The measurement of low pt particles with the PHOBOS spectrometer relies in the precise knowledge of energy loss and straggling in silicon pad detectors. The energy loss information is used for both tracking and particle identification. The quality of particle identification in the momentum range slightly below minimum ionizing is largely determined by a precise knowledge of energy straggling in thin detectors. Phobos’ aim of achieving a pt acceptance as low as possible also requires us to measure and identify stopping particle.

Although predictions based on the Bethe-Bloch formula are probably reliable, no published data exist for pions and kaons at lower momenta. The Bethe-Bloch formla can provide us only with a crude estimate of the energy loss, as it primarily describes the energy loss for thick absorber and does not give an estimate of the energy straggling. Additionally the response of Phobos’ thin silicon pad detectors to stopping pions and kaons is unknown.

In order to obtain this for Phobos vital knowledge we would like to

1. measure the energy loss and straggling for pions and kaons in the momentum range of 100 MeV/c to 750MeV/c
2. measure the signal spectrum for stopping pions and kaons

We completed the construction tests of the Spectrometer Type 1 modules, which will later be installed as the first four planes in the spectrometer. These modules are fully operational and can be used for the measurement.

We will be able to make measurements at higher energies in the B2 beam line, which will serve as reference and calibration point in the minimum ionizing momentum range, but the B2 beam line does not allow us to obtain low momentum measurements. We contacted Brad Tippens of the Crystal Ball collaboration and they have been supportive for us to make some measurements by placing our silicon detector modules behind the crystal ball. E913 is capable of providing us with a low momentum pion and kaon beam, where we can reach the lower momenta with a degrader, and can provide us with a TOF measurement. Unfortunately the crystal ball collaboration is not enthusiastic that we use a degrader as this interferes with some of their measurements.

To complete the needed series of measurements we would need dedicated running for 3 days after the conclusion of the E913 program. We would be very appreciative if this could be possible.

For the planned dE/dx-measurement we will use four Spectrometer Type 1 Modules. One module consists of two silicon sensors which carry a total of 3000 pads. The total active area for each module is 22 by 139mm2. The pad size is 1x1mm2, which gives us sufficient two-dimensional position information to allow full track reconstruction through all four planes.

We plan to arrange the module perpendicular to the beam with with a spacing of about 1cm along the beam, as illustrated in figure 1. In this arrangement we minimize charge sharing effect between pads and most signals will be contained in a single pad. The Phobos Spectrometer Type 1 modules are optimized for precise energy loss measurement, the energy resolution of the module was measured to be 4.3 keV. Each module is equipped with calibration electronics, which allows us to monitor and calibrate the gain of the system over the full dynamic range.

Each detector plane will give us an independent measurement of energy loss and straggling by sampling the pulse height for every pad. With this measurement we can study the signal spectrum for a given particle type and momentum in each plane.

By correlating the signals of four planes we will study the particle identification properties of silicon pad detectors. For particles, which cross all four planes, we can use the truncated mean of four measurement to obtain the characteristic energy loss and test Kaon to Pion separation at different momenta. Stopping particle can be identified by measuring their kinetic energy, which is given by the sum of all signals, and the energy loss in the planes prior to the stopping plane. The same techniques will be used in Phobos for particle identification in the spectrometer.

The momentum range covered by the C6 beam line matches the main momentum range of PHOBOS and is thus ideal for this test. Furthermore the C6 beam line of the E913 experiment can provide us in its kaon beam setting with a simultaneous kaon and pion beam at identical momenta in a composition approximately expected for PHOBOS.

The beam line momentum accuracy of +/-4% is already sufficient for our measurement. The particle rate of 10⁴ paricles/spill in the K- beam setting is sufficient to provide us with the neccessary data in reasonable short time. Additionally the E913 experiment has agreed to provide us with signals of their installed time-of-flight system which we plan to use for particle identification and further momentum definition.

3.1) Momentum range of 350 to 700MeV/c:

We plan to investigate detector behaviour in the momentum range of 350 to 700MeV/c with a direct kaon beam at four different momentum settings (350, 450, 550, 700). The second measurement point corresponds to minimum ionizing pions, which serves a cross calibration to our test bench measurements.

