Response of microphone meteorite detectors to impact of high velocity particle

Friichtenicht, J. F.

English

NASA Technical Reports Server

k' 
8 
4158-6010-TU-000 
TECHNICAL REPORT 
RESPONSE OF MICROPHONE METEORITE DETECTORS TO THE 
IMPACT OF HIGH VEWITY PARTICLES 
(THRUI 
May 1965 
GPO PRICE $ 
OTS PRICE(S) $ 
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Prepared for 
National Aeronautics and Space Administration 
Washington, D. C. 20546 
Contract N o .  NASw-936 
4.2 3& 
f/ Prepared by -
J .  F .  Fr i ichtenicht  D. B. Langmuir 
Director 
PHYSICAL ELECTRONICS LABORATORY 
Physical  Research Divis ion 
TRW Space Technology Laboratories 
One Space Park, Redondo Beach, California 
https://ntrs.nasa.gov/search.jsp?R=19650015386 2020-03-16T23:54:31+00:00Z
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RESPONSE OF MICROPHONE METEORITE DETECTORS TO THE 
IMPACT OF HIGH VELOCXTY PARTICLES* 
I. INTRODUCTION 
Most of t he  d i r e c t  measurements on the  p rope r t i e s  of micro- 
meteoroids have u t i l i z e d  microphone de tec to r s .  These c o n s i s t  of 
p e z o e l e c t r i c  c r y s t a l s  which a r e  mechanically coupled t o  m e t a l l i c  
p l a t e s  o r  "sounding boards''. The c r y s t a l  responds t o  mechanical 
v i b r a t i o n s  induced i n  the  sounding board by t h e  impact of a micro- 
meteori te .  The output of t h e  microphone is an e l e c t r i c a l  s i g n a l  
with an amplitude proport ional  to t he  magnitude of t h e  mechanical 
v ib ra t ions .  Despite the  widespread use of t h i s  type of sensor,  
t he  o v e r a l l  response of the  de tec tor  to dynamic p rope r t i e s  of the  
meteoroids has not been firmly e s t ab l i shed .  In  t h e i r  comprehensive 
compilation of meteori te  data ,  Alexander, e t  a l l  assumed a momentum 
dependence based on the r e s u l t s  of tes ts  conducted a t  very low 
impact v e l o c i t i e s .  A momentum dependence has a l s o  been repor ted  
f o r  impact v e l o c i t i e s  up t o  about 4 kdsec.  In c o n t r a s t ,  the  
t h e o r e t i c a l  treatment of Stanyukovich3 and t h e  experiments of 
Denardo using l a r g e  p a r t i c l e s  a t  v e l o c i t i e s  up t o  7 . 8  km/sec 
suggest  an energy dependence 
2 
4 
;This r e p o r t  g ives  t h e  r e s u l t s  of t e s t s  using very small  
p a r t i c l e s  with impact Veloci t ies  up t o  7 . 5  ludsec. Although the  
r e s u l t s  a r e  not  completely d e f i n i t i v e ,  they show t h a t  t he  o v e r a l l  
response of a microphone de tec tor  l ies  somewhere between t h e  
momentum and energy dependences suggested by o the r  experimenters. 
_- 
11. m R I W T A L  APPARATUS AND PROCEDURES 
5 - High speed p a r t i c l e s  from the STL e l e c t r o s t a t i c  acce le ra to r  
w e r e  used f o r  t h e  measurements described i n  t h i s  r e p o r t .  In  t h i s  
device,  small  p a r t i c l e s  a re  charged e l e c t r i c a l l y  by an induct ive 
process6 and a r e  in j ec t ed  i n t o  t h e  acce le ra t ing  f i e l d  of a 2- 
mil l ion  v o l t  Van de Graaff generator.  The f i n a l  ve loc i ty  a t t a i n e d  
* 
This  work conducted under NASA Contracts NAS5-763 and NASw-936. I 
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by t h e  p a r t i c l e  is given by 
where Va is the  acce le ra t ing  voltage and m and q a r e  the p a r t i c l e  
mass and charge. As shown i n  Ref. 6, the q/m of the  p a r t i c l e s  is 
propor t iona l  t o  the  r ec ip roca l  0% p a r t i c l e  rad ius .  Consequently, 
t he  e l e c t r o s t a t i c  method of acce lera t ing  p a r t i c l e s  is appl icable  
only t o  small  p a r t i c l e s .  Under optimum condi t ions,  a 1 micron 
diameter i ron  p a r t i c l e  achieves a f i n a l  ve loc i ty  of about 7 km/sec. 
