Measurement of cosmic rays
KOZ
Natural background radiation comes from two primary sources: cosmic radiation and
terrestrial sources. The worldwide average background dose for a human being is about 2.4
millisievert (mSv) per year.
The Earth, and all living things on it, are constantly bombarded by radiation from outer space.
This radiation primarily consists of positively charged ions from protons to iron nuclei
derived from the sun and from other sources outside our solar system. This radiation interacts
with atoms in the atmosphere to create secondary radiation, including X-rays, muons, protons,
alpha particles, pions, electrons, neutrinos and neutrons.
Cosmic rays are energetic particles originating from space that impinge on Earth's
atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are
helium nuclei (alpha particles) and about 1% are electrons (beta minus particles). The term
"ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam
of particles.
The variety of particle energies reflects the wide variety of sources. The origins of these
particles range from energetic processes on the Sun all the way to as yet unknown events in
the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far
higher than the 1012 to 1013 eV that man-made particle accelerators can produce.
Observations have shown that cosmic rays with an energy above 10 GeV (10 x 109 eV)
approach the Earth’s surface isotropically (equally from all directions); it has been
hypothesized that this is not due to an even distribution of cosmic ray sources, but instead is
due to galactic magnetic fields causing cosmic rays to travel in spiral paths. This limits
cosmic ray’s usefulness in positional astronomy as they carry no information of their direction
of origin. At energies below 10 GeV there is a directional dependence, due to the interaction
of the charged component of the cosmic rays with the Earth's magnetic field.
Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic
rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary
cosmic rays can interact with interstellar matter to create secondary cosmic rays. The sun also
emits low energy cosmic rays associated with solar flares. The exact composition of primary
cosmic rays, outside the Earth's atmosphere, is dependent on which part of the energy
spectrum is observed. However, in general, almost 90% of all the incoming cosmic rays are
protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons. The
remaining fraction is made up of the other heavier nuclei which are abundant end products of
star’s nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not
abundant nuclear synthesis end products, or products of the Big Bang, primarily lithium,
beryllium and boron. These light nuclei appear in cosmic rays in much greater abundance
(about 1:100 particles) than in solar atmospheres, where their abundance is about 10-7 that of
helium.
This abundance difference is a result of the way secondary cosmic rays are formed. When the
heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei,
collide with interstellar matter, they break up into lighter nuclei (in a process termed cosmic
ray spallation), into lithium, beryllium and boron. It is found that the energy spectra of Li, Be
and B falls off somewhat steeper than that of carbon or oxygen, indicating that less cosmic ray
spallation occurs for the higher energy nuclei presumably due to their escape from the galactic
magnetic field. Spallation is also responsible for the abundances of Sc, Ti, V and Mn elements
in cosmic rays, which are produced by collisions of Fe and Ni nuclei with interstellar matter.
The flux (flow rate) of cosmic rays incident on the Earth’s upper atmosphere is modulated
(varied) by two processes; the sun’s solar wind and the Earth's magnetic field. Solar wind is
expanding magnetized plasma generated by the sun, which has the effect of decelerating the
incoming particles as well as partially excluding some of the particles with energies below
about 1 GeV. The amount of solar wind is not constant due to changes in solar activity over
its regular eleven-year cycle. Hence the level of modulation varies in autocorrelation with
solar activity. Also the Earth's magnetic field deflects some of the cosmic rays, which is
confirmed by the fact that the intensity of cosmic radiation is dependent on latitude, longitude
and azimuth. The cosmic flux varies from eastern and western directions due to the polarity of
the Earth’s geomagnetic field and the positive charge dominance in primary cosmic rays; this
is termed the east-west effect. The cosmic ray intensity at the equator is lower than at the
poles as the geomagnetic cutoff value is greatest at the equator. This can be understood by the
fact that charged particle tend to move in the direction of field lines and not across them. This
is the reason the Aurorae occur at the poles, since the field lines curve down towards the
Earth’s surface there. Finally, the longitude dependence arises from the fact that the
geomagnetic dipole axis is not parallel to the Earth’s rotation axis.