3.2) Momentum range below 350 MeV/c:

The momentum range below 350 MeV/c for kaons is accessible for us in the C6 beam line by using a momentum copper momentum degrader. The degrader allows us to slow the initial particles down to a momentum range where they will occasionally stop in the detector. This technique is regularly used by the E913 collaboration for calibration of the crystal ball. We plan to use a copper degrader with for different thicknesses in front of our silicon detector. The degrader thicknesses and initial beam energy are chosen to produce kaons with momenta of 300, 250, 200, 100 MeV/c. The E913 collaboration has provided us with simulation software for the current beam line arrangement to optimize the degrader.

The C6 pion beam and the additional use of the degrader will allow us to access the low momentum range for pions again down to a momentum range of stopping pions. We plan to carry out measurements at 200, 150, 100, 75 and 50 MeV/c, to be achieved with one constant pion beam energy and four degrader thicknesses.

Summary table of beam and degrader settings:

The above beam setting will allow us to determine the energy loss and straggling properties of thin silicon detectors for kaons and pions in the momentum range from stopping to minimum ionizing.

The E913 has agreed to provide us with signals of their already installed TOF scintilators for trigger and TOF measurement. Test bench measurement of our TDC system yielded a time resolution of 112ps. Figure 2 illustrates the arrangement of scintilators relative to our detector and degrader.

Trigger Mode 1:

For higher particle momenta for runs without degrader we plan to trigger on coincidences of scintilator 1,3 and 4 (sc1*sc3*sc4) which will select particles traversing the entire detector system. The TOF measurement of tsc4-tsc1 will provide us particle identification.

Trigger Mode2:

At lower particle momentum achieved with the degarder, we will trigger on coincidences of scintilator 2,3 and 4 (sc2*sc3*sc4), which selects the low momentum particles traversing all four silicon planes. The TOF measurement of tsc4-tsc3 will determine the particle momentum and type after the degrader.

Trigger Mode 3:

For the measurement of stopping particles, we plan to trigger on signals of scintilator 2 and 3 while using scinitlator 4 as veto counter (sc2*sc3*not(sc4)). This trigger mode will enrich our sample of stopping kaons and pions in the detector. The particle kinteic energy will be given in this run mode through the signal sum in the silicon detector.

The spectrometer modules used for the tests are final modules as they will be installed in the Phobos experiment. The readout of the modules is based on a system of VME ADC boards which were designed and constructed dedicated for tests with silicon detectors covering a very large dynamic range, as the Phobos detectors do.

The readout system consists of 2 VME ADC boards with four analog input channels each. Each silicon detector will be readout by 12 VA-HDR1 VLSI chips mounted on the module, which are serially connected to one VME ADC input channel. Data digitization is done inside the VME ADC boards with on 12-bit ADCs. The digitized data are readout via a standard VME-PCI interface to a single 200MHz Pentium II PC using National Instrument LabWindows software. Control of the chip readout is provided by a VME-Viking control module which allows serial readout of chips’ channels with up to 10MHz. The supply voltages are regulated, filtered and distributed to the modules by an additional VME module.

The system provides the readout for all 12000 pads of the test system. First tests with the above system have confirmed the necessary dynamic range and linearity of the ADC system. The readout system in the configuration can handle non zero-suppressed data, as we plan to use during the tests, for 12000 channels at 25Hz in continuous operation and about 45 Hz in AGS spill mode.

We plan to take data in 10 runs for the kaon beam setting, which are 4 runs for different kaon momenta without degrader in trigger mode 1, 4 runs with degrader in trigger mode 2, and 2 runs with degrader in trigger mode 3 (stopping kaons). Similar we plan to record 8 runs for the pion beam setting, which are 1 run without degrader in trigger mode 1, 4 runs with degrader in trigger mode 2 and 3 runs with degrader in trigger mode 3 (stopping pions).

For runs in trigger mode 1 or 2 (13 runs) we require a total 40k events/run, for runs with stopping trigger (5 runs) we require 100k events/run. We expect to achieve an average readout rate of 10Hz, reflecting the AGS 50% duty cycle and our data acquisition rate of 20Hz in continuous mode. We estimate the necessary effective beam-on time for the measurements to be 40 hours including beam momentum selection. Additional 10 hours are required for detector and trigger adjustment.

Plans for a tentative beam schedule: