The focal plane polarimeter measures the polarization of protons in the hadron spectrometer detector stack. When the protons pass through a carbon analyzer, the nuclear spin-orbit force leads to an azimuthal asymmetry in scattering from carbon nuclei, if the protons are polarized. The particle trajectories, in particular the scattering angles in the carbon, are determined by pairs of front and rear straw chambers, a type of drift chamber.
The front straw chambers are separated by about 114 cm, and are located before and after the gas Cerenkov detector. The second chamber is followed by scintillator 2, which is in turn followed by the polarimeter carbon analyzer. The rear chambers, chambers 3 and 4, are separated by 38 cm and are immediately behind the carbon analyzer.
The carbon analyzer consists of 5 carbon blocks. Each block is split in the middle so that it may be moved into or out of the proton paths, so that the total thickness of scattering carbon may be adjusted. The block thicknesses, from front to rear, are 9", 6", 3", 1.5", and 0.75". The block positions are controlled through EPICS; the controls may be reached through the Hall A / hadron spectrometer / detectors menus. Particles passing through the carbon analyzer can be absorbed in it. Due to this attenuation, the blocks should generally be left open, so that there is no attenuation effect on non FPP experiments that require S3 information.
The straw chamber planes are designated as X, U, and V planes. The central ray defines the z axis. X wires measure position along the dispersive direction. The UV coordinate system is created by a 45 degree rotation in the transverse plane of the XY coordinate system, with +U between the +X and +Y axes, and +V between the +Y and -X axes.
The straw chamber operation is described in the following paragraphs.
When a charged particle passes through the chamber in typical Jefferson Lab operating conditions, there will be about 30 primary ionizations of gas molecules. Positive high voltage of about 1.8 - 1.9 kV is applied to the wire in the center of each straw. Electrons from the ionizations drift towards the wire. When the electrons get within about 100 of the wire, the gain in energy between collisions with gas molecules is sufficient that gas molecules are further ionized in collisions. This leads to an avalanche, and a gain of about 105 per primary ionization under the conditions in which the FPP is run.
The movement of the positive and negative ions leads to a voltage drop
on the wire, or equivalently to a negative analog signal.
The analog signal is about 20 ns long, with a (negative)
peak current of about 40 A, and propagates towards each end of the straw.
At one end of each straw is a board that supplies high voltage
(see Figure ![[*]](images/icons.gif/cross_ref_motif.gif){kind=icon}
);
impedance matching on this board, with a 1500 pF capacitor and a 370 resistor, reduces reflection of the signal.
The other end of each straw is connected to a readout board, that amplifies,
discriminates, and multiplexes the input signals -
(see Figure ).
At the readout end, the signal is ``coupled to ground'' through a 1500
pF capacitor followed by 310 + 50 resistors.
In parallel with the 50 resistor are diodes to limit the
signal size, to prevent damage to the readout board circuitry.
An amplifier samples the signal over the 50 resistor.
The amp gain is about -10 mV/A, resulting in a +400 mV signal
to a comparator.
A threshold voltage input to the readout board is put over a voltage divider
consisting of 1500 + 10 resistors.
For the typical 4 V threshhold applied to the board, the comparator
puts out a logical pulse when the 400 mV (peak) signal rises above the
4 V / 151 = 26 mV threshold.
One-shots are then used to fix the width of the logical pulse for each channel
- the one shot width is fine tuned by the use of high precision resistors in
an RC circuit; these resistors are mounted in sockets so as to be easily
replaced if the need arises.
An OR circuit then combines eight individual straw outputs into a single
electronics channel.
The positive high voltage is supplied by a LeCroy 1458, located in rack 6 on the upper electronics level. Each of the four FPP chambers has six separate HV sections, supplied by six HV channels of the 1458. Internally, within the Faraday cages, the high voltage is distributed to stacks of high voltage / test pulser boards, through which it is connected to each straw via a 1 M 1/4 watt resistor. As additional protection, a 25 M resistor is added to the high voltage line into the chamber. If a straw shorts to ground, this would lead to a 70 A current and 0.1 W power loss in the 25 M resistor; this would not trip the power supply with standard trip values of 110 A. Documentation of how the high voltage is distributed within each chamber can be found in the FPP logbooks; this knowledge should not be needed for normal operations.
The readout cards require a high-current low-voltage power supply and a low-current low-voltage power supply for a threshold level. The readout electronics are mounted on the chamber, shielded within Faraday cages. The high-current power supplies were built by the Rutgers University Department of Physics & Astronomy Electronics Shop. These supplies are set to provide sufficient current at 5 V for the boards to which they are hooked up. No adjustments, except for turning the supplies on / off, should be needed in normal operation. There are voltage setting, current limiting, and overvoltage protection potentiometers within the boxes; adjustment information is given in the FPP logbooks. The low current supplies are Hewlett-Packard 6111A supplies. The 6111As can provide up to 1 A for voltage from 0 to 20 V. The supplies are currently hooked up through the rear panel to a DAC in the data acquisition panel; front panel controls on the supplies are disabled, execpt for the on/off switch. The voltage is controlled through an EPICS FPP threshold window, that is accesses through the Hall A / hadron spectrometer / detectors menus. The high-current supplies are not computer controlled. All supplies are mounted in the detector stack.
The multiplexed logical signals from the chambers have amplitudes
smaller than ECL levels, to prevent noise at the chamber.
These signals are fed to level shifter boards
(see Figure ),
located in the FPP rack on
the lower electronics level of the detector stack, on the beam right side.
A high-current 5 V power supply for the level shifter boards is
located at the bottom of the same rack.
The boards convert the signals to ECL standard levels.
The level shifter outputs
are connected to the starts of LeCroy Model 1877 Fastbus TDC's, located
in the lower electronics level on the beam left side.
The TDCs measure both leading and trailing edge times to allow demultiplexing.
The TDCs are subsequently stopped by the overall event trigger, and
are read out by the CODA aquisition software.
The data are histogramed online by the DHIST software.
In-depth offline data analysis requires the ESPACE software.
The chamber gas is presently a combination of argon and ethane, about 63% and 37% by weight. Gas is routed from the Hall A gas shed mixing system to the gas panel located on the lower electronics level of the space frame, and subsequently to the FPP chambers. The gas system is shared with the VDCs. A detailed description of the system has been written by Howard Fenker. See
In addition, the chambers are outfitted with a test pulser capability. A pulse is introduced into an 8 channel (16 wire) twisted pair cable on each chamber, which connects to the high voltage boards, at the opposite ends of each straw from the readout boards. The pulse is resistively coupled through a 20 k resistor to the ground leg of a 1500 pF capacitor, and thence into the straws. After propagating through the straw, the pulse enters the readout board. A pulse of about 1 V amplitude in the twisted pair cable is sufficient to provide a few mV signal into the readout boards, resulting in a logical output signal. The system may be used to test the functionality of each readout channel and / or the continuity of the high voltage wire in each straw. The system currently is only implemented for manual operation, except that data may be read out through CODA. This procedure requires some familiarity with trigger logic and setup, should only be done by experts, and is not documented here.