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AMS will have two missions in space. The first mission will be 10 days on the space shuttle (STS). For the second mission AMS will be carried on the shuttle up to the international space station (ISS) for a 3 to 5 year stay.
The AMS detector has five major elements: A permanent magnet, a silicon microstrip tracker, a scintillator time of flight hodoscope, scintillator anticoincidence counters and an aerogel Cerenkov threshold counter. In addition there are electronics and support structure and interfaces.
After sketching the physics of AMS and the AMS mission profiles, brief mechanical descriptions and brief functional descriptions are given of these elements in turn.
Terrestrial creatures have been observing neutral particles, specifically photons (light rays, radio waves, etc), from outside the earth ever since ever when. Improved observations of these neutral particles, both from the surface of earth and above the earth have led to many important discoveries and form the basis for our understanding of the composition and creation of our universe.
However, until now, it has been extremely difficult to make the corresponding measurements of charged particles, because they interact in the atmosphere. AMS is designed to do just that, and do it much better than has been possible.
The physics goals of the first AMS mission concentrate on the measurement of extraterrestrial anti-protons. In 10 days AMS will record more than 10 times the total number of anti-protons observed to date. The spectrum of these antiprotons will indicate or set limits on the nature of the missing, or dark, matter which is postulated to compose 90% of our universe.
The second mission will improve on these measurements and will also search for complex antimatter nuclei (anti-Helium, anti-Carbon, etc). The observation of such would indicate large regions of our cosmos are made of antimatter. From current observations, there are no indications that such regions should exist and the general feeling is they "ought" not to exist. However, no theory of the origin of the universe during the big bang can satisfactorily explain their nonexistence. Essentially, during the big bang there were equal amounts of matter and antimatter. Now we can't see any significant amounts of antimatter. The AMS experiment will help to answer where it might have gone.
This high inclination and altitude are vital for the AMS physics goals, as it provides time near to the geomagnetic poles. Away from these poles the earth's magnetic field bends away low energy charged particles before they can reach the experiment.
During the mission, and after an initial 4 to 5 hour checkout phase, AMS will have 100 hours of dedicated data taking time. Current (Aug 97) flight planning indicates about 25 hours of this will be before the rendezvous and the balance afterwards. To operate the AMS electronics and detectors require about 700 Watts of power from the shuttle. Depending on the orientation of the shuttle, up to another 1000 Watts may be required to power heaters which will keep the parts of the experiment in their operational ranges. AMS will also need to be commanded from the ground, especially during the checkout phase, and the maximum rate at which commands can be sent up is one every 2 seconds. Monitoring data will be generated by AMS and transmitted at a net rate of 10K bits/second during the 70% of the time the S-Band "down-link" is active. The science, or event, data will be generated at the much higher rate of 1M bits/second averaged over an orbit and piped down the Ku-Band. The weight of the payload is 9200 lbs and it will be mounted in the rear of the shuttle payload bay:
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The SpaceHab module and MIR docking equipment (not shown) will be mounted in front of AMS.
In addition to the physics goals mentioned above, this mission will serve to exercise our command and data interfaces, measure backgrounds to allow the trigger and detectors to be tuned for the the next mission, and verify the AMS thermal modelling.
The second mission begins with AMS being carried on shuttle flight STS-121, also known as ISS-21-UF4, to the ISS at the beginning of the year 2002 (under constant review). After docking with ISS, AMS, with the USS, will be lifted out of the payload bay by the shuttle robotic arm and handed off to the station robotic arm. It will then be translated out to the starboard truss of the station and positioned on an external payload attach site:
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AMS will remain on ISS for 3 to 5 years, after which it will be dismounted and returned to earth. It is planned to consume not more than 1000 Watts and produce data at the same rate as during the shuttle mission. Based on measurements made during the shuttle mission, additional detectors will be included to ensure the fulfilment of the expanded physics goals.
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| Figure 1. |
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| Note: the colors of each element correspond to the AMS color coding conventions. |
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| Note: The Aerogel Counter as shown is incorrect, see Figure 1 for a more correct view. |
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The permanent magnet is in the shape of a cylindrical shell, with height 800mm, inner radius 514mm and outer radius 600mm. For orientation, the detector is described with the magnet cylinder axis vertical. It is made of blocks of the magnetic material Nd-Fe-B with a strength of 46 MGOe (colored yellow in figure 1). The weight of the blocks is 1900 kgs. The careful arrangement of the magnetization vectors of the blocks around the cylinder creates a dipole field, where the magnetic field within the bore of the magnet reaches a fairly uniform 1.5 KGauss perpendicular to the bore. Outside the bore the field quickly drops off, reaching the level of a few Gauss less than a couple of meters from the center. The magnetic material is enclosed in an Al support structure (colored black). This is made up of thin cylindrical inner and outer skins and thicker flanges above and below the magnet. The OD of the flanges protrudes beyond the magnet OD, as do various reinforcement hoops and mounting points. The magnetic material weighs 1900 kgs. The use of a permanent magnet is one of the key design concepts of AMS, obviating the need for power and auxillary services, for example cyrogenics, required by a super conducting magnet.
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The magnet cylinder axis defines the "z" axis, with "up" being "+z". The direction of the field within the magnet defines the "+x" axis. The "y" axis is taken to form a right handed coordinate system. When situated in the shuttle, these axes are the same as the shuttle axes, with "+x" towards the tail of the shuttle, "+z" out of the payload bay and "+y" to port.
