Precision Cosmic Ray physics with space-born experiment

Abstract

More than 100 years after their discoveries, cosmic rays have been extensively studied, both with balloon experiments and with ground observatories. More recently, the possibility of mounting detectors on satellites or on the International Space Station has allowed for a long duration (several years) continuous observation of primary cosmic rays, i.e. before their interaction with the earth atmosphere, thus opening a new regime of precision measurements. In this review, recent results from major space experiments, as Pamela, AMS02 and Fermi, as well as next generation experiments proposed for the International Space Station, for standalone satellites or for the yet to come Chinese Space Station, will be presented. The impact of these experiment on the knowledge of Cosmic Ray propagation will also be discussed.

Full text

EPJ Web of Conferences 121, 03001 (2016) DOI: 10.1051/epjconf/201612103001
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Precision Cosmic Ray physics with
space-born experiment
Marco Incaglia
Istituto Nazionale di Fisica Nucleare (INFN), Pisa, Italy
Abstract. More than 100 years after their discoveries, cosmic rays have
been extensively studied, both with balloon experiments and with ground
observatories. More recently, the possibility of mounting detectors on
satellites or on the International Space Station has allowed for a long
duration (several years) continuous observation of primary cosmic rays,
i.e. before their interaction with the earth atmosphere, thus opening a new
regime of precision measurements. In this review, recent results from major
space experiments, as Pamela, AMS02 and Fermi, as well as next generation
experiments proposed for the International Space Station, for standalone
satellites or for the yet to come Chinese Space Station, will be presented. The
impact of these experiment on the knowledge of Cosmic Ray propagation
will also be discussed.
1. Introduction
Charged Cosmic Rays cover many orders of magnitude, both in flux and in energy range.
Therefore many different techniques have been established to study them over the whole
spectrum. In particular, large areas have to be covered in order to observe cosmic rays with
energy above 1015 eV, where 1 particle/m2/year is expected1.
With current techniques, observations at such energies and above can only be performed
on ground2.
On the other hand, particle fluxes at energies 1 GeV–1 TeV are of order
1 particle/m2/second and experiments having dimensions 1m
3can be used. This has
allowed the deployment of some detectors in space, either as free-flyers – PAMELA, FERMI-
GLAST – or as payloads mounted on the International Space Station (ISS) – AMS02 and, in
the near future, CALET, ISS-CREAM.
The clear advantage of such detectors, with respect to ground based ones, is that they are
sensitive to the primary CR component, where by primary I mean “before interacting with
the earth atmosphere”, they can reach a much higher precision on the energy determination,
in particular for electromagnetic particles, and on the chemical composition. Moreover, if
equipped with a magnet, they are sensitive to anti-particles. On the other hand, due to the
ae-mail: marco.incagli@pi.infn.it
1Here I am referring to the integral flux. The flux of particular species (nuclei, leptons, ...) can vary by orders of
magnitude, as we will see in the next sections.
2A noticable exeption to this paradigma is the possibility of observing the fluorescence light, emitted by very
energetic air showers (>1020 eV), from a telescope orbiting at an altitude of 400 km. Several such experiments
have been proposed in the past – EUSO, JEM-EUSO, KLYPVE, OWL; as of now, only a small prototype
(MINI-EUSO) has a good chance of being launched on the International Space Station in the near future.
C
The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the
Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).

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large cost of space missions, such detectors are limited both in mass and in geometrical
dimensions, which results in a severe limitation in the energy range due to the steeply falling
CR flux.
As an intermediate step between ground and space detectors, balloon experiments have
to be mentioned. Flying at an altitude of 37 km, with only 5 gr/cm2of residual atmosphere
above, these experiments share similar advantages as space ones, at a much lower cost.
However balloon flights last at most 3 weeks, so the integration time is quite limited.
