UCLA-COSMIC/1999-02
Optimization of OWL-AirWatch
Optics & Photo-Detectors
Katsushi Arisaka
University of California, Los Angles
Department of Physics and Astronomy
Los Angles, California 90095
arisaka@physics.ucla.edu
December 29, 1999
Abstract
The OWL-AirWatch is a space-based, next-generation fluorescence detector to study ultra
high-energy cosmic rays and neutrinos. It will require state-of-art mega-pixel photon detectors
with a single photon detection capability. This document summarizes the general principle of
the detector and the specifications of the photon detectors. Several possible candidates of
photon detectors that satisfy such requirements are presented.
This report (MS Word file, 1.4MB) is available at
http://www.physics.ucla.edu/~arisaka/owl/arisaka_owl.doc
-1-
Table of Contents
1. Basic Concept of OWL -AirWatch Experiment . . . . . . . . . . . . . . . . . . 3
2. Derivation of Scaling Laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.
2.2.
2.3.
2.4.
2.5.
2.6.
Formula for Signals
Formula for Noise, SNR and Energy Threshold
Formula for Photo-detectors
Angular Resolution
Covered Area and Aperture
Summary
3. Expected Performance of Current Design . . . . . . . . . . . . . . . . . . . . . 11
3.1.
3.2.
3.3.
3.4.
3.5.
Basic Principles of Detector Optimization
Delta Launch Vehicles
AirWatch Design
Multi-OWL Design
Comparison with Other Previous/Ongoing Experiments
4. Requirement of the Photo-Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1.
4.2.
4.3.
Physical Dimensions
Signal Sensitivity
Time and Other Properties
5. Comparison of Existing Photo-Detectors . . . . . . . . . . . . . . . . . . . . . . 31
5.1. Vacuum Based Devices
5.2. Solid State Devices
5.3. Hybrid Devices
6. Candidate Photo-Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.1.
6.2.
6.3.
6.4.
Metal-channel PMT
Flat Panel PMT
Katsushi's Dream Detector
Summary and R&D plan
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
-2-
1. Basic Concept of OWL -AirWatch experiment
The OWL-AirWatch is a space-based, next generation fluorescence detector to study ultra highenergy cosmic rays and neutrinos. It is expected to achieve the following [Red Book].
1) The aperture will be of order 106km2sr, an order of magnitude larger than the proposed
Telescope Array and also two orders of magnitude bigger than the Pierre-Auger.
2) The energy threshold should be below 1020eV to study the energy spectrum around the ZGK
cut off and above.
3) Good angular resolution of order ~ 1o is desirable.
4) Extremely large volume and aperture for neutrino events compared to any other previous
experiments for the energy around and above 1020eV is expected.
The baseline design shown in Figure.1 is currently under consideration.
Owl-AirWatch Detector
Cosmic Ray
FOV  60o
H  500km
Cosmic Ray Shower
Ground
Atmosphere (~10km)
 600km
Figure.1 Basic concept of OWL-AirWatch experiment
1) A satellite will be launched most likely by a Delta III rocket to a trajectory of ~500km high.
2) It consists of a wide Field-of-View (~ 60o), large area (2~5m diameter), light collector made
by a mirror or a Fresnel lens.
3) Each photon detector views ~ 1km by ~1km of the atmosphere.
-3-
Such a space-based detector has the following major advantages over the ground-based, cosmic
ray detectors such as Fly's Eye, HiRes or proposed Telescope Array.
1) Fluorescence photons propagate mostly in vacuum space. Thus attenuation by atmosphere is
much less than the case of the ground-based detectors.
2) Distance from signals to a detector is the same within a factor of two, wherever cosmic rays
hit in the detector aperture. Thus the conversion factor from signal to energy is more or less
constant, unlike the ground detectors where a huge correction of 1/R2 (as well as correction
for atmospheric attenuation) is mandatory. It means uniform, well-defined energy threshold
can be easily achieved. The dynamic range required for readout is also directly related the
dynamic range of signals of interest.
3) The boundary of the detector aperture is simply defined by the Field of View of the wideangle optics, independent of the energy of the cosmic rays unlike ground-based detectors
where it is energy dependent and, to some-extent, atmospheric-condition dependent.
4) Signals are precisely registered at (t, x, y, z = 0) when cosmic ray showers hit the ground.
This solves the ambiguity of direction inside of the Detector Shower Plane (DSP). Therefore
stereo-view is unnecessary unlike the ground-based detectors. As a matter of fact, one can
consider data samples in the OWL-AirWatch as charged particle tracks measured by a TPC
(Time Projection Chamber) in high-energy experiments. A sampling rate of 1MHz (i.e.
1sec clock speed) corresponds to 300m sampling in space, which is adequate.
5) We observe only photons emitted backwards. Thus contamination by Cherenkov lights is
negligible, unlike ground-based detectors. Actually the Cherenkov light will be detected
from the reflection on the ground after the shower front collides on it. This can be used as a
powerful time stamp described above.
6) By changing the altitude of the OWL, we can optimize the aperture and the energy threshold
rather easily. Both aperture and energy threshold increase with OWL altitude.
On the other hand, there are some disadvantages. The distance from the space, ~500km, is at least
ten times longer than the distance from ground-based detectors. Thus attenuation by 1/R2 is
expected to be two orders of magnitude larger. In addition, limited size of mirrors would make the
signal size even smaller. Therefore careful optimization of optics and photon detectors is most
critical to ensure the good sensitivity for interesting physics.
This document first describes the basic relation between geometry/optics and the expected signal,
the noise level as well as the angular resolution. Several useful analytical formulas will be
introduced as scaling laws in terms of detector parameters. These formulas are the basis for a
detailed comparison of various photo-detector candidates.
-4-
2. Derivation of Scaling Laws
2.1 Formula for Signals
Generally speaking, the fluorescence signal, S, at the shower-max, generated by the cosmic ray of
energy E (eV), detected by a photon detector (in unit of photo-electron), is given by the following
[HiRes].
S ( pe)  4220( photons) 
E (eV )
 ( R)
 AT 2 (km2 / m 2 / n sec)
20
10
R
(1)
Where A (m2) is the area of the light collector,  is the quantum efficiency, T (nsec) is the gate
width, R (km) is the distance from the signal source to the detector, (R) is the correction factor
for light attenuation from the source to the surface of the photon detector.
To apply this formula to the OWL-AirWatch configuration, let's consider a cosmic ray
shower with the incident angle of  from the zenith, and angle  measured from the R-vector. As
shown in the figure below, let's assume that: the height of the satellite is H (km), the view angle
from the detector to the cosmic ray shower (measured from the zenith) is , each photo-detector
has a field of view of , the physical track length inside of  is Ltrack, the track length
projected onto the plane perpendicular to the R-vector is L.
Owl-AirWatch Detector