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The silicon microstrip tracker (colored red in Figure 1) is arrayed in six horizontal planes. The four inner planes, T2 to T5 in the figure, are within the magnet bore and are support by a Carbon fiber (Cf) cylindrical shell. Two larger outer planes, are just above, T1, and below, T6, the bore of the magnet, supported by square tubes. As shown in the figure below, each plane is tessellated with 4x7 cm silicon wafers of thickness 300 microns arranged end to end in "ladders". The ladders are mounted side by side on feet to the panels, with the length of the ladders running in the "x" direction. The inner planes have 14 ladders, the outer 17. Each wafer has microstrips etched on both sides with the strips on each side running orthogonally, and strips within the ladder connected, for the strips running along the ladder, end to end and for the strips running crosswise, via a a kapton foil printed with a diagonal circuit pattern. The two sides are denoted "S" and "K", the side with the kapton foil, respectively. Except for the outermost ladders in each plane, they are split in the middle. To fill out the circle, the half-ladders are of varying length, containing from 8 to 15 wafers.
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| Silicon wafers tesslating the inner (upper right) and outer (lower left) tracker planes. |
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For the STS mission about half the required silicon wafers will be available, so only the center most ladders (darker red in Figure 1, light blue in drawing ams0820a) will be installed.
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There are four planes of time of flight (TOF) scintillators (colored green in Figure 1, only two planes are shown in drawing ams0822). Two, S1 and S2, are located above T1 above the magnet and the other two, S3 and S4, below T6 below the magnet. The pairs of planes are each mounted on a 10cm thick Al honeycomb panel. Each plane consists of 14 scintillator paddles of thickness 1cm, width 11 cm, arranged with a 1 cm overlap. The paddles of S1 and S4 run parallel to the "x" direction, S2 and S3 to the "y" direction. The lengths of the paddles vary to fill out a circle. Each end of each paddle is viewed by 3 photomultiplier tubes (PMT).
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The 16 anticoincidence scintillator counters (ACC, dark blue in Figure 1) are arranged like barrel staves in a 1 cm thick cylindrical shell just inside the magnet ID and just outside the inner tracker Cf support shell. The counters are readout with flexible light guides made of optical fibers viewed at each end of each counter by one PMT.
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| The Aerogel Threshold Cerenkov (ATC) detector |
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The two planes of the aerogel threshold Cerenkov counter are located below S4 (ATC, colored brown in Figure 1). Each is made up of 10x10 cm**2 cells of 7 cm thick aerogel. Each cell is viewed by one PMT. The cells are arranged in an 8x10 matrix on the upper layer and an 8x11 matrix on the lower layer. To avoid "cracks" in the coverage the two layers are offset about half a cell in both the "x" and "y" directions.
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The detectors have front end electronics mounted on or near the detectors. However the bulk of the electronics is located in crates (shown black and various colors in figure 1) mounted outside the magnet.
The top of the experiment is covered with a Low Energy Particle Shield (LEPS) shaped like a cap, hat, hood. Likewise a Skirt is wrapped around the bottom of the experiment.
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| The USS (silvery) shown with the ground handling equipment (GHE, blue). |
The experiment rests in the Unique Support Structure (USS). Vertical I-Beams interfacing the magnet structure to the USS and everything is mounted to the magnet flanges, except the ATC, which has separate mounting feet directly to the USS. Some of the electronics is also mounted directly to the USS. The USS, which is made up Al box beams and circular truss elements, fits across the shuttle payload bay.
Altogether, the experiment plus the USS weight totals 9200 lbs for the STS mission.
The purpose of the magnet is to bend charge particle which traverse the magnet bore. Coming from the top, positively charged particles (positrons, protons, nuclei) curve in one direction and negatively charged particles (electrons, antiprotons, anti-nuclei) bend in the other. The degree to which they bend, that is the radius of curvature, depends on the "rigidity" of the particle, and the rigidity depends on the momentum and inversely on the charge-magnitude.
The tracker allows the measurement of this rigidity by registering the passage of a charged particle in each of the six planes. The strips on the "S" side of the wafer measure the "x" coordinate of the trajectory and the "K" strips measure the "y" coordinate. The "z" coordinate is known from the location of the tracker planes. In addition, the amount of energy deposited in each plane by the particle passage allows a determination of the charge magnitude of the particle.
The TOF counters also measure the energy deposition and so provide another measurement of the charge magnitude. By dividing the length of the trajectory between different layers by the difference in the signal times in those layers, the particle velocity can be determined. In addition, the coincidence of fast signals from counters in several layers is used to indicate when a particle has passed through the detector, and this indication is used to "trigger" the precise readout of all the detector elements.
Signals in the ACC counters indicate that a particle entered, or exited, the detector not though the TOF counters but through the magnet. As these particles cannot be precisely analyzed, signals from these counters are used to reject such particles.
The ATC supplements these measurements by providing a direct measurement of whether the particle velocity is above of below a threshold value.
The electronics has various functions. The trigger recognizes that a particle of interest has passed through the detector. Subdetector readout chains digitize the detector signals. The data acquisition system collates all the signals from the passage of this particle in to an event. The power system supplies the electronics and the detectors with the electricity required for them to operate. The heater system maintains the detector elements within operational temperature ranges. Monitoring and control electronics collect the various operational parameters as well as operating the other electronics, either in response to the measured parameters or in response to commands. Data links interface the transmission of commands to, and parameters and event data from, the detector.
The LEPS and Skirt lessen the rate of spurious signals induced by low energy particles, especially electrons. The lowest portions of the experiment are protected by the material in the belly of the shuttle.
Away from the detector, support equipment constitutes the other end of these data links. Online software running on this equipment monitors the data, records it, and makes it available to the operators. The operators also use the online software on the support equipment to generate and transmit commands to the detector. Offline analysis works from these records to reconstruct the event and perform detailed physics analysis.
AMS top level links for your convenience:
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