If you would like more information about the PAL instrument, please e- mail Bob Hull, Technician Specialist: rhull@nmsu.edu.
Tracking System
The current tracking system consists of 8 multi-wire proportional chambers
(MWPC) and 2 drift chambers. These modules provide 12 position
measurements in the x-axis (the axis along which the magnet will
influence particle trajectories) and 8 measurements in the y-axis.
The PAL balloon payload data system has evolved from a simple relay- controlled operation to an on-board advanced minicomputer system. With improvements in our payload have pushed our data rate to new limits every flight. The present PAL flight computer system consists of a Digital Equipment Corporation (DEC) VAX 3100 processor with its "Q-bus" backplane mounted in a custom chassis. Special cooling and DC power systems are necessary for flight operation. The system is located on the baseplate of the gondola, where the magnetic field is 150 Gauss. The 2 gigabyte hard disk drive and 4 mm tape drives are mounted in a 1/16" steel box to provide magnetic shielding. The CPU (32 bit architecture) operates at 22 Mhz. The custom real-time flight code is stored on disk but executes from RAM. While on the ground, the on-board computer is part of a Local Area Network to allow remote updates of the real-time software.
The interface from the computer to the payload data system in via Kinetic Systems CAMAC controllers. Four CAMAC crates hold the electronics modules that acquire the science and engineering data. Using a parallel bus connection, the crate controllers manage data gathering from the various detectors and sensors in payload.
During high latitude flights, the event rate averages 400 cosmic rays
per second at the flight altitude of 120,000 ft. Typical events consist of
300 16-bit data words. A 24-hour flight will generate approximately 2
gigabytes of data. The formatted data is stored on-board on DAT tape and
hard disk. RF bandwidth restrictions have limited our ability to transmit
all our data to the ground. A sample of the science and engineering values
is telemetered for operational verification. Our command system provides an
RS-232 terminal connection to the flight computer's console interface. By
modulating a ground transmitter with the serial command stream (2400 baud
uplink) and mixing the CPU response with the downlink telemetry, the closed
loop operating system allows direct interaction with the payload computer
during the flight.
The main function of the calorimeter is to detect electromagnetic
showers for the purpose of identifying electron and positron events. The
calorimeter consists of 8 silicon wafer readout planes interleaved with 8
tungsten absorber layers one radiation length thick. Each of the readout
planes contains and x and y readout layer. Each sampling plane has 256
readout channels (128 for each view) and provides both position and energy
loss (dE/dx) information (Aversa, 1995). Each of the readout channels can
be calibrated using a sample of minimum ionizing particles (MIPS). The ADC
distribution for each strip is fitted using a semi-Gaussian plus Landau
distribution. Using this procedure, the individual strips have a signal to
noise ratio of better than 15, for MIPS.
For flights from areas of high geomagnetic cutoff (e.g. Ft. Sumner, NM) the
primary purpose of the TOF is to identify albedo (upward moving) particles.
With this system, downward moving particles are separated from upward moving
particles by more than 20 sigma. The TOF system is made up of two planes
consisting of two scintillator paddles each. The paddles are viewed at each end
by Hamamatsu R2490-05 phototubes. Ultraviolet-transmitting plexiglass light
pipes are used to maximize the amount of light collected from each paddle. This
system has a time resolution of 360 ps. The R2490 phototubes employ a mesh
dynode configuration which allows them to function in high magnetic fields
without shielding. Using this tube technology allows for considerable weight
savings over tube designs that require heavy iron shielding.
