The state of cosmic ray physics is reviewed. It is concluded that the nonexistent lunar magnetic field, the low lunar radiation background, and the lack of an atmosphere on the Moon provide an excellent environment for the study of high energy primary cosmic rays

Linsley, John

English

NASA Technical Reports Server

I 
N89- 15817 
COSMIC-RAY DETECTORS ON THE MOON 
John Linsley 
Department of Physics and Astronomy 
and Institute for Astrophysics 
University of New Mexico, Albuquerque, New Mexico 87131 
The Place of Cosmic-Ray Studies in Astronomy 
In the context of astronomy, cosmic rays are charged particles - usually protons or 
electrons - that have been accelerated through the action of naturally occurring electric fields, or 
they are  neutral secondaries of such particles. Presumably, the electric fields are a consequence of 
changing magnetic fields. The high energy of the particles shows that these fields are remarkably 
strong and extensive. Cosmic rays provide direct and conclusive evidence that particle acceleration is 
a ubiquitous and energetically very important feature of the astronomical landscape, which increases 
in importance as one leaves the familiar territory inhabited by ordinary stars to explore the extreme 
conditions present in supernova explosions, near collapsed stars, and near active galactic nuclei. 
Although the cosmic rays studied directly near the Earth a re  almost all protons or other nuclei 
until recently, the indirect evidence for cosmic rays in distant parts of the universe has shown only 
that high-energy electrons are  present. This evidence comes from synchrotron radiation. Analogous 
evidence for the presence of high-energy nuclei, hence for the fields that have accelerated them, is 
given by gamma rays and neutrinos arising from the decay of pions produced in hadronic 
interactions. 
Cosmic rays also provide the only directly accessible sample of matter from outside the solar 
system. Since they can travel very great distances, they may reveal the existence of mirror-image 
galaxies consisting entirely of antimatter. Carbon-14 and other “cosmogenic” nuclides provide 
information about the activity of the Sun in prehistoric times, whereas, on shorter time scales, 
cosmic-ray modulation is an  important source of information about conditions in the heliosphere far 
from the ecliptic plane. 
The Scale and Charact& of Cosmic-Ray Research 
The current scale and character of worldwide cosmic-ray research is shown in table I in the 
form of a program summary of the Nineteenth International Cosmic Ray Conference, which took 
place in La Jolla, California, in the summer of 1985. These conferences have been held every 2 years 
since the first one in Kracow, Poland, in 1947. Like most conferences, they have grown, so that the 
attendance has been about 500 in recent years. The number of contributed papers has been about 
1000; thus, even with a limit of 4 pages each, one is faced every 2 years with 10 or more thick volumes 
of conference proceedings. The number of professional scientists engaged in this work is about 1000. 
This number may be compared to Virginia Trimble’s estimate that, currently, there are about 7000 
professional astronomers worldwide. 
In 1985, about one-fourth of the sessions (those listed under “High-energy interactions”) 
concerned the application of cosmic rays to the study of particle physics rather than astronomy. Some 
of this activity, but not all, will dry up in the next few decades because of competition from manmade 
accelerators (colliding beam machines). A large percentage of the papers (almost 80%) concerned 
observations (called “experiments” in this context) and the manner of performing them. 
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Advantages and Disadvantages of the Moon as a Base for Cosmic-Ray Observations 
Advantages 
Some advantages of the lunar surface as a location for performing cosmic-ray observations 
follow. 
1. Absence of atmosphere 
2. Weak magnetic field 
3. Low radiation background 
4. Vast extent 
5. Enormous mass, for use as shielding, as tarwet material, in construction, and as raw 
material for manufacturing 
6.  Stability 
7. Low gravity 
8. High vacuum 
9. Variety of environments (within a small range) by choice of latitude 
The absence of a n  atmosphere has overwhelming importance. As humans, we tend to take for 
granted the atmosphere on planet Earth. We overlook the fact that  its transparency to very-low- 
energy electromagnetic quanta is exceptional, a consequence of the capability of these quanta to 
propagate through the atmosphere in the form of waves. However, to higher energy electromagnetic 
quanta and to fast particles, our atmosphere is a formidable barrier; at sea level, it is equivalent to 
33 feet of water. All the cosmic rays one observes a t  the Earth's surface consist of secondary particles, 
a greatly altered remnant of those striking its'atmosphere from above. 