The nuclei that make up cosmic rays are able to travel from their distant sources to the Earth
because of the low density of matter in space. Nuclei interact strongly with other matter, so
when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric
gases. These collisions, in a process known as a shower, result in the production of many
pions and kaons, unstable mesons which quickly decay into muons. Because muons do not
interact strongly with the atmosphere and because of the relativistic effect of time dilation
many of these muons are able to reach the surface of the Earth. Muons are ionizing radiation,
and may easily be detected by many types of particle detectors such as bubble chambers or
scintillation detectors. If several muons are observed by separated detectors at the same
instant it is clear that they must have been produced in the same shower event.
When cosmic ray particles enter the Earth's atmosphere they collide with molecules, mainly
oxygen and nitrogen, to produce a cascade of lighter particles, a so-called air shower. The
general idea is shown in the figure which shows a cosmic ray shower produced by a high
energy proton of cosmic ray origin striking an atmospheric molecule.
This image is a simplified picture of an air shower: in reality, the number of particles created
in an air shower event can reach in the billions, depending on the energy and chemical
environment (i.e. atmospheric) of the primary particle. All of the produced particles stay
within about one degree of the primary particle's path. Typical particles produced in such
collisions are charged mesons (e.g. positive and negative pions and kaons); one common
collision is:
Cosmic rays are also responsible for the continuous production of a number of unstable
isotopes in the Earth’s atmosphere, such as carbon-14, via the reaction:
Cosmic rays kept the level of carbon-14 in the atmosphere roughly constant (70 tons) for at
least the past 100,000 years, until the beginning of aboveground nuclear weapons testing in
the early 1950s. This is an important fact used in radiocarbon dating which is used in
archaeology.
This is a random process on the atomic level, in that it is impossible to predict when a given
atom will decay, but given a large number of similar atoms, the decay rate, on average, is
predictable.
The decay of an unstable nucleus is entirely random and it is impossible to predict when a
particular atom will decay. However, it is equally likely to decay at any time. Therefore, given
a sample of a particular radioisotope, the number of decay events –dN expected to occur in a
small interval of time dt is proportional to the number of atoms present. If N is the number of
atoms, then the probability of decay (– dN/N) is proportional to dt:
Particular radionuclides decay at different rates, each having its own decay constant (λ). The
negative sign indicates that N decreases with each decay event. The solution to this first-order
differential equation is the following function:
Where N0 is the amount of N at time zero (t = 0). The second equation recognizes that the
differential decay constant λ has units of 1/time, and can thus also be represented as 1/τ,
where τ is a characteristic time for the process. This characteristic time is called the time
constant of the process. In radioactive decay, this process time constant it also the mean
lifetime for decaying atoms. Each atom "lives" for a finite amount of time before it decays,
and it may be shown that this mean lifetime is the arithmetic mean of all the atoms' lifetimes,
and that it is τ, which again is related to the decay constant as follows:
The previous exponential function generally represents the result of exponential decay. It is
only an approximate solution, for two reasons. Firstly, the exponential function is continuous,
but the physical quantity N can only take non-negative integer values. Secondly, because it
describes a random process, it is only statistically true. However, in most common cases, N is
a very large number and the function is a good approximation.
A more commonly used parameter is the half-life. Given a sample of a particular
radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. The half
life is related to the decay constant as follows:
This relationship between the half-life and the decay constant shows that highly radioactive
substances are quickly spent, while those that radiate weakly endure longer. Half-lives of
known radionuclides vary widely, from more than 1019 years (such as for very nearly stable
nuclides, e.g. 209Bi), to 10-23 seconds for highly unstable ones.
The Bethe-Bloch formula describes the energy-loss by ionization of swift charged particles
(protons, alpha particles, atomic ions, but not electrons) traversing matter.
Charged particles moving through matter interact with the electrons of atoms in the material.
The interaction excites or ionizes the atoms. This leads to an energy loss of the traveling
particle. The Bethe formula which was found by Hans Bethe in 1930, describes the energy
loss per distance traveled (or the stopping power of the material traversed):
where
β
v
E
x
c
ze
e
me
n
I

=v/c
velocity of the particle
energy of the particle
distance travelled by the particle
speed of light
particle charge
charge of the electron
rest mass of the electron
electron density of the target
mean excitation potential of the
target
permittivity of free space
Here, the electron density of the material can be calculated by
, where ρ is the
density of the material, Z,A the atomic number and mass number, resp., and NA the Avogadro
number.
