Examples of Time of Fight Detectors

• Variety of methods for measuring time it takes for a particle to travel through a medium

• A time of flight (TOF) detector is a particle detector which can discriminate between lighter and heavier elementary particles of same momentum using their time of flight between two scintillators

• One Method- The first of the scintillators activates a clock upon being hit while the other stops the clock upon being hit. If the two masses are denoted by m1and m2 and have velocities v1 and v2 then the time of flight difference is measured where L is the distance between the scintillators. The approximation is in the relativistic limit at momentum p and c denotes the speed of light in vacuum. Usually, δt is the apparatus' time resolution

ALICE Experiment• Measures the particles produced in collisions so the evolution

of the system in space and time can be reconstructed. • Subdetectors- Ensemble of cylindrical detectors embedded in

the magnetic field ( bending the trajectories) measures at many points the passage of particles carrying electric charge.

• Particle identification- TOF measures with a precision better than a tenth of a billionth of a second, the time that each particle takes to travel from the vertex to reach it, so that one can measure its speed

• Includes a Muon spectrometer, HMPID measures the faint light patterns generated by fast particles and the TRD measures the special radiation very fast particles emit when crossing different materials, thus allowing to identify electrons.

• Charged particles in the intermediate momentum range are identified in ALICE by the Time Of Flight (TOF) detector

• The time measurement with the TOF, in conjunction with the momentum and track length measured by the tracking detectors is used to calculate the particle mass

The goal of ALICE is to study •The QCD thermodynamics via the measurement of π, K and p transverse momentum distributions and particle ratios on an Event-by- Event basis.• Signatures of QGP (Quark Gluon Plasma) formation via open charm and φ meson production.

ALICE’s TOF Detector

• The TOF detector has a cylindrical shape, covering polar angles between 45 degrees and 135 degrees over the full azimuth.

• It has a modular structure with 18 sectors; each of these sectors is divided into 5 modules along the beam direction.

• The modules contain a total of 1638 detector elements (MRPC strips), covering an area of 160 m2 with 157248 readout channels (pads).

Detector Description• The detector chosen for the ALICE TOF is the

Multigap Resistive Plate Chamber (MRPC), developed within the CERN LAA project.

• The detector element is a long MRPC strip with an active area of 7.4 × 120 cm2.

• It has 96 readout pads of 2.5 × 3.5 cm2 arranged in two rows.

• It consists of 2 stacks of glass, each with 5 gas gaps of 250 μm; spacers made of nylon fishing line keep the distance between the glass plates fixed.

Operations• The MRPC –( Multigap Resistive Plate Chambers) is a stack of

resistive glass plates. A high voltage is applied to the external surfaces of the stack. Further out there are pickup electrodes.

• A charged particle ionizes the gas and the high electric field amplifies this ionization by an electron avalanche.

• The resistive plates stop the avalanche development in each gap; they are however transparent to the fast signal induced on the pickup electrodes by the movement of the electrons.

• So the total signal is the sum of the signals from all gaps (the reason for many gaps is to achieve high efficiency), whereas the time jitter of the signal depends on the individual gap width (the reason for narrow gaps is to achieve good time resolution).

• The pattern of hit strips gives a quick measure, which is then used by the trigger to make immediate decisions about whether the data are worth keeping. RPCs combine a good spatial resolution with a time resolution

Electronics• The signal from the MRPC must be amplified

and discriminated and the time measured with an accuracy matching the intrinsic time resolution of the detector.

For the readout the HPTDC (High Performance Time to Digital Converter), a chip developed by the CERN EP/MIC group, with bin width of 25 ps. Time-to-

digital converters register the arrival of a single ion at discrete time "bins"; thresholding discriminates between noise and ion arrival events.

For the Front End the NINO ASIC, developed recently by the CERN LAA project, which combines a fast amplifier,

discriminator and stretcher.

Detector PerformanceThe time resolution of the TOF MRPC is in the 50 ps range, as can be seen in the figure showing the time distribution after slewing correction. A typical efficiency and time resolution plateau as a function of the high voltage is more than 2 kV long before the onset of streamers, with efficiency reaching 99.9 %.

π, K and p mass peaks clearly show up in the momentum range up to 3 GeV/c, in a simulation of 250 Pb-Pb events generated with HIJING for magnetic field B = 0.4 T

CDF II Collider at FermiLab

CDF System

Time of Fight at CDF• CDF II experiment incorporated TOF into the detector to

provide charge kaon identification to improve neutral B meson flavor determination.

• Expected TOF of 100 ps, the system will provide 2 standard deviation separation between K± and π± for momenta of p<1.6 GeV/c

• TOF was added to improve particle identification capabilities.• TOF is to identify the b (particle/antiparticle) flavor of B

hadrons produced in the collisions• An enhancement of the b flavor identification is crucial to

improve the statistical precision in CP violation measurements and oscillations.

• Determining b flavor (content of b or b’) of B mesons• Quantify the power of a b flavor tag is the total tag effectiveness

εD^2, where ε is the tag efficiency and D is the tag dilution which is related to the probability P that the tag dilution is correct; D=2P-1

• CDF developed several methods of “tagging” that will be incorporated into its second run

• Same Side Tags- exploit charge correlation of particles produced in association with hadronization (formation of hadrons from Quarks and Gluons) of a b quark into a B meson

• Opposite Side Tags-identifies flavor of the second B hadron of interest

• Identification of charged Kaon can be used as an opposite tag b/c B’ (anti) hadrons produce more K- than K+ through a decay sequence BeautyCharmStrange

• Kaon identification also enhances effectiveness of same side tagging. B (B’) mesons produce in association with K+ (K-)

Significant fraction of kaons from the B hadron are in the identification range of TOF. Opposite side kaon was not

established in the first CDF run.

Combining it with the same side tagging ( Run I), TOF is expected to double the flavor tag effectiveness of B/ B- mesons

• Particle identification ( kaon Identification) with TOF is performed by measuring the difference ( time of flight) in the time of arrival of the particle at the scintillator with the collision time t(o). The mass m can be determined from the momentum p, the path length L, and the time of flight t

TOF System• 216 Bars of Bicron-408 Scintillators chosen for

a long attenuation length• Bars are 279 cm X4 cm X 4 cm installed at a

radius of 138 cm from the beam in 4.7 cm of radial space between the main drift chamber and cryostat of the superconducting solenoid

• 432 Photomultiplier tubes (PMT) operating in the 1.4 T magnetic field are attached to each end of the scintillator bars.

• Signal form from anode and dynode stage of PMT. PMT is fed into a preamplifier which drives the signal to the electronics

• From the preamplifier the signal follows 2 paths, one for timing and the other for charge measurements.

• First path the signal is fed into a discriminator and the output serves as the start signal for the time to amplitude conversion circuit.

• Second path used to correct the time measurement for pulse height dependence

TOF Results• TOF system with 20 130 cm bars was installed inside the CDF

solenoid at the end of the Run I to learn how to achieve the best timing resolution (100 ps) from a full system.

• Flight time resolution average over all the tubes was 220-250 ps from the fit of the TOF mass, which is a low resolution. It was rectified in the full system