Another form of shielding is provided by the Earth's magnetic field. Except near the geomag- 
netic poles, this field prevents charged particles from reaching the Earth's surface unless their energy 
is higher than 1 to 10 GeV, three orders of magnitude above the energy of charged particles produced 
by radioactivity . 
It follows that, with a few notable exceptions, observations of primary cosmic rays must be 
performed currently using balloons, rockets, or space vehicles. Thus, for the majority of observations 
listed in table I, the Moon is a n  excellent site. The exceptions include, of course, some of the solar and 
heliospheric observations requiring special locations in the solar system. The other exceptions are  
observations of nuclei, gamma rays, neutrinos, and interactions of these particles, in the highest 
energy range, for which the Earth's atmosphere is used as a n  amplifying device in the production of 
extensive air  showers. 
In a number ofcases, the Moon is not just  a n  alternative to artificial satellites. Its other 
advantages - of vast extent, enormous mass, and low level of background radiation - will enable 
performing cosmic-ray observations on the Moon that otherwise would be physically impossible or 
economically impractical. One of these cases, neutrino astronomy, has been described in another 
56 
paper; others a re  described in the final portion of this paper, following some comments about 
scientific style and its possible implications with respect to cosmic-ray detectors on the Moon. 
Disadvantages 
Some disadvantages of the lunar surface as a base for performing cosmic-ray observations are  
the high cost of transport from Earth and the absence of an atmosphere and of oceans. 
The Scientific Backdrop 
Since they first began, cosmic-ray studies have manifested a special style which may still 
influence the manner in which cosmic-ray physicists react to opportunities afforded by the 
establishment of permanent bases on the Moon. Cosmic rays were discovered not in the laboratory 
but a t  great heights by placing ionization chambers in manned balloons (Hess 1912). The 
extraordinary penetrating power that distinguishes cosmic rays from other forms of natural radiation 
was demonstrated not in the laboratory but by lowering ionization chambers to suitable depths in 
mountain lakes at different altitudes in the California Sierras (Millikan 1923-26). Proof that 
primary cosmic rays are  mostly charged particles (Clay 1928-33, A. H. Compton 1930-36) and that 
their charge is positive (T. H. Johnson, L. Alvarez, Rossi and DeBenedetti 1933) was obtained not in 
the laboratory but by using the Earth’s magnetic field, observing first the latitude effect and then the 
east-west effect. Proof that cosmic-ray energies can measure a t  least 1015 eV (Auger e t  al. 1938) 
depended on the fact that particles with this much energy generate enormous atmospheric cascades, 
called extensive air  showers. Considering just the solid angle, any air  shower experiment a t  sea level 
makes essential use of a cone-shaped chunk of atmosphere weighing some 1010 kg. This mass 
explains why the lack of an  atmosphere can be a disadvantage for locating cosmic-ray detectors on the 
Moon. 
The greatest contribution of cosmic-ray studies to science came from discoveries leading to a 
new branch of physics, particle physics, now performed almost entirely using giant particle 
accelerating machines. Some of these discoveries, of the positron, of ymesons or muons, of charged n- 
mesons or pions, of kaons, and of hyperons, were made in a conventional laboratory setting. Others 
involved going out of conventional laboratories and setting up unique new bases on the highest 
accessible mountains: on Mt. Evans in the Unitedstates, on the Jungfraujoch, Aiguille du Midi, and 
Testa Grigia in the Alps, on Chacaltaya in Bolivia, and in the Pamirs of central Asia. Going to the 
other extreme, the discovery of cosmic neutrinos was made using the deepest accessible mines, a t  
Kolar Gold Field in India and in South Africa. 
These accomplishments illustrate a certain scientific style. The physicists who performed 
these feats were equipped with an especially broad range of knowledge and skills. But they also were 
marvelously opportunistic, using whatever was available in the environment, and later on, using 
whatever new vehicles were introduced to gain access to new environments. The discovery of the 
Van Allen radiation belts was the first discovery using a space vehicle. There are  important cosmic- 
ray detectors still functioning on the Explorer spacecraft, which a re  now reaching the boundary of 
interplanetary space. 