In this review I will focus on space experiments discussing the general characteristics
(Sect. 2) with a specific reference to geometrical acceptance and background rejection
(Sect. 3). I will then discuss the physics case and the recent results from the two major
experiments focused on charged cosmic rays: PAMELA and AMS02 (Sects. 4 and 5). As
there are specific presentations on photon physics, in these proceedings, I will not discuss
here results from the FERMI experiment, although it will be mentioned while describing
the general characteristics of space experiments. Finally I will briefly mention the detectors
foreseen for the near or the more far future (Sect. 6).
2. Space detectors
Although several techniques can be (and have been) used for building or designing space
detectors, three general classes of experiments can be identified:
1. magnetic spectrometers (àlaAMS02);
2. pair-conversion telescopes (àlaFERMI);
3. calorimeters (àlaATIC or CREAM, but also CALET, DAMPE or ISS-CREAM) with
a possible enphasis on hadrons (nuclei) or leptons (e±,) which can, on their turn, be
more specialized on hadrons or on electromagnetic showers.
Of course, different combinations of the different techniques are possible.
2.1 Magnetic spectrometers
Spectrometers allow for the measurement of the particle momentum and of the sign of its
charge. As a consequence, it is possible to have access to anti-particles, in particular to
positrons and anti-protons, but also, at least in principle, to anti-deuteron or anti-helium
nuclei.
Moreover, by combining the momentum with the measurement of the velocity ,
performed either through Cherenkov or through Time Of Flight (TOF) techniques, it is
possible to determine the nuclear mass number Avia the relation:
A=RZe
mNc (1)
where mNis the nucleon mass3, thus giving access to isotopes (3He,4He,9Be,10 Be, ...).
A review of the techniques used for isotope identification requires a specific discussion of
TOF or Cherenkov detectors, which goes beyond the scope of this generic introduction.
With current technologies, two solutions are possible for magnets: permanent magnets
and superconducting magnets working at the superfluid helium temperature. Several
interesting R&D are in progress to develop superconducting magnets operating at higher
3It is easy to write an expression in which the neutron and the proton masses appear explicitely instead of the
average mN. However, the error introduced by using this approximation is much smaller than the one due to the
momentum or velocity measurements.
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temperatures ([1]). However these techniques have still to be fully developed, and then it will
be necessary to test them for space. So this possibility will not be discussed any further.
Permanent magnets are heavy and their magnetic field is limited to values of 1 kGauss;
superconducting magnets are hard to operate in space and require a reservoir of Helium to
be carried on board. For example, in the first project of the AMS02 experiment a dewar with
3500 l of Helium was foreseen. With this amount of helium, the lifetime of the experiment
was planned to be 28 months ([2]).
2.2 Pair-conversion telescopes
Apair-conversion telescope is a detector with a dedicated tracking stage having a thickness of
1-2 radiation lengths, normally obtained by interleaving silicon active material with tungsten
layers, in which photons interact converting in an e+e−pair. The two leptons are precisely
traced and their energy is measured by the calorimeter located below. So these detectors are
mostly dedicated to precision physics at energies which can be as low as 1 MeV, which
is the threshold energy for electron pair production. As a matter of fact, electrons produced
with a momentum of few MeV undergo multiple scattering in the passive layers and their
momentum cannot be effectively measured. So the lower energy limit is really 100 MeV. An
example of such an experiment is FERMI ([3]). To reach the 1 MeV limit, a modified structure
in which the passive layers are replaced by additional active silicon has been proposed in ([4]).
These type of detectors have an excellent angular resolution, or Point Spread Function
(PSF). However the presence of the heavy tracker introduces some complexity in the system
which has some impact in the Field Of View (FOV) and in the energy resolution.
2.3 Calorimeters
Calorimeter-based detectors are specialized in measuring the nuclear charge and the energy
of electromagnetic showers. The limit on the total weight and dimensions does not allow to
build hadronic calorimeters as on ground. However it is nevertheless possible to measure the
energy of hadrons by using some special techniques, as described later for the ISS-CREAM
experiment.
