How STACEE works

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     The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new experiment dedicated to the study of GeV/TeV gamma rays from astrophysical sources. STACEE uses as its collection mirror the large field of solar heliostats at the National Solar Thermal Test Facility near Albuquerque, New Mexico, maintained by Sandia National Laboratories. Although the heliostats have been designed for the collection and concentration of sunlight for solar energy research, the STACEE collaboration has demonstrated that it is also possible to use these mirrors at night to collect the quick flashes of blue Cherenkov light that result from gamma-ray air showers. These gamma rays originate from astrophysical sources at galactic and extragalactic distances. As gamma rays hit the Earth's atmosphere they create an extensive shower of high-energy particles. These particles go faster than the speed of light in air, and thus emit light, or Cherenkov radiation (similar to the sonic boom emitted when a body goes faster than the speed of sound). The radiation (at optical and ultraviolet wavelengths) lasts for only a few nanoseconds at a time and is imperceptible by the human eye. By reflecting the Cherenkov photons to secondary mirrors on the central tower, the flashes can be recorded by a camera of photomultiplier tubes, which will turn the light into a measurable electric signal. The very large mirror area can be exploited to detect gamma rays at an energy threshold significantly lower than achieved by current instruments, thus providing a means of opening an important window in the gamma-ray spectrum.

Click on portions of the drawing to learn about STACEE components:
Click for Gamma-ray info.Click for Atmospheric Cherenkov Effect infoClick for Information on Heliostat MirrorsClick here to learn about STACEE's secondary optics Click here to learn about STACEE's electronics

The solar tower technique:
1) Cherenkov light is produced by an electronmagnetic cascade in the upper atmosphere.
2) Heliostat mirrors are used to track an astrophysical source and reflect Cherenkov light onto the Solar Tower.
3) A secondary mirror in the tower focuses light from each heliostat onto a separate photomultiplier tube.

Elements of the STACEE detection process: (Figures can be enlarged by clicking on them)

Gamma Rays

The Electromagnetic Spectrum: short wavelength (high energy) is on the left, long wavelength (low energy) is on the right.
STACEE is sensitive to the range of gamma-ray energies from 50-500 GeV. This region of the electromagnetic spectrum lies beyond that currently explored by satellite experiments and below the threshold of existing imaging Cherenkov telescopes.
STACEE targets gamma-ray sources like pulsars, supernova remnants, and active galaxies. This all sky map shows the gamma-ray sources detected by the EGRET detector on board the Compton Gamma-Ray Observatory. Some gamma-ray sources, like the active galaxy Mrk 421, have their peak energy output in the 50-500 GeV range. Thus STACEE is ideally suited to observe these sources.
Click here to see STACEE's data on the Crab Nebula Click here to see STACEE's data on the blazar Mrk 421

Atmospheric Cherenkov Effect:

High energy gamma rays and cosmic rays initiate particle cascades as they enter the Earth's atmosphere. Cherenkov light from these air showers can be detected at ground level. Since gamma rays create a unique cascade pattern it is possible to discern between gamma rays and the more abundant background cosmic rays. The graphs at left compare shower structure between gamma rays and protons. The vertical axis is height in the atmosphere (in km). The horizontal axis is lateral position on the group (in units of 100 m). The color coding is gamma, e+/e-, µ+-, hadrons.


STACEE uses the heliostats at the National Solar Thermal Test Facility (NSTTF) near Albuquerque, New Mexico, to collect Cherenkov light from air showers. The 37 square meter heliostat mirrors (at left) reflect the Cherenkov light to smaller secondary mirrors on the tower (at right). The heliostat mirrors create a gamma-ray telescope with enormous collecting area that makes it possible to explore the new energy range for gamma rays. By using the exiting NSTTF facility, observations in this new 50-500 GeV window can be made at a fraction of the cost of a satellite telescope.

Secondary Optics:

STACEE uses five secondary mirrors to direct light from each heliostat onto a single light-sensitive detector. Three mirrors, about 2 m in diameter, are located on the upper level of the tower (figure at left) and two 1.1 m diameter mirrors are located on the lower level of the tower (below).

The final stage of the STACEE optics concentrates the light from the heliostat image onto a photomultiplier tube. This is accomplished using a Dielectric Total Internal Reflecting Concentrator (DTIRC). The DTIRC only accepts light below a certain incident angle to enter the photomultiplier tube.


Photomultiplier tube pulses are AC-coupled, amplified (x100) and sent down 45 m of RG-213 cable to the STACEE electronics room. These pulses are then sent to a 1 GHz Flash-ADC Digitizer system. Copies of the pulses are sent to discriminators to form the digital trigger.

Trigger System:

STACEE uses custom built VME boards that have programmable time-of-flight delays to form a two level multiplicity trigger. The delay signals compensate for differences in light travel time from the shower to the PMTs. These delays are continually updated to account for the movement of the target across the sky. Delays have a 1 ns resolution and the trigger window is a few nanoseconds. The middle figure shows a digitized Cherenkov pulse from the FADC for a single channel from the STACEE-48 project. The graph at right shows the trigger rate of STACEE-48 as a function of the discriminator threshold (a single photoelectron is about 25 mV). Data taken with randomized delay settings (green on the graph) show that triggers caused by fluctuations in night sky brightness become negligible above 110 mV. STACEE triggers below 5 photo electrons per tube.

Simulations from STACEE-48:

STACEE-48 was an initial version with the experiment that ran in 2001. The triggering probability of simulated showers is used to calculate the effective collection area of the array (Top Plot). Convolving this area with an assumed source spectrum of E-2 yields an energy threshold of 120 GeV for a source at zenith (Bottom Plot).

Shower Reconstruction in STACEE-48:

The arrival direction of the shower is reconstructed by fitting the photon arrival times (corrected for light travel time from the heliostats) to a spherical wavefront. The top figure shows the distribution of Chi-square-values for this fit. The bottom green plot shows the fit residuals for a single channel - the RMS deviations are typically ~1 ns. The lower blue plot shows the difference in reconstruction angles between two independent halves of the array. The typical reconstruction error should be approximately half of this difference or ~0.15 degrees.



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