29th International Cosmic Ray Conference Pune (2005) 00, 101–104

The Telescope Array Low Energy Extension: the Tower Detector
G. B. Thomson , and the TA/TALE Collaboration(a) Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA

Presenter: G. B. Thomson (thomson@physics.rutgers.edu) usa-thomson-G-abs4-he15-poster
The Telescope Array Low Energy Extension (TALE) is a series of detectors to be added to the Telescope Array(TA) experiment to increase its aperture at high energies and extend its energy coverage to lower energies. Oneof the TALE detectors is a “tower” detector. This is a fluorescence detector designed for sensitivity to low  energy cosmic rays, fromtoeV. To achieve this low-energy sensitivity the detector’s mirrors will
be larger than those of any previous fluorescence experiment, and they will point higher in the sky than thoseof HiRes, TA, or Auger.

1.

Introduction

The TA and TALE experiments are being built by a collaboration of research groups from Japan and theUnited States. The experiment is designed to study the spectrum, composition, and anisotropy of ultrahighenergy cosmic rays, over a wide energy range, with a large aperture, and with good control of systematic
uncertainties.
The experiment will be located in Millard County, Utah. It will consist of TA detectors (a ground array of576 scintillation counters separated by 1.2 km, and three fluorescence detectors overlooking it), two TALE
fluorescence detectors, and an infill array. The TALE fluorescence detectors will be located a distance of 6 km
from two of the TA fluorescence detectors. This spacing is optimum for studying the ankle of the cosmic rayspectrum with a nearly flat stereo aperture. All these fluorescence detectors will observe at elevation angles
from 3 to 31 degrees above the horizon.
Associated with one of the TALE detector sites will be a “tower detector” which will observe at higher elevations, from 31 to 71 degrees. The tower detector will have larger mirrors and be able to detect cosmic rays  from about
eV toeV. A ground array of about 110 scintillation counters, of 400 m spacing, will bedeployed in front of the tower detector for hybrid coverage at low energies.
Important physics occurs in this energy range. The galactic - extragalactic transition occurs here: at the lower
end the cosmic ray flux is dominated by galactic sources, and at the upper end extragalactic sources dominate.
HiRes-MIA [1] and HiRes stereo [2] data show a change in composition in this energy range, from heavy tolight, indicative of this transition. One can perform a composition-tagged analysis to study galactic sources bypicking out the heavy part of the composition, and extragalactic using the light part.
  In the region below abouteV most extragalactic cosmic rays come from redshifts greater than z=1.5.
Two candidate sources, QSO’s and AGN’s, show a break in their evolution near this redshift [3], in that QSOand AGN luminosity densities stop increasing with redshift and become almost flat at larger redshifts. If theseastronomical objects are sources of ultrahigh energy cosmic rays, then this break in source intensity should bevisible in the cosmic ray spectrum also, where it should provide a feature in the extragalactic spectrum in the 
middle of theeV decade. An experiment that can detect cosmic rays from well below this point up to theenergy of the GZK suppression is needed to study this question, which is a test of the nature of the sources ofultrahigh energy cosmic rays. Fits to the spectrum measured by TA and TALE, taking galactic and extragalactic
sources into account, should be sensitive to this break.
Together the TA and TALE detectors will make an experiment that has good sensitivity over four orders of

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G. B. Thomson et al.

magnitude in energy, from well below the second knee through the area of the GZK suppression [4]. The sameevents will seen by different detectors and cross normalization of energy scales will be possible.

2.

The TALE Tower Detector

Fluorescence detectors such as those deployed by the HiRes, TA, and Auger experiments, that observe fluorescence light from showers at elevation angles up to 31 degrees, suffer a bias for cosmic ray showers below an energy of abouteV. This is because lower energy events can be seen only if they are close to the detector,and the point of maximum shower development,   , most often occurs above the elevation angle coverageof the detector. Since seeing   is important for good energy resolution, events must be selected where  is visible, and the bias results.
To see lower energy showers with good statistics and good energy resolution one must align one’s fluorescence
detectors to look higher in elevation angle. An example of this was the HiRes prototype - MIA hybrid experiment [1], where the fluorescence detectors were arranged to observe from 3 to 71 degrees in elevation. This experiment observed down to an minimum energy of about
eV. To go even lower in minimum energy one
must improve one’s signal to noise ratio, for example to collect more light by using larger mirrors. Thus thestrategy we adopt in the TALE tower detector is to build mirrors three times larger in area than HiRes’, point
them from 31 to 71 degrees in elevation, and deploy them with one of the TALE detectors, whose mirrors point
lower in the sky, to provide overall coverage from 3 to 71 degrees.

3.