The main characteristic of calorimeters, however, is clearly to maximize the geometrical
acceptance, by dedicating all the mass budget to the detector.
Comparing calorimeters to gamma-converters and to magnetic spectrometers, it is
possible to note that the pointing capability for photons is 0.5◦, more than enough
for anisotropy studies, and that, although not sensitive to anti-particles, calorimeters can
accurately study the total e++e−flux, thus being able to detect sharp variations in the
spectrum induced by a potential DM source.
3. Statistics vs. Acceptance and the issue of background
The CR flux decreases very rapidsly with the energy. As an example, Fig. 1shows the number
of electrons (e++e−) and of positrons expected above a given energy for several geometrical
acceptances, indicated in the plot by different lines.
The two colored dashed and dotted lines approximately indicate the geometrical accep-
tances of FERMI and AMS02 experiments. The numbers represent the effective geometrical
acceptance integrated over one year, where effective means that all inefficiencies, including
dead time, trigger efficiency and so on, are accounted for. For an apparatus having the size of
Fermi, which is currently the high energy space detector with the largest acceptance, at most
100 electron events per year are expected at energies E =2–3 TeV. This number rapidly
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Figure 1. Number of events above a given energy (efficiency cuts not considered) for different
geometrical exposures. The red dotted and the green dashed lines appreoximately represent the
geometrical acceptance of the AMS02 and the FERMI experiment, respectively.
Figure 2. AMS02 and FERMI detectors in scale. Different dimensions and different geometrical
acceptances are evident.
decreases for a detector whose FOV is limited by the presence of the magnet. Figure 2shows
the two detectors on the same scale, putting in evidence the different choices made.
However statistics is not the whole story. Different channels, in particular anti-particles,
have fluxes which can be suppressed by several orders of magnitude with respect to the most
common ones. Figure 3shows the flux, as a function of energy, for some particle types with
respect to protons. In this plot, three regions can be identified: ep-rejection, charge confusion
and photons.
• ep-rejection: electrons suffer from a proton background of 102−103, depending on the
energy, while for positrons this background is 104; as a consequence, to measure
the electron spectrum in the TeV region with a precision better than 10%, an electron-
proton (ep) rejection of 10−4−10−5must be achieved, while still keeping a high signal
efficiency to keep under control the statistical error.
• charge confusion: protons are approximately 4 orders of magnitude more abundant
than anti-protons. Therefore the sign of the charge has to be determined with such a
level of precision in order to avoid the charge confusion effect, i.e. the wrong sign
reconstruction.
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Figure 3. Cosmic ray flux as a function of energy relative to protons for several species.
• photons: to measure photons, a proton suppression factor of order ppm is required.
All these constraints must be considered while designing experiments for space.
4. The physics case: Cosmic Rays and Dark Matter search
Many topics may be studied in space experiments. The two main ones are: Cosmic Rays (CR)
origin and propagation and Dark Matter (DM) searches.
4.1 Cosmic Rays origin and propagation
The reference theoretical framework which is widely used to interpret CR nuclear data is
based on the following main assumptions/approximations:
• CR diffusion is treated as isotropic: the interstellar diffusion tensor is assumed to be a
scalar independent from rigidity, the so called diffusion coefficient D;
• the diffusion coefficient is assumed to be spatially uniform and (in most cases) to have
a simple power-law dependence on the particle rigidity: D()∝;
• all primary nuclear species share the same single (or broken) power-law acceleration
shape up to E1015 eV, the so called knee;
• CR sources are continuosly distributed in the Galactic disk.
With these approximations, the main experimental features of primary and secondary CRs are
reasonably well reproduced (fluxes, ¯
p/p ratio, primaries/secondaries spectra, ...). However
new and more precise experimental data are in tension with the predictions of this standard
scenario and call for a more detailed and complete theoretical framework.
