The measurements regarding the proton flux indicates a deviation after a few hun-dreds of GeV from the spectral index of 2.7 expected from the first order diffusion model (see Fig. 1.19). The current explanations of this break include an additional production of protons by nearby sources [37], acceleration in SNR not only trough forward but also with reverse shocks [38], or modifications to the diffusion model [39]. After 1 TeV it seems that the existing measurements are different between each other and after 10 TeV the statistical uncertainty starts to be predominant (see Fig. 1.20). DAMPE can help in shedding more light on this measurement in the high energy range.
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Figure 1.18: Electron plus positron flux measured by DAMPE [28] compared to several exper-iments [29–32].
Figure 1.19: Comparison of the proton flux measured by several experiments [40–44] multiplied by E2.7k as function of the kinetic energy. Picture taken from [40].
CHAPTER 1. COSMIC RAYS IN THE UNIVERSE
Figure 1.20: Comparison of the proton flux multiplied by E2.6 as a function of the energy measured by several experiments [40,41], [44], [47–49] in the medium-high energy range. Picture taken from [47].
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Chapter 2
Dark Matter Particle Explorer
To help to have a better understanding of the DAMPE detector, it is useful to recall briefly the main experiments that used similar techniques and measured cosmic rays directly. As shown in section 1.4, EAS shower experiments are not sensible to the primary charge charge of cosmic rays, therefore the first attempt to detect directly the primary cosmic-ray charge was to build a particle detector and connect it to a balloon, so that it could fly high in the atmosphere and measure directly the charge of the incoming cosmic ray trough the ionization energy lost in the material.
Advanced Thin Ionization Chamber (ATIC)
The ATIC experiment [50] was built in order to fly with a ballon for several times in Antartica and to measure, high in the atmosphere, the charge of cosmic rays through a silicon pixel detector [51]. The detector is also equipped with a carbon target placed after the silicon matrix and a bismuth germanate crystal calorimeter (BGO) to measure the particle energy (absorbtion depth of 17.9 radiation length). From 2000 until 2007 it flew 3 times for a total of 57 days and could measured the spectrum of cosmic-ray nuclei from 50 GeV to more than 100 TeV [41]. An all-electron measurement was possible as well [52]. A scheme and a picture of the ATIC detector are shown in Fig. 2.11.
Cosmic Ray Energetic and Mass experiment (CREAM) and ISS-CREAM The CREAM detector [53] has a redundant measurement of the primary cosmic-ray charge and energy in the range [1012,1015] eV (see the scheme of the original instrument in Fig. 2.2 on the left, relative to the CREAM-I flight). CREAM flew for six times between 2004 and 2010 accumulating 161 days of flight time. It consists of a Transi-tion RadiaTransi-tion Detector (TRD) (informaTransi-tion on the relativisticγof the particle) which combined with a Cherenkov Detector (information of the mass of the particle) provides a measurement of the energy. Moreover, it is equipped with a tungsten/scintillator
1Pictures taken from the ATIC official websitehttp://atic.phys.lsu.edu.
CHAPTER 2. DARK MATTER PARTICLE EXPLORER
Figure 2.1: The ATIC detector in a scheme (left) and on a picture (right) before one of its flights.
Figure 2.2: The CREAM-I instrument in a scheme [53] (left) and on a picture before one of its flights (right).
sampling calorimeter to have an additional and independent measurement of the par-ticle energy. The charge is measured in several parts: first, the parpar-ticle encounters a Timing-based scintillator Charge Detector (TCD), then the Charge Detector (CD), a Silicon Charge Detector (SCD) and the scintillating fibers Si with i=0,..3. A picture taken when CREAM was integrated in the balloon before one of its flights is shown in Fig. 2.22 on the right. Results from the CREAM-III flight reporting the measured fluxes for proton and helium are available in [48]. For the CREAM-III flight the TRD was removed and a Cherenkov camera was added [48]. The CREAM experiment was space qualified [45], [46] in order to be launched and operated as an external module on the International Space Station from August 22 2017 (ISS-CREAM).
2Pictures taken from the CREAM official websitehttps://cosmicray.umd.edu/cream/.
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2.1 Space detection of cosmic rays
Ballon experiments are quite challenging by itself but there was a really strong de-mand for a detector that could acquire cosmic rays outside of the atmosphere for many years. To explore such a possibility, the Alpha Magnetic Spectrometer (AMS-01) flew to space in a Space Shuttle for 11 days in 1998 [54] demonstrating that a complex detector could be launched and operated in space.
Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics (PAMELA)
Following the possibility opened by AMS-01, the PAMELA detector was launched with a Soyuz-U rocket on June 15 2006 from the Baikonur cosmodrome (Kazakhstan). A review on the detector construction details and results from ten years of operation are re-ported in [55]. A scheme of the various component of the detector and a figure represent-ing the host of PAMELA, the Resurs DK1 satellite, is shown in Fig. 2.3. With three Time
4 THE PAMELA COLLABORATION O. ADRIANI ETC.
Fig. 1.: The PAMELA instrument: a schematic overview of the apparatus.
Furthermore PAMELA is well suited to conduct studies of cosmic-ray acceleration and propagation mechanisms in the Galaxy, solar modulation effects, the emissions of Solar Energetic Particles (SEPs) inside the heliosphere and investigate the particles in the Earth’s magnetosphere.
The PAMELA instrument as a whole, including sub-detectors, is described in Section 2. The Resurs satellite and Ground Segment are presented in Section 3. Qualification tests of the apparatus are shown in Section 4. PAMELA main scientific results are presented in Section 5, divided in several subsections to better explain the different topics and data analysis methods. Conclusions are finally drawn in Section 6.
2. – The PAMELA apparatus
The PAMELA apparatus is composed of the following subdetectors, arranged as shown in Figure 1: a Time-of-Flight system (ToF: S1, S2, S3), an anticoincidence system (CARD, CAT, CAS), a permanent magnet spectrometer, an electromagnetic calorimeter, a shower tail catcher scintillator (S4), a neutron detector. The apparatus is m high, has a mass of kg and an average power consumption of W. The acceptance of the instrument is cm sr and the maximum detectable rigidity is TV. Spillover effects limit the upper detectable antiparticle momentum to 190 GeV/c ( 270 GeV/c) for antiprotons (positrons).
PAMELA sub-detectors are described with more detail in the following paragraphs.
However, a fully comprehensive description can be found in [14].
TEN YEARS OF PAMELA IN SPACE 9
Fig. 6.: Left: A sketch of the Resurs DK1 satellite which hosts the PAMELA experiment in a Pressurized Container. Right: the reception antenna at NTs OMZ.
More details about PAMELA instrument, including the data acquisition and the trigger system, are given in [14].
3. – The Resurs DK1 satellite and the NTs OMZ ground segment
The Resurs DK1 satellite class is manufactured by the Russian space company TsSKB Progress to perform multi-spectral remote sensing of the Earth’s surface and acquire high-quality images in near real-time. Data delivery to ground is realized via a high-speed radio link. The satellite has a mass of 6.7 tonnes and a height of 7.4 m. The solar array span is 14 m. The satellite is three-axis stabilized with an axis orientation accuracy of 0.2 arcmin and an angular velocity stabilization accuracy of 0.005 /s.
On the Resurs-DK1 satellite employed for PAMELA, a dedicated Pressurized Con-tainer (PC) was built and attached to the satellite to hold the instrument, as clearly visible in Figure 6 (left). The container is cylindrical in shape and has an inside di-ameter of about 105 cm, a semi-spherical bottom and a conical top. It is made of an aluminum alloy, with a thickness of 2 mm in the acceptance of PAMELA. During launch and orbital maneuvers, the PC is secured against the body of the satellite. During data-taking it is swung up to give PAMELA a clear view into space.
PAMELA was successfully launched on June 15th 2006 from the cosmodrome of Baikonur in Kazakhstan by a Soyuz-U rocket. The orbit was non-sunsynchronous, with an altitude varying between 350 km and 600 km at an inclination of 70.0 . In September 2010 the orbit was changed to a nearly circular one at an altitude of about 550 km, and it remained so until the end of the mission.
PAMELA was first switched-on on the June 20th2006. After a brief period of com-missioning, the instrument has been in a continuous data-taking mode since July 11th 2006.
Data downlinked to ground has proceeded regularly during the whole mission. Figure
Figure 2.3: The PAMELA instrument in a scheme (left) and a figure of the satellite that host the detector (right). Pictures taken from [55].
Of Flight (TOF) detectors, a 16 radiation length electromagnetic calorimeter, a mag-netic spectrometer (silicon tracker plus a 0.48 T magnet), the primary goal of PAMELA is to measure anti-matter and light-nuclei spectra, like its name says. PAMELA was the first experiment able to measure accurately the positron fraction in cosmic rays [56]
and has observed for the first time a hardening in the light elemental cosmic-ray energy spectra [44].
