Cosmic ray astrophysics, an active area at the cutting edge of basic research,
investigates particles (or rays) which arrive at the earth's surface from
sources beyond our sun (or even beyond our galaxy). Project GRAND is an
extensive air shower array which studies these rays in two energy bands
depending on the trigger selected: from 30 to 300 GeV (GeV is giga, or billion
electron volts, eV), and from 100 to 100 000 TeV (TeV is teva, or trillion eV).
Gamma Ray Astrophysics at Notre Dame
The original goal in the construction of this array was to study stellar point
sources of gamma rays. This study requires: good angular resolution, good
particle identification, and adequate area together with sufficient running
time to gather statistics. The idea of using tracking detectors provides
superior angular resolution and simultaneous particle identification; this
project has pioneered the use of tracking detectors in cosmic ray research. PWC
detectors also allow the array to be used for additional studies. including a
new primary goal: measuring the cosmic rays in the energy region from 30 to 300
The GRAND array is located in a 100 m x 100 m field adjacent to the Notre Dame
campus at 222 m elevation, 86o W and 42o N. The 64 stations are
arranged in an 8 x 8 grid with 14 m separation between stations. The
electronics trailer at the center of the field is a former astronaut debriefing
and isolation chamber, and was obtained from NASA government surplus. The
southwest quadrant was the first constructed; these stations were placed 0.6 m
into the ground for natural cooling in the summer. The cooling works well, but
moisture problems suggested the placement of the remainder of the array above
ground with added insulation.
Each station is enclosed by a 2.4 m x 2.4 m x 0.9 m high hut. Inside are eight
planes of proportional wire chambers (PWCs) and a 50 mm thick steel absorber
plate. The chambers have been surveyed to a precision of 0.1 deg. The
temperature of the huts is controlled with heaters to keep them > 17 deg C in
the winter; dehumidifiers control the humidity < 55%. The eight planes are
arranged in xy pairs; the pairs are separated vertically by 200 mm and are 1.2
sq-m in area. The top three pairs measure the xz and yz angle of the track; the
pair below the 50 mm steel absorber plate are used for muon identification.
Muons are distinguished from electrons by demanding that a muon track be
single hits in each of the 3 pairs above the steel plus the pair below, all of
which are aligned. This simple algorithm is 96% accurate for muons. Muon
identification is performed for each track.
IV. Proportional Wire Chambers (PWCs)
George Charpak received the Nobel prize for inventing the PWC detector which is
used extensively in other fields of physics and medicine; Project GRAND has
pioneered its adaptation for cosmic ray arrays. Construction design utilizes
mass production techniques to economically manufacture precision detectors. The
cells in each plane are 14 mm wide and 1100 mm long separated 9.5 mm from the
high voltage planes. The PWCs are filled with a gas mixture of 80% argon and
20% carbon dioxide and operate at 2600 V.
V. Electronics and Data Acquisition System
The huts contain 8 PWC planes. A plane has 80 cells; each cell has an amplifier,
shift register memory, and summer whose outputs are used for coincidence input
to form a self-trigger for the station. A station trigger causes the status of
its 640 cells to be stored in local shift register memory. The trigger is
comprised of an x-OR-y signal from each pair of planes; the top 3 PWC pair of
signals are placed in 3-fold coincidence. This 3-fold coincidence records over
99% of the cosmic ray tracks and introduces < 1% noise.
The trailer receives a timing signal from each hut which triggered on a track.
A coincidence circuit is set for N simultaneous huts, where N can be set to 1,
2, 3, etc. A coincidence causes a train of clock pulses to be sent to all the
stations which clocks the data serially from their shift register memory to the
trailer. The data from all the stations are read out to the online computer in
the trailer in a total of 70 microseconds. The time of the trigger, accurate to
1 millisecond, is recorded along with the rest of the trigger's data.
The data acquisition system "computer" records the data. There is a single muon
trigger (lower energy band). The search for one-and-only-one-hit pattern in each
of the 8 planes of a station; the successful wire coordinates are stored in
memory. The computer stores 900 tracks in memory and then writes them to a
disk. Single muon data are stored at a rate of 960 muons per sec; each disk
holds several months of data. With N=3 shower data (higher energy band), the
data acquisition system takes data for cosmic ray showers.
All 64 stations of the array were completed in spring, 1996. Soon thereafter, a
record heavy rain flooded ten of the stations. Recovered from this flooding;
all 64 stations are now working and taking data. Permanent drainage has been
added on the south and west sides of the array which has prevented flooding
The most recent publications were presented at the International
Cosmic Ray Conference in Beijing, China in August of 2011. The titles
"Atmospheric Effects on Muon Flux at Project GRAND", J. Poirier
and T. Catanach, Proc 32nd ICRC, (2011)
"Periodic Variations in Muon Flux at Project GRAND", J. Poirier and T.
Catanach, Proc 32nd ICRC, (2011)
"Status Report on Project GRAND", J. Poirier, C. D'Andrea, C. Barry,
T. Catanach, L. Cheng, T. Wilson and C. Swartzendruber, Proc 32nd ICRC, (2011)
"A Measurement of Secondary Muon Angular Distribution with High
Statistics", C. D'Andrea and J. Poirier, Proc 32nd ICRC, (2011)
These papers appear in the proceedings of this conference (published)
or copies can be obtained upon request.
A complete listing of publication references from Project GRAND is
available under Publications.
VII. Construction and Operating Funds
The construction of this project was funded by the National Science Foundation
and Notre Dame together with private contributions totaling about a million
dollars. With government restrictions on funding, the NSF is not able to
provide even the small funds required to operate the completed array. The major
operating expense is the argon gas which flows through the detectors at a cost
of approximately $200/week. We are thus now seeking private contributions, to
be matched by university funding, to operate GRAND to gather the data for which
it was constructed and reap the secrets of nature now open to its view.