Higher v e l o c i t i e s  can be achieved with smaller p a r t i c l e s ,  but the  
momentum of smaller p a r t i c l e s  is reduced correspondingly. The 
l i m i t i n g  s e n s i t i v i t y  of the  microphone d e t e c t o r s  prohib i ted  the  
use of higher ve loc i ty  p a r t i c l e s  f o r  these  experiments. 
The charge and veloci ty  of each p a r t i c l e  from the  acce ler -  
a t o r  are determined p r i s r  t o  inpaca; by measuring t h e  magnitude and 
dura t ion ,  respec t ive ly ,  of a voltage s i g n a l  induced on a c y l i n d r i c a l  
d r i f t  tube of known capacitance and length which t h e  p a r t i c l e  
t r a v e r s e s  a x i a l l y .  The charge is given by q = CVi where Vi is the  
magnitude of t he  induced voltage pulse and C is the  capacitance of 
t he  d r i f t  tube. The veloc i ty  is simply v = l/t where t is the  
t i m e  requi red  t o  pass through a cy l inder  of length a .  Knowing t h e  
acce le ra t ing  voltage,  the  mass of the p a r t i c l e  can be computed 
from Eq. (1). 
The s i g n a l  on t h e  d r i f t  tube was amplif ied and displayed on 
one t r a c e  of a dual  beam osci l loscope w h i l e  the s i g n a l  from the  
microphone d e t e c t o r  was displayed on t h e  o ther .  Both traces were 
t r igge red  by t h e  leading edge of t he  voltage pulse from the 
velocity-charge de t ec to r .  By taking account of t he  f l i g h t  t i m e  
from the  charge de t ec to r  t o  t h e  po in t  of impact, one can be 
assured t h a t  both s i g n a l s  were produced by t h e  same p a r t i c l e .  
The c r y s t a l  transducer acquired for these tes ts  was t y p i c a l  
of those used i n  f l i g h t  experiments. It cons is ted  of a c y l i n d r i c a l l y  
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shaped lead z i r cona te  c r y s t a l  encapsulated i n  an aluminum can. 
Preliminary tests showed t h a t  the microphone was not  s u f f i c i e n t l y  
s e n s i t i v e  t o  be used d i r e c t l y  w i t h  p a r t i c l e s  from the  acce le ra to r .  
A s  a consequence of t h i s ,  t he  c r y s t a l  was removed f r o m  the  can and 
suspended i n  a rubber grommet with i t s  a x i s  p a r a l l e l  t o  the  beam 
of p a r t i c l e s  from the  acce lera tor .  The s i g n a l  from t h e  c r y s t a l  
w a s  der ived from a pickup loop a t  one end of the c r y s t a l .  The 
f ace  of the  c r y s t a l  exposed t o  t h e  beam of p a r t i c l e s  was grounded. 
Two types of impact su r f aces  were used, The f i r s t  was a l a y e r  of 
conducting s i l v e r  pa in t  w h i l e  the  o ther  was a polished aluminum 
d i s c  cemented t o  t h e  c r y s t a l .  Since there was no apparent 
d i f f e rence  i n  response f o r  the d i f f e r e n t  sur faces ,  the r e s u l t s  
from both were combined i n  the da ta  ana lys i s .  