R
H


Ltrack
Ground
Cosmic Ray Shower
L
Figure.2 Various geometrical parameters between the OWL-AirWatch and Cosmic rays.
-5-
Table.1 below summarizes the notation of parameters and their default values in the following
analysis.
Notation
Default
Value
Energy of Cosmic Ray
E
1020 eV
Satellite Height from the ground
H
500km
FOV
60o
D
3m
fstop
1.0
R
-
 R 
0.5
The incident angle the cosmic ray from the zenith
Viewed angle from the detector to the cosmic ray shower
(measured from the zenith)
Incident angle of the cosmic ray (measured from the R-vector)

-

-

-
Field of View of Photo-detector Pixel

0.1o
Quantum Efficiency of Photo-detector

0.2
Integration Time of Flash ADC
T
1sec
Parameter Description
Total Field of View the detector mirror
Diameter of Mirror
F-stop of Mirror
Distance between the Cosmic-ray Shower and the Detector
Light Attenuation Factor after traveling the distance R
Table.1 Notation of parameters and their default values
There are several simple geometrical relations such as,
H
 cos ,
R
L
 cos 2     sin  ,
Ltrack
L
  .
R
(2)
Taking these relations and A  D 2 4 (where D is the diameter of the mirror), the signal given
by Equation (1) is conveniently expressed by,
 E  500km   D    ( R)  T 

 11.8( pe)  cos    20 
 
 

 10 eV  H   3m   0.1  1 sec 
2
S T
2
-6-
2
(3)
Furthermore, once the shower traveling time in the FOV is integrated over, T can be taken as
Ttrack 
Ltrack
L
R  
H  


.
c
c  sin  c  sin 
c  cos  sin 
1
 H   
 2.9(  sec) 



cos   sin   500km  0.1 
(4)
Here, c is the speed of light ( c  0.3km /  sec ).
The track length in , Ltrack, is also given by
Ltrack  870(m) 
1
 H   



cos  sin   500km  0.1 
(5)
The projected length of Ltrack onto the plane perpendicular to R-vector is simply,
L  870(m) 
1  H   



cos   500km  0.1 
(6)
Using Equation (4) for T in Equation (1), one can obtain the following expression as well.
S   4220( photons) 
E (eV )
FOV  ( R)
 A 

(km2 / m 2 / n sec)
20
c  sin  R
10
cos  E  500km  D    ( R)   
 34.2( pe) 



 


sin   10 20 eV  H  3m   0.1  0.1 
2
(7)
2.2 Formula for Background, SNR and Energy Threshold
The dark sky noise has recently been measured by the Italian AirWatch group by a balloon flight,
which is ~400 photons/m2/sr/nsec [Catalano].
Considering the area A(m2), the single pixel solid angle (deg2) and integration time of T (nsec),
then the background noise level, B, is given by,
B( pe)  0.12( photons)  AT(m 2 / n sec/ deg 2 ) .
This can be re-expressed by,
 D     T   
  
 1.7( pe)  
 

 3m   0.2  1 sec  0.1 
2
BT
-7-
2
(8)
The important criterion to separate signal from noise is the Signal to Noise Ratio (SNR).
Generally speaking, SNR is given by S  . In the simplest case,  has two contribution: Poisson
statistics of signal itself,  signal  S , and Poisson statistics of the dark sky noise,  dark  B .
Thus,
   signal2   dark 2  S  B
SNR 
S


S
SB
(9)
Once the signal becomes significantly larger than the noise, SNR simply becomes 1
result, the SNR (after T of integration) can be expressed by,
 500km  D   E   ( R)  T 

SNRT  3.4  cos  

  20


 H  3m   10 eV  0.1  1 sec 
S . As a
(10)
One can solve the above equation in terms of energy to obtain a formula for the energy
threshold. Assuming that typical SNR required to distinguish signals from noise is 2 (i.e.
4 photoelectrons), the energy threshold, Eth, is given by,
2
2
1
 SNR 
 H   3m 
Eth  3.4 1019 eV  

 
 

2
 2  cos   500km   D 
2
 0.1  1 sec 



  ( R)  T 
(11)
2.3 Formula for Photon Detectors
The pixel size of the photon detector, detector(mm), is given by,
d etector  f stop  D  
 f stop  D   

 5.2(mm)  
  
 1  3m  0.1 
(12)
Here, fstop is the f-number of the mirror. The overall dimension of the photon detector area, Detector,
is given as a function of the FOV by,
Detector  f stop  D  FOV
 f stop  D  FOV 

 3.1(m)  

 
 1  3m  60 
(13)
-8-
Lastly, the number of pixels of photon detector is simply given by,
  FOV 
# Pixels   

4   
2

 FOV   0.1 


 2.8  10 5  
  
 60    
2
(14)
2
2.4 Angular Resolution
Angular resolution is one of the most important factors in Astronomy to identify point sources. In
case of the Owl-AirWatch, it is given by the accuracy of track reconstruction. Roughly speaking,
it is related to the accuracy of the track width measurement, width, divided by the observed track

length, Ltotal. (i.e.    width ).
Ltotal
What is width? In the Owl-AirWatch experiment, it is determined by the Field of View of the
photon-detector pixel size in one direction, and by the clock speed of the Flash ADC in the other
direction. These discrete measurements give a three-dimensional box in space, given by L 
L  LT. Therefore a typical value of width is given by,
 width 
Lwidth
,
2.3
Lwidth  2L  LT .
2
2
In addition, it is necessary to take into account the multiple data samples. With #hits, the
resolution is expected to improve by a factor of # hits . However the hits near the center of the
shower track make little contribution to the angular resolution. Thus it is reasonable to reduce the
effective number of hits by 2/3 or so. Finally, the angular resolution is given by,
 
Lwidth
Ltotal  2.3  # hits  0.7
Ltotal
.
Lwidth
By combining the above three equations, one can derive the following formula for the angular
resolution.
Here, the number of hits is roughly given by # hits 
 2L 2  L 2

T
   0.52rad  
Ltotal

3
2



(15)
-9-
10km
, where  is the zenith angle of the
cos 
Cosmic ray shower. Under the condition of LT  L (which can be satisfied by speeding up
the FADC clock), Equation (15) becomes,
As the final step, Ltotal can be replaced by Ltotal 
3

 L  2
   1.3  cos     
 1km 

 cos   H  
 1. 2   

 
 cos  500km  0.1



(16)
3
2
If our design goal is  = 1o for H = 500km (for small angles  and ), the requirement on the
detector parameters would be L < 840m and  < 0.089o.
2.5 Covered Area and Aperture
The whole area viewed by the detector can be expressed as a function of the Field of View of the
whole detectors, FOV, as shown below.
Area    H  tan FOV 2
2
 H   tan FOV 2  
 2.6  10 (km )  
 