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle
(two protons and two neutrons bound together into a particle identical to a helium nucleus)
and transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less.
For example:
23892U
→
23490Th
+
42He2+
although this is typically written as:
238U
→ 234Th
+ α
An alpha particle is the same as a helium-4 nucleus, and both mass number and atomic
number are the same. Alpha decay is a form of nuclear fission where the parent atom splits
into two daughter products. Alpha decay is fundamentally a quantum tunneling process.
Unlike beta decay, alpha decay is governed by the strong nuclear force.
Alpha particles have a typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy,
i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05c.
Because of their relatively large mass, +2 charge and relatively low velocity, they are very
likely to interact with other atoms and lose their energy, so their forward motion is effectively
stopped within a few centimeters of air.
Being relatively heavy and positively charged, alpha particles tend to have a very short mean
free path, and quickly lose kinetic energy within a short distance of their source. This results
in several MeV being deposited in a relatively small volume of material. This increases the
chance of cellular damage in cases of internal contamination. In general, external alpha
radiation is not harmful since alpha particles are effectively shielded by a few centimeters of
air, a piece of paper, or the thin layer of dead skin cells. Even touching an alpha source is
usually not harmful, though many alpha sources also are accompanied by beta-emitting
radiodaughters, and alpha emission is also accompanied by gamma photon emission. If
substances emitting alpha particles are ingested, inhaled, injected or introduced through the
skin, then it could result in a measurable dose.
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of
radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing
radiation also known as beta rays. The production of beta particles is termed beta decay. They
are designated by the Greek letter beta (β). There are two forms of beta decay, β− and β+,
which respectively give rise to the electron and the positron.
An unstable atomic nucleus with an excess of neutrons may undergo β− decay, where a
neutron is converted into a proton, an electron and an electron-type antineutrino (the
antiparticle of the neutrino):
n → p + e− + νe
This process is mediated by the weak interaction. The neutron turns into a proton through the
emission of a virtual W− boson. At the quark level, W− emission turns a down-type quark into
an up-type quark, turning a neutron (one up quark and two down quarks) into a proton (two
up quarks and one down quark). The virtual W− boson then decays into an electron and an
antineutrino.
Beta decay commonly occurs among the neutron-rich fission byproducts produced in nuclear
reactors. Free neutrons also decay via this process. This is the source of the copious amount of
electron antineutrinos produced by fission reactors.
Unstable atomic nuclei with an excess of protons may undergo β+ decay, also called inverse
beta decay, where a proton is converted into a neutron, a positron and an electron-type
neutrino:
p → n + e+ + νe
Beta plus decay can only happen inside nuclei when the absolute value of the binding energy
of the daughter nucleus is higher than that of the mother nucleus.
Inverse beta decay is one of the steps in nuclear fusion processes that produce energy inside
stars.
Gamma rays (denoted as γ) are a form of electromagnetic radiation or light emission of
frequencies produced by sub-atomic particle interactions, such as electron-positron
annihilation or radioactive decay. Gamma rays are generally characterized as electromagnetic
radiation having the highest frequency and energy, and also the shortest wavelength (below
about 10 picometers), within the electromagnetic spectrum. Gamma rays consist of high
energy photons with energies above about 100 keV. Gamma rays were discovered by Paul
Villard, a French chemist and physicist, in 1900, while studying uranium.
Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays. The
distinction between the two terms, however, depends on the source of the radiation, not its
wavelength; X-ray photons are generated by energetic electron processes, gamma rays by
transitions within atomic nuclei.
Due to their high energy content, gamma rays can cause serious damage when absorbed by
living cells.
When a gamma ray passes through matter, the probability for absorption in a thin layer is
proportional to the thickness of that layer. This leads to an exponential decrease of intensity
with thickness. The exponential absorption holds only for a narrow beam of gamma rays. If a
wide beam of gamma rays passes through a thick slab of concrete, the scattering from the
sides reduces the absorption.
Here, μ = nσ is the absorption coefficient, measured in cm−1, n the number of atoms per cm3
in the material, σ the absorption cross section in cm2 and d the thickness of material in cm.
In passing through matter, gamma radiation ionizes via three main processes: the
photoelectric effect, Compton scattering, and pair production.