This detector allows for the separation of the lighter (and hence higher velocity) electrons, positrons, muons and pions from the heavier (slower moving) antiprotons and protons in the 4-20 GeV range. PAL has used a gas Cherenkov counter in 4 previous flights: CRL7 (1979), CRL8 (1986), MASS89 ( 1989), and MASS91 (1991). During this period, the instrument has undergone several changes and upgrades. Presently, the system incorporates a 1 meter pathlength of Freon 12 gas viewed by four 12.7 cm (5 in.) photomultiplier tubes. A four section pyramidal mirror is used to focus the light on to the phototubes. By using a low light level LED pulse, incorporated in the detector, a single photoelectron peak can be resolved for each quadrant of the mirror. This allows for an absolute calibration and photoelectron scale to be set. Using this information, we find that a fully relativistic particle will produce 15 photoelectrons. This number decreases for portions of the mirror where the reflecting geometry is not as favorable (e.g. at the edges and the corners).
The efficiency for the detection of a muon's Cherenkov light is given by
the fraction of muon events above threshold accompanied by a Cherenkov
signal and is found to be more than 98%. The accidental muon rate, muons
below threshold that are accompanied by an accidental Cherenkov noise pulse
, has been found to be less than 0.2%. This makes the Cherenkov detector an
excellent instrument for both antiproton and positron measurements.
The transition radiation detector has been used to discriminate positrons
from protons. It was flown in the 1993 TS93 flight. The TRD is made
of ten modules each consisting of a carbon fiber radiator followed by a
multiwire proportional chamber. In order to achieve a
proton-electron rejection factor of the order of 10^-3 with a strict limitation
on power consumption to about 40 mW per chamber channel, INFN collaborators
developed low power consumption "cluster counting" electronics.
NaF Ring Imaging Cherenkov
The NaF RICH used in the 1994 CAPRICE flight was the first balloon-borne ring
imaging Cherenkov counter capable of measuring charge one particles.
This instrument, built by the Royal Institute of Technology (Sweden), used
a solid NaF
radiator and was designed for the measurement of low energy antiprotons.
In this detector, 1 cm of NaF (refractive index = 1.4)
is followed by a helium-filled space which
allows expansion of the Cherenkov disk and formation of a ring of Cherenkov
photons. The Cherenkov ring is then transmitted through a quartz window into
an MWPC filled with methane, isobutane and TMAE
(tetra-dimethyl-amine-ethylene). The MWPC utilizes a segmented cathode readout
(8 mm squares) to record x-y locations of the particle passage and the
conversion locations for the Cherenkov photons. The detector size is 50 cm by
50 cm. This corresponds to about 4000 readout pads. The electronics for the
NaF RICH detector are based on the AMPLEX chip. The Royal Institute of
Technology will incorporate this proven readout technology in the new Gas Ring
Imaging Cherenkov (GRICH).
Aerogel Cherenkov
An innovative large-size aerogel detector developed by the
California Institute of Technology and used in the 1992 IMAX flight.
Brass Calorimeter
This streamer tube calorimeter detected electromagnetic
showers for the purpose of identifying electron and positron events for
MASS89, MASS91, and MASS92.
It uses brass as a radiation absorber and wire support.
The instrument is gas tight. An isobutane mixture and
pure isobutane have been used as the ionizing gas.
Shower Counter
Used in 1979 CRL7 flight.
Future instrumentation:
Gas Ring Imaging Cherenkov
The GRICH detector represents the next major step in particle detectors for
balloon-borne experiments. The GRICH detector, with an appropriate gas
radiator, will allow antiprotons to be separated from the background of
electrons, pions and muons in the 2.7 - 50 GeV energy range. This detector
incorporates a photosensitive readout system that provides information on the
position of each photon. Using the position of the Cherenkov photons, the
opening angle of the Cherenkov light cone can be determined, and hence the
velocity of the incident particle can be very accurately determined. This
detector has several advantages over conventional Cherenkov detectors. The
GRICH detector is relatively insensitive to the statistical fluctuation in the
number of Cherenkov photons that are collected. Almost all of the "noise"
photons produced from gamma rays can be removed since these photons are
produced at random angles with a very small probability that they will fall in
the Cherenkov light cone of the incident particle. For a given radiator, the
range of measurable velocities for the ring imaging count is much wider since
the upper velocity limit is determined by the position resolution of the
detector and not by the point at which the Cherenkov light saturates.