Among the observables which are in tension with the standard CR description, some of
the ones which are relevant for space experiments are:
•p/He ratio: ATIC, CREAM and PAMELA found different spectral indexes for proton
and Helium CRs. Preliminary AMS02 results seem to confirm this difference. This is
hardly explainable in terms of Fermi acceleration theory, which does not differentiate
between elements at ultra-relativistic rigidities.
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•CR spectra hardening: PAMELA found a sharp spectral breaking of pand He spectra at
the same rigidity pbreak 240 GeV. Although AMS02 preliminary data do not confirm
this result, it surely allows for a better connection with CREAM data in the 10−500 TeV
energy range. Since a spectral breaking is not predicted by Fermi acceleration theory,
this hardening may either require to abandon the continuos source distribution limit
and/or to introduce major changes in the way CR propagation is treated.
•anisotropy and secondary to primary ratio: an anisotropy in CR arrival direction is
expected, with a larger contribution from the galactic center with respect to the external
part of the galaxy. Models which predict a diffusion coefficient >0.5, favored by
recent B/C ratio data, severely overpredict this anisotropy.
These observational issues call, on one hand, for an improved description of the CR
production and propagation and, on the other hand, for more experimental data with improved
resolution to further constraint the theory. The main role of space experiments, in this respect,
is there capability of discriminating the different components by measuring the Zof nuclei
by dE/dx in different materials, tipically scintillators or tracker silicon layers, or by other
techniques (e.g. the amount of photons observed in the rings produced by particle crossing
radiators in RICH counters).
4.2 Dark Matter identification
There is now a overwhelming experimental evidence of the presence of a large component of
a new type of particles, generally called Dark Matter (DM), whose mass makes up 80%
of the mass budget of the universe. Although there is no evidence of the energy scale of
this new form of matter, which could range from invisible to macroscopic objects, there is
a particular solution which has several attractive features: the Weakly Interacting Massive
Particles (WIMP). There is a singular coincidence between the parameters of the Standar
Model of particle physics and of the Cosmological Model to provide valid DM candidates at
the electroweak scale (1 TeV) with a cross section typical of weak processes.
These particles are being studied in different processes:
• the annihilation, or indirect, channel, in which to DM particles annihilate to produce
two standard ones. This channel is the relevant one for space experiments;
• the interaction, or direct channel, is pursued in underground laboratories which look for
the interaction of the WIMP wind, caused by the earth motion in the solar system, and
in the galaxy in general, with ordinary ultra-pure matter;
• the production in collisions of Standard Model particles, as studied in colliders.
The annihilation channel can be detected by an excess of particles with respect to the ones
produced in standard Cosmic Rays. It is normally expected that the DM annihilation provides
a particle-antiparticle pair, therefore, being antiparticles much rarer than particles in CRs,
cosmic rays, the eventual excess would be more relevant in the positron or the antiproton
channel than in the correspondants particles.
For this reason, it was proposed in the 90s that monoenergetic positrons from
halo annihilations could be a significant and distinctive signal for massive dark-matter
particles [7]. The signature would be an increase in the positron fraction (e+/(e++e−)) with
respect to the one expected from the interaction of primary cosmic rays with the interstellar
medium. To detect positrons a magnetic detector is required, which led to the construction
and set in orbit first of the PAMELA experiment, on a dedicated satellite, and then of AMS02,
installed on the ISS.
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Figure 4. Positron fraction measured by PAMELA experiment.
5. The magnetic spectrometers: PAMELA and AMS02
The two detectors are described in detail in several publications (e.g. [5,6]). The main
structure is a permanent magnet with a dipole field, in both cases with intensity 1kG,
equipped with a set of silicon tracking layers, followed by an electromagnetic calorimeter
having a thickness of 17 X0(X0=radiation length). Both detectors are also equipped
with a Time Of Flight trigger system and a set of Anti-coincidence counters to reject
tracks entering the apparatus from the sides. AMS02, in addition, has a Transition Radiation
Detector, for electron-proton (or positron-proton) rejection, and a Ring Image Cherenkov
Counter, which can measure the particle velocity and can provide information on isotope
composition. However the main difference between the two detectors are the dimensions
and, as a consequence, the Geometrical Acceptance: PAMELA weighs 500 kg and has
an acceptance A20 cm2sr, while AMS02 payload weighs 7 tons4and its effective
geometrical acceptance (i.e. after selection) is of 450 m2sr. PAMELA was launched in
June 2006; AMS02 in May 2011.
