Ground based gamma- ray detection
Experimental techniques for particle pysics
Outline
Overview of ground based gamma-ray experiments and techniques
High Energy Stereoscopic System (H.E.S.S)
The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE)
Milagro
Summary
Overview of Cosmic-ray and gamma- ray experiments
Atmospheric Cherenkov experiments
Telescopes and telescope systems:
CANGAROO at Woomera, Australia [Collaboration between Australia and Nippon for a GAmma Ray Observatory in the Outback].
CAT [Cherenkov Array at Thémis]
CLUE [C(h)erenkov Light Ultraviolet Experiment] at the HEGRA site on La Palma (operational from 1997 to 2000)
HEGRA Cherenkov Telescopes on La Palma, Canary Islands (operational until Sep. 2002)
PACT [Pachmarhi Array of C(h)erenkov telescopes] at the High Energy Gamma Ray Observatory at Pachmarhi, India
TACTIC [TeV Atmospheric Cerenkov Telescope with Imaging Camera] at Mt. Abu, Rajasthan, India.
Telescope Array (for Cherenkov and fluorescence light)
Whipple Gamma-Ray Telescope on Mt. Hopkins, Arizona New telescope projects:
CANGAROO-III system of four 10 m telescopes in Australia (operational since March 2004).
H.E.S.S. [High Energy Stereoscopic System], four 13 m telescopes in Namibia
MAGIC (a 17 m telescope on La Palma, Canary Islands, operational since 2003)
MACE [Major Atmospheric Cerenkov Telescope Experiment] (project in India)
VERITAS [Very Energetic Radiation Imaging Telescope Array System] USA Solar power facilities as light collectors:
CACTUS [Converted Atmospheric Cherenkov Telescope Using Solar-2]
CELESTE [CErenkov Low Energy Sampling and Timing Experiment] at Thémis, France
GRAAL [Gamma-Ray Astronomy at ALmeria] near Almeria, Spain (operational 1998-2001)
STACEE [Solar Tower Air Cherenkov Experiment] at Sandia Labs, New Mexico Cherenkov counter arrays:
AIROBICC (non-imaging counters in the HEGRA array)
Atmospheric fluorescence experiments
ASHRA [All-sky Survey High Resolution Air-shower detector]
Auger Project Fluorescence Group
EUSO [Extreme Universe Space Observatory ]
HiRes The High Resolution Fly's Eye Cosmic Ray Detector OWL [Orbiting Wide-angle Light collectors]
Telescope Array (for Cherenkov and fluorescence light)
Air shower experiments with particle detectors
AGASA [Akeno Giant Air Shower Array]
ARGO-YBJ: new experiment under construction in Tibet
ASCE [Air-Shower Core Experiment] (Sydney, operational 1989-1991)
Buckland Park Extensive Air Shower Array (Australia) (operational 1994-1998)
CASA [Chicago Air Shower Array] (operational 1990-1998)
CRT [Cosmic Ray Tracking] (prototypes, operational 1992-1996)
EAS-TOP experiment (Italy, above the Gran Sasso underground laboratory, until April 2000)
Haverah Park (former experiment of Leeds University, operational until 1993)
GRAND [Gamma Ray Astrophysics at Notre Dame] (an array of tracking detectors)
GREX [Gamma Ray Experiment] array (Haverah Park, operational 1986-1995)
HEGRA [High Energy Gamma Ray Astronomy] (operational 1988-2002)
KASCADE [KArlsruhe Shower Core and Array DEtector]
MILAGRO (Water Cherenkov experiment near Los Alamos)
Pierre Auger Project (originally also known as the Giant Airshower Detector Project).
SPASE 2 [South Pole Air Shower Array]
SUGAR [Sydney University Giant Air shower Recorder] (was operational from 1968 to 1979)
Overview of Cosmic-ray and gamma- ray experiments
Some of the observation techniques used in ground based gamma-ray experiments
Imaging atmospheric Cherenkov telescope (IACT)
Water Cherenkov experiment
Solar collector experiment
IACT
Image the very brief flash of Cherenkov radiation (~ ns)
Continuous emission of Cherenkov radiation and the light is emitted in the energy range of UV-light to X-rays
Resemble optical telescopes, where the light is reflected to the camera in the focal plane by mirrors
Threshold energy ~ 1/(detector area)^(1/2)
The Cherenkov photons detected by PMTs in the Camera are used to reconstruct the gamma-ray
From the recorded image of the Cherenkov radiation the initial particle can be
determined, and also its direction and energy
Problem, how to differ CRs from gamma-rays?
IACT
How to differ between gamma-rays events and cosmic ray (CR) background!
Gamma-ray images are aligned with the source since they are parallel with the optical axis
Random orientation of the Cherenkov radiation from CRs
The size and shape of the images are different
The H.E.S.S. phase I Telescopes
Fast facts about the H.E.S.S.:
H.E.S.S. is situated in the Khomas Highland of Namibia at 1.8 km above see level
Four telescopes arranged in a square with 120 m sides
Energy range ~ 0.1 to 1 TeV with a energy resolution of ~15%
Angular resolution of 0.1 ° for a single event
Can explore gamma-ray sources with intensities of a few thousandth parts of the flux of the Crab nebula
Can only observe on moonless nights, about 1000 h per year
All four telescopes were operational in December 2003
Main motivations for building H.E.S.S:
Increases the sensitivity compared to other experiments by a factor of 10
Lowers the energy threshold to 100 GeV
Detects gamma-ray sources 100 times faster than previous experiments
The Mirror
360 round facet mirrors, each with a diameter of 600±1mm
Focal length of a round facet mirror is 15.00±0.25 m
Total mirror area is 107 m² for each telescope
Reflectivity: at least 80% between 300 and 600 nm
Material: aluminized optical glass, thickness ≈ 15 mm
Point spread function: 80% of light in 1 mrad diameter
H.E.S.S
The Camera
The Cameras have a field of view (FOV) 5° (about 10 times the diameter of the moon)
Each camera provides 960 image elements (pixels) and they cover an area of 1,4 m²
Each pixel consist of a photomultiplier tube (PMT) and has a FOV of 0.16°
To get a PMT to trigger, a signal larger than a given threshold have to be achieved (typically a few photo-electrons)
An effective coincidence window for a pixel of 1.5 ns is used to effectively reduce random triggers from the night sky
The camera is triggered by a coincidence of 3 to 5 pixels
H.E.S.S
Central trigger system and data acquisition
Once a camera triggers on a shower, the central trigger station is alerted. If two or more telescopes trigger simultaneously, the analog signal in the telescope is digitalized, processed and read out (few 10 μs to 10 ms)
For non-coincident events the telescope read-out electronics is cleared after a few μs it is ready for the next event
The DAQ collects and combine data from the telescopes and monitor the instruments, and perform a first analysis.
