- Email: [email protected]
Primary velocity dependence of impact ejecta parameters
Planet. ti Sci. Vol.26.p~.469 to471. @Per'@monRcrsLtd..197s.PrintcdinNorthemIrcland
PRIMARY
VELOCITY EJECTA
DEPENDENCE PARAMETERS
OF IMPACT
G. EIGDDORN” Max-Flanck-Institut fiir Kerphysik, Heidelberg, Federal Republic of Germany (Received in find form 15 Nouember 1977) Almbnet-From the light emitted during impacts of secondary particles produced during hypervelocity primary impacts, the velocities and relative masses of these ejecta were determined as a function of the angle between the ejection direction and the target surface. The velocity of the ejecta increases with increasing impact velocity and decreasing ejection angle. The ratio of the maximum ejecta velocity to the primary impact velocity decreases with increasing impact speed. The main fraction of the secondary particles is ejected in rather small angular intervals of about lo” width in elevation. The ejection angle of the main fraction of the ejecta mass increases with increasing impact velocity.
INTRODUCllON with light gas guns performed by Gault and Heitowit (1963) and Schneider (1975) yielded information about secondary particles produced during hypervelocity particle impact. Gault and Heitowit (1963) photographed the impact event with high speed cameras and measured from these pictures the ejecta velocity. Schneider (1975) counted craters on secondary targets produced by ejecta with velocities greater than 3 km s-t. From the crater size and angular distribution, an angular mass distribution of secondary particles was obtained. Similar experiments at a dust accelerator were performed by McDonnell et al. (1976) using a lunar rock as primary target. Earlier experiments at the dust accelerator (Eichhom, 1975) showed that information about secondary particles can be. obtained from the light flash produced by the ejecta. In this paper further measurements using this method will be presented. Experiments
EXPERIMENTALARRANGEMENT
A 2 W Van de Graaf dust accelerator was used to accelerate iron particles (Fechtig et al., 1972) onto a gold target with different selected velocities. The light was detected with photomultipliers of type EMI9558QB. Figure 1 shows the arrangement of target and photomultiplier. A glass cylinder was placed in front of the photomultiplier as a secondary target. The front side of this cylinder was shadowed except zones corresponding to a distinct * Present address: Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, U.S.A. 469
1: target 2:photomultiplier FIG.
1
3:glasacytinder &particle beam
1.
THEARRANGEMENTOFTHEPRIMARY (1) ANDSECONDARY(3)TARGETANDTHEPHOTOMULTiPLIER (2).
Secondary particles ejected from the primary target at ejection angles u produce light by hitting the front side of a glass cylinder (3). The side walls of the glass cylinder are silver coated to guide as much of the light as possible to the photomultiplier. To select ejecta at specified ejection angles a the front side of the glass cylinder is shaded with the exception of a zone around the specified angle.
ejection angle ((Y) measured with respect to the target surface. The zones had about 10” width in elevation. The smallest angle at which ejecta could be detected was 5.6”, the largest one 80”. This range was subdivided in seven intervals. Figure 2 shows the light flash recorded in one of these intervals (zone around ar = 50“). As shown in an earlier paper, the light signal consists of two parts. The first part (a) is due to the light produced during the primary particle impact. The second part (b) is produced by the impacts of ejecta produced during the primary impact. The difference between the detection times of the light bursts due to the primary (a) and secondary (b) impacts were combined
470
G.
Ercruio~~
FIG. 2. THE OSCILLOSCOPE RECORDING
OFITIEOWIPUT OF APHOTOMIJLTIPLIERSENSINGTHELlGHTPRQDLJCEDBYIRON PARTlCLESIMPACMNGAGOLDTARGETATAPRIMARYVELOCITY v =5.2 kin/s AMD SELECTEQ EIEtXION ANGLES BETWEENo!= 44O ANDo!=%*.
The first part (a) is due to the primary flash; the second part (b) is caused by the light produce d by ejecta impacting the glass cylinder in the unshaded zone tentered at cy= SO”. with the known distance between primary and secondary target to obtain the velocity of the fastest ejecta in this angular interval. The mean ejecta velocity was determined from the time between the first flash and the maximum of the second flash. This technique is only sensitive for ejecta with velocities greater than about 1 km s-l, and all conclusions must take into account this limitation.