 500km   tan 30

2
5
2
2
(17)
The aperture is the Area times the solid angle, where the typical solid angle of a fluorescence
detector such as Fly's Eye is approximately  steradians. Thus the aperture is given by,
Aperture    Area(km2 str ) .
A typical duty factor of the ground-based fluorescence experiment is an order of 10%, because it
is only operational during the moon-less, clear night. Since a space based experiment overviews
the sky underneath, wherever it goes (including over the bright cities or cloudy sky), the duty
factor might be somewhat lower. But let's assume 10% duty factor, the effective aperture
becomes,
Apertureeff  0.1  Area
 0.32  Area
(18)
 Duty _ factor  Area 
 3.2  10 5 (km 2 str )  
 6 2 
0.10

 10 km 
- 10 -
One of the prime goals of the OWL-AirWatch experiment is the detection of ultra high-energy
neutrino interactions. The target volume for neutrino interactions is, in fact, enormous. It is
convenient to express the weight of the atmosphere in terms of water equivalent volume in unit of
km3, because people are taking about 1km3 size detector such as the ICECUBE for the next
generation neutrino astronomy. Since the atmosphere is equivalent to a 10m thick layer of water,
the target volume for neutrino, Volume, and effective volume including the duty factor, Volumeeff,
is given by,
Volume  0.01 Area(km3 )
(19)
Volumeeff  1  10 3  Area(km3 )
 Duty _ factor  Area 
 100(km3 )  
 6 2 
0.10

 10 km 
(20)
2.6 Summary
In summary, Table.2 below shows various scaling laws with the notation in Table.1. These
formulas will be used for detector optimization in later sections.
3. Expected Performance of Current Design
3.1 Basic Principles of Detector Optimization
There are three basic physical quantities of prime interest:
1) Effective Aperture (after the correction of duty factor) should be at least ten times larger
than the Auger or the Telescope Array. Since they have about 7,000km2str, the goal of the
OWL-AirWatch should be 70,000 km2str or more.
2) Energy threshold should be well below 1020eV. It is desirable to keep it lower than the GZK
cut off energy of ~31019eV. Such a low energy threshold ensures the detection of the
shoulder of the power spectrum around the cut-off energy, which provides a decisive
calibration point of the absolute energy scale.
3) Angular resolution should be better than 1o, as 1o is the design value of the Auger
experiment.
The default values in Table.1 and 2 have been chosen by considering these requirements and their
technical feasibility as of today. As a matter of fact, if one takes the default values as listed, the
Effective Aperture is 83,000km2str, the Energy threshold is 3.41019eV and the Angular
resolution is 1.2o. Although the angular resolution is rather poor and the threshold is a bit high,
the default values in Table 2 more or less satisfy our design goals.
- 11 -
Parameter Description
Photo-detector Pixel
Dimension
Dimension of Photodetector Total Area
Formulas
 f stop  D   

d etector  5.2(mm)  
  
1

 3m  0.1 
 f stop  D  FOV 

Detector  3.1(m)  

 
 1  3m  60 
Track Length in T
(= cT )
Track Length in 
L  870(m) 
Shower Max Signal
at E = 1020 eV in T
 E  500km   D 
S T  11.8( pe)  cos 2    20 
 

 10 eV  H   3m 
2
Total Number of Pixels



2

 FOV   0.1

# Pixels  2.8  10  



 60   
 T 

LT  300(m)  
 1 sec 
5
1  H   



cos   500km  0.1 
2
2
  ( R)  T 



 0.1  1 sec 
cos  E  500km  D    ( R)   
 34.2( pe) 



 


sin   10 20 eV  H  3m   0.1  0.1 
2
Shower Max Signal
at E = 1020 eV in 
S 
Dark Sky Noise
 D     T   
  
BT  1.7( pe)  
 

 3m   0.2  1 sec  0.1 
SNR in T (for B << S)
 500km  D   E   ( R)  T 

SNRT  3.4  cos  

  20 

 H  3m   10 eV  0.1  1 sec 
Energy Threshold
1
 SNR 
 H   3m 
Eth  3.4  10 eV  

 
 

2
 2  cos   500km   D 
Angular Resolution
(for LT << L)
 cos   H    2
   1.2   

  
 cos   500km  0.1 
2
2
2
2
19
2
 0.1  1 sec 



  ( R)  T 
3
Area viewed by the
Whole Detector
Effective Aperture
Effective Neutrino
Target Volume
 H   tan FOV 2 
Area  2.6  10 (km )  
 


 500km   tan 30

 Duty _ factor  Area 
Apertureeff  3.2  10 5 (km 2 str )  
 6 2 
0.10

 10 km 
 Duty _ factor  Area 
Volumeeff  100(km3 )  
 6 2 
0.10

 10 km 
2
5
2
Table.2 Summary of useful scaling laws
- 12 -
2
3.2 Delta Launch Vehicles
Unlike ground base experiments, a space-based experiment must be launched either by a rocket or
a Space Shuttle. This fact severely constrains the size, weight as well as power consumption.
Thus before we discuss the current baseline design of the OWL-AirWatch detector, I would like
to review a possible launch vehicle and its constraint.
Standard Launch Vehicles as of today are Delta Rocket series by the Boeing Company. Among
them, recently developed Delta III is the largest and the most advanced one with reasonable
launch cost. Its technical information is available in [Delta III]. Table 3 from this document
shown below is typical Mission Capabilities. It can launch up to 8,292kg of a spacecraft to LowEarth Orbit (LEO), or 3,810kg to Geosynchronous Transfer Orbit (GTO).
Figure 3 shows the Delta III spacecraft envelope with 4.0-m (13.1-ft)-diameter fairing for twostage configuration. As is shown here, the largest available diameter for optics is 3.75m without
deployable (or inflatable) mechanism.
Table.3 Typical Delta III Mission capabilities (from [Delta III] )
- 13 -
Figure.3 Delta III Spacecraft Envelope, 4.0-m (13.1-ft)-dia Fairing, Two-Stage Configuration
- 14 -
Figure.4 Delta III Vehicle, Two-Stage Circular Orbit Altitude Capability
Figure. 5 Typical Delta III LEO Mission Ground Trace
- 15 -
Figure 4 shows relation between the circular orbit altitude (in km) and the spacecraft mass (in kg).
Even with one-burn direct insertion method, more than 5000kg can be launched to the low orbit
of 500km. With two-burn Hohmann Transfer, the Delta III is capable of launching more than
7000kg.
Lastly, Figure 5 shows a typical ground trace of the Low-Earth Orbit (LEO) mission, after
launched from the Cape Canaveral Air Station in Florida. The latitude stays within +-30 degree.
In the following section, two existing designs, the AirWatch and the Multi-OWL will be analyzed
by applying the scaling laws to see how well these designs perform.
3.3 AirWatch Baseline Design
One of the working examples is the baseline design currently under consideration jointly by the
Italian, AirWatch group and the US OWL group. The detector concept is shown in Figure 6.
Their parameters are listed in Table.4.
The optics system is currently under extensive optimization by David Lamb and others at
University of Alabama, Huntsville [Lamb]. According to their latest study, the optics consists of
double Fresnel lenses of 3.5m diameters and a 2.5m-diameter entrance pupil. The F-stop is 1.3,
which gives the focal plane diameter of 3.36m. We are still trying to make the diameter of the
entrance pupil as large as possible, and the f-stop number as small as possible, while maintaining
the reasonable image size (i.e. 0.1o) for the wavelength of our interest. A possibility of flat Fresnel
lenses and flat focal plane has been considered as well. However such simplification appears to
degrade the image quality to unacceptable range, especially with a small f-stop number.
One can see that the current dimensions are carefully chosen from a practical point of view. With
light attenuation factor of 0.5, the expected signal at E = 1020eV is 5.6 photoelectrons per FADC
gate width of 833nsec. Actually the traveling time of air showers on each pixel is longer than the
L
974m
FADC gate and given by Ttotal   
 3.2 sec
c
300m /  sec
If one takes this in the above calculation, the signal in one pixel at shower maximum becomes 22
photoelectrons, which is clearly detectable without any doubt.
The angular resolution is poorer than we wish. This is primary due to the large pixel size 7mm
that is given by the dimension of the existing photon detector (shown later). In any case, the
optics simulation shows that it is difficult to achieve a significantly smaller spot size than this
value due to various constraints. On the other hand, the number of pixels is reduced to 200k,
which helps to reduce the weight and power consumption of the focal plane.
The effective aperture is 83,000km2str assuming 10% duty factor. Our intention is not to
compromise this size, as it is our primary interest to cover at least ten times larger area than the
Auger or the Telescope Array.
- 16 -
Deployable Light Shield
4m
Fresnel Lens
Entrance Pupil
Shutter
Calibration Light
Source
4m
Fresnel Lens
Focal Plane
Battery/Electronics etc.
etc.
Payload Attach
Fitting
etc.
Deployable Solar Panels
etc.
3.5m
Figure.4 Conceptual design of the baseline OWL-AirWatch detector
- 17 -
Notation
AirWatch
Double Fresnel
H
500km
FOV
60o
Diameter of Entrance Pupil
D
2.5m
Diameter of Fresnel Lens
-
3.5m
fstop
1.3