The important advantages offered by the Moon no doubt will be exploited for the study of 
cosmic rays. However, this opportunistic style makes it hard to predict the form of cosmic-ray 
detectors on the Moon. Most of the earliest detectors will be similar to instruments already in use. 
Others will take advantage of unrelated activities in progress on the Moon a t  later times. Planners 
should be warned, perhaps, that cosmic-ray scientists are used to not paying for some essential parts 
57 
of their equipment such as mountains, mines, or the overlying atmosphere. Planners must guess first 
what other investigators will be doing and then make guesses about how cosmic-ray scientists might 
avail themselves of the new research environment. Some of these guesses follow. 
Cosmic-Ray Detectors on the Moon 
Most studies of solar and heliospheric cosmic rays and of cosmic-ray phenomena require long- 
term observations using relatively small, lightweight detectors. Studies of gamma-ray bursts are  
similar in this regard. The lunar surface is an  attractive location for work in these fields, but there is 
no obvious way to enhance the value of this work through the application of lunar resources. 
The kind of cosmic-ray research that is likely to be performed on the Moon in the early stages of 
lunar base development is research in the low-energy range on constituents such as ultraheavy 
nuclei, positrons and electrons, gamma rays, antiprotons and antinuclei, and certain secondary 
isotopes useful as cosmic-ray clocks. These constituents share the property of being very sparse; 
therefore, detectors need to be very large, although they can be lightweight in relation to size. The 
detectors are  well suited to modular design and for incremental assembly. By basing detectors of this 
kind on the Moon rather than on a space station, money may be saved in the areas of deployment, 
mechanical support, and some aspects of maintenance. 
At higher energies, the intensity decreases; thus, the need for large detectors becomes even 
greater. I t  also becomes difficult to measure the energy of the particles and to differentiate between 
particles. As a rule, a target of some kind in which the particles undergo collisions with stationary 
nuclei must be provided. The detector must be very heavy and very large. Since unprocessed lunar 
soil is entirely adequate for use as a target and a n  absorber, the Moon has a n  overwhelming 
advantage over space stations as a base for detectors of every sort of very-high-energy (VHE) 
cosmic rays. 
Illustrative Examples 
Figure 1 shows the manner in which a detector of high-energy (greater than 1 TeV) gamma 
rays, electrons, and charge-resolved nuclei might be incorporated in the protective shield of a 
structure intended primarily for manufacturing or research of some kind unrelated to cosmic rays. 
The structure is imagined to be roughly cylindrical, 20 m in diameter and 50 m long. For the 
protection of personnel from the intense low-energy cosmic rays, it  has been shielded with a 5-m-thick 
layer of lunar soil. I have imagined that duringconstruction of the shield, several layers of 
lightweight gas-filled counters were deployed a t  intermediate depths. A similar layer of counters lies 
on top of the shield, and a final layer is attached on the inside to the ceiling of the tank. 
The uppermost layer of counters measures the charge of individual incoming particles. The 
first layer of soil beneath it acts as a target in which the particles initiate cascades. The higher the 
incident energy, the larger the cascades will be and the further they will penetrate. The number of 
particles reaching successive layers of counters provides a measure of the primary energy and also 
serves to discriminate between incident hadrons (nuclei) and leptons (electrons) or gamma rays. 
The product of detection area and solid angle is several thousand mz-steradians; thus, the 
cosmic-ray counting rate will be several thousand per year for particles with an  energy greater than 
1016 eV. Useful results on cosmic-ray composition will be obtained to a maximum energy of 
1017 eV per particle, the energy spectrum of electrons will be measured up to about 1014 eV, and 
gamma rays from point sources will be observed up  to a t  least 1014 eV. , 
58 
Lightweight counters can be constructed from metallized plastic foil as indicated in figure 2, 
and the cylindrical shape of the individual cells is maintained by the pressure of the filling gas. The 
tendency for distortion produced by the pressure of the overlying soil is reduced by the low gravity of 
the Moon, so the filling pressure does not have to be excessively great. Perhaps the filling gas, 
typically consisting mainly of argon, can be obtained as a byproduct of the gas extraction plants some 
authors have envisioned for generating hydrogen on the Moon. 