0
IO 20 30 +o 50 80 70 EJECTION ANGLE DEGREE3
80
00
FIG. 4. THE MAXIMUM EJECTA VELOCITY FOR THREE WIFFERENTIhtI'ACTvELOCITIESASAFUNffIONOFTHEEIECIXON ANGLEWITHRESPECTTOTHETARGETSURFACE.
velocities have a maximum near X5” ejection angle and decrease with increasing angle to about 2 km s-l. At ejection angles greater #an 70”, the ejecta velocities increase again. In each angular interval, the ejecta velocity increases with increasing impact speed. The maximum secondary velocity varies from 13 to 22 km s-l depending on the primary velocity. Since the light produced by the ejecta is near the detection limit of the photomultiplier, the errors of the calculated velocities are rather large. They ape estimated to be about 30%. Figure 5 shows the ratio between secondary and
ejecta
As mentioned above, a maximum and a mean ejecta velocity can be calculated. These velocities are shown in Figs. 3 and 4 for different angular intervals and three different impact velocities. The
+: U4.2 KM S -’ 0: U-5.2 KM S -’ X: U-7.9 KM S -’
0
IO 20 30 if 50 60 70 EJECTION RNGLE [DEGREE]
80
so
FIG. 3. THE~~~AVEL~~F~R~REEDI~~~ IMPACTVELOCfflESASAFUNCITONOFTHEBJECI-IONANGLE WITHRESPECrTOTHETARGETSURFACE.
FIG.~. ~ERA~OOF~E~E~A~L~~A~~EPRIMARY~OC~PLO~DAGA~ST~PRI~ARYV~L~~.
The curve and hvo points are calculations and measurements of Gault er al. (1968).
471
Velocity dependence of impact ejecta parameters primary velocity as a function of the primary impact speed. Also shown are calculated and measured values of Gault et al., 1968. The measured values in both cases are somewhat lower than the values calculated by Gauit et al., but show a similar dependence on the primary velocity. EJEffA The
ejecta ejecta
MASS DHTRlBUTlON
integrated light energy (E) produced in each angular interval is a function mass and velocity (Eichhorn, 1974):
by the of the
E - mv4.R By normalizing the measured light energy to the mean ejecta velocity, a quantity is obtained which is proportional to the ejecta mass. By normalizing this quantity in each angular interval to the sum over all intervals, a relative mass distribution can be calculated. Such distributions for three impact velocities as a function of the ejection angle are shown in Fig. 6. The ejecta are concentrated in mainly one interval for impact velocities of 3 and 8 kms-‘. For 5 km s-l impact velocity, the mass is distributed over two ranges. This may be due to the fact that the mean ejection angle is at the boundary between these two intervals. The mean ejection angle of the secondary particles increases with increasing impact speed in the range between 40 and 60”. Gault and Heitowit (1963) and McDonnell et al. (1976), using silicate rock targets, observed the main ejecta mass at angles greater than 80”. 100
FE .. AU --:U-3.2KPlS-’ -:U+.2KMS-’ ---:U-7.RKMS-’ r-1 I I I I
0
FIG.
6.
THE
THE DIFFERENT
:--~.I
, , , ,
: / ’ i : : :
MASS
ANGULAR IMPACT
DISTRIBUTION
INTERVALS
P & z
: : : : :
IO 20 30 40 50 60 70 EJECTION RNGLE [DEGREE3 RELATIVE
.o r
: :
CONCLUDING
REMARKS
By measuring both primary and secondary impact generated light flashes, the velocity and angular distribution of the ejecta can be determined. The ejecta velocities and the ejection angle of the main ejecta mass increase with increasing impact velocity. The ratio between the secondary and primary velocity decreases from 4 at 3 km s-’ to 2.8 at 8 km SC’. These results are in good agreement with calculations and measurements of Gault et al. (1968). The main ejecta mass was observed to be concentrated in rather small angular intervals around ejection angles from 40 to 60”. The differing results of Gault and Heitowit (1963) and McDonnell et al. (1976), who detected the main ejecta mass at angles greater than 75”, may be due to the different target materials. Future measurements and calculations should permit determination of absolute ejecta masses and energies. REFERENCES
80
80
OF EJECTA
FOR THREE
VELOCITIES.