0.2
cos 2    R 
0.5
Integration Time
T
833nsec
Field of View of Photo-detector Pixel

0.11o
Photo-detector Pixel Dimension
detector
7.0mm
Dimension of Photo-detector Total Area
Detector
3.3m
Total Number of Pixels
#Pixels
200K
Track Length in  (at  = 90o)
L
974m
Track Length in T (= cT )
LT
250m
Shower Max Signal at E = 1020eV (in T)
ST
5.6pe
Shower Max Signal at E = 1020eV (in )
S
22pe
Night Sky Noise (in T)
BT
1.3pe
SNRT
2.4
Energy Threshold (for SNR = 2)
Eth
7.11019eV
Angular resolution

1.4o
Area
2.6105km2
Effective Aperture
Apertureeff
8.3104km2str
Effective Neutrino Target Volume
Volumeeff
260km3
Parameter Description
Satellite Height
Total Field of View
F-stop of Mirror
Quantum Efficiency
Light Attenuation Factor
Signal to Noise Ratio at E = 1020eV (in T)
Area Viewed by the Whole Detector
Table.4 Baseline Parameters of AirWatch Detector under consideration.
- 18 -
3.4 Multi-OWL Design
An idea of Multiple OWLs has been proposed by Y. Takahashi as the way to expand the Field of
View by another order of magnitude from the Single-OWL [Takahashi]. It would consist of six
OWL-AirWatch detectors at the orbital height of ~1000km as shown below. The axis of each
detector would be tilted by 30o from the zenith. As a result, the overall Field of View of such a
detector array would expand to 120o.
As shown in Figure 6, thanks to the finite size of the Earth, the Multi-OWL would cover the
entire horizon seen by the detector. As a result, the area coverage and the aperture become
remarkably large.
On the other hand, there are several shortfalls.
1) Unless much larger mirrors are deployed, due to its high altitude as well as 1/R2 factor,
Energy threshold would become too high.
2) Events near the horizon would be too far, and the time stamps of (t, x, y, z = 0) no longer
provide strong constraint of the event geometry due to a poor measurement of x or y. In other
words, the situation becomes similar to the ground-based detector.
3) Because of large 1/R2 factor, event quality (such as energy and angular resolutions) would
change depending on R.
To compensate for these shortfalls, the following modifications over the single OWL/AirWatch
are required.
1) To maintain the angular resolution, the pixel size must be reduced by more than a factor of
two.
2) Diameter of mirror needs to be enlarged, say, to the order of 5m.
3) It is highly desirable to develop new type of photon detectors with high quantum efficiency
as high as 50% (as described in Section 6.3).
For the three dimensional reconstruction of neutrino events, we might want to arrange the six
OWLs in stereo view as shown Figure 7.
- 19 -
Multi-OWL Detector
H=1000km
60o
6680km
R=6380km
30o
30o
Earth
Side View
3340km
Top View
Figure.6 Concept of Multi-OWL consisting of six OWLs at 1000km high.
- 20 -
Multi-OWL Detector
60o
H=1000km
2000km
R=6380km
Earth
Side View
3500km
2000km
Top View
Figure.7 A possible stereo view arrangement of six OWLs at 1000km heigh
- 21 -
Parameter Description
Notation
Number of Detectors
Satellite Height
Total Field of View
Effective Diameter of Mirror
F-stop of Mirror
Quantum Efficiency
Light Attenuation Factor
Integration Time
Field of View of Photo-detector Pixel
Photo-detector Pixel Dimension
Dimension of Photo-detector Total Area
Total Number of Pixels
Track Length in  (at  = 90o)
Track length in T (= cT )
Shower Max Signal at E = 1020 eV (in T)
Night Sky Noise (in T)
Energy Threshold (for SNR = 2)
Angular resolution
Area Viewed by the Whole Detector
Duty Factor
Effective Aperture
Effective Neutrino Target Volume (Water eq.)
N
H
FOV
D
fstop
Single-OWL
Low
500km
1000km
Area
Duty_factor
Apertureeff
Volumeeff
60o
60
2.5m
1.3
0.2
0.5
833nsec
0.11o
7mm
3.3m
200K
974m
1.95km
250m
5.6pe
1.8pe
1.3pe
19
7.110 eV
1.4o
2.6105km2
2.81020eV
3.8o
1.0106km2
10%
8.310 km str 3.3105km2str
260km3
1040km3
4
2
120o
5m ( 6)
1.0
0.5
0.05 ~ 0.5
833nsec
0.036o
3.2mm
5m ( 6)
15M
630m ~ 1.3km
250m
7 ~ 17pe
0.2 ~ 17pe
1.3pe
19
2~510 eV
2~201019eV
0.74o
0.74 ~ 5.6o
5.5106km2
2.5107km2
10%
6
2
1.810 km str
8.0106km2str
5,500 km3
25,000 km3
Table.5 Baseline Parameters of proposed Single- and Multi-OWL detector.
- 22 -
Multi-OWLs
Mono
6
1000km
o