One event from a camera can be reduced to 1,5 kB, hence an expected trigger rate of the four telescope system of 1 kHz which yields a data rate of 6 MB/s. A typical observation night will result in approximately 100 GB of event data to be handled
H.E.S.S
Gamma-ray reconstruction
The combined image from the telescopes of the observed Cherenkov light from the particle showers is shown in the upper picture
The energy of the incident gamma-ray is proportional to the intensity and can be extracted from the Cherenkov light together with measured atmospheric parameters using Monte Carlo
simulations (determined with Infrared radiometers, ceilometer, optical telescope and a weather station)
To generate a sky map of the gamma- rays a computer program takes the upper image of the air shower and determine its direction
The origin is plotted as a point on the sky map, and many of those points combined provide an image of the source which can be seen in the two bottom pictures
H.E.S.S
STACEE
Fast facts about STACEE:
Located at Sandia National laboratories, Albuquerque, New Mexico
Complete detector operating since spring 2002
Uses air Cherenkov technique
Energy range from 50-500 GeV
Angular resolution 0,18 ° and energy resolution 30%, both at 100 GeV
FOV ranges from 0.5° (for the most distant
Primary optics
64 heliostat mirrors (large steerable mirrors) reflect Cherenkov light onto the Solar Tower
Each mirror has an collecting area of 37 m² (Total mirror area ~2300 m²)
Back aluminized glass
25 segments, 4·4 feet each
80% reflectivity of visible light
Accurate to 0.05 °
STACEE
Secondary optics
(Tower)
Three mirrors with diameter of 2.0 m at top level
Two 1.1 m diameter mirrors at lower level
Each heliostat mirror is mapped onto one PMT
Uses Dielectric Total Internal
Reflecting Concentrator (DTIRC) to reduce noise and to correct for
different FOV for the heliostats
STACEE
Shower reconstruction
The energy of the incoming gamma-ray is determined from the intensity of the Cherenkov light, which is done off-line
Photons arrival times are fitted to a
spherical wavefront to determine arrival direction which is also done off-line
The difference in reconstructing angles between two independent halves of the array is presented in the blue graph.
The typical reconstruction error is
~0.15 °⇒ angular resoution ~0.15 °
The background subtraction is done by observing an “empty” piece of the sky during an equal amount of time as the on target observation and then
subtracting
STACEE
Vital to find the shower core to determine the energy and direction of the gamma-ray
Milagro Gamma-Ray Observatory
Fast facts about the Milagro:
Located near Los Alamos, 2630m above sea level
Running since January 2000 (2002)
Observes the Cherenkov radiation from the extensive air shower (EAS) that have made it to the ground
FOV is almost the entire sky
Operate both during night and day
Uses a Water Cherenkov EAS Array technique
The first EAS detector which has a peak sensitivity near 1 TeV with a energy resolution of 75%
Angular resolution 0.45 °
Air Cherenkov telescope (ACT) versus EAS Array
Milagro was designed to combine the low
energy threshold of ACTs with the large FOV and high duty cycle of EAS array
High (>90%) Low (5-10%)
Duty Cycle (On time)
Large (>45o) Small (~2o)
FOV
Moderate (>50%) Excellent (>99.7%)
Background Rejection
High (>50 TeV) Low (<200 GeV)
Energy Threshold
Extensive Air Shower Array Air Cherenkov
Telescope
Milagro
The Water Cherenkov detector
The central pond:
A pond of 80m x 60m x 8m filled with water and covered with a light-tight cover
Effective area ~5,000 m²
Has 450 PMTs in the top layer and 273 PMTs in the bottom layer
The PMTs are on a 2.8m x 2.8m grid so that nearly all of the particles that enter the water can be detected
The outrigger array:
Each of the 175 detectors surrounding the Milagro pond is a 1500 gallon water tank with an area of
~4.6m² and 1m high.
The tanks are lined with the reflective material Tyvek and a PMT that looks down into the tank
Outriggers span ~ 40,000 m²
The outriggers determined more efficiently the core of the shower, which not only improves the angular resolution, it also allows for the determination of the shower energy and improves the gamma hadron separation
Milagro
Reel time data and shower reconstruction
Record events at a rate of ~ 1700 Hz
Data rate ~ 2.5 MB/sec
All raw data must be reconstructed in real time because of the difficulties of large amount of data
On long timescales an aggressive background rejection is used
Reconstruction of the gamma-ray sky map is done by determining the core position, incident direction and shower size
The energy of the original gamma-ray is determined from the number of detectors hit and how much light they were hit with
Milagro
Summary
Observing gamma-rays indirect through Cherenkov radiation is an effective
technique
Most ground based experiments apply this technique in one way or another