This may be due to the fact that the main mass is ejected in these cases with velocities smaller than plates which cannot be de1 km s-’ as spallation tected by the technique we used. Another possibility is that metal targets do not yield low-speed spallation material. Theoretical calculations of the impact process for Fe impacting anorthosite performed by O’Keefe and Ahrens (1976) also showed that only a very small fraction of the ejecta has velocities above 2.4 km s-l. However, their calculated mass and velocity distribution for the ejecta are quite different from the experimental distributions obtained in this work. These differences may be due also to the different target materials.
IN
DIFFERENT
Eichhorn, G. (1976). Analysis of the hypervelocity impact process from impact flash measurements. Planet. Space Sci. 24, 771. Fechtig, H., Gault, D. E., Neukum, G., and Schneider, E. (1972). Natutwissenschaften 59, 15 1. Gault, D. E. and Heitowit, E. D. (1963). The partition of energy for hypervelocity impact craters formed in rock. Hypervelocity Zmpact Symp. 2, 419. Gault, D. E., Quaide, W. L. and Overbeck, V. R. (1968). Impact cratering mechanics and structures. Shock Metamorphism of Natural Materials, 87-99 (1968). McDonnell, J. A. Jr., Flavill, R. P. and Carey, W. C. (1976). The micrometeroid impact crater communication distribution and accretionary populations on lunar rocks: exnerimental measurements. Proc. 7th Lunar Sci. Conf. 3, iim. O’Keefe, J. D. and Ahrens, T. J. (1976). Impact ejecta on the moon. Proc. 7th Lunar Sci. Cord. 3. DD. 3007-3025. Schneider, E. (1975). Impact ejecta’ex&ding lunar escape velocity. The Moon 13, 173.
PRIMARY
VELOCITY EJECTA
DEPENDENCE PARAMETERS
OF IMPACT
G. EIGDDORN” Max-Flanck-Institut fiir Kerphysik, Heidelberg, Federal Republic of Germany (Received in find form 15 Nouember 1977) Almbnet-From the light emitted during impacts of secondary particles produced during hypervelocity primary impacts, the velocities and relative masses of these ejecta were determined as a function of the angle between the ejection direction and the target surface. The velocity of the ejecta increases with increasing impact velocity and decreasing ejection angle. The ratio of the maximum ejecta velocity to the primary impact velocity decreases with increasing impact speed. The main fraction of the secondary particles is ejected in rather small angular intervals of about lo” width in elevation. The ejection angle of the main fraction of the ejecta mass increases with increasing impact velocity.
INTRODUCllON with light gas guns performed by Gault and Heitowit (1963) and Schneider (1975) yielded information about secondary particles produced during hypervelocity particle impact. Gault and Heitowit (1963) photographed the impact event with high speed cameras and measured from these pictures the ejecta velocity. Schneider (1975) counted craters on secondary targets produced by ejecta with velocities greater than 3 km s-t. From the crater size and angular distribution, an angular mass distribution of secondary particles was obtained. Similar experiments at a dust accelerator were performed by McDonnell et al. (1976) using a lunar rock as primary target. Earlier experiments at the dust accelerator (Eichhom, 1975) showed that information about secondary particles can be. obtained from the light flash produced by the ejecta. In this paper further measurements using this method will be presented. Experiments
EXPERIMENTALARRANGEMENT
A 2 W Van de Graaf dust accelerator was used to accelerate iron particles (Fechtig et al., 1972) onto a gold target with different selected velocities. The light was detected with photomultipliers of type EMI9558QB. Figure 1 shows the arrangement of target and photomultiplier. A glass cylinder was placed in front of the photomultiplier as a secondary target. The front side of this cylinder was shadowed except zones corresponding to a distinct * Present address: Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, U.S.A. 469
1: target 2:photomultiplier FIG.