The s i g n a l  f r o m  t h e  microphone was f e d  t o  a wide band 
ampl i f ie r .  The waveform a t  t h e  output of t h e  ampl i f ie r  was an 
exponent ia l ly  decreasing o s c i l l a t o r y  s i g n a l  with frequency of 
about 100 kc a s  i l l u s t r a t e d  i n  Fig.  1-a. The s i g n a l  shown r e s u l t e d  
from the  impact of a glass bead dropped from a height  of a few 
cent imeters .  To reduce some of t he  low frequency noise  problems 
a s soc ia t ed  with mounting the  microphone i n  the  t e s t  chamber, the 
amplif ied s i g n a l  was fed  through a r e l a t i v e l y  l o w  Q resonant 
c i r c u i t  tuned t o  the  resonant frequency of t h e  c r y s t a l .  The 
c i r c u i t  is shown in Fig. 1-b and a t y p i c a l  s i g n a l  is i l l u s t r a t e d  
i n  Fig.  1-c. The amplitude of t h e  microphone s i g n a l  w a s  measured 
a t  t h e  point  of maximum excursion from the base l i n e .  Since the  
c i r c u i t r y  is composed e n t i r e l y  of l i n e a r  elements and t h e  f r e -  
quencies are f ixed,  it can be shown t h a t  t h e  measured maximum 
amplitude is always proportional to t h e  amplitude of the f i r s t  
ha l f  cyc le  a t  t he  output of t h e  c r y s t a l .  
111. RESULTS 
Most of the  hypotheses regarding the  response of m i c r o -  
phone detectors can be reduced t o  the  f o r m  V = K mv where t h e  
value of K depends upon the  nature of the impact, m and v a r e  
01 
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I 
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1 J 
WIDE BAND 
AMPLIFER - - 
T 
Figure 1-a 
CRYSTAL 
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Figure 1-b 
Figure 1-c 
o s c l L L o s ~  I
Figure 1. Schematic Diagram of t h e  c i r c u i t  used i n  t h e  Micro- 
phone Detector Tests. The waveform a t  the t o p  of t h e  
f igu re  is obtained a t  the  output of t he  ampl i f i e r  
while t h e  waveform a t  t he  bottom is a t  t h e  output 
terminal  of t h e  tuned c i r c u i t .  
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t h e  p a r t i c l e  mass and v e l o c i t y ,  and 3~ is an undetermined exponent. 
There are e s s e n t i a l l y  three  types of impacts t o  be considered. 
These a r e  e las t ic  impacts, completely i n e l a s t i c  impacts, and in-  
e las t ic  impacts enhanced by t h e  expulsion of material f r o m  t h e  
crater which increases  the  momentum imparted t o  the  c r y s t a l . ;  For 
e l a s t i c  impacts I( = 2 and a - 1. For t h e  completely i n e l a s t i c  
case IC and a would both be unity.  The t h i r d  type of impact is 
probably the  most appl icable  in t he  present  context  and is cer- 
t a i n l y  the  m o s t  d i f f i c u l t  t o  i n t e r p r e t .  R. L. Bjork, quoted by 
Dubin, contends t h a t  t h e  momentum con t r ibu t ion  from expel led 
ma te r i a l  may be a s  much a s  two and one-half times the momentum of 
t h e  impacting p a r t i c l e  a t  70 km/sec. Although it is probably an 
overs impl i f ica t ion  of the  case, t h e  assumption has been made t h a t  
K is a constant  over t h e  experimental range and t h a t  t he  ve loc i ty  
dependence is contained completely by t h e  exponent a. 
c- 
7 
The da ta  are presented i n  t w o  forms. In Fig. 2 t he  m i c r o -  
phone output s i g n a l  is norml ized  t o  the  p a r t i c l e  m a s s  and p l o t t e d  
a s  a func t ion  of impact ve loc i ty .  Since the  p a r t i c l e  parameters 
a r e  near t h e  l i m i t i n g  s e n s i t i v i t y  of t he  microphone system, t h e  
da t a  po in t s  e x h i b i t  considerable s c a t t e r  which p r o h i b i t s  p rec i se  
measurements. However, t he  t rend of the  da ta  is obvious. A l i n e  
drawn through the  da t a  po in t s  by eye a s  shown y i e l d s  a value f o r  
cx. of 1.7. This is i n  con t r a s t  t o  a = 1 f o r  momentum dependence 
and a = 2 f o r  an energy dependence. 
