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The AMS silicon tracker
ELSEVIER
The AMS Silicon
Nuclear
Physics
B (Proc.
Suppl.) 113 (2002) 139-146 www.elsevier.com/localelnpe
Tracker
W.J. Burger INFN-Sezione di Perugia I-06100 Perugia, Italia The Alpha Magnetic Spectrometer (AMS) is designed to operate on the International Space Station (ISS) for a long duration measurement (3 yr) of the cosmic ray spectra in the rigidity range from N 0.1 GV to several TV. At the detector level, the principal innovation is the use of a large area (7m’) silicon microstrip tracker. A first version of the AMS was flown on the NASA spa.ce shuttle Discovery in June 1998. A description of the detector and a brief review of selected performance results from the test flight are presented.
1. Introduction Magnetic spectrometers have contributed significantly over the last 30 years to our understanding of cosmic ray origin and propagation. The balloon-borne instruments attain altitudes where the atmospheric overburden (- 5g/cm2) is comparable to the material thickness of the The dynamic range of detector components. a given spectrometer can be characterized by the maximum detectable rigidity (MDR), defined as the rigidity value corresponding to 100% Typical MDR values of measurement error. the different superconducting magnets equipped with gas-filled tracking devices range between 50330 GV [l-6]. I n g eneral, the balloon flights take place over different sites on the North American continent with local vertical rigidity cutoffs between N 0.5 to 4.5 GV. The measurements of the spectra of the dominant cosmic ray components, protons (90%) and helium (10%) have been used to constrain models where the underlying physics is described in terms of initial acceleration in the source region, followed by particle propagation (and eventual reacceleration) in the interstellar medium [7-111. The observed flux is corrected for energy losses and inelastic interactions in and above the detector. The interstellar flux is obtained after a demodulation of the top-of-the-atmosphere (TOA) flux, which takes into account the diffusion, convection, and adiabatic deceleration in the solar
wind of the cosmic rays detected at the Earth. A comparison of the fluxes measured with different levels of solar activity provide a test of the modulation theory. Although good agreement is obtained for the shape of the primary proton and helium nuclei spectra, discrepancies (lo-30%) are observed between the absolute fluxes quoted by the different experiments. A characteristic specific to a magnetic spectrometer is the ability to determine the sign of the charge. This feature is used to distinguish the relatively rare electrons (< 1%) from the dominant proton component. The detection capability for electrons, and positrons (- 0.1% relative abundance), is enhanced by the use of ancillary detectors such as shower counters [12], and for the most recent measurements, transition radiation devices; imaging calorimeters, and Ring Imaging Cherekov (RICH) detectors, often combined in the same experiment [13-161. The most extensive measurements have been provided by the High-Energy Antimatter Telescope (HEAT) [IS] which has reported results for both electrons and positrons between l-50 GV, corresponding to an estimated MDR for the spectrometer of 170 GV. The measurement of the electron component of the cosmic radiation provides complementary information of the physics processes affecting the particle propagation in the Galaxy. In the absence of hadronic interactions, the energy losses are dominated by snychroton radiation in the
0920-5632/02/$ - see front matter 0 2002 Elsevier Science B.V. All rights reserved. PI1 SO920-5632(02)01 X33-9
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WJ Burger/Nuclear
Physics B (Proc. Suppl.) 113 (2002) 139-146
galactic magnetic field and inverse Compton scattering. Due to the presence of the cosmic microwave background, the latter process excludes the possibility for electrons to traverse intergalactic distances. The present experimental results indicate the positron flux is consistent with secondary production originating from nuclear cosmic-ray interactions in the interstellar medium, while the electron flux is dominated by electrons accelerated in sources which may or may not be the same as for hadrons. The measurements are limited by the maximum flight time of the balloons (- 24 hr). An increase in statistics is required to define the acceleration mechanism at the source(s). Measurements at higher energies may reveal the existence of new types of galactic sources. A mass spectrometer’ provides an unambiguous detection of antimatter. The most recent results for the antiproton flux have been reported by BESS [17] in the range of 0.2-4.2 GV, and by CAPRICE [6] between 3-49 GV. The measured flux from the two experiments is compatible with a purely secondary origin of the antiprotons, with a characteristic peak in the kinetic energy spectrum at N 2 GeV. Above the peak, the sensitivity to an eventual signal indicating a new physical source is limited by statistics: the CAPRICE results are based on 31 antiprotons. At lower energies, the BESS results represent a total of 848 antiprotons recorded during the years 1993-1998. However, at the lower energies the predicted flux is sensitive to uncertainties in the secondary production cross section and solar modulation. It has been suggested that the effects produced by the solar modulation could distinguish a primary component of antiprotons from interstellar secondaries [18], which implies a series of measurements throughout the 11 yr solar cycle, or longer (22 yr [19]). The most statistically significant experimental limit for antimatter is provided by antihelium searches. The present limit from the balloonborne spectrometers was reported by BESS based IIn general, the particle velocity is determined by a timeof-flight measurement accompanied, in some experiments, with a second determination based on the Cherenkov light emitted in a selected medium.
on the helium nuclei observed in the period 19932000: < 7 x lop7 in the range of 1 to 14 GV [20]. 2.
