osmic Ray Velocity and Electric Charge Measurements in the AMS experimen



The AMS02 detector
==================

AMS \[@bib:ams\] (Alpha Magnetic Spectrometer) is a precision spectrometer designed to search for cosmic antimatter, dark matter and to study the relative abundance of elements and isotopic composition of the primary cosmic rays with an energy up to $\sim$1TeV. It will be installed in the International Space Station (ISS), in 2008, where it will operate for more than three years.

The spectrometer will be able to measure the rigidity ($R\equiv pc/ |Z| e$), the charge ($Z$), the velocity ($\beta$) and the energy ($E$) of cosmic rays within a geometrical acceptance of $ \sim$0.5m$^2$.sr. Figure \[fig:ams\] shows a schematic view of the AMS spectrometer. At both ends of the AMS spectrometer exist the Transition Radiation Detector (TRD) (top) and the Electromagnetic Calorimeter (ECAL) (bottom). Both will provide AMS with capability to discriminate between leptons and hadrons. Additionally the calorimeter will trigger and detect photons. The TRD will be followed by the first of the four Time-of-Flight (TOF) system scintillator planes. The TOF system \[@bib:ams02-tof\] is composed of four roughly circular planes of 12cm wide scintillator paddles, one pair of planes above the magnet, the upper TOF, and one pair bellow, the lower TOF. There will be a total of 34 paddles. The TOF will provide a fast trigger within 200ns, charge and velocity measurements for charged particles, as well as information on their direction of incidence. The TOF operation at regions with very intense magnetic fields forces the use of shielded fine mesh phototubes and the optimization of the light guides geometry, with some of them twisted and bent. Moreover the system guarantees redundancy, with two photomultipliers on each end of the paddles and double redundant electronics. A time resolution of 130ps for protons is expected \[@bib:ams02-note\].\
The tracking system will be surrounded by veto counters and embedded in a magnetic field of about 0.9Tesla produced by a superconducting magnet. It will consist on a Silicon Tracker \[@bib:ams02-tracker\] made of 8 layers of double sided silicon sensors with a total area of $\sim$6.7m$^2$. There will be a total of $\sim$2500 silicon sensors arranged on 192 ladders. The position of the charged particles crossing the tracker layers is measured with a precision of $\sim$10$\mu$m along the bending plane and $\sim$30$\mu$m on the transverse direction. With a bending power (BL$^2$) of around 0.9T.m$^2$, particles rigidity is measured with an accuracy better than 2% up to 20GV and the maximal detectable rigidity is around 1-2TV. Electric charge is also measured from energy deposition up to Z$\sim$26.\
The Ring Imaging <span>Čerenkov</span> Detector (RICH) \[@bib:buenerd\] will be located right after the last TOF plane and before the electromagnetic calorimeter. It is a proximity focusing device with a dual radiator configuration on the top made of a low refractive index aerogel 1.050, 2.7cm thick and a central square of sodium fluoride (NaF), 0.5cm thick. Its detection matrix is composed of 680 photomultipliers and light guides and a high reflectivity conical mirror surrounds the whole set. RICH was designed to measure the velocity of singly charged particles with a resolution $\Delta\beta/\beta$ of 0.1$\%$, to extend the charge separation up to iron, to contribute to $e/p$ separation and to albedo rejection.\
Particle identification on AMS-02 relies on a very precise determination of the magnetic rigidity, energy, velocity and electric charge. Velocity of low energy particles (up to $\sim 1.5$ GeV) is measured by TOF detector while for kinetic energies above the radiator thresholds (0.5GeV for sodium fluoride and 2GeV for aerogel) the RICH will provide very accurate measurements; a target resolution of $\sim$1$\%$ and $\sim$0.1$\%$ for singly charged particles is expected, respectively for sodium fluoride and aerogel radiators. The electric charge is measured by the silicon tracker and TOF detectors through $dE/dx$ samplings and on the RICH through the <span>Čerenkov</span> ring signal integration. Charge identification, at least, up to the iron element is expected.

\[fig:ams\]

Velocity measurement
====================

TOF measures the crossing time between two scintillator planes and extracts the velocity through $\beta$=$\Delta$L/$\Delta$t. The time of flight resolution for two scintillators, tested in a test beam at CERN in October 2003 with fragments of an indium beam of 158GeV/c/nuc, is shown in figure \[fig:thc\] as function of the particle charge. One of the tested scintillators had bent and twisted light guides (C2) while the other one had bent light guides (C3). A time resolution of 180ps was estimated for this conservative configuration. However, as the measurement in AMS-02 will be done with four independent measurements, the time resolution which can be inferred is of the order of 130ps for a minimum ionizing particle.\
In the RICH detector the velocity of the particle, $\beta$, is straightforwardly derived from the <span>Čerenkov</span> angle reconstruction ($\cos\theta_c=\frac{1}{\beta~n}$). Two reconstruction methods were developed: a geometrical method based on a hit-by-hit reconstruction and a method based on a likelihood fit to the pattern of the detected photons. The best value of $\theta_c$ will result in the former case from the average of the hit velocities after hit clusterization and in the latter from the maximization of a Likelihood function $L(\theta_c)$ given by,