The da ta  from Fig. 2 are r e p l o t t e d  i n  Fig. 3 t o  more 
adequably demonstrate the  departure  f r o m  t h e  commonly assumed 
momentum dependence. Here the magnitude of t h e  microphone output 
s i g n a l  is p l o t t e d  aga ins t  p a r t i c l e  momentum. An independent 
momentum c a l i b r a t i o n  point  was obtained by dropping p l a s t i c  beads 
weighing approximately 100 micrograms on t h e  f ace  of t h e  c rys t a l  
from a known height .  This was done i n  a i r ,  but no drag c o r r e c t i o n s  
were made nor was t h e  c o e f f i c i e n t  of r e s t i t u t i o n  taken i n t o  account. 
However, the computed momentum was good to about 10% which is 
s u f f i c i e n t  f o r  comparison purposes. Using t h i s  point  and the  
. 
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10.0 
PART IC LE VELOCITY (KM/S EC) 
Figure 2.  Amplitude of Microphone s i g n a l  normalized to  p a r t i c l e  
mass p lo t ted  a s  a funct ion of impact v e l o c i t y .  
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I I I I I 
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o r i g i n  a s  t h e  o ther ,  t h e  e las t ic  impact l i n e  corresponding t o  
the t r a n s f e r  of two u n i t s  of momentum was drawn on the  graph. 
Nearly a l l  of t h e  da t a  po in t s  obtained w i t h  high ve loc i ty  p a r t i c l e s  
l i e  above the  e las t ic  impact l i n e .  This i n d i c a t e s  t h a t  t h e  ejected 
ma te r i a l  imparts a t  l e a s t  a s  much (and i n  m o s t  cases  more) momentum 
than does the  impacting p a r t i c l e  over the  ve loc i ty  range covered. 
IV. SUMMRY 
Although considerable modification of a t y p i c a l  microphone 
detector was requi red  i n  order t o  obta in  measurable s i g n a l s ,  t h e  
r e s u l t s  of the measurements described above a r e  q u i t e  general  i n  
nature .  
cannot be charac te r ized  a s  simply momentum or energy dependent.; In 
order  t o  more f u l l y  a s s e s s  the s i t u a t i o n ,  a much more detai led 
s tudy should be conducted. On the  other hand, the  empir ical  ap- 
proach, such as t h a t  u t i l i z e d  here, provides information which 
should be considered i n  t h e  ana lys i s  of da ta  from f l i g h t  experi- 
ments. 
The k e s u l t s  imply t h a t  the  response of microphone d e t e c t o r s  
vw 
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REFERENCES 
1. W. M. Alexander, C. W. Mecracken, L. Secretan, and O.E. Berg. 
Paper presented a t  the Cospar Meeting, Amsterdam, May, 1962. 
(To be published.)  A l s o  avai lable  a s  NASA TN D-1669, 1962. 
2. G.  I). Anderson, e t  a l ,  Proc.  Third Symposium on Hypervelocity 
Impact, F. Genevese Ed., Armour Research Foundation, I l l i n o i s  
I n s t i t u t e  of Technology, Chicago, 1959, p. 361. 
3. K. P. Stanyukovich, A r t i f i c i a l  Earth S a t e l l i t e s ,  V o l .  4, 
Plenum Press, New York, 1961 9 P *  292. 
4 .  B. Pat Denardo, NASA Technical Note TN D-1210, 1962. 
5 .  J. F, Fr i ich ten ich t ,  Rev. Sci. I n s t r . ,  Vol. 33 (1962) p. 209. 
6 .  H. Shelton, C. D. Hendricks, Jr., and R. F. Wuerker, J. Appl. 
Phys., Vol. 31 (1960) p. 1243. 
7 .  M. Dubin, Space Research, Proc. of t h e  F i r s t  In te rna t iona l  
Space Science Symposium, Interscience Publ ishers ,  Inc., 
N e w  York, 1960, p. 1042. 


Response of microphone meteorite detectors to the impact of high velocity particles

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