Designing the Experiment
The August 5, 1995, mission aboard the Kuiper Airborne Observatory was an excellent opportunity to measure the relationship between cosmic ray intensities and altitude. This may provide a measure of the increased exposure to cosmic rays by passengers on commercial air flights as well as high altitude pilots or astronauts. Continued ground-based experiments by students will extend this data to address many other variables.
Cosmic ray research began in 1911 when researchers, attempting to explain how shielded electroscopes were spontaneously discharging, realized that we are constantly being bombarded by some sort of "rays" from the "cosmos". Subsequent research has shown that these "rays" are actually particles and are divided into two kinds, "primary" and "secondary".
Primary cosmic rays are known to be subatomic particles arriving from outer space which have high energies as a result of their extremely high velocities. About 87% of primary cosmic rays are protons (hydrogen nuclei), and about 12 percent are alpha particles (helium nuclei). Most of these primary cosmic rays have energies of 0.3 GeV (3x108 eV) which corresponds to a velocity of 2/3 the speed of light. However, some "primary" cosmic rays have energies up to 1020 eV. For comparison, the highest energy particles produced in labs here on earth is on the order of 1012 eV.
Very few of these primary particles survive passage through our atmosphere. Here at the surface of the earth, only about 3% of all rays are still primary. Collisions of these primary particles with atoms in our atmosphere produce showers of a variety of energetic particles. These particles in turn decay into other particles. The end result is here under the atmosphere we mostly see muons. Muons are subatomic particles that, like electrons have charges of -1 but have rest masses about 200 times that of the electrons. Essentially, muons are hyper electrons on steroids. Compared to an electron, when a muon slams into something, it is like a collision with a Mack truck on the freeway instead of a schwinn bicycle in the parking lot!
According to the Particle Data book, 75% of all particles at sea level are muons with average energies of about 2 GeV. At this energy, muons lose energy at the rate of 1.3 MeV/g/cm2. The density of air is 0.001 g/cm3 and for comparison, iron is 8 g/cm3. The range (or penetration depth) in air is 1.5 x 106 cm (about 9 miles), on the order of the thickness of our atmosphere. These same muons, on the other hand, have a range in iron of about 190 cm. In other words, shielding us from cosmic rays would require almost 2 meters of iron! This is clearly impractical.
At 41,000 ft we were flying at an altitude of 12.5 km or 7.8 miles.
Based on the ICAO standard Atmosphere model (ref. CRC handbook):
… The composition of the atmosphere varies little from sea level with the exception of water vapor (which is virtually absent above 8-18 km) and carbon dioxide.
… The temperature varies linearly with altitude down to about -50 deg C at 12 km.
… The pressure and density vary approximately as exponential relations with altitude as follows:
where: r = density (kg/m3)
P= air pressure (kg/m2)
h= altitude (km)
Particle Data Group, "Review of Particle Properties", Phys. Rev. D50, part 1, 1994
Harpell, Langeveld, et.al., "The CCRT: an inexpensive cosmic ray muon detector", SLAC-TN-95-1, Stanford Linear Accelerator Center
Willy Langeveld, pc, 7/26/95
"Cosmic Rays", Encarta Æ, Copyright © 1994, Microsoft Corp.
"Handbook of Chemistry & Physics", 50th edition, CRC
Charged particles can be detected by use of "Scintillator" panels. These are sheets of a special plastic that scintillates or sparkles whenever a charged particle passes through it. These sparkles are extremely faint so we must completely block off all other sources of light by wrapping the scintillator panel in many layers of black tape and enclosing a very sensitive detector for light called a "Photomultiplier" or PM tube. Whenever a charged particle passes through the panel it will produce a few photons of light which will in turn, cause the PM tube to produce a tiny pulse of electricity.
We are constantly being bombarded with charged particles from many sources besides cosmic rays. We can distinguish between cosmic ray muons and other charged particles by the fact that muons have much more energy than most other particles and they are coming down from outer space instead of up from the ground. Any particle that can pass through both panels when they are oriented vertically is assumed to be a cosmic ray muon.
The detector consists of two scintillator panels and the electronics needed to count the number of pulses that occur simultaneously in both panels. The panels are oriented one above the other so that only particles coming from above can pass through both panels. The complete design is detailed in the SLAC technical note referenced above.
Muon intensity may vary not only with the altitude and angle from vertical but also with latitude and time of night (i.e. the relative position of the sun and the earthís magnetosphere). Because the KAO is moving and the earth is turning we will be unable to hold these variables constant. Therefore we must record any changes in these variables and use subsequent experiments to determine what, if any, effect they have.
I received clearance and installed the CCRT in the KAO after the Saturday, 8/5/95, 1 pm briefing. Baseline measurements were run during the 8 pm pre-flight briefing. Power was turned off during taxi, takeoff, and dead legs. Takeoff occured at 9:15 pm Saturday, 8/5/95. Data taking resumed at 9:53 pm when we leveled out at 37,000 feet. The counter was cycling so fast that I quickly modified the experiment plan from counts per 15 minute interval to measuring the time needed to fill the counter once (256 counts) or four times (1024 counts). The CCRT was maintained in a vertical position. That is its orientation was perpendicular to the aircraftís floor. Except briefly during turns, takeoff, and landing; this was parallel to the earthís vertical.
At least every 15 minutes I recorded the:
… test voltages (both PM base voltages, and the +6 / -6 supply voltages),
… flight data: the altitude, latitude, longitude
… any comments about conditions
I planed to continue the data at least 1 hour after landing. However, again, the power was turned off upon beginning descent and remained off after landing.
The voltages were set and maintained to optimum levels determined prior to leaving home (see procedure in SLAC technical note as well as the CCRT Calibration Data sheet).
Since the CCRT is designed to count (c) discrete independent events over a time period (t), the counts should follow a Poisson distribution. The desired result of each measurement will be a time rate,
The cosmic ray rate is subject to a continuous random variation over time, successive measurements will more closely follow a gaussian distribution. However, since measurements were made in a variety of modes and time intervals, each has a different standard deviation. Because of this, the best estimator of the long term rate will be an rms average, weighted by the standard deviation.
The results have been graphed (see Data Analysis). The error bars are ± one standard deviation. The last data item of each set is the average of the preceeding items. I believe the graphs show good consistancy of the data with the exception of the first few measurements made on board the KAO at 37,000 ft. I think it reasonable to eliminate the extreme data point for reasons given in the flight log.
On page 6, I have plotted the average data at each elevation as well as a best-fit exponential curve approximating the data. The data certainly appears to fit an exponential curve, however the significance of the derived exponential parameters is at this time unknown.