cos    R 
T


Multi-OWLs
Stereo
1
2
detector
Detector
#Pixels
L
LT
ST
B
Eth
Single-OWL
High
Table.5 is the baseline parameter of Single- and Multi-OWLs with various configurations. For
Single-OWL, I assumed two different heights: 500km and 1,000km. For Multi-OWLs, I assumed
two different configurations shown in Figure.6 and Figure.7. At the same time, the mirror
diameter and the quantum efficiency are increased to 5m and 50% respectively, hoping that such
technology will become feasible when time comes. As a result, the energy thresholds are
maintained to be around 1020eV, and angular resolution is maintained to be an order of 1o. The
aperture is progressively improved step by step.
3.5 Comparison with Other Previous/Ongoing Experiments.
Finally we are ready to compare the OWL-AirWatch experiment with other similar experiments.
Table. 6 shows such comparison between past, ongoing and future experiments. Several remarks
can be made:
1) Even at low altitude of 500km with single detector, the OWL has more than ten times larger
effective aperture than the Auger or Telescope Array.
2) In each step of OWL, the aperture is enlarged by a factor of five. Monocular operation of the
Multi-OWL at 1,000km provides the effective aperture of 8,000,000km2str; 1,000 times larger
than the Auger (or TA) and 100 times larger than the single OWL at 500km. This clearly
demonstrates that the OWL is the open-ended project with many possibilities and
improvements.
3) The energy threshold of the OWL is several times higher than that of the Auger or TA. The
Auger and TA will systematically study the Energy spectrum just below GZK cut off and
around the cut off. Thus the OWL is optimized to study the spectrum above the GZK cut off,
after the super GZK events are established by the Auger and TA.
4) The angular resolution of the OWL is comparable to other experiments, thanks to the small
pixel size which effectively views the same segment of atmosphere; an order of 1km square.
In summary, it is safe to say that the OWL is a well-thought, next-generation experiment after the
Auger and TA with an order of magnitude larger aperture.
- 23 -
Experiments
Method
Notation
Covered
Area
Duty
Factor
Area
Unit
Effective
Aperture
- Apertureeff
Effective Energy
Neutrino
ThreVolume
shold
Volumeeff
Eth
Angular
Resolution
Cost
Start
Year

-
-
km2
%
km2str
km3
eV
degree
$M
-
Fly's Eye
Fluorescence
300
10
100
0.2
~1017
~0.5o
~0.5
1986
AGASA
Ground
100
100
250
0.1
~1017
~1o
~1
1992
HiRes
Fluorescence
3,000
10
700
1.0
~1018
~0.5o
~5
1999
Auger (one-site)
Ground
3,000
100
7,000
3.0
~1019
~1o
2004
~50
Auger (hybrid)
Hybrid
Telescope Array
Fluorescence
ICECUBE
Cherenkov
3,000
10
700
0.3
~1019
~0.5o
21,000
10
6,000
21.0
~1019
~0.5o
~80
~2005
-
100
-
1.0
~1012
~1o
~80
~2005
2004
(1019)
OWL
Single, Low
Fluorescence
260,000
10
83,000
260.0
~7
~1.4o
Single, High
Fluorescence
1,040,000
10
330,000
1,040.0
~28
~3.8o
Multi, Stereo
Fluorescence
5,500,000
10
1,800,000
5,500.0
2~5
~0.7o
~2006
~200
~2008
?
~1000
Multi, Mono
Fluorescence
25,000,000
10
8,000,000
25,000.0
2~20
0.7~6o
Table.6 Comparison between various past, ongoing and future experiments
- 24 -
?
4. Requirement of the Photo-Detectors
Developing photo-detectors of the OWL-AirWatch is technically one of the most challenging
projects of its own. It is basically a mega-pixel devise that covers several meter-squares of the
area. It must have single photoelectron sensitivity as well. This section goes through the basic
specifications in some details.
4.1 Physical Dimensions
The pixel size determines the sampling rate of cosmic-ray showers. As is shown in the Table.2, it
is primarily related to the angular resolution of the shower reconstruction. If one requires angular
resolution of  = 1o, from Equation (16), the requirement on the field of view of each pixel
becomes,
  0.11 
cos   500km    



cos   H  1 
(21)
By inserting this into Equation (10), one can obtain the formula for the optimal pixel size.
d etector  6.0(mm) 
cos  f stop  D  500km    



 
cos   1  3m  H  1 
(22)
The sampling rate is also important for the reconstruction of the shower profile which is related to
the energy resolution and determination of the Shower Maximum position. An order of L =1km
sampling rate is desirable, and it actually gives the similar requirement as Equation (19), shown
below.
 500km  L 
  0.11  cos  


 H  1km 
(23)
In addition to above optical consideration, the following mechanical specification are of great
importance.
1) Minimum dead space. For a continuous field of view, dead space between pixels as well as
photo-detector modules must be minimized. Generally speaking, dead space between pixels is
easy to reduce, but between modules is difficult due to mechanical structure. Less than 10%
area is desirable, but 20% would be acceptable, assuming one module consists of large
number (>64) of pixels.
- 25 -
2) The weight is a major concern in space. Assuming the total weight of the detector is an order
of three tons, and allowed weight for photon detector is less than 10% of total weight (i.e. less
than 300kg) , the weight per pixel should be less than 1.5gram. To be conservative, less than
1gram per pixel seems more desirable.
3) The focal plane is likely to be curved, either concave or convex depending on optics. Its
curvature is not severe, but the photo-detector unit must be flexible enough to follow the
curvature.
4.2 Signal Sensitivity
In the ideal case, Poisson Statistics of the number of observed photons governs the Signal to
Noise Ratio (SNR) of photo-detectors.
SNR 
S


S pe
S pe
 S pe .
(24)
In reality, however, several modifications to this equation are need.
1) We must consider the Poisson statistics of the collected photoelectrons. The number of
photo-electrons, Spe, is given by S pe    C ol  S  , where  is the quantum efficiency, Col is
the collection efficiency for photoelectrons and S is the number of incident photons.
2) The Poisson statistics is further modified by the Excess Noise Factor (ENF). ENF is defined
as the increase of the 2. (i.e. output2 = ENFintput2). In case of photon detectors, the ENF is
given by the formula below, where n stands for the multiplication factor of the n'th dynode.
As shown later, for typical PMTs, n is 5~10, while it is about two for the fine mesh and
MCP. As for solid-state device, the photo diode has ENF of one, but the APD has two or
greater than two.
1
1
1
ENF  1  

(25)
1 1  2
 1   2  n
3) Lastly, there is an additional contribution from the Equivalent Noise Charge (ENC). A
typical amplifier has about 1000e- of ENC. This noise must be normalized by the gain of the
photo-detector, G so that it can be compared in the unit of photoelectrons.
Taking all these factors into account, Equation (9) and (25) must be combined and modified as
follows.
- 26 -
 2   signal2   dark 2   ENC 2
 signal  ENF   C ol  S 
(26)
 dark  ENF   C ol  B
 ENC 
ENC
G
Here B is the incident photons caused by the dark sky noise.
By substituting (26) into (24), SNR becomes,
SNR 

