Muon system

Eng Up Kedr

The major purpose of the muon identifier is a reliable isolation of muons produced in e+e- annihilation, with a considerable number of hadrons. A highly efficient identification of events with a single and two muons is required to study semileptonic decays of B, D, and F-mesons, to establish the psi-meson yield and to analise tau+tau- -events.

The muon identifier for the detector KEDR has a thickness of the hadron absorber of 6-7 nuclear lengths. After each layer of absorber there is a layer of streamer tubes. The system comprises 8 cylindrical octants covering 0.68 of 4 pi (47o< theta < 133o, the angle is determined by center of 2-nd layer).

The total amount of the substance-absorber is determined by desiderable coefficient of suppression of a hadron background for hard muons. The pions and kaons are capable of imitating muons either due to the probability to pass through the matter without nuclear interaction, or to the decay into a muon and a neutrino. The latter is possible if the angle between the direction of the muon and hadron motions is so small that the precision of coordinate measurements does not allow a kink in the track to be identified and the measured momentum does not contradict the range of a muon path.

Pions create a higher background for muon identification than kaons because in the multihadron dacays the number of charged kaons is several times smaller than that of pions. A marked kink in the track during the kaons decay is more probable than in the case of pion. Moreover, the kaon passed without interaction has a noticeable shorter ionization range of the pass in comparison with the muon with the same momentum (32-34 cm of iron). In view of this, if iron is divided into layers whose thickness is less than 25 cm (with possible angles of incidence taken into account), then the kaon is not capable to imitate the muon on account of the interaction-free path.

The probability of muon imitation due to the pion decay is determined by length of flight in a matter of low density, i.e. by the radius of the identification system (60 cm in KEDR) as well as by the accuracy of tracking to find the kink. For the pions, the probability of an interaction-free passage through a matter decreases with increasing the total thickness of the absorber. The chosen thickness enables to make this probability lower than the muon imitation probability due to pion decay.

The whole of the absorber can pass only muons whose energy is higher than the threshold one (E=1.7-2.1 GeV), on account of ionization losses. For the hadron background suppression, in case of identification of muon whose energy is lower than the threshold, are necessary intermediate sensitive layers. These offer the possibility to evaluate the particle energy and to distinguish them with respect to the range.

The first sensitive layer allows the information of neutral hadrons (K and neutrons) to be gained.

In the chosen variant, the iron of the octants is divided into 4 layers of 23, 23, 23 and 20 cm thick. A sensitive layer consisting of streamer tubes is placed after each absorber. Streamer tubes are assembled in modules. Total number of tubes in three layers is 2288, total number of channels is 544, total sensitive volume is 12 m2.

The stainless steel tubes of 40 mm diameter with 0.3 mm wall thickness are 4.5 m length. At the ends, such a tube is closed by plastic plugs along the axis of which brass pins are placed to fix the anode wire.

Blowing is performed through special holes in the plugs. The work has been made on a choice of the diameter of the anode wire, material for the cathode and the gas mixture. Using the gas Ar-CO2-n-pentane (2:1:1), under normal pressure the following characteristics of the streamer signal have been obtained: a rise time about 5 ns, a total width of 80 ns, mean pulse height 1 mA, pulse height spectrum FWHM 80%.

One electronics channel is applied for the definition of the longitudinal coordinate in 4 tubes connected to a chain. The electronics channel incorporates two discriminators, a time stretcher and direct counting TDC. A chain of tubes is a line along which the signal propagates. The discriminators are located directly at the ends of a chain and connected to the time stretcher by a twisted pair cable. The time stretchers are also connected to the TDC by twisted pair cable of 20 m long. The calibration signals of a generator may be applied to the ends of the chain and to the places of tube connections. Thus, one can perform a continuous monitoring of the status of electronics.

The spatial resolution in direction transversal to the tubes is determined by their size and location in the layers and is 40 mm. The longitudinal coordinate resolution was measured with a set of 6 endcap modules registrating cosmic rays particles. The averaged on all channels resolution was found to be 33 mm. The averaged on all channels resolution for 2 middle tubes of a chain (a chain consists of 4 tubes) was 23 mm (HV 4000 V). The relative resolution taking into account the total length of a chain 16.5 m is equal to 0.2 % and 0.15 % correspondingly.

Last modified on 20-06-2001