Another interesting possibility is to measure the intensity of ultra-high-energy (UHE) 
antiprotons, using the Earth’s magnetic field to separate them from the much more abundant 
protons. Antinuclei accelerated in antigalaxies, if they exist, can enter the Milky Way galaxy more 
easily if their energy is ultrahigh. To accomplish the separation, one would use a horizontal cosmic- 
ray telescope located near the Moon’s limb so as to point a t  all times in the direction of the Earth. 
In general, highly relativistic particles with a charge Z times the charge of an electron, and 
energy E (electronvolts), will be deflected by a transverse magnetic field Bl (gauss) through a n  angle 
0 given by 
where the integration is over the path of the particle. For particles grazing the Earth’s limb in the 
equatorial plane, the magnetic field integral equals 4 X 10s G-cm, some 400 times greater than the 
field integral of a superconducting magnet facility proposed for the U.S. Space Station. The energy 
coverage would therefore extend to energies that are higher by about the same factor. 
A consequence of this deflection is that the Earth’s cosmic-ray shadow, due to particles being 
intercepted by the Earth,  will be displaced through an  angle given by the equation. For a given 
particle energy, the shadows produced by protons and antiprotons will fall symmetrically on opposite 
sides of the geometrical shadow. 
The telescope I envision, shown schematically in figure 3, would consist of a large ionization 
calorimeter for measuring E and of widely spaced drift chambers for finding particle trajectories. The 
ionization chamber would consist of gas-filled counters interleaved with bins of lunar soil. Electrons 
and antiprotons of the same energy would be distinguishable by virtue of their different cascade 
profiles; electron-initiated cascades are  purely electromagnetic, whereas cascades initiated by 
antiprotons are  hadronic. The calorimeter wouldbe used concurrently, in combination with other 
counters not shown here, for other purposes, such as gamma-ray astronomy in the VHE-UHE bands. 
TABLE 1.- PROGRAM OF THE 19TH INTERNATIONAL COSMIC RAY CONFERENCE, 
LA JOLLA, CALIFORNIA, AUGUST 11-23,1985 
[Numbers of sessions devoted to the various topics] 
GALACTIC AND EXTRAGALACTIC COSMIC RAYS . . . . . . . . . . . . . . . . . . . .  
Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Gammarays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11 
Nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9 
Electrons, positrons, antiprotons . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
Searches for monopoles, quarks, etc. . . . . . . . . . . . . . . . . . . . . . . . .  1 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Techniques and Instrumentation . . . . . . . . . . . . . .  
SOLAR AND HELIOSPHERIC COSMIC RAYS AND 
COSMIC-RAY PHENOMENA . . . . . . . . . . . . . . . . . . . . . .  
. . . . . . . . . . . . .  
. . . . . . . . . . . . .  
37 
24 
7 
6 
21 
Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16 
Solar flare particles and gamma rays . . . . . . . . . . . . . . . . . . . . . . .  4 
Solarneutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
Interplanetary acceleration, Jovian electrons . . . . . . . . . . . . . . .  1 
Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7 
Intensity gradients in the heliosphere . . . . . . . . . . . . . . . . . . . . . .  2 
1 Cosmogenic nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
I Techniques and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
HIGH-ENERGY INTERACTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18 
Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10 
Hadronic interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5 
Cascades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
Secondary leptons (muons) and leptonic interactions . . . . . . . . .  2 
Theory and Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5 
Techniques and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
60 
Figure 1.- A large ionization calorimeter built into the shielding of a manufacturing facility or a 
laboratory on the Moon. The dashed lines represent layers of gas-filled ionization counters. 
61 
0 a 0 0 
0 0 0 
Figure 2.- Cross section of a lightweight gas counter panel made of metallized plastic foil (with the 
usual anode wires). To save cargo space, the panels would be deflated and pressed flat during 
shipment . 
DRIFT CHAMBERS 
CALORIMETER 
TOWARD 
EARTH 
Figure 3.- A cosmic-ray charge analyzer designed to be located on the Moon and to use the Earth’s 
dipole as a magnet. 
62 


Cosmic-ray detectors on the Moon

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