Semiconductor projectile impact detector

A semiconductor projectile impact detector for use in determining micrometeorite presence as well as its flux and energy comprises a photovoltaic cell which generates a voltage according to the light and heat emitted by the micrometeorites upon impact with the cell. A counter and peak amplitude measuring device are used to indicate the number of particules which strike the surface of the cell as well as the kinetic energy of each of the particles.

FIELD OF THE INVENTION 
This invention relates to a method and device for the detection and 
investigation of cometary coma environments by determining the number 
density of the particulates in the coma, and also the kinetic energy of 
each of the particles. 
BACKGROUND OF THE INVENTION 
Due to the increased amount of space travel which has been conducted in the 
last few years, it has been discovered that many microscopic particulates 
and cometary comas travelling at great speeds are prevalent in deep space. 
These particulates which, for example, are present on the tails of comets, 
are of great interest to scientists as a tool in helping to unravel the 
many mysteries of deep space, as well as space engineers who study these 
particles in order to be able to design an adequate capsule, rocket or 
space vehicle skin which would be impervious to the repeated impacts of 
these small microscopic particulates. 
Prior art detectors include the use of optical devices and pressurized 
meteor detectors. Optical devices are unsatisfactory in the environment of 
interest because these devices tend to become light limited in the 
vicinity of a comet. Pressurized meteor detectors have limited application 
because they tend to respond only to particles having relatively high 
impact velocity and become inoperative after one impact. 
Other prior art devices utilize a solar battery or photovoltaic cell such 
as is disclosed in U.S. Pat. No. 2,944,250 to Outt. This patent shows a 
method of determining meteor flux by employing a number of series and 
parallel connected solar batteries. One of these batteries is utilized to 
indicate the missile roll rate and missile aspect angle of attack, while 
the other batteries are used as a means for measuring the integrated 
eroding effects of the high velocity meteorite particles. This patent 
gathers information regarding mass per unit time, particle number per unit 
time and particle-size distribution needed for calculating effects on 
skins of missiles and space vehicles by measuring the deterioration in the 
output of a solar battery due to the sandblasting effect of the cell by 
the micrometeorites. A first reading is taken at T.sub.1 and a second 
reading is taken at T.sub.2. The difference in the intensity of these 
readings therefore gives rise to meteor flux during the time period 
T.sub.2 -T.sub.1. Therefore, since the output of Outt gives an integrated 
reading, it cannot measure the exact number of impacts nor can it measure 
the kinetic energy of each of the particulates. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to overcome the defects 
of the prior art as indicated above. 
Another object of the present invention is to produce a projectile impact 
detector which may be used in deep space. 
Another object of the present invention is to produce a projectile impact 
detector which utilizes a photovoltaic cell. 
A further object of the present invention is to produce a projectile impact 
detector which counts the number of particulate impacts. 
Yet another object of the present invention is to produce a projectile 
impact detector which can measure the kinetic energy of each of the 
particulates which strike the detector. 
These and other objects of the present invention are accomplished by a 
device which utilizes a P/N type photovoltaic cell to measure the number 
density of particulates in a cometary coma. This cell is sensitive to the 
light and heat generated by a hypervelocity impact by a projectile giving 
rise to a voltage across the cell which can be measured by appropriate 
instrumentation. This instrumentation includes a counting device for 
counting the number of voltage pulses as well as a peak amplitude 
measuring device for measuring the peak amplitude of each of the voltage 
pulses. Since the light intensity generated by the impacting projectile is 
proportional to the kinetic energy of the projectile, the recordation of 
the peak amplitude of the voltage pulses generated by the photovoltaic 
cell enables the energy of each projectile to be computed. Therefore, if 
velocity information exists for the particulates, the particulate mass can 
be determined.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows the projectile impact detector which includes a photovoltaic 
cell 10 composed of P type semiconductor material 12 and N type 
semiconductor material 14 such as silicon, germanium or gallium arsenide. 