1
3:glasacytinder &particle beam
1.
THEARRANGEMENTOFTHEPRIMARY (1) ANDSECONDARY(3)TARGETANDTHEPHOTOMULTiPLIER (2).
Secondary particles ejected from the primary target at ejection angles u produce light by hitting the front side of a glass cylinder (3). The side walls of the glass cylinder are silver coated to guide as much of the light as possible to the photomultiplier. To select ejecta at specified ejection angles a the front side of the glass cylinder is shaded with the exception of a zone around the specified angle.
ejection angle ((Y) measured with respect to the target surface. The zones had about 10” width in elevation. The smallest angle at which ejecta could be detected was 5.6”, the largest one 80”. This range was subdivided in seven intervals. Figure 2 shows the light flash recorded in one of these intervals (zone around ar = 50“). As shown in an earlier paper, the light signal consists of two parts. The first part (a) is due to the light produced during the primary particle impact. The second part (b) is produced by the impacts of ejecta produced during the primary impact. The difference between the detection times of the light bursts due to the primary (a) and secondary (b) impacts were combined
470
G.
Ercruio~~
FIG. 2. THE OSCILLOSCOPE RECORDING
OFITIEOWIPUT OF APHOTOMIJLTIPLIERSENSINGTHELlGHTPRQDLJCEDBYIRON PARTlCLESIMPACMNGAGOLDTARGETATAPRIMARYVELOCITY v =5.2 kin/s AMD SELECTEQ EIEtXION ANGLES BETWEENo!= 44O ANDo!=%*.
The first part (a) is due to the primary flash; the second part (b) is caused by the light produce d by ejecta impacting the glass cylinder in the unshaded zone tentered at cy= SO”. with the known distance between primary and secondary target to obtain the velocity of the fastest ejecta in this angular interval. The mean ejecta velocity was determined from the time between the first flash and the maximum of the second flash. This technique is only sensitive for ejecta with velocities greater than about 1 km s-l, and all conclusions must take into account this limitation.
0
IO 20 30 +o 50 80 70 EJECTION ANGLE DEGREE3
80
00
FIG. 4. THE MAXIMUM EJECTA VELOCITY FOR THREE WIFFERENTIhtI'ACTvELOCITIESASAFUNffIONOFTHEEIECIXON ANGLEWITHRESPECTTOTHETARGETSURFACE.
velocities have a maximum near X5” ejection angle and decrease with increasing angle to about 2 km s-l. At ejection angles greater #an 70”, the ejecta velocities increase again. In each angular interval, the ejecta velocity increases with increasing impact speed. The maximum secondary velocity varies from 13 to 22 km s-l depending on the primary velocity. Since the light produced by the ejecta is near the detection limit of the photomultiplier, the errors of the calculated velocities are rather large. They ape estimated to be about 30%. Figure 5 shows the ratio between secondary and
ejecta
As mentioned above, a maximum and a mean ejecta velocity can be calculated. These velocities are shown in Figs. 3 and 4 for different angular intervals and three different impact velocities. The
+: U4.2 KM S -’ 0: U-5.2 KM S -’ X: U-7.9 KM S -’
0
IO 20 30 if 50 60 70 EJECTION RNGLE [DEGREE]
80
so
FIG. 3. THE~~~AVEL~~F~R~REEDI~~~ IMPACTVELOCfflESASAFUNCITONOFTHEBJECI-IONANGLE WITHRESPECrTOTHETARGETSURFACE.
FIG.~. ~ERA~OOF~E~E~A~L~~A~~EPRIMARY~OC~PLO~DAGA~ST~PRI~ARYV~L~~.
The curve and hvo points are calculations and measurements of Gault er al. (1968).