5.1 Positron excess and anti-proton flux
The first clear evidence of an excess of positrons in the flux of Cosmic Rays was published by
PAMELA in April 2009 on Nature ([5]) and it is represented in Fig. 4. The observed spectrum
is incompatible with a secondary production, which is expected to smoothly decrease as in
the solid line of Fig. 4. Therefore, either a significant modification in the acceleration and
propagation models for cosmic rays is needed, or a primary component is present.
A modification in the propagation of secondary cosmic rays has been proposed by several
authors ([8,9]). The proposed effect, which could explain the observed “excess”, is that a
fraction of the secondary positrons are produced inside the same astrophisical source which
accelerate the primaries, such as supernova remnants. The flux of positrons produced at the
shock is naturally harder than the standard one and could explain the rise in the observed
4Almost half of this weight is due to the support system which connects the detector to the ISS.
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positron fraction. The same mechanism, however, predicts a similar effect also on the ratio
¯
p/p and B/C (Boron over Carbon), excess which, at the moment, has not been observed5.
Alternatively, the excess could be due to the existence of a primary source which,
tipically, produces e+,e−pairs. A very intriguing possibility is that these pairs are produced
by the annihilation of DM particles, as suggested in [7]. However there are also more
standard scenarios in which positrons are produced by a nearby astrophysical source; for
example a pulsar could emit energetic photons that, interacting with the strong magnetic field
surrounding the neutron star, decay in an e+e−pair [12].
Although this excess generated a lot of excitement in the community, in view of the
potential DM explanation, the measurement of the anti-proton flux by the same collaboration
showed no excess with respect to standard secondary production [13]. While this result is
well compatible with the pulsar hypothesis, a well behaving DM particle would instead
produce a sizeable proton-antiproton flux, unless some ad hoc mechanism is invoked. An
additional difficulty in the DM explanation is the magnitude of the effect, as it exceeds what
is expected by the thermal cross section requiring sizeable boost effects. Several ways out
have been proposed (see, e.g., [14]or[15]), but to discriminate between the different possible
explanations, more precise measurements extending at higher energies are necessary.
5.2 The AMS02 experiment
A new measurement of the positron and electron fluxes, as well as of the positron fraction,
has been recently performed by the AMS02 experiment with a much higher statistics.
Details are given in [16]. Here we note that positron events suffer from two different
backgrounds:
1. ep-rejection: protons which are wrongly reconstructed as electromagnetic showers
(protons are 4 orders of magnitude more abundant than positrons);
2. charge confusion: electrons whose charge has been wrongly identified in the tracker
and are reconstructed as positrons.
In AMS02 electrons are distinguished from protons by three different, and in large part
independent, detectors:
• the Transition Radiation Detector (TRD) located on top of AMS02 measures the
- electrons emitted by relativistic particles when crossing the surface between materials
with different dielectric constant. This effect is summarized in a likelihood estimator, as
shown in Fig. 5(a);
• the ECAL measures the 3D shower shape and, by using a multivariate approach,
distinguishes electrons from protons as shown in Fig. 5(b);
• finally, by comparing the total energy deposited in ECAL with the tracker momentum,
leptons peak at E/p =1, while hadrons are shifted to zero, having a sizable leakage in
the ECAL (Fig. 5(c).