Although most solar batteries or photovoltaic cells which are used as 
power sources include a cover plate and interference filter that 
selectively filters certain wavelengths thereby preventing the solar 
battery from overheating, the cell of the present device does not include 
such a plate or filter so that the impacting projectiles 16 travelling 
toward the photovoltaic cell 10 actually impact the semiconductor 
material. If the impact detector is used to monitor particle occurrence in 
outer space, the cell 10 is affixed to the outer skin of the missile, 
capsule or the space vehicle. However, the invention should not be 
construed to be so limited, since it can be appreciated that it may be 
used in inner space environments; such as laboratories, where particle 
size and energy information are desired, or in situations where the outer 
space environment is simulated. 
A pair of "AC" coupling capacitors 18 are used so that only the energy of 
impacts above some threshold value are obtained. These capacitors are 
necessary to effectively filter out any extraneous noise or impacts of 
very minute particles which are of no interest. They also serve to block 
the D.C. component of the photovoltaic cell. Differential amplifier 20 is 
used to strengthen the signals obtained from the impacting of the 
projectiles onto the cell, as well as to detect the voltage produced by 
the cell. 
The signal output 30 of the differential amplifier 20 may either be 
transmitted to a suitable storage means 22, such as magnetic tape which is 
present in the missile, or it can be directly transmitted by a standard 
telemetry circuit 32 to a processing device 24 containing a counter 26 and 
a peak amplitude measuring device 28. The counter 26 is used to count the 
number of impacts on the photovoltaic cell 10, and the peak amplitude 
measuring means 28 is used to measure the peak voltage of each of the 
voltage pulses generated by the impact of the particulates. If the signal 
30 is stored in the storage means 22, once the missile or other capsule is 
recovered, the information stored therein is then directly transferred 
through appropriate circuitry 36 to the measuring device 24. 
The present invention operates in the following manner a single or a series 
of photovoltaic cells are affixed to the outer skin of an outer space 
vehicle such as a capsule or missile so that the properties of the 
particulates present in deep space can be analyzed, or if laboratory study 
is desired, the cells themselves are simply introduced into the 
environment of interest. The impact of projectiles 16 onto the 
photovoltaic cell 10 generates light and heat which is proportional to the 
kinetic energy of the projectile, and therefore is also proportional to 
the voltage generated by the cell 10. This voltage passes through AC 
coupling capacitors 18 and differential amplifier 20 so that only the 
number of impacts and energy of impacts above some threshold value are 
obtained. The generated signal 30 is then either transmitted directly to a 
data measuring device 24 or is stored in a suitable storage means 22 and 
then transmitted to the measuring device 24 at a later time. 
The measuring means 24 contains a counter 26 and peak amplitude measuring 
device 28. The counter 26 counts the number of impacts of projectiles and 
the peak amplitude measuring device 28 indicates the voltage generated by 
each of the impacts and therefore also indicates the kinetic energy of 
each of the projectiles. If, therefore, the velocity of each of these 
projectiles can be determined, the mass of the respective projectile can 
be ascertained. 
The present invention has been tested in laboratory environments by 
bombarding the photovoltaic cell with particles from a light gas gun using 
helium as the accelerating gas. It was found that good readings were still 
obtained even though the cell was struck many times. 
While this invention has been described in conjunction with a missile 
travelling in outer space, the present device can also be used for 
detecting simulated micrometeorite impacts in the laboratory. This usage 
would be for the design of spacecraft equipment in order to render it 
insensitive to meteorite impact below some threshold value. Furthermore, 
if used on a spacecraft utilizing a photovoltaic cell arrangement for 
spacecraft power, this same arrangement may be utilized as a large area 
meteoroid detector. 
Furthermore, while this invention has been described in great detail, it 
will be obvious to those skilled in the art that various changes may be 
made without departing from the scope of the invention and that the 
invention is not to be considered limited to what is shown in the drawing 
and described in the specification.