AMS
The Alpha Magnetic Spectrometer (AMS) is designed to operate three years on the International Space Station (ISS). The programmed observation time of the large acceptance spectrometer (0.5 m%r) represents a significant improvement for the flux determinations of the rare cosmic rays components and the antimatter search. The measurements made at the 400 km orbital altitude of the ISS are not affected by the corrections needed to extract the TOA fluxes from the data collected at the balloon altitudes (40 km). The atmospheric corrections represent a non-negligible contribution to the systematic uncertainty for the flux measurements of the rarest cosmic-ray components, positrons and antiprotons. A simplified version of the detector (AMSOl), consisting of a 0.14 T permanent magnet, time-offlight and veto scintillator paddles, a silicon microstrip tracker, and an aerogel Cherenkov detector, was flown on the shuttle Discovery in June 1998 (NASA flight STS-91). The primary objective of the flight was the validation of the technical design. The successful operation of the detector during the lo-day flight resulted in high statistic measurements of the proton, helium, electron and positron fluxes, as well as a new limit for antihelium: < 1.1 x 10m6 in the rigidity range l140 GV [21]. The results from the test flight data are summarized in Ref. [22]. The wide range in latitude (f51.7”) of the shuttle orbit provided a comprehensive map of the particle fluxes around the Earth. The rigidity measurement and the knowledge of the incident direction of the particle have been used to distinguish in the observed spectra, the contributions of the primary particles and atmospheric secondaries [23]. The secondary fluxes show reasonably good agreement with the fluxes predicted by different atmospheric interaction models which use as input the measured primary proton flux [24-261. The knowledge of the primary proton and helium
KJ Burger/Nuclear
Physics B (Proc. Suppl.)
113 (2002) 139-146
141
fluxes [27], as well as the under geomagnetic cutoff, secondary proton flux, are important for the interpretation of neutrino oscillation results based on the observation of atmospheric neutrinos. 3.
Silicon Tracker
The silicon tracker [28] is composed of 41.360 x 72.045 x 0.300 mm3 double-sided silicon microstrip sensors. The n-doped sensors are biased via the punchthrough technique and p+ blocking strips, implanted on the n-side, are used to minimize the influence of surface charge on the position measurement obtained from the ohmic side. The sensor design uses capacitive charge coupling with (implantation) readout strip pitches of 27.5 (110) pm for the y coordinate pside (bending direction), and 52 (208) pm for the x coordinate n-side. The silicon sensors are grouped together, for readout and biasing, in ladders of different lengths to match the cylindrical geometry of the AMS magnet. The cylindrical design optimizes the ratio acceptance-to-weight. Figure 1 shows the principal elements of the ladder and the main components of the readout hybrids. The readout strips of the silicon sensors are ac-coupled to the low noise, high dynamic range Vahdr chips [29] via 700 pF capacitor chips. Thin flexible upilex films are used to connect the signal strips of the sensors to the readout chips. In this manner, the hybrids are placed perpendicular to the detection plane, thus minimizing the material in the acceptance. Figure 2 shows the assembled AMSOl tracker. The support structure is divided into three sections: a carbon fiber cylindrical shell which supports the planes 2 to 5 located inside the magnet, and two carbon fiber flanges which support the exterior planes 1 and 6. The hybrids are mounted on carbon fiber-metal cooling bars which evacuate the heat generated by the frontend electronics to the exterior. The distance between planes 1 and 6 is one meter. The tracker planes are composite material structures composed of a low density, aluminum honeycomb interior surrounded by two carbon fiber layers. The material thickness of an interior
Figure 1. The principal components ladder.
of the silicon
plane, including ladders, represent 0.65% of a radiation length. The AMSOl tracker was equipped for the shuttle flight with 57 ladders, distributed in the six planes and representing a total silicon area of 2.1 m’. The AMS tracker is the first application in space of the high precision silicon technology developed for position measurements in ground accelerator experiments [30]. The high modularity, low voltage levels (< 100 V), and gas-free operation of the devices are well suited for operation in space. The position resolution of a silicon microstrip sensor (< 10pm) represents a factor of - 10 improvement with respect to the different gas devices used by the balloon-borne superconducting magnetic spectrometers. The principal disadvantages of the microstrip detectors are the cost and the relative high density of the silicon, resulting in a configuration with a smaller number of higher precision measurements with respect to the gas tracking systems. Silicon microstrip sensors were employed originally as vertex detectors in collider experiments to provide a few high precision position measurements near the interaction point. In this application, the number of sensors is reduced. The track reconstruction over a larger volume is provided by the position measurements in more traditional gas chambers.
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K!J Burger/Nuclear
Figure 2. View of the assembled tracker.
Physics B (Proc. Suppl.)
AMSOl silicon
The AMS application differs considerably. The tracking information is provided uniquely by the silicon, which implies a large surface area and higher interstrip capacitances, given the constraints imposed by limited power and the desire to limit the material thickness. The higher noise in the silicon must be compensated by a low-noise readout for reliable track reconstruction. In view of the marginal increase of the plane hermeticity, and the very significant complication of the mechanical design, there is no overlap between the ladders in the planes of the tracker. 4. STS-91
Performance
Results
No degradation of the tracker performance was observed after the shuttle launch. All ladders functioned normally throughout the flight with noise and dark current levels comparable to those measured on the ground. The principal influence on the tracker performance was the temperature variation of the detector during the lo-day flight. The flight temperatures are shown in Fig. 3.