$$L(\theta_c) = \prod_{i=1}^{nhits} p_i^{n_i}  \left[ r_i(\theta_c) \right].
\label{eq:likelihood}
\vspace{-0.3cm}$$

where the probability of a hit belonging to a <span>Čerenkov</span> ring of angle $\theta_c$ ($p_i$) is function of the closest distance of the hit to the <span>Čerenkov</span> pattern ($r_i$). In both methods the hits position is weighted by the detected signal $n_i$. For a more complete description of the method see \[@bib:NIM\]. The resolution expected for $\beta\sim$1 singly charged particles crossing the aerogel radiator is around $4\,mrad$ while for those crossing the NaF radiator is around $8\,mrad$. The accuracy of the velocity determination improves with the charge. Figure \[fig:thc\] (right plot) shows the evolution of the velocity resolution for indium beam fragments of $158 ~GeV/c/n$ detected with a RICH prototype corresponding to $1/6$ of the final RICH detector. The radiator plane was placed perpendicular to the beam direction. The photon expansion length between the radiator and the detection matrix was adjusted to 42.3cm, allowing fully contained <span>Čerenkov</span> rings.

<span>cc</span>

Charge measurements
===================

As it was said previously, TOF and tracker measure the charge (Z) through $dE/dx$ samplings. Figure \[fig:chg\] a) shows the charge measurement from the anode signal of one of the TOF counters (C2) tested in ion beam at CERN in 2003, in principle the most unfavourable one. Charge separation up to the aluminium is visible. Figure \[fig:chg\] b) shows the combined measurements for 4 or more ladders on the tracker K side. The ion species can be distinguished up to Z=26.

RICH charge measurement is based on the fact that the number of <span>Čerenkov</span> photons produced in the radiator depends on the particle charge through $N_{\gamma} \propto Z^2 L \sin^2\theta_c$ where $L$ is the radiator thickness. Once the total number of photoelectrons ($N_{p.e}$) associated to a <span>Čerenkov</span> ring is computed one has to correct it by the photon ring overall efficiency in order to derive the charge. The uncertainty on charge determination results from two distinct contributions. One of statistical nature independent of the nuclei charge and depending essentially on the amount of <span>Čerenkov</span> signal detected for singly charged particles ($N_{p.e} \sim 10$). Another one of systematic nature scaling with the charge and coming essentially from non-uniformities on the radiator plane and photon detection efficiency. The RICH goal of a good charge separation in a wide range of nuclei charges implies a good mapping and monitoring of the potential non-uniformities present on the detector.

Charge peaks reconstructed with the RICH prototype for data taken at CERN during October 2003, are shown in figure \[fig:chg\] c). The RICH configuration included an aerogel radiator of n=1.05 and 2.5cm thick. A charge resolution for helium events slightly better than $\Delta Z \sim 0.2$ was observed together with a systematic uncertainty of $1\%$. A clear charge separation up to Z=28 was achieved. For a more complete description of the charge reconstruction method see \[@bib:NIM\]. Finally figure \[fig:chg\] d) presents the comparison of charge measured by tracker and RICH. Ions can be distinguished and identified up to Z=26, an excellent correlation is obtained.

Acknowledgements
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I would like to express my acknowledgements to the AMS collaboration for giving me the opportunity to attend the Lake Louise Winter Institute 2005. I also express my gratitude to the Fundação da Ciência e Tecnologia for the financial support to participate in the meeting.

Conclusions
===========

AMS is a spectrometer designed for antimatter and dark matter searches and for measuring relative abundances of nuclei and isotopes.The velocity of singly charged particles is expected to be measured with a precision of 0.1$\%$ and charge separation up to iron is attainable. Real data analysis was done with data collected with prototypes of TOF, tracker and RICH in test beams at CERN, in October 2002 and 2003\

<span>99</span> S. P. Ahlen et al., Nucl. Instrum. Methods A <span>**350**</span>,34 (1994).\
V. M. Balebanov et al., <span>*AMS proposal to DOE*</span>, approved April 1995. D. Casadei et al., Nuclear Physics B (Proc. Suppl.) <span>**113**</span>,133 (2002). The AMS collaboration, AMS on ISS, *http://ams.cern.ch/AMS/AMS.pdf*. W. J.Burger, Nuclear Physics B (Proc. Suppl.) <span>**113**</span>,139 (2002). M. Buenerd, Proceedings of the Fourth Workshop on Rich Detectors (RICH02) June 5-10, 2002, Pylos, Greece. .

{'source': 'arXiv_out/0801'}