S pe
 signal   dark 2   ENC 2
2
(27)
  Col  S
ENF  C ol ( S  B )  ( ENC G ) 2
In physics experiments, the energy resolution is commonly used instead of the SNR, and it is
given by,

1


E SNR
ENF  C ol ( S  B )  ( ENC G ) 2
  Col  S
(28)
From Equation (27) and (28), one can conclude the following.
1) In order for (ENC/G)2 to be negligible, the intrinsic gain of the photo-detector (G) must be
much larger than the readout noise (ENC). A typical ENC is an order of 1,000e- for fast
integration (<100nsec), and about 300e- for slow integration (~1sec). Thus the Gain must be
at least 3,000. Higher than 10,000 is more desirable to be conservative.
2) The dark sky noise (B) must be significantly smaller than the signal (S). To be safe, let's
assume that B < S /3. From Equation (3) and (8), this condition can be expressed by,
 500km   E   ( R) 
  0.09   cos   
   20 

 H   10 eV  0.5 
(29)
It is interesting to see that Equation (29) actually coincides with the requirement from the angular
resolution of 1o given in Equation (21) and (23). Once the above conditions, 1) and 2) are
satisfied, Equation (27) can be simplified to,
SNR 
  Col  S
(30)
ENF
- 27 -
At this point, to achieve superior sensitivity, the following becomes very important.
3) Quantum Efficiency () and Collection Efficiency (Col) must be as high as possible.
Generally speaking, Quantum efficiency is the single most important parameter of the signal
detection in any apppication. Although the higher the better, 25% is the practical number
based on conventional bi-alkali photo cathode. With a solid state photo-cathode, it is expected
to be improved to 50% level in the near future.
4) Excess Noise Factor (ENF) must be as close as possible to unity. From our previous
experience, to clearly observe single photoelectron peaks, it must be smaller than 1.1.
In addition, there are several requirements so that inherent sensitivity is not compromised.
1) The window glass must be transparent to the UV fluorescence light from Nitrogen excitation,
whose wavelengths are 337, 357 and 397nm. 90% transmittance is desirable and 80% is the
minimum requirement.
2) The intrinsic dark pulse rate should be much less than the dark sky noise rate, which is
typically 1MHz. The order of 10kHz would be good.
3) Pixel to pixel uniformity of photo-cathode and photoelectron collection efficiency must be
reasonably good; fluctuation less than 10% is desirable, and 20% is the maximum tolerance.
4) Anode uniformity (i.e. Gain uniformity) on the other hand is less important, since the detector
can count the number of photoelectrons, as far as the single photoelectron level is calibrated
pixel by pixel. Less than 20% non-uniformity is desirable, but up to 50% can be tolerated in
our past experience (such as RICH detector at HERA-B).
5) Cross talk between pixels should be reasonably small. Less than 2% is desirable, but 5% can
be tolerated.
6) Only modest dynamic range (pulse linearity) is required. The largest signal with a ~1sec gate
would be 1,000 photoelectrons or so. With another order of magnitude of safety factor,
linearity up to 10,000 photoelectrons level desirable.
4.3 Other properties
In addition to the above specifications, the following items need to be considered.
1) As far as response speed such as rise time, fall time and pulse width are concerned, the
requirement comes from the fact that the readout electronics requires photon counting. A
study by the Italian group shows that shorter than 10nsec pulse width is required [Catalano].
Typical time response of photomultipliers satisfies this requirement.
- 28 -
2) Power consumption by the HV power supply is a major concern specific to space based
experiments. Assuming total power of 1KW in whole detector and a 10% allowance for
photon detectors, power budget per pixel is
1kW  0.1
 500 W .
2  10 5
3) After-pulses should not contribute to the signal level. Lass than 1% are desirable.
4) Long-term stability for 10 years operation is required. Since the expected dark sky noise level
is of order 2MHz, the cathode dark current is
1.6  10 19 C  2  10 6 / sec  0.32 pA / pixel .
Assuming Gain of 106, the anode dark current is given by,
 G 
I dark  320nA / pixel   6 
 10 
The dark pulse rate and the dark current of photon-detector itself should be kept much lower
than this level. Assuming 10 years of operation with 20% duty factor, the accumulated charge
per pixel is,
 T
 Duty _ factor  G 

Q  2Coulomb / pixel  
 6  .
0.2
 10 
 10 years 
5) The total cost of photon detectors must be reasonable. Assuming $10M total is acceptable, the
cost per pixel should be less than $50.
Based on the argument above, specifications for photon detectors can be summarized as shown in
Table 6.
- 29 -
Parameter Description
Notation
Pixel Size
Total Number of Pixels
Total Photo-Cathode Area
Physical Dimension of One unit
Window Transparency at 350nm
detector
#Pixels
Acathode
Dunit
-
Weight per Pixel
Dead Space
W
Quantum Efficiency
Cathode Non-uniformity
Anode Non-uniformity
Photo-electron Collection Efficiency
Cross Talk between Pixels
Excess Noise Factor
Intrinsic Gain