I13 (2002) 139-146
The shuttle Discovery was docked with the Russian space station Mir during the first half of the flight. The AMS detector, mounted in the shuttle bay, was placed between Mir and the body of the shuttle, thus shielded from the Sun and the radiation reflected by the Earth. A decrease of the AMS temperature was observed until the middle of day five of the mission, when Discovery separated from Mir, and the shuttle bay wits oriented to point in the zenith direction for the second half of the flight dedicated to the AMS data collection. The constant temperature rise observed at the zenith position required a change of the shuttle orientation in order to limit the temperatures of the AMS electronics, typically lo-15’C higher than the temperatures shown in Fig. 3, which correspond to the average values recorded at the tracker planes. The tracker noise levels and dark currents display, as expected, a clear correlation with the temperature changes, lower temperature implying lower thermal noise. In terms of electrical performance, the tracker signal-to-noise’ on the pside varied between 7:l and 8:l during the flight. The signal-to-noise of the n-side of the silicon sensors was considerable worse (4:l). The poor nside performance was due to the small ratio of the implantation-to-readout strip pitches in the original design (26:208 pm), which resulted in a loss of charge collected across the readout gap. The performance is recovered for the same readout strip pitch, by doubling the implantation strip pitch. The tracker provides a measure of the energy loss in addition to the position of the particle. The electronics was designed to provide charge identification for Z 5 10. The shuttle flight data shows a clear charge separation for Z 5 8 and a linear response of the readout electronics for Z 5 6. The AMSOl online trigger was not optimized for the detection of the higher charge nuclei; in particular, the veto condition resulted in an important loss for charges above Z = 4 [31]. The tracker plane efficiencies (p-side) for Z = 1 particles (- 80%) varied during the flight fol2defined as the cluster charge divided by the root-meansquare of the pedestal widths of the member strips
KJ Burger/Nuclear
Physics B (Proc. Suppl.)
lowing the signal-to-noise (temperature) evolution. The contributions to the tracker plane inefficiencies are the geometric inefficiency due to the inter-ladder and inter-sensor distances (1.4%), the insensitive region of the silicon sensors (5.6%), readout channels characterized by a non-gauss&r noise behaviour which were suppressed in the readout (3%), and non-working channels (9%). The latter, which were defined by their low occupation levels for 2 = 1 particles, represent a negligible contribution to the inefficiency for the higher charge nuclei. Further details on the electrical performance can be found in Ref. [28]. The tracker mechanical performance can be evaluated in terms of stability and precision visd-vis temperature, launch accelerations, and outgassing in space. The thermal behaviour of the structure can be modelled rather accurately in function of the thermal properties of the chosen materials and the expected thermal loads. The tracker was designed to survive temperature variations of 20 f 40°C. The effects of the launch on the structure were modelled and checked, at the level of the plane, with a ground vibration test of a quarter-plane prototype equipped with a functional ladder. The influence of the flight temperatures on the tracker mechanical stability was evaluated by studying the behaviour of the track residuals. Five and six hit, high rigidity tracks (2 1OGV) associated with identified protons, are refitted to determine the residual at each plane by removing in turn, the hit in question, and comparing the hit position with t,he position predicted by the refit. The results shown in Fig. 3 correspond to the yresiduals at plane 4. Each point represents the average residual value computed every 5 minutes. The residuals at the other plane positions display a similar behaviour. A comparison of the residual displacements and flight temperatures yields an upper limit of < 0.3nm/“C for the temperature dependence, corresponding to an overall displacement of - 5pm for the 15°C variation observed during the flight. In the analysis, the field map values of the permanent magnet have been corrected using the field variation recorded during the flight, -1.3 x 10e4 T/“C.
113 (2002) 139-146
143
Mission Elapsed Time (Days)
Figure 3. Flight temperatures (top) and flight track residuals (bottom) as a function of the Mission Elapsed Time.
5. Alignment The final alignment uncertainty reflects the performance of the mechanical design in terms of precision. The silicon sensors are fabricated with a submicron precision for the alignment of the implanted and metalized structures, i.e. strips, guard rings, and fiducial reference marks. The reference marks provide the relative positions of the sensors in the ladder, and of the ladders in the plane. An in situ alignment with tracks was not possible with the AMSOl permanent magnet. The tracker was aligned after the fight with tracks in fixed momentum particle beams. The alignment determines the ladder positions in the planes and the plane positions in the support structure. The alignment of the silicon sensors in the ladder (< 5nm) was guaranteed by the mechanical precision of the ladder jigs and the cutting precision of the silicon wafers [32]. The novel approach simplifies considerably the ladder fabrication and reduces the equipment cost of the assembly lines. The ladder positions on the planes were mea-
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Burger/Nuclear
Physics B (Proc. Suppl.) 113 (2002) 139-146
sured before and after the flight with an accuracy better than 5(10) pm for the in-plane (outof-plane) distances. A comparison of the preflight and postflight metrology provides information on the dimensional changes produced by the space vacuum.3 A comparison of the postflight metrology ladder positions with those obtained from the beam alignment ,allows an estimate of the alignment uncertainty at the plane level. The results of the latter comparison are shown in Fig. 4.
Differences in ladder center posiFigure 4. tions (z,y,z) on the planes (particle beam alignment minus postflight metrology values) . The quoted numbers are the central values and one sigma widths of the gaussian fits.