Tower Detector Simulations

The detectors being simulated are shown in Figure 1. The diameter of the mirror is 4.0 m, with a 7.50 m radiusof curvature. Winston cones will be used to collect the light onto the phototubes, and a drawing of the Winston
cone is shown in the inset in this figure.
We have used the full Monte Carlo simulation for the HiRes-II detector, including analysis cuts, to simulate
the TALE tower detector. HiRes-II is the newer of the two detectors in the High Resolution Fly’s Eye (HiRes)
experiment, which has a flash ADC readout system and is deployed on top of Camel’s Back Ridge in the U.S.
Army Dugway Proving Ground in Utah. The agreement between this Monte Carlo and the HiRes-II data wasexcellent, so we believe that the prediction for the tower detector will be excellent as well.
Figure 2 shows the arrangement of detectors used for the simulation. A full two-ring detector (identical toHiRes-II) observing from 3 to 31 degrees in elevation, plus 15 large mirrors observing between 31 and 71
degrees was used.
Figure 3 shows the aperture of several TA and TALE detectors: the TA surface detector (SD), the TALE tower
detector, with the HiRes-II detector monocular aperture for comparison. Figure 4 shows the number of events
that will be collected per year by these three detectors.
It can be seen from these figures that the TALE tower detector has a minimum energy that is an order ofmagnitude lower than a standard fluorescence detector like those of the HiRes, TA, and Auger experiments.
The tower detector will produce a good-statistics measurement of the flux and composition of cosmic rays from   toeV.
We plan to deploy an infill array of 110 scintillation counters in front of the TALE tower detector. These counters will be in a grid of spacing 0.4 km, 1/3 of that of the TA surface detector. These counters will be used forhybrid observations with the tower detector. This will improve the energy resolution for spectrum measure-The TALE Tower Detector

103

Figure 1. Drawing of the large mirrors that will be used for
the TALE Tower Detector. The inset shows the outline of
the Winston cones to be used to channel light to the phototubes.

Figure 2. How the TALE detector might look. The mirrors
in the outermost two rings (that observe from 3 to 31 degrees in elevation) are identical to HiRes mirrors, and themirrors in the inner rings are the large mirrors that will be
used for the TALE Tower Detector.

Figure 3. The aperture of TALE tower detector and theTA surface detector (SD). The monocular aperture of theHiRes-II detector is shown for comparison.

Figure 4. Histogram of the energy of events that will becollected per year by the TALE tower detector and the TA
surface detector (SD). The number of events collected peryear by the HiRes-II detector in monocular mode is shownfor comparison. The symbols used are the same as in theprevious figure.

104G. B. Thomson et al.

ments, and will be particularly important for measuring   of showers (and hence their composition). A
hybrid measurement of  has better resolution than a monocular measurement by about a factor of 2.

4.

Conclusions

The TALE tower detector with its infill array, the TALE 6-km stereo detector [5], the TA fluorescence detectors,
and the TA surface detector will be a powerful experiment for the study of ultrahigh energy cosmic rays.
The physics of ultrahigh energy cosmic rays in the energy range above
eV is very rich. Three spectralfeatures are present, in contrast to the single feature in the spectrum at all lower energies. In addition thegalactic - extragalactic transition occurs here.
The composition of the primaries at the lower end of this energy range should be heavy and of galactic origin,then at higher energies make a transition to become light and of extragalactic origin. This will occur justwhere the TALE detectors are at their optimum efficiency. The highest energy events should continue this lightcomposition.
QSO’s and AGN’s, two types of astronomical objects which are expected to be sources of extragalactic cosmicrays, show a break in their evolution at a redshift of about 1.5. The effect of this break should appear in the 
flux of cosmic rays in the middle of theeV decade. To understand cosmic ray sources it is important tosearch for this break. An experiment to carry out this search must cover a wide energy range, from an order ofmagnitude below the break, all the way to the threshold for the GZK suppression.
The TALE tower detector will extend the fluorescence technique to lower energies than previously explored.
 eV is perfect for fits to the cosmic ray spectrum to search for the break in theThe minimum energy ofextragalactic spectrum from QSO and AGN sources. The composition of cosmic rays will be studied by theTALE tower detector (and by the TALE 6-km stereo detector) in a model-independent way.

5.

Acknowledgements

We thank colleagues from the AGASA, HiRes, and TA collaborations for valuable discussions. The Telescope
Array is being constructed by the support of Grant-in-Aid for Scientific Research (Kakenhi) on the Priority
Area ”The Highest Energy Cosmic Rays” by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We also thank the University of Utah, the State of Utah, and the Utah Economic Development
Board for their generous support.

References[1] T. Abu-Zayyad et al., Astrophys. J. 557, 686 (2001)[2] R.U. Abbasi et al., Astrophys. J. 622, 910 (2005)[3] B.J. Boyle and Roberto Terlevich, astro-ph/9710134.
A.J. Barger et al., submitted to Astron. J., astro-ph/0410527.
[4] K. Griesen Phys. Rev. Lett. 16, 748 (1966).
G.T. Zatsepin and V.A. Kuzmin, JETP. Lett. 4, 78 (1966).
[5] The Telescope Array Low Energy Extension (1): the 6-km Stereo Detector, C.C.H. Jui et al., to appear inthe Proc. of the 29th ICRC, Pune (2005).