Details of the analysis are in [16]. The resulting positron fraction is shown in Fig. 6, which
extends the previous measurements up to 500 GeV (upper edge of last bin). The excess is
confirmed with high precision; moreover, a clear flattening in the fraction occurs starting from
200 GeV. This behaviour is compatible both with the pulsar and with the DM explanation,
as in both cases a cutoff is expected. In the first case the cutoff is due to the energy of the
emitted photons, while in the second one the upper limit is determined by the DM mass. A
5See, however, [11] for a recent review which includes recent preliminary AMS02 data on B/C.
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Figure 5. Electron-proton rejection in AMS02 through a) Transition Radiation Detector b)
Electromagnetic Calorimeter c) Energy/Rigidity ratio.
Figure 6. Positron fraction measured by AMS02 for energies above 10 GeV, where the fraction starts
to increase. AMS02 shows, for the first time, a clear flattening of the fraction starting at 200 GeV.
possibility to discriminate between the two scenarios is shown in Fig. 7in which the AMS02
results are extrapolated under two different models: typical DM-like models show a sharper
cutoff with respect to pulsar-like ones. However this conclusion is not so firmly established
(see, for example, [17]).
The separate fluxes have also been measured [18] and are shown in Fig. 8, where the
positrons have been scaled up by a factor 10 in order to be compared with the electrons
(positron scale on the right axis). A comparison between the two histograms shows how the
effect of this eventual primary source has a different impact on the two components. Due to
the higher statistics, the electron flux has been extended up to 700 GeV.
Although a more detailed information on the electron and positron spectrum is certainly
useful, only a multi-messenger approach may provide sufficient information to disantangle
among the different explanations. On this respect, several other results are expected from the
AMS02 collaboration:
• proton and helium flux
• B/C, C/O and B, C, O fluxes
• anti-protons
• light nuclei and Nitrogen (Li, Be, N)
• photons
• heavier nuclei
• isotopes (3He/4He and 10 Be/9Be)
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Figure 7. Projected statistical sensitivity of AMS02 experiment after 12 years of data taking. The lower,
green line represents the expected background from secondaries. The brown middle line includes a DM
contribution with mass 700 GeV; the upper blue line represents a Pulsar like model. These models are
only representative.
Figure 8. AMS02 flux for electrons (left axis) and positron (right axis).
• anti-D
• anti-He.
All these measurements are useful either as a direct probe of DM, as anti-protons and,
possibly, anti-deuteron, or as a constraint to the propagation of CR in the insterstellar medium,
as B/C or as the isotope ratio.
6. Future experiments
6.1 Near future
Within 2017, three more space experiments are foreseen, all of calorimetric type:
CALET [19], ISS-CREAM [20], DAMPE [21].
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The first two will be mounted in 2015/2016 on the Japanese module of the International
Space Station, while DAMPE is supposed to be launched at the end of 2016 on board of a
chinese satellite.
CALET and DAMPE have very similar characteristics: the main detector is a thick
electromagnetic calorimeter of 30 X0, with very good energy resolution (1% at 1 TeV),
pointing capability (0.5 degrees) and an excellent ep-rejection (at least 10−4), with a heavy
tracker in front in which photons can convert. In CALET the tracker active material is made
of 1 mm scintillating fibers, so the main focus is on the identification of electromagnetic and
hadronic showers, while DAMPE uses silicon wafers, with more emphasis on photon pair
production. Both detectors have very good Z discrimination capabilities. The geometrical
acceptance is 0.1–0.2 m2sr, thus a factor of 4 larger than AMS02, but still an order of
magnitude smaller than FERMI.
ISS-CREAM, instead, is a modified version of the CREAM experiment, who had six
succesful antartic flights on aerostatic balloons between the years 2004 and 2010, for a
cumulative exposure of 162 days. In spite of the success of the balloon missions, the
experiment integrated less than half a year of equivalent data for a space mission, therefore
the possibility of installing the apparatus on the ISS would boost the physics potential, at least
in terms of accumulated statistics. The CREAM experiment extends the direct measurement
of cosmic-ray composition to energies of hundreds of TeV, at which CRs are capable of
generating gigantic air showers which have been observed on the ground, thereby providing
calibration for indirect measurements. A specific technique, which uses a preshower carbon
detector half interaction length thick to induce hadronic cascades, is used to reach such
energies with a thin calorimeter (thin in terms of hadronic showers). So, differently from
previous experiments, this one is more focused on the precise measurements of the energy
dependance of elemental spectra at the highest energies.