Specifications
Minimum
Ideal
~ 6mm
~ 2  105
~ 9m2
> 2.5cm
> 5cm
> 80%
> 90%
< 1.5gram
< 20%
< 1gram
< 10%
Col
ENF
G
> 20%
< 20%
< 50%
> 80%
< 5%
< 1.1
> 3,000
> 50%
< 10%
< 20%
> 90%
< 2%
1.0
> 105
Dynamic Range
Equivalent Noise Charge
ENC
~1,000
< 1000e-
~10,000
< 300e-
Transit Time Spread for Single Photo-electron
Rise Time and Fall Time
Pulse Width
Readout Speed per Detector Unit (16~64ch)
Intrinsic Dark Current per Pixel
Intrinsic Dark Pulse Rate per Pixel
After Pulse
TTS
Idark
-
< 1nsec
< 5nsec
< 10nsec
< 0.5nsec
< 2nsec
< 5nsec
-
Life Time
Power Consumption of HV Power Supply per Pixel
Cost per Pixel
-
<1sec
< 1nA
< 10kHz
< 1%
>10 Years
< 500W
< $50
Table.7 Specifications for the OWL-AirWatch photo-detectors.
- 30 -
< 200W
< $20
5. Comparison of Existing Photo-Detectors
Before we go into specific photon detector candidates, it is probably a good idea to systematically
survey various types of existing and recently developed photon detectors. Detailed discussion can
be found by my review talks in several detector conferences listed in [Arisaka.1, Arisaka.2].
In table 8, I have listed all possible detector candidates, together with required specifications in
the OWL-AirWatch for convenience. Photon detectors can be categorized in three groups:
Vacuum based, Solid State and Hybrid. The parameters of candidates are typical values and not
necessary optimized for our specific application. A good value is highlighted by Bold, and a fatal
value is highlighted by Underline in the Table 8. At the end of the table, each detector is graded
by A to F. A detector with at least one fatal value receives F grade.
5.1 Vacuum Based Devices
The most commonly used vacuum based detector is a photomultiplier, widely used by many
applications in high energy or astro-particle experiments. Major characteristics are high intrinsic
gain with single photon count capability, high speed, but poor quantum efficiency. Variety of
dynode structures for position sensitivity is available such as Metal Channel Plate, Micro Channel
Plate (MCP) and Fine Mesh. By adopting these dynodes, multi pixel PMT with pixel size from
2mm to 1cm has become commercially available in 90's.
5.2 Solid State Devices
Another branch of photon detectors is a solid-state device mainly made by Silicon PIN junction. It
is extremely linear with high quantum efficiency, but low (or no) intrinsic gain and rather slow
time response (per unit sensitive area.) It has been extensively used for energy measurement in
calorimeters where light intensity is high enough. Pixelization is trivial on a silicon wafer and a
CCD is the best example in this category.
There have been several attempts to improve gain as shown in table; Among them, Avalanche
Photon Diode (APD), Metal Resistive Semiconductor (MRS) and Visible Light Photon Counter
(VLPC) are listed here. Unfortunately, there is still no suitable candidate for our purpose, either
because of slow readout speed, low gain or small pixel size.
5.3 Hybrid Devices
Hybrid devices combine vacuum and solid state in one system. Photoelectrons are emitted into
vacuum from photo cathode, and after acceleration by 10kV or so, they bombard a solid state
device. Depending the type of solid-state device, it is named as HPD (Hybrid Photo dynode),
HAPD (Hybrid Avalanche Photon Diode), ISPA (Imaging Silicon Pixel Array) or EBCCD
(Electron Bombarded CCD).
- 31 -
16~64
<256
64
~1M
~2.5
~2.5
5
~30
25
40
70
>1000
1~10
~.01
.5~5
~1
~1
<256
~1M
<16
<16
<16
~10
~1
~2
~2
~2
2~10
2~10
.1~2
.01~.1
3
<256
<256
~100K
~1M
256
~2
~2
~30
~2
5
>20
>50
%
Col
>80
>90
%
54
~50
10
~30
~20
~20
~20
~20
<10
<10
<10
<10
<10
~10
~10
~10
~10
~10
40
40
100
100
50
~40
~40
~30
~30
~10

>5
>10
-
ENF
<1.1
1.0
-
G
>3000
>105
-
<20
<10
%
80
70
80
90
10
10
-
1.2
1.5
1.2
1.5
~106
~106
~106
103 ~ 6
~50
~20
~50
~10
~80
~80
~80
~60
~60
100
100
100
10
100
1
1
2
-
1.0
1.0
>2
1.0
1.0
1
1
<100
104
104
<10
<10
~10
~10
~10
~20
~20
~20
~20
~50
90
90
90
90
>90
1000
1000
1000
1000
1000
1.0
1.0
1.0
1.0
1.0
1000
105
1000
1000
~105
<10
~10
<10
<10
<10
Table.8 Comparison of various photo-detectors (Bold: Good, Underline: Fatal)
- 32 -
Anode Nonuniformity
2~4
2~10
5.6
.01~1
~6
~6
mm