 Photoelectric Effect: This describes the case in which a gamma photon interacts with
and transfers its energy to an atomic electron, ejecting that electron from the atom.
The kinetic energy of the resulting photoelectron is equal to the energy of the incident
gamma photon minus the binding energy of the electron. The photoelectric effect is
the dominant energy transfer mechanism for x-ray and gamma ray photons with
energies below 50 keV (thousand electron volts), but it is much less important at
higher energies.
 Compton Scattering: This is an interaction in which an incident gamma photon loses
enough energy to an atomic electron to cause its ejection, with the remainder of the
original photon's energy being emitted as a new, lower energy gamma photon with an
emission direction different from that of the incident gamma photon. The probability
of Compton scatter decreases with increasing photon energy. Compton scattering is
thought to be the principal absorption mechanism for gamma rays in the intermediate
energy range 100 keV to 10 MeV. Compton scattering is relatively independent of the
atomic number of the absorbing material.
 Pair Production: By interaction with the electric field of a nucleus, the energy of the
incident photon is converted into the mass of an electron-positron pair. Energy in
excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the
kinetic energy of the pair and the recoil nucleus. At the end of the positron's range, it
combines with a free electron. The entire mass of these two particles is then converted
into two gamma photons of at least 0.51 MeV energy each (or higher according to the
kinetic energy of the annihilated particles)
The secondary electrons (and/or positrons) produced in any of these three processes
frequently have enough energy to produce much ionization themselves.
Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum
shielding. Gamma rays can only be reduced by much more substantial barriers, such as a very
thick layer of lead.
Neutron radiation is a kind of ionizing radiation which consists of free neutrons.
Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the
same way protons, photons, and electrons do (exciting an electron) because neutrons have no
charge. However, neutron interactions are largely ionizing, for example when neutron
absorption results in gamma emission and the gamma subsequently removes an electron from
an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more
traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are
more penetrating than alpha radiation or beta radiation. In some cases they are more
penetrating than gamma radiation, which is impeded in materials of high atomic number. In
hydrogen, a low energy neutron may not be as penetrating as a high energy gamma.
Neutron radiation protection relies on radiation shielding. In comparison with conventional
ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and
slowed (absorbed) by light nuclei, so a large mass of hydrogen-rich material is needed.
Neutrons readily pass through most material, but interact enough to cause biological damage.
Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe
and dangerous radiation available. The most effective materials are eg. water, polyethylene,
paraffin wax, or concrete, where a considerable amount of water molecules is chemically
bound to the cement. Waxes, Plastics, Water, Oil, etc. all contain large amounts of Hydrogen
atoms. The light atoms serve to slow down the neutrons by elastic scattering, so they can then
be absorbed by nuclear reactions. However, gamma radiation is often produced in such
reactions, so additional shielding has to be provided to absorb it.
A Geiger-Müller tube (or GM tube) is the sensing element of a Geiger counter instrument that
can detect a single particle of ionizing radiation, and typically produce an audible click for
each. It was named for Hans Geiger who invented the device in 1908, and Walther Müller
who collaborated with Geiger in developing it further in 1928. It is a type of gaseous
ionization detector with an operating voltage in the Geiger plateau.
A Geiger-Müller tube consists of a tube filled with an low-pressure (~0.1 Atm) inert gas such
as helium, neon or argon, in some cases in a Penning mixture, and an organic vapor or a
halogen gas and contains electrodes, between which there is a voltage of several hundred
volts, but no current flowing. The walls of the tube are either metal or the inside coated with
metal or graphite to form the cathode while the anode is a wire passing up the center of the
tube.
When ionizing radiation passes through the tube, some of the gas molecules are ionized,
creating positively charged ions, and electrons. The strong electric field created by the tube's
electrodes accelerates the ions towards the cathode and the electrons towards the anode. The
ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way,
creating an avalanche of charged particles.
This results in a short, intense pulse of current which passes (or cascades) from the negative
electrode to the positive electrode and is measured or counted.
Most detectors include an audio amplifier that produce an audible click on discharge. The
number of pulses per second measures the intensity of the radiation field. Some Geiger
counters display an exposure rate, but this does not relate easily to a dose rate as the
instrument does not discriminate between radiation at different energy.