6.2 Medium term experiments
6.2.1 GAMMA-400
GAMMA-400 [22] is a russian-italian experiment which will fly on board of the russian
Navigator spacecraft. The experiment will be initially installed on a highly elliptical orbit
(with apogee 300.000 km, perigee 500 km and an inclination of 51.4◦), with 7 days orbital
period; the orbit will then be gradually changed, in 5 months, to a circular one with a radius
of 200.000 km radius. At such an orbit the Earth will not cover a significant fraction of the
sky, as is usually the case for the Low Earth Orbit (LEO) of the experiments mentioned so
far. Also the geomagnetic effect on charged particles will become negligible, which allows
for a clean study of the low energy part (E<10 GeV) of the spectrum.
The launch is foreseen for 2020. The project started with the main goal of improving
FERMI results on photon physics. Therefore the apparatus has a converting tracker and a
calorimeter having an energy resolution of 1% at 1 TeV. To reach this energy resolution, the
calorimeter has to be thicker than the FERMI one, with the consequence that the geometrical
acceptance is almost a factor of 2 lower.
However, the project has been further developed and now the proposed calorimeter uses
the CaloCube technique [23], which allows to use not only the upper face but also the lateral
ones to detect CRs, with the consequence that the geometrical acceptance is increased by a
factor 5, at least for charged particles.
The introduction of a highly segmented homogeneous calorimeter made with CsI(Tl)
cubes, with improved energy resolution and extended geometrical factor, coupled to the
improvement of the converter-tracker structure make GAMMA-400 an excellent dual
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instrument, optimized both for photons (in the 100 MeV–1 TeV energy range) and charged
cosmic rays (electrons up to 10 TeV and high energy nuclei up to the knee region).
6.2.2 The chinese space station
China has recently announced an ambitious space program timetable with the goal of
establishing its first space station around 2022. As part of this program, a major space
experiment for Cosmic Ray detection is foreseen. Two possible designs are currently under
study: a calorimetric detector surrounded by a pair-conversion tracker, similar in many
aspects to GAMMA-400 but with a wider geometrical coverage, and a magnetic spectrometer.
The calorimetric apparatus option is more advanced, with a collaboration already building
up around the design of HERD (High Energy cosmic Radiation Detection), while the
magnetic spectrometer option, possibly an AMS03 mission, is still very preliminar.
Although at the moment it is too preliminary to make detailed comments about these
possible detectors, it is evident the potentiality of such an observatory.
7. Conclusions
Many important and useful informations can be acquired by studying cosmic rays, both on
their origin and propagation mechanism but also, possibly, on new physics, in particular Dark
Matter.
Due to the large spread in energy and in flux, many different techniques have been
developed. Among these, in the recent years space experiments have been characterized by
the unique possibility of studying with great care the energy range from few hundred MeV
up to hundreds of TeV, at least for protons.
Space experiments have the unique opportunity of measuring anti-particles and isotopes,
and have a greatly improved capability of discriminating different nuclear charges Z. Several
important experiments are still collecting data: PAMELA, which is almost at the end of its
life after 8 years of glorious mission, AMS02 and FERMI. Many new experiments have been
proposed or are being built for the near and for the far future.
Only through a comparison of many different channels, the so-called “multi-messenger”
approach, it will be possible to constraint the propagation of standard CRs and thus put severe
limits on, or exclude some models of, Dark Matter.
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EPJ Web of Conferences 121, 03001 (2016) DOI: 10.1051/epjconf/201612103001
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