accordingly, the tracking turn-off time for each of the eight analog converter circuits 251A-258A will be controlled independently of the others.

It will be further noted that the ADC enable voltage 209 is held low for a different longer preselected time interval following the start of the start clock signal 101. This clamps the ramp voltage 241 at zero to prevent spurious outputs from appearing from any of the analog converter circuits 25lA-258A. Further, it is expected that this different longer time interval during which the ADC enable voltage 209 is held low, will be substantially longer than the period during which all ions of interest from the target electrode 60 are collected.

Referring now to FIGS; 9A and B, there may be seen a simplified but detailed schematic representation of circuitry which may be employed to function as the collector circuitry 67 and charge-sensitive amplifiers 65-66 depicted in FIG. 5. Referring to FIG. 9A in detail, it will be noted that the circuitry illustrated therein is conventionally suitable for use as the two

chargesensitive amplifiers

65 and 66, and that both

such preamplifiers

65 and 66 are therefore identical in design and function. However, it will also be noted that a pair of grids 61A and Bare also preferably included with the collector electrode 61 for reasons which will be readily apparent to those with experience in this art.

As hereinbefore stated, charge splitting is used to divide the charge received by the collector electrode 61 between the two identical charge-sensitive preamplifiers depicted in detail in FIG. 9A. A suitable splitting ratio has been found to be 1000 to 1, as compared with the IOO-to-l ratio which is preferably employed for the two charge-

sensitive preamplifiers

62 and 63 employed with the target electrode 60.

The balance of the collector circuitry 67 depicted generally in FIG. 5, may be seen in FIGS. 9A and 98 to be composed of post amplifiers and gain control switches of conventional design. It should be further noted, however, that the selection of the particular one of the two

preamplifiers

65 and 66 depends on the state of the voltage at the input terminals 673 and 67C, in order to provide the gains desired for the post amplifiers depicted in FIG. 9B. Accordingly, FIG. 9A is directed to the illustration of examplary circuitry suitable for the gain control switch portion of the collector circuitry 67 in FIG. 5, as well as the two charge-

sensitive amplifiers

65 and 66. On the other hand, FIG. 9B is directed primarily to circuitry which is conventional but nonetheless suitable for use as the post amplifier portion of the collector circuit 67.

When a plasma in thermal equilibrium at absolute temperature T contains several species of atoms and ions, the degree of ionization of each species whose ionization energy is E, is related to T according to Sahas equation (cf. Sutton and Sherman 1965): n,. lh'n a], "I'm p 'I' where: n,, m, and n,,' are the numbers per unit volume of electrons, ions of species 5, and neutral atoms of species s, respectively; C is a constant; m", n is the ratio u,"

/u,," in which u, and u,, are respectively the internal partition functions of ions and neutrals of species s; and k is Boltzmanns constant. The Saha equation shows that the fractional ionization of a given species of atom depends on the ratio of the ionization energy of that species to the mean thermal energy of the plasma. For reasonably low plasma temperatures, only a small fraction of the atoms are ionized (except for those with very low ionization potentials). In order to use the impact ionization effect to reliably determine the relative abundance of the elements in cosmic dust and meteoroids, a measure of the plasma temperature must be obtained. If the impact target contains two known elements in known concentration, the measurement of the relative ion signals of these two elements can be reduced to plasma temperature by taking the ratio of the Saha equation. v

Further modifications and alternative embodiments will be apparent to those skilled in the art in view of this description, and, accordingly, the foregoing specification is considered to be illustrative only.

We claim:

1. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode means having two known elements in known concentration, responsive to random impact of said particulate matter to emit a mixture of electrons and ions of target material and particulate matter,

grid means for suppressing said electrons from said target electrode,

time-of-flight means for counting said ions from elements of the target and the ions from the particulate matter as a function of their charge-to-mass ratio, and

register means for deriving a functional indication of the rate of occurrence of each of said ions of a particular mass whereby with utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the elements of the particulate matter.

2. The apparatus described in claim 1, wherein said time-of-flight means further includes a collector electrode arranged a predetermined distance from said target electrode and grid means for receiving said ions, and

1 means for establishing a field-free region between said collector electrode and said grid means.

3. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode having two known elements in known concentration arranged for impact with said particulate matter,

electron suppression means adjacent said target electrode,

a collector electrode spaced a preselected distance from said suppression means and cooperating with said target electrode to define a field-free region therebetween,

timing means for determining the time-of-flight of ions from elements of the target and the ions from the particulate matter of said particulate matter traveling to said collector electrode after impact with said target electrode, and

display means responsive to said timing means for providing a recordable mass-dependent indication of the number of ions traversing said distance during a determined time interval whereby by utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the elements of the particulate matter. 4. The apparatus described in claim 3, wherein said collector electrode further includes a phosphor for deriving scintillations in functional relationship to said ions emanating from said target electrode, and photoelectric means responsive to said scintillations for deriving electrical signals functionally related in magnitude to said scintillations. 5. The collector electrode described in claim 4, further including electron generating means responsive to said traversing ions for producing functionally related secondary electrons, and accelerating means for receiving and accelerating said secondary electrons into said phosphor. 6. The apparatus described in claim 5, further including indicating means for deriving from said electrical signals a recordable indication functionally related to the mass of said particulate matter impacting on said target electrode and producing said traversing ions. 7. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode of having two known elements in known concentration arranged in a high vacuum atmosphere to emit ions of said target material and said particulate matter in response to impact by said particulate matter, target amplifier means for generating a first timing signal functionally related to said impact of said particulate matter, and a sensitive ion detector spaced a predetermined distance from said target electrode in said high vacuum atmosphere for receiving said ions emitted by said target electrode and for deriving therefrom a second timing signal functionally related to the time-of-flight of said ions from said target electrode and an indicating signal representing the number of said impacting ions,

from elements of the target and the ions from the particulate matter,

whereby by utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the ele ments of the particulate matter.

8. The apparatus described in claim 7, wherein said ion detector includes accelerating means for increasing the energy of said ions to velocities which are dependent substantially only on the mass of said ions.

9. The apparatus described in claim 8, wherein said ion detector further includes a collector electrode means spaced a predetermined distance from said target electrode, and

means for establishing a field-free condition between said target and collector electrodes.

10. The apparatus described in claim 9, further including indicating means connected with said collector electrode for deriving an observable signal which is functionally indicative of the number and atomic mass of said ions.

11. The apparatus described in claim 7, wherein said ion detector includes an ion collector assembly arranged a predetermined distance from said target electrode,

an ion detector assembly in conjunction with said ion collector assembly, and

means directing the particulate matter to strike the target at an angle thereto.

Claims

1. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode means having two known elements in known concentration, responsive to random impact of said particulate matter to emit a mixture of electrons and ions of target material and particulate matter, grid means for suppressing said electrons from said target electrode, time-of-flight means for counting said ions from elements of the target and the ions from the particulate matter as a function of their charge-to-mass ratio, and register means for deriving a functional indication of the rate of occurrence of each of said ions of a particular mass whereby with utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the elements of the particulate matter.

2. The apparatus described in claim 1, wherein said time-of-flight means further includes a collector electrode arranged a predetermined distance from said target electrode and grid means for receiving said ions, and means for establishing a field-free region between said collector electrode and said grid means.

3. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode having two known elements in known concentration arranged for impact with said particulate matter, electron suppression means adjacent said target electrode, a collector electrode spaced a preselected distance from said suppression means and cooperating with said target electrode to define a field-free region therebetween, timing means for determining the time-of-flight of ions from elements of the target and the ions from the particulate matter of said particulate matter traveling to said collector electrode after impact with said target electrode, and display means responsive to said timing means for providing a recordable mass-dependent indication of the number of ions traversing said distance during a determined time interval whereby by utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the elements of the particulate matter.

4. The apparatus described in claim 3, wherein said collector electrode further includes a phosphor for deriving scintillations in functional relationship to said ions emanating from said target electrode, and photoelectric means responsive to said scintillations for deriving electrical signals functionally related in magnitude to said scintillations.

5. The cOllector electrode described in claim 4, further including electron generating means responsive to said traversing ions for producing functionally related secondary electrons, and accelerating means for receiving and accelerating said secondary electrons into said phosphor.

6. The apparatus described in claim 5, further including indicating means for deriving from said electrical signals a recordable indication functionally related to the mass of said particulate matter impacting on said target electrode and producing said traversing ions.

7. Apparatus for investigating the character of particulate matter traveling at high energies, comprising a target electrode of having two known elements in known concentration arranged in a high vacuum atmosphere to emit ions of said target material and said particulate matter in response to impact by said particulate matter, target amplifier means for generating a first timing signal functionally related to said impact of said particulate matter, and a sensitive ion detector spaced a predetermined distance from said target electrode in said high vacuum atmosphere for receiving said ions emitted by said target electrode and for deriving therefrom a second timing signal functionally related to the time-of-flight of said ions from said target electrode and an indicating signal representing the number of said impacting ions, from elements of the target and the ions from the particulate matter, whereby by utilization of the Saha equivalent equation the plasma temperature is ascertainable and thereby the true relative abundance of the elements of the particulate matter.

9. The apparatus described in claim 8, wherein said ion detector further includes a collector electrode means spaced a predetermined distance from said target electrode, and means for establishing a field-free condition between said target and collector electrodes.

11. The apparatus described in claim 7, wherein said ion detector includes an ion collector assembly arranged a predetermined distance from said target electrode, an ion detector assembly in conjunction with said ion collector assembly, and means directing the particulate matter to strike the target at an angle thereto.