There is good overall agreement between the two determinations for the in-plane ladder positions (zy). However, the relatively large average difference (30pm) observed in the out-ofplane direction (z) indicates that the beam alignment is less sensitive to the reduction of the ladder heights on the plane, a phenomenon which is clearly seen in the comparison of the preflight and 3The metrology is performed at a fixed temperature.
postflight metrology data (AZ ,-+ 50 pm). The lower postflight ladder heights are a consequence of the dimensional changes of the Airex foam support of the ladder structure (Fig. 1) due to outgassing in the space vacuum. The Airex foam was chosen for its relatively low hygroscopicity. In the mechanical design, the foam is free to move in the out-of-plane direction. The in-plane displacements are limited by the fixation of the ladders to the planes. The fixation pieces provide a greater flexibility for displacements in the z direction, i.e. along the ladder length and parallel to the direction of the magnetic field. The larger dispersion observed in the y bending direction (Fig. 4) indicates an influence of the permanent magnet on the beam alignment results. The disagreement observed for the ladder heights is symptomatic of the limited precision of the z position measurements. A less accurate measurement (50-100 pm uncertainty) of the plane positions in the tracker support structure was made before the flight. However, the installation of the tracker in the magnet requires the removal of the exterior planes. The dimensional changes of the Airex foam are relatively small with respect to the differences observed in the z positions of the planes after the beam alignment. The maximum .z displacement observed between the beam alignment and preflight plane positions was N 0.7 mm for plane 6.4 The data of Fig. 4 are used to estimate the effect of the alignment uncertainty on the rigidity resolution. The AMSOl proton rigidity resolution (2 4 hit tracks) is simulated with position measurement resolutions of 40 and 10 pm for the CC and y coordinates respectively.5 The alignment uncertainty is introduced by displacing the recorded track positions by the observed difference between the beam alignment and postflight metrology ladder positions prior to the curvature fit. Figure 5 shows the effect of the alignment on the rigidity resolution. The resolution obtained 4The beam alignment optimizes the plane positions with respect to plane 1. 5The AMSOl z resolution varied with charge: 40 (30) pm for 2 = l(2)
WJ Burger/Nuclear
Physics B (Proc. Suppl.) 113 (2002) 139-146
with the inclusion of the alignment uncertainty reproduces the resolution used for the physics analysis, The latter is based on a full detector simulation which reproduces the test beam performance [21] with a position resolution of 17um for the y coordinate (40pm for x).
AMSO 1
F t
-
2
*.:
145
signal-to-noise performance should be improved by a further optimization of the ladder assembly. During the fabrication of the AMSOl ladders, a 30-50% increase in dark current was observed for nearly half the ladders after assembly. An estimate of the AMS02 proton rigidity resolution (2 5 hit tracks) is presented in Fig. 6. Position resolutions of 30 (10) pm were used for the x (y) coordinates with a silicon detection efficiency of 90%. The alignment uncertainty is based the AMSOl results (Fig. 4). In principle, the presence of the superconducting magnet will allow a field-free in situ alignment minimizing the influence of the alignment on the rigidity resolution.
”
‘j
.:*
B
E LT
.’ .,.*
.-
Ah602
.:’ .,...*.* ,..*
-I
10
” --
,,/
...’
_,...-~-’
Figure 5. Rigidity resolution for the AMSOl six plane, six layer tracker and 0.14 T permanent magnet. Vertical lines denote the MDR.
6. AMSOZ The final version of the detector (AMSOS) which will be installed on the ISS includes two modifications which affect the tracker performance: a superconducting magnet and an increase in the number of silicon layers (six to eight). The superconducting magnet represents a considerable technical challenge. An increase in the number of track measurements is obtained by removing one of the interior tracker planes, and equipping both sides of the three remaining internal planes with ladders. In addition to the change of the n-side implantation pitch mentionned previously, the general
Figure 6. Rigidity resolution for the AMS02 five plane, eight layer tracker and 0.8 T superconducting magnet. Vertical lines denote the MDR.
7. Conclusion
The AMS silicon tracker presents a twofold challenge: the extension of the silicon microstrip technology to a large area detector and its application in space. The test flight results demon-
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K1 Burger/Nuclear Physics B (Proc. Suppl.) 113 (2002) 139-146
strate both the successful adaptation of the devices to the space environment and the feasibility of large area detectors. AMS is a high visibility experiment. The relevance of the ISS for basic science has been debated since the project was proposed [33]. The highly improbable possibility to observe primordial antimatter in our Galaxy has been eloquently evoked [34]. Nevertheless, the AMS experiment inscribes in a general trend, the “rediscovery” of cosmic rays by the particle physics community after a long period of “latency”, dictated certainly by scientific and technical considerations, but not completely independent of political and economic influences [35]. The trend also implies a change in the manner to apprehend science, the return to observations of natural phenomena as opposed to experiments conducted in the laboratory. Thirty years later... “The observation of just one or two well verified, unambiguous anti-nuclei in the incident cosmic-ray flux would have profound astrophysical significance, because most present day cosmologies do not take account of significant amounts of antimatter.” [36]
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
L.H. Smith et al., Ap. J., 180 (1973) 987. R.L. Golden et al., NucI. Instr. and Method., 148 (1978) 179. A.D. Tomasch et al., Nucl. Instr. and Method. A, 294 (1990) 627. S.W. Barwick et al., Nucl. Instr. and Method A, 400 (1997) 34. Y. Ajima et al., Nucl. Instr. Method. A, 443 (2000) 71. M. Boezio et al., Ap. J., 561 (2001) 787. E.S. Seo et al., Ap. J., 378 (1991) 763. R. Bellotti et al., Phys. Rev. D, 60 (1999) 052002-l. M. Boezio et al., Ap. J., 518 (1999) 457. W. Menn et al., Ap. J., 533 (2000) 281. T. Sanuki et al., Ap. J., 545 (2000) 1135. R.L. Golden et al., Ap. J., 287 (1984) 622. R.L. Golden et al., Ap. J., 436 (1994) 769. R.L. Golden et al., Ap. J., 457 (1996) L103. M. Boezio et al., Ap. J., 532 (2000) 653.