Alpha Magnetic Spectrometer (AMS-02)
The Alpha Magnetic Spectrometer is an astroparticle detector operating as an ex-ternal module on the International Space Station since May 2011. AMS-02 can perform
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CHAPTER 2. DARK MATTER PARTICLE EXPLORER
accurate measurements of the spectra of charged cosmic rays from 0.5 GeV to few TeVs.
A scheme of the detector and a picture of AMS-02 on the ISS are shown in Fig. 2.43.
Figure 2.4: The AMS-02 instrument in a scheme (left) and a picture of AMS on the ISS (right).
As depicted in Fig. 2.4, the core of the instrument is a permanent magnet generating a field of about 0.14 T within a cylindrical shaped volume (diameter and height∼1 m) surrounding a precision tracking device made by double-sided silicon micro-strip detec-tors [57]. Seven layers of silicon detecdetec-tors inside the magnet volume (Inner Tracker) and two layers outside the field volume allow to reconstruct the tracks of charged particles and measure the particle magnetic rigidity. At both ends of the magnet, two segmented scintillator planes are placed to measure the velocity of charged particles (ToF). The An-tiCoincidence scintillator Counter (ACC) is a barrel of scintillation counters surrounding the Inner Tracker and providing a veto signal against particles entering the detector side-ways. The Ring Imaging Cherenkov (RICH) is located below the magnet and measures the particle velocity and charge. The Transition Radiation Detector (TRD), placed on top of AMS, ensures the e/p separation. The Electromagnetic CALorimeter (ECAL), on the bottom of AMS, allows an accurate discrimination between electromagnetic particles and hadrons, and provides energy measurements for leptons and photons. AMS-02 can indirectly infer the mass of the cosmic-ray, and therefore it is dedicated also to the search for antihelium nuclei (AMS-01 started this search in [58]). In fact, the probability to produce antihelium by spallation of primary cosmic rays on the ISM is extremely low in our galaxy [59], and antihelium nuclei are direct proof of the existence of antimat-ter domains in the Universe. Since the beginning of the data acquisition, 3He and4He candidates were found; the background estimation in this analysis is ongoing and with more statistics a definitive statement will be possible. The latest results on the proton and the helium energy spectra are reported in [40] and [60]. Other recent measurements related to the acceleration mechanism of cosmic rays are reported in [61–63].
3The picture is taken from the official AMS websitehttps://ams.nasa.gov.
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CALorimeteric Electron Telescope (CALET)
CALET is a Japan-led international experiment launched on August 19 2015 on the Japanese module of the International Space Station (ISS). The designed mission life time is two years, which has been extended to five years. CALET consists of three main parts from top to bottom: a scintillator array that acts as charge detector (CD), a fine-grained preshower imaging calorimeter (IMC) used to trace the incoming cosmic-ray direction, and a total absorbtion calorimeter (TASC). The total thickness of the detector corresponds to 30 radiation lengths and 1.3 interaction lengths (more details are available in [64]). A scheme of CALET with a simulated electron entering the detector (picture taken from [65]) and a picture of the position of CALET on the ISS are shown in Fig. 2.5.
Recent results from CALET regarding the combined electron plus positron measurement are available in [66].
Figure 2.5: The CALET instrument in a scheme (left) and on a picture on the ISS (right).
NUCLEON
The NUCLEON detector [47] is a Russian particle physics detector launched on December 28 2014 to a sun-synchronous orbit at an altitude of 475 km as an additional payload of the Russian satellite Resource-P 2. NUCLEON is designed to allow two independent energy measurements: one through an ionization calorimeter (IC), which is widely used in the space detection of cosmic-rays, and one with a kinematic lightweight energy meter (KLEM). The latter, a pioneer technique, is based on measuring the energy of the primary particle by determining the angular distribution of secondaries produced in the interaction of the impinging cosmic ray on the spectrometer. NUCLEON is composed by a charge measurement system (ChMS), a carbon target, six planes for the energy measurement system based on this new KLEM method, three double-layer planes for the scintillator trigger system, and a small aperture calorimeter of 25×25 cm2 (IC).
A scheme of the experiment is shown in Fig. 2.6 and very recent results are available in [67].
CHAPTER 2. DARK MATTER PARTICLE EXPLORER
Figure 2.6: Layout of the NUCLEON experiment in section. The particle encounters first the two planes of the charge measurement system (1), a carbon target (2), six planes for the KLEM tracker system (3) interleaved with three planes for the scintillator trigger systems (4) and followed by the ionization calorimeter (5). Picture taken from [47].