471
Velocity dependence of impact ejecta parameters primary velocity as a function of the primary impact speed. Also shown are calculated and measured values of Gault et al., 1968. The measured values in both cases are somewhat lower than the values calculated by Gauit et al., but show a similar dependence on the primary velocity. EJEffA The
ejecta ejecta
MASS DHTRlBUTlON
integrated light energy (E) produced in each angular interval is a function mass and velocity (Eichhorn, 1974):
by the of the
E - mv4.R By normalizing the measured light energy to the mean ejecta velocity, a quantity is obtained which is proportional to the ejecta mass. By normalizing this quantity in each angular interval to the sum over all intervals, a relative mass distribution can be calculated. Such distributions for three impact velocities as a function of the ejection angle are shown in Fig. 6. The ejecta are concentrated in mainly one interval for impact velocities of 3 and 8 kms-‘. For 5 km s-l impact velocity, the mass is distributed over two ranges. This may be due to the fact that the mean ejection angle is at the boundary between these two intervals. The mean ejection angle of the secondary particles increases with increasing impact speed in the range between 40 and 60”. Gault and Heitowit (1963) and McDonnell et al. (1976), using silicate rock targets, observed the main ejecta mass at angles greater than 80”. 100
FE .. AU --:U-3.2KPlS-’ -:U+.2KMS-’ ---:U-7.RKMS-’ r-1 I I I I
0
FIG.
6.
THE
THE DIFFERENT
:--~.I
, , , ,
: / ’ i : : :
MASS
ANGULAR IMPACT
DISTRIBUTION
INTERVALS
P & z
: : : : :
IO 20 30 40 50 60 70 EJECTION RNGLE [DEGREE3 RELATIVE
.o r
: :
CONCLUDING
REMARKS
By measuring both primary and secondary impact generated light flashes, the velocity and angular distribution of the ejecta can be determined. The ejecta velocities and the ejection angle of the main ejecta mass increase with increasing impact velocity. The ratio between the secondary and primary velocity decreases from 4 at 3 km s-’ to 2.8 at 8 km SC’. These results are in good agreement with calculations and measurements of Gault et al. (1968). The main ejecta mass was observed to be concentrated in rather small angular intervals around ejection angles from 40 to 60”. The differing results of Gault and Heitowit (1963) and McDonnell et al. (1976), who detected the main ejecta mass at angles greater than 75”, may be due to the different target materials. Future measurements and calculations should permit determination of absolute ejecta masses and energies. REFERENCES
80
80
OF EJECTA
FOR THREE
VELOCITIES.
This may be due to the fact that the main mass is ejected in these cases with velocities smaller than plates which cannot be de1 km s-’ as spallation tected by the technique we used. Another possibility is that metal targets do not yield low-speed spallation material. Theoretical calculations of the impact process for Fe impacting anorthosite performed by O’Keefe and Ahrens (1976) also showed that only a very small fraction of the ejecta has velocities above 2.4 km s-l. However, their calculated mass and velocity distribution for the ejecta are quite different from the experimental distributions obtained in this work. These differences may be due also to the different target materials.
IN
DIFFERENT
Eichhorn, G. (1976). Analysis of the hypervelocity impact process from impact flash measurements. Planet. Space Sci. 24, 771. Fechtig, H., Gault, D. E., Neukum, G., and Schneider, E. (1972). Natutwissenschaften 59, 15 1. Gault, D. E. and Heitowit, E. D. (1963). The partition of energy for hypervelocity impact craters formed in rock. Hypervelocity Zmpact Symp. 2, 419. Gault, D. E., Quaide, W. L. and Overbeck, V. R. (1968). Impact cratering mechanics and structures. Shock Metamorphism of Natural Materials, 87-99 (1968). McDonnell, J. A. Jr., Flavill, R. P. and Carey, W. C. (1976). The micrometeroid impact crater communication distribution and accretionary populations on lunar rocks: exnerimental measurements. Proc. 7th Lunar Sci. Conf. 3, iim. O’Keefe, J. D. and Ahrens, T. J. (1976). Impact ejecta on the moon. Proc. 7th Lunar Sci. Cord. 3. DD. 3007-3025. Schneider, E. (1975). Impact ejecta’ex&ding lunar escape velocity. The Moon 13, 173.