16. M.A. DuVernois 296.
et al., Ap. J., 559
(2001)
17. T. Maeno et al., Astropart. Phys., 16 (2001) 121.
18. T. Mitsui et al., Phys. Lett. B, 389 (1996) 169. 19. J.W. Bieber et al., Phys. Rev. Lett., 83 (1999) 674.
20. M. Sass et al., “A search for antihelium with the BESS spectrometer”, Proceedings of the 27th International Cosmic Ray Conference, Hambourg (2001) 1711. 21. J. Alcaraz et al., Phys. Lett. B, 461 (1999) 387.
22. M. Aguilar et al., Physics Reports, 366 (2002) 331.
et al., J. Geophys. Res., 107 23. E. Fiandrini (2002) A6. 24. L. Derome et al., Phys. Lett. B, 489 (2000) 1. 25. V. Plyaskin, Phys. Lett. B, 516 (2001) 213. 26. P. Lipari, Astropart. Phys., 16 (2002) 295. Spectrum to 1 27. T.K. Gaisser et al.,“Primary TeV and Beyond”, Proceedings of the 27th International Cosmic Ray Conference, Hambourg (2001) 1. Cimento - Vol. 112 28. J. Alcaraz et al., Il NZLOVO A, N. 11 (2000) 1325. 29. B. Alpat et al., Nucl. Instr. and Method. A, 446
(2000) 552.
30. M. Acciarri et al., Nucl. Instr. and Method. A 351 (1994) 300. “AMS Results for Particle 31. W.J. Burger, Charge Z > l”, Proceedings of the Vulcan0 Workshop 2000, Vulcan0 (2000) 445. 32. W.J. Burger, Nucl. Instr. and Method. A 435 (1999) 202. 33. T. Beardsley, “Science in the Sky”, Scientific America (June 1996) 36. 34. G. Tarle and S.P. Swordy, ‘Cosmic Antimatter”, Scientific America (April 1998) 30. 35. C. Allegre, La defaite de Platon, Fayard (1995). 36. A. Buffington et al., Nature, 236 (1972) 335.
The AMS Silicon
Nuclear
Physics
B (Proc.
Suppl.) 113 (2002) 139-146 www.elsevier.com/localelnpe
Tracker
W.J. Burger INFN-Sezione di Perugia I-06100 Perugia, Italia The Alpha Magnetic Spectrometer (AMS) is designed to operate on the International Space Station (ISS) for a long duration measurement (3 yr) of the cosmic ray spectra in the rigidity range from N 0.1 GV to several TV. At the detector level, the principal innovation is the use of a large area (7m’) silicon microstrip tracker. A first version of the AMS was flown on the NASA spa.ce shuttle Discovery in June 1998. A description of the detector and a brief review of selected performance results from the test flight are presented.
1. Introduction Magnetic spectrometers have contributed significantly over the last 30 years to our understanding of cosmic ray origin and propagation. The balloon-borne instruments attain altitudes where the atmospheric overburden (- 5g/cm2) is comparable to the material thickness of the The dynamic range of detector components. a given spectrometer can be characterized by the maximum detectable rigidity (MDR), defined as the rigidity value corresponding to 100% Typical MDR values of measurement error. the different superconducting magnets equipped with gas-filled tracking devices range between 50330 GV [l-6]. I n g eneral, the balloon flights take place over different sites on the North American continent with local vertical rigidity cutoffs between N 0.5 to 4.5 GV. The measurements of the spectra of the dominant cosmic ray components, protons (90%) and helium (10%) have been used to constrain models where the underlying physics is described in terms of initial acceleration in the source region, followed by particle propagation (and eventual reacceleration) in the interstellar medium [7-111. The observed flux is corrected for energy losses and inelastic interactions in and above the detector. The interstellar flux is obtained after a demodulation of the top-of-the-atmosphere (TOA) flux, which takes into account the diffusion, convection, and adiabatic deceleration in the solar
wind of the cosmic rays detected at the Earth. A comparison of the fluxes measured with different levels of solar activity provide a test of the modulation theory. Although good agreement is obtained for the shape of the primary proton and helium nuclei spectra, discrepancies (lo-30%) are observed between the absolute fluxes quoted by the different experiments. A characteristic specific to a magnetic spectrometer is the ability to determine the sign of the charge. This feature is used to distinguish the relatively rare electrons (< 1%) from the dominant proton component. The detection capability for electrons, and positrons (- 0.1% relative abundance), is enhanced by the use of ancillary detectors such as shower counters [12], and for the most recent measurements, transition radiation devices; imaging calorimeters, and Ring Imaging Cherekov (RICH) detectors, often combined in the same experiment [13-161. The most extensive measurements have been provided by the High-Energy Antimatter Telescope (HEAT) [IS] which has reported results for both electrons and positrons between l-50 GV, corresponding to an estimated MDR for the spectrometer of 170 GV. The measurement of the electron component of the cosmic radiation provides complementary information of the physics processes affecting the particle propagation in the Galaxy. In the absence of hadronic interactions, the energy losses are dominated by snychroton radiation in the
0920-5632/02/$ - see front matter 0 2002 Elsevier Science B.V. All rights reserved. PI1 SO920-5632(02)01 X33-9
140
WJ Burger/Nuclear
Physics B (Proc. Suppl.) 113 (2002) 139-146
galactic magnetic field and inverse Compton scattering. Due to the presence of the cosmic microwave background, the latter process excludes the possibility for electrons to traverse intergalactic distances. The present experimental results indicate the positron flux is consistent with secondary production originating from nuclear cosmic-ray interactions in the interstellar medium, while the electron flux is dominated by electrons accelerated in sources which may or may not be the same as for hadrons. The measurements are limited by the maximum flight time of the balloons (- 24 hr). An increase in statistics is required to define the acceleration mechanism at the source(s). Measurements at higher energies may reveal the existence of new types of galactic sources. A mass spectrometer’ provides an unambiguous detection of antimatter. The most recent results for the antiproton flux have been reported by BESS [17] in the range of 0.2-4.2 GV, and by CAPRICE [6] between 3-49 GV. The measured flux from the two experiments is compatible with a purely secondary origin of the antiprotons, with a characteristic peak in the kinetic energy spectrum at N 2 GeV. Above the peak, the sensitivity to an eventual signal indicating a new physical source is limited by statistics: the CAPRICE results are based on 31 antiprotons. At lower energies, the BESS results represent a total of 848 antiprotons recorded during the years 1993-1998. However, at the lower energies the predicted flux is sensitive to uncertainties in the secondary production cross section and solar modulation. It has been suggested that the effects produced by the solar modulation could distinguish a primary component of antiprotons from interstellar secondaries [18], which implies a series of measurements throughout the 11 yr solar cycle, or longer (22 yr [19]). The most statistically significant experimental limit for antimatter is provided by antihelium searches. The present limit from the balloonborne spectrometers was reported by BESS based IIn general, the particle velocity is determined by a timeof-flight measurement accompanied, in some experiments, with a second determination based on the Cherenkov light emitted in a selected medium.