The usual form of tube is an end-window tube. This type is so-named because the tube has a
window at one end through which ionizing radiation can easily penetrate. The other end
normally has the electrical connectors. There are two types of end-window tubes: the glassmantle type and the mica window type. The glass window type will not detect alpha radiation
since it is unable to penetrate the glass, but is usually cheaper and will usually detect beta
radiation and X-rays. The mica window type will detect alpha radiation but is more fragile.
Most tubes will detect gamma radiation, and usually beta radiation above about 2.5 MeV.
Geiger-Müller tubes will not normally detect neutrons since these do not ionise the gas.
However, neutron-sensitive tubes can be produced which either have the inside of the tube
coated with boron or contain boron trifluoride or helium-3 gas. The neutrons interact with the
boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and
tritium ions and electrons. These charged particles then trigger the normal avalanche process.
The G.M. tube must produce a single pulse on entry of a single particle.It must not give any
spurious pulse and recover quickly to the passive state.But unfortunately the positive Ar ions
that eventually strike the cathode become neutral Ar atoms in an excited state by gaining
electrons from the cathode. The excited atoms return to the ground state by emitting photons
and these photons cause avalanches and hence spurious pulses.
To prevent the current from flowing continuously there are several techniques to stop, or
quench the discharge. Quenching is important because a single particle entering the tube is
counted by a single discharge, and so it will be unable to detect another particle until the
discharge has been stopped, and because the tube is damaged by prolonged discharges.
External quenching uses external electronics to remove the high voltage between the
electrodes. Self-quenching or internal-quenching tubes stop the discharge without external
assistance, and contain a small amount of a polyatomic organic vapor such as butane or
ethanol; or alternatively a halogen such as bromine or chlorine.
If the diatomic gas(quencher) is introduced in the tube, the positive Ar ions, during their slow
motion to the cathode, would have multiple collisions with the quencher gas molecules and
transfer their charge and some energy to them. Thus neutral Ar atoms would reach the
cathode. The quencher gas ions in their turn reach the cathode, gain electrons thereform and
move into excited states. But these excited molecules lose their energy not by photon
emission but by dissociation into neutral quencher molecules. No spurious pulses are thus
produced.
Measuring goals:
1) Introducing the unisotropy of cosmic ray and the effects of building material to the
rays: the Geiger-Müller tubes set in coincidence mode are placed on the top of each
other, later located side by side, looking at to the sky.
Repeating the above experiment in the lab, located in the basement.
2) Examining the penetrating property of the gamma rays within different types of
materials.
3) Measuring the efficient range of the radioactive rays with GM tubes.
4) Statistical approach: counting the time between two decays and the number of decays
in a given time period.
Measuring procedure and analysis:
1) The Geiger-Müller tubes should be set to coincidence mode. First the tubes are to be
placed on each other, later side-by-side, and the measurement should take 15-20
minutes in open air or at a place with open view to the sky. The experiment should be
repeated in the laboratory, located in the basement, also.
In coincidence mode, the detector reports if only the avalanche of charged particles
takes places in the same time in all the tubes. The cause for the ionisation in the GM
tubes are the higly powered particles coming from cosmos, or those induced secondary
rays (cascade). We can predict the direction of the cosmic radiation based on the
special allocation of the GM tubes.
Determine the direction of the radioactive particles.
2) Penetration of gamma-rays. We will shadow a radioactive source with several layer of
a material (like wood or metals). Different types of materials (in different thickness)
are placed between the GM tube and the radioactive source, each experiment should
be repeated three times.
Determine the decrease of the intensity of the radiation, and based on your experience,
determinate also the type of the radiation!
Do not forget to count the background also, and do not leave out from the
calculations!
You should use the gm-plu10.exe or gmplus11.exe programs for this experiment.
3) Examine the absorbtion coefficient of the different types of materials which were
placed between the GM tube and the radioactive source!
Determine the amount of the different types of materials which is needed to the
absolute decreasement of the intensity.
You should use the gm-plu10.exe or gmplus11.exe programs for this experiment.
4) You should save the data of the GM tubes which counted low-intensity and highintensity radioactive source also. These data are needed to write the laboratory report.
You should use the gm_tim29.exe or gm_34.exe programs for this experiment.
Draw the probability density distributions based on these data and examine that it
accounts for Poisson or Gauss distribution.