on the helium nuclei observed in the period 19932000: < 7 x lop7 in the range of 1 to 14 GV [20]. 2.
AMS
The Alpha Magnetic Spectrometer (AMS) is designed to operate three years on the International Space Station (ISS). The programmed observation time of the large acceptance spectrometer (0.5 m%r) represents a significant improvement for the flux determinations of the rare cosmic rays components and the antimatter search. The measurements made at the 400 km orbital altitude of the ISS are not affected by the corrections needed to extract the TOA fluxes from the data collected at the balloon altitudes (40 km). The atmospheric corrections represent a non-negligible contribution to the systematic uncertainty for the flux measurements of the rarest cosmic-ray components, positrons and antiprotons. A simplified version of the detector (AMSOl), consisting of a 0.14 T permanent magnet, time-offlight and veto scintillator paddles, a silicon microstrip tracker, and an aerogel Cherenkov detector, was flown on the shuttle Discovery in June 1998 (NASA flight STS-91). The primary objective of the flight was the validation of the technical design. The successful operation of the detector during the lo-day flight resulted in high statistic measurements of the proton, helium, electron and positron fluxes, as well as a new limit for antihelium: < 1.1 x 10m6 in the rigidity range l140 GV [21]. The results from the test flight data are summarized in Ref. [22]. The wide range in latitude (f51.7”) of the shuttle orbit provided a comprehensive map of the particle fluxes around the Earth. The rigidity measurement and the knowledge of the incident direction of the particle have been used to distinguish in the observed spectra, the contributions of the primary particles and atmospheric secondaries [23]. The secondary fluxes show reasonably good agreement with the fluxes predicted by different atmospheric interaction models which use as input the measured primary proton flux [24-261. The knowledge of the primary proton and helium
KJ Burger/Nuclear
Physics B (Proc. Suppl.)
113 (2002) 139-146
141
fluxes [27], as well as the under geomagnetic cutoff, secondary proton flux, are important for the interpretation of neutrino oscillation results based on the observation of atmospheric neutrinos. 3.
Silicon Tracker
The silicon tracker [28] is composed of 41.360 x 72.045 x 0.300 mm3 double-sided silicon microstrip sensors. The n-doped sensors are biased via the punchthrough technique and p+ blocking strips, implanted on the n-side, are used to minimize the influence of surface charge on the position measurement obtained from the ohmic side. The sensor design uses capacitive charge coupling with (implantation) readout strip pitches of 27.5 (110) pm for the y coordinate pside (bending direction), and 52 (208) pm for the x coordinate n-side. The silicon sensors are grouped together, for readout and biasing, in ladders of different lengths to match the cylindrical geometry of the AMS magnet. The cylindrical design optimizes the ratio acceptance-to-weight. Figure 1 shows the principal elements of the ladder and the main components of the readout hybrids. The readout strips of the silicon sensors are ac-coupled to the low noise, high dynamic range Vahdr chips [29] via 700 pF capacitor chips. Thin flexible upilex films are used to connect the signal strips of the sensors to the readout chips. In this manner, the hybrids are placed perpendicular to the detection plane, thus minimizing the material in the acceptance. Figure 2 shows the assembled AMSOl tracker. The support structure is divided into three sections: a carbon fiber cylindrical shell which supports the planes 2 to 5 located inside the magnet, and two carbon fiber flanges which support the exterior planes 1 and 6. The hybrids are mounted on carbon fiber-metal cooling bars which evacuate the heat generated by the frontend electronics to the exterior. The distance between planes 1 and 6 is one meter. The tracker planes are composite material structures composed of a low density, aluminum honeycomb interior surrounded by two carbon fiber layers. The material thickness of an interior
Figure 1. The principal components ladder.
of the silicon
plane, including ladders, represent 0.65% of a radiation length. The AMSOl tracker was equipped for the shuttle flight with 57 ladders, distributed in the six planes and representing a total silicon area of 2.1 m’. The AMS tracker is the first application in space of the high precision silicon technology developed for position measurements in ground accelerator experiments [30]. The high modularity, low voltage levels (< 100 V), and gas-free operation of the devices are well suited for operation in space. The position resolution of a silicon microstrip sensor (< 10pm) represents a factor of - 10 improvement with respect to the different gas devices used by the balloon-borne superconducting magnetic spectrometers. The principal disadvantages of the microstrip detectors are the cost and the relative high density of the silicon, resulting in a configuration with a smaller number of higher precision measurements with respect to the gas tracking systems. Silicon microstrip sensors were employed originally as vertex detectors in collider experiments to provide a few high precision position measurements near the interaction point. In this application, the number of sensors is reduced. The track reconstruction over a larger volume is provided by the position measurements in more traditional gas chambers.
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Figure 2. View of the assembled tracker.
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AMSOl silicon
The AMS application differs considerably. The tracking information is provided uniquely by the silicon, which implies a large surface area and higher interstrip capacitances, given the constraints imposed by limited power and the desire to limit the material thickness. The higher noise in the silicon must be compensated by a low-noise readout for reliable track reconstruction. In view of the marginal increase of the plane hermeticity, and the very significant complication of the mechanical design, there is no overlap between the ladders in the planes of the tracker. 4. STS-91
Performance
Results
No degradation of the tracker performance was observed after the shuttle launch. All ladders functioned normally throughout the flight with noise and dark current levels comparable to those measured on the ground. The principal influence on the tracker performance was the temperature variation of the detector during the lo-day flight. The flight temperatures are shown in Fig. 3.
I13 (2002) 139-146
The shuttle Discovery was docked with the Russian space station Mir during the first half of the flight. The AMS detector, mounted in the shuttle bay, was placed between Mir and the body of the shuttle, thus shielded from the Sun and the radiation reflected by the Earth. A decrease of the AMS temperature was observed until the middle of day five of the mission, when Discovery separated from Mir, and the shuttle bay wits oriented to point in the zenith direction for the second half of the flight dedicated to the AMS data collection. The constant temperature rise observed at the zenith position required a change of the shuttle orientation in order to limit the temperatures of the AMS electronics, typically lo-15’C higher than the temperatures shown in Fig. 3, which correspond to the average values recorded at the tracker planes. The tracker noise levels and dark currents display, as expected, a clear correlation with the temperature changes, lower temperature implying lower thermal noise. In terms of electrical performance, the tracker signal-to-noise’ on the pside varied between 7:l and 8:l during the flight. The signal-to-noise of the n-side of the silicon sensors was considerable worse (4:l). The poor nside performance was due to the small ratio of the implantation-to-readout strip pitches in the original design (26:208 pm), which resulted in a loss of charge collected across the readout gap. The performance is recovered for the same readout strip pitch, by doubling the implantation strip pitch. The tracker provides a measure of the energy loss in addition to the position of the particle. The electronics was designed to provide charge identification for Z 5 10. The shuttle flight data shows a clear charge separation for Z 5 8 and a linear response of the readout electronics for Z 5 6. The AMSOl online trigger was not optimized for the detection of the higher charge nuclei; in particular, the veto condition resulted in an important loss for charges above Z = 4 [31]. The tracker plane efficiencies (p-side) for Z = 1 particles (- 80%) varied during the flight fol2defined as the cluster charge divided by the root-meansquare of the pedestal widths of the member strips
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lowing the signal-to-noise (temperature) evolution. The contributions to the tracker plane inefficiencies are the geometric inefficiency due to the inter-ladder and inter-sensor distances (1.4%), the insensitive region of the silicon sensors (5.6%), readout channels characterized by a non-gauss&r noise behaviour which were suppressed in the readout (3%), and non-working channels (9%). The latter, which were defined by their low occupation levels for 2 = 1 particles, represent a negligible contribution to the inefficiency for the higher charge nuclei. Further details on the electrical performance can be found in Ref. [28]. The tracker mechanical performance can be evaluated in terms of stability and precision visd-vis temperature, launch accelerations, and outgassing in space. The thermal behaviour of the structure can be modelled rather accurately in function of the thermal properties of the chosen materials and the expected thermal loads. The tracker was designed to survive temperature variations of 20 f 40°C. The effects of the launch on the structure were modelled and checked, at the level of the plane, with a ground vibration test of a quarter-plane prototype equipped with a functional ladder. The influence of the flight temperatures on the tracker mechanical stability was evaluated by studying the behaviour of the track residuals. Five and six hit, high rigidity tracks (2 1OGV) associated with identified protons, are refitted to determine the residual at each plane by removing in turn, the hit in question, and comparing the hit position with t,he position predicted by the refit. The results shown in Fig. 3 correspond to the yresiduals at plane 4. Each point represents the average residual value computed every 5 minutes. The residuals at the other plane positions display a similar behaviour. A comparison of the residual displacements and flight temperatures yields an upper limit of < 0.3nm/“C for the temperature dependence, corresponding to an overall displacement of - 5pm for the 15°C variation observed during the flight. In the analysis, the field map values of the permanent magnet have been corrected using the field variation recorded during the flight, -1.3 x 10e4 T/“C.
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143
Mission Elapsed Time (Days)
Figure 3. Flight temperatures (top) and flight track residuals (bottom) as a function of the Mission Elapsed Time.
5. Alignment The final alignment uncertainty reflects the performance of the mechanical design in terms of precision. The silicon sensors are fabricated with a submicron precision for the alignment of the implanted and metalized structures, i.e. strips, guard rings, and fiducial reference marks. The reference marks provide the relative positions of the sensors in the ladder, and of the ladders in the plane. An in situ alignment with tracks was not possible with the AMSOl permanent magnet. The tracker was aligned after the fight with tracks in fixed momentum particle beams. The alignment determines the ladder positions in the planes and the plane positions in the support structure. The alignment of the silicon sensors in the ladder (< 5nm) was guaranteed by the mechanical precision of the ladder jigs and the cutting precision of the silicon wafers [32]. The novel approach simplifies considerably the ladder fabrication and reduces the equipment cost of the assembly lines. The ladder positions on the planes were mea-
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sured before and after the flight with an accuracy better than 5(10) pm for the in-plane (outof-plane) distances. A comparison of the preflight and postflight metrology provides information on the dimensional changes produced by the space vacuum.3 A comparison of the postflight metrology ladder positions with those obtained from the beam alignment ,allows an estimate of the alignment uncertainty at the plane level. The results of the latter comparison are shown in Fig. 4.
Differences in ladder center posiFigure 4. tions (z,y,z) on the planes (particle beam alignment minus postflight metrology values) . The quoted numbers are the central values and one sigma widths of the gaussian fits.
There is good overall agreement between the two determinations for the in-plane ladder positions (zy). However, the relatively large average difference (30pm) observed in the out-ofplane direction (z) indicates that the beam alignment is less sensitive to the reduction of the ladder heights on the plane, a phenomenon which is clearly seen in the comparison of the preflight and 3The metrology is performed at a fixed temperature.
postflight metrology data (AZ ,-+ 50 pm). The lower postflight ladder heights are a consequence of the dimensional changes of the Airex foam support of the ladder structure (Fig. 1) due to outgassing in the space vacuum. The Airex foam was chosen for its relatively low hygroscopicity. In the mechanical design, the foam is free to move in the out-of-plane direction. The in-plane displacements are limited by the fixation of the ladders to the planes. The fixation pieces provide a greater flexibility for displacements in the z direction, i.e. along the ladder length and parallel to the direction of the magnetic field. The larger dispersion observed in the y bending direction (Fig. 4) indicates an influence of the permanent magnet on the beam alignment results. The disagreement observed for the ladder heights is symptomatic of the limited precision of the z position measurements. A less accurate measurement (50-100 pm uncertainty) of the plane positions in the tracker support structure was made before the flight. However, the installation of the tracker in the magnet requires the removal of the exterior planes. The dimensional changes of the Airex foam are relatively small with respect to the differences observed in the z positions of the planes after the beam alignment. The maximum .z displacement observed between the beam alignment and preflight plane positions was N 0.7 mm for plane 6.4 The data of Fig. 4 are used to estimate the effect of the alignment uncertainty on the rigidity resolution. The AMSOl proton rigidity resolution (2 4 hit tracks) is simulated with position measurement resolutions of 40 and 10 pm for the CC and y coordinates respectively.5 The alignment uncertainty is introduced by displacing the recorded track positions by the observed difference between the beam alignment and postflight metrology ladder positions prior to the curvature fit. Figure 5 shows the effect of the alignment on the rigidity resolution. The resolution obtained 4The beam alignment optimizes the plane positions with respect to plane 1. 5The AMSOl z resolution varied with charge: 40 (30) pm for 2 = l(2)
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with the inclusion of the alignment uncertainty reproduces the resolution used for the physics analysis, The latter is based on a full detector simulation which reproduces the test beam performance [21] with a position resolution of 17um for the y coordinate (40pm for x).
AMSO 1
F t
-
2
*.:
145
signal-to-noise performance should be improved by a further optimization of the ladder assembly. During the fabrication of the AMSOl ladders, a 30-50% increase in dark current was observed for nearly half the ladders after assembly. An estimate of the AMS02 proton rigidity resolution (2 5 hit tracks) is presented in Fig. 6. Position resolutions of 30 (10) pm were used for the x (y) coordinates with a silicon detection efficiency of 90%. The alignment uncertainty is based the AMSOl results (Fig. 4). In principle, the presence of the superconducting magnet will allow a field-free in situ alignment minimizing the influence of the alignment on the rigidity resolution.
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Figure 5. Rigidity resolution for the AMSOl six plane, six layer tracker and 0.14 T permanent magnet. Vertical lines denote the MDR.
6. AMSOZ The final version of the detector (AMSOS) which will be installed on the ISS includes two modifications which affect the tracker performance: a superconducting magnet and an increase in the number of silicon layers (six to eight). The superconducting magnet represents a considerable technical challenge. An increase in the number of track measurements is obtained by removing one of the interior tracker planes, and equipping both sides of the three remaining internal planes with ladders. In addition to the change of the n-side implantation pitch mentionned previously, the general
Figure 6. Rigidity resolution for the AMS02 five plane, eight layer tracker and 0.8 T superconducting magnet. Vertical lines denote the MDR.
7. Conclusion
The AMS silicon tracker presents a twofold challenge: the extension of the silicon microstrip technology to a large area detector and its application in space. The test flight results demon-
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strate both the successful adaptation of the devices to the space environment and the feasibility of large area detectors. AMS is a high visibility experiment. The relevance of the ISS for basic science has been debated since the project was proposed [33]. The highly improbable possibility to observe primordial antimatter in our Galaxy has been eloquently evoked [34]. Nevertheless, the AMS experiment inscribes in a general trend, the “rediscovery” of cosmic rays by the particle physics community after a long period of “latency”, dictated certainly by scientific and technical considerations, but not completely independent of political and economic influences [35]. The trend also implies a change in the manner to apprehend science, the return to observations of natural phenomena as opposed to experiments conducted in the laboratory. Thirty years later... “The observation of just one or two well verified, unambiguous anti-nuclei in the incident cosmic-ray flux would have profound astrophysical significance, because most present day cosmologies do not take account of significant amounts of antimatter.” [36]
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