Intrinsic Gain
<20
<10
%
Excess Noise
Factor
W
g
Mortification
factor ()
Wight per
Unit
>2.5
>5
cm
detector
Quantum
Efficiency at
300nm
Photoelectron
Collection
Efficiency
Dimension
of one Unit
Dead Space
#Pixel per
Unit
>16
>64
Pixel Size
Notation
Specs (Minimum)
Spec (Ideal)
Unit
Vacuum Device
Metal Cannel PMT
MCP PMT
Flat Panel PMT
Image Intensifier
Solid State Device
PIN Photo Diode
CCD
APD
MRS
VLPC
Hybrid Device
HPD
HAPD
ISPA
EBCCD
Katsushi's Dream
Final Grade
Cost per
Pixel
Readout
Time per
Unit
Rise/Fall
Time
Transit Time
Spread
Signal to Noise Ratio
(S = #Incident Photons)
Remarks
Notation
Specs (Minimum)
SNR
S 6.0
TTS
<1
<5
<1
<50
A-
Specs (Ideal)
Unit
Vacuum Device
Metal Channel PMT
S 2.0
-
<0.5
nsec
<2
nsec
<1
sec
<20
$
A+
-
S 6.0
0.3
1
<1
40
A_
Most practical solution as of today.
MCP PMT
S 8.6
0.1
0.5
<1
50
B
Poor resolution than Metal Channel.
Flat Panel PMT
S 6.0
0.3
1
<1
?
A
Best practical solution of tomorrow.
Image Intensifier
Solid State Device
PIN Photo Diode
S 8.3
-
-
>1000
5
F
Pixel too small, bulky, slow readout.
S 1.3 ~ 1000000 / S 
2
5
<1
10
F
No Gain.
CCD
S 1.3 ~ 1000 / S 
-
-
>1000
<1
F
Pixel too small, no Gain, slow readout.
APD
S 2.5 ~ 100 / S 
2
5
<1
50
C
Gain too low.
MRS
S 16
2
5
<1
?
F
Poor photoelectron collection, too noisy.
VLPC
Hybrid Device
HPD
S 1.6
2
5
<1
50
C
Pixel a bit too small, Liquid He, costly.
S 5.6 ~ 10 / S 
2
5
<1
20
B
Gain a bit too low.
HAPD
S 5.6
2
5
<1
50
A
Excellent device, but only single pixel.
ISPA
S 5.6
2
5
<1
?
B+
Bulky, could be improved. Stay tuned.
EBCCD
S 5.6
-
-
>1000
?
F
Katsushi's Dream
S 2.0
2
5
<1
??
A+
- 33 -
Pixel too small, low readout
Ideal device but does not exist yet!
Advantages of such devices are their conceptual simplicity, uniform response with large dynamic
range, and flexibility for pixelization. On the other hand, it still inherits poor quantum efficiency
as a vacuum device. Every 3.6V of acceleration in vacuum yields another electron-hole pair, thus
10kV acceleration produces an intrinsic gain of ~ 3000. With additional gain of 10 ~ 100 by APD,
HAPD can have high gain of ~ 105, enough for single photon counting without any amplifier.
Recently, the LHC-CMS experiment has adopted multi-pixel HPD developed by DEP for a
hadron calorimeter [Cushman]. The LHC-b experiment is also considering a large-area multipixel HPD for RICH (Ring Imaging Cherenkov). Such a device would be very attractive for
OWL-AirWatch, if it could reduce the dead-area.
6. Candidate Photo-Detectors
6.1 Metal-channel PMT
Based on the argument above, the best commercial device as of today is chosen to be the
Hamamatsu Metal-channel Plate PMT, R7600 series. This is a metal packaged, square PMT with
outer dimension of 25.7mm x 25.7mm. Various multi-pixel versions are available; either one, four,
16 or 64. For our purpose, the 16-pixels version, called R7600-M16 fits all our minimum
specifications.
The cross sectional view of the front face of this device is given in Figure. 8.
Anode Pixel
Photo Cathode
4.1mm
17.5mm
22mm
0.5mm
4mm
4.1mm
Figure. 8 Front view of Hamamatsu R7600-M16.
- 34 -
25.7mm
Figure. 9 Mechanical structure and Catalog Specifications of Hamamatsu R7600-M16
- 35 -
Figure. 10 Quantum Efficiency, Gain and Time Response of Hamamatsu R7600-M16
- 36 -
Figure. 11 Linearity, Uniformity and Cross-talk of Hamamatsu R7600-M16. The cross talk in
Figure 7 above is measured by shining the central pixel at a level of 100%.
- 37 -
To show typical characteristics of this PMT, specifications and various plots from the catalog are
shown in Figure. 9 ~ 11. They are taken from the catalog of [H6568] which is the assembled
version of R5900-M16. (R5900 is the previous model of R7600 with larger outer dimension, thus
larger dead-space.)
A major problem of this type of device is a large dead area. In the R7600 series, the effective
4mm  4mm  16
 0.39 . In reality, 0.5mm of the dead space
photo-cathode area is given by
25.7 mm  25.7 mm
between pixels is not dead but gives signal into adjacent pixels. By taking this into account, the
2
 17.5mm 
more practical effective area is given by 
  0.46 , still less than a half.
 25.7mm 
To avoid the dead space, segmented Winston-cone type light collector shown in Figure 12 can be
developed. Preliminary Monte Carlo Simulation shows that an order of 50-60% light collection
efficiency can be achieved with a standard reflector of 90% reflectivity [Kimura]. The entrance
of the light corrector would be covered by UV band-path filter to reject visible-IR part of dark sky
noise.
Surface of
R7600-M16
4mm
2.57cm
Entrance Surface
of Light Guide
7mm
2.8cm
2cm
UV filter
Light
Guide
2cm
R7600-M16
PMT
2.57cm
Figure. 12 R7600-M16 PMT with light collector to avoid dead area.
- 38 -
Another challenge is how to cover a curved focal plane by the flat surface of PMTs. As the spot
size is of the same order as pixel size due to chromatic aberration, it is conceivable to cover the
focal plane by segmented flat panels as shown in Figure 13 below. Here the panel size is 67.2cm,
corresponding to 96 pixels (or 24 PMTs). Although more detailed optical and mechanical studies
are required, this approach appears feasible. The Super-Panel can be sub-divided into four SubPanels of 48 x 48 pixels that correspond to one unit of Trigger/electronics design by the Italian
group [Catalano].
Super-Panel
(96x96 pixel)
Sub-Panel
(48x48 pixel)
67.2cm
3.36m
3.5m
Figure. 13 A possible layout of flat panels to cover the curved focal plane.
- 39 -
6.2 Flat Panel PMT
The Flat Panel PMT is the next generation PMT under development at Hamamatsu which would
replace the R7600 series [Yoshizawa]. It is expected to become commercially available in mid
2000. Figure 14 shows the picture of a prototype and the conceptual cross sectional view.
Window (2.8 mm t)
Bialkali Photocathode
Metal Channel Dynode (10-stage)
Anode
Tip off
tube
Pixel
Figure. 14 A picture of the Flat Panel PMT and its structure.
- 40 -
As shown here, the outer dimension is 50.5mm square with effective area of 49.7mm square,
2
 49.7 mm 
yielding the effective area of 
  0.97 , remarkable improvement over R7600! If this is
 50.5mm 
the case, the inefficient light collector is no longer inevitable.
The first product will have 64 pixels with 5.6mm pixel diameter. The specifications provided by
Hamamatsu are given in Table 9. The basic characteristics are expected to be similar to R7600 in
terms of quantum efficiency, gain, time response etc.. Fortunately the pixel size well matches our
requirement for the OWL-AirWatch experiment. Therefore once this becomes available, one Flat
Panel PMT can naturally replaces four of R7600-M16.
The only concern is a rather thick glass window. It is currently 2.8mm, and may need to reduce to
~1mm level to avoid unwanted optical cross talk inside. Otherwise, this device would make an
ideal focal plane for our application.
Parameter
Spectral Response
Photo-cathode Material
Material
Window
Thickness
Structure
Dynode
Number of Stages
Supply Voltage
Gain
Number of Pixels
Pixel Size and Pitch
Effective Area
Dimensional Outline
Weight
Description/Value
300 to 600
Bialkali
Borosilicate glass
2.8
Metal channel Dynode
10（12）
1000
106
8x8
5.6 x 5.6 / 6.0
49.7 x 49.7
50.5 x 50.5 x 12.4
70
Table. 9 Specifications of the Flat Panel by Hamamatsu.
- 41 -
Unit
nm
mm
V
mm
mm
mm
gram
6.3 Katsushi's Dream Detector
Even though multi pixel PMTs such as R7600 and the Flat-panel PMT satisfy our specifications,
it is still far from the ideal device; Quantum efficiency is poor (20 ~ 25%), and the gain is not
uniform from pixel to pixel (by a factor of 2 ~ 3). To overcome such disadvantages, I have been
proposing "Katsushi's Dream Detector" on many occasions [Arisaka.1, Arisaka.2]. The concept
is shown in Figure 15.
This is a multi-pixel, Hybrid APD with Solid State photo cathode having ~50% quantum
efficiency, housed in a ceramic square case of the Flat-Panel PMT size. Assuming that finer
segmentation will become important for the second-generation OWL-AirWatch experiments, 256
square pixels with 3mm size are assumed here. To achieve 50% of quantum efficiency at the wave
length of 300 ~ 400nm, InGaN is under consideration for the photo cathode. Front-end readout
electronics is directly attached behind the APD array with signal processing digital electronics,
driving a single optical fiber to send out digital signals. This greatly simplifies the mechanical
complexity of feed-though.
Once it is realized, such a device could easily replace the Flat-Panel in the future. We plan to
continue necessary R&D in a close collaboration with industries.
Glass Window
(1mmt)
InGaN Photo Cathode
APD Array
(16 x 16 = 256 Pixel)
Readout Electronics
HV
LV
47.6mm•
Optical Fiber for
Signal Readout
50.5mm•
Figure. 15 Conceptual cross sectional view of Katsushi's Dream Detector.
- 42 -
Ceramic
Case
6.4 Summary and R&D plan
So far, I have listed three candidates from a realistic one to a dream one. For fair comparison, all
the important parameters are summarized in Table 10 together with the specifications. In this
table, poor parameters are underlined.
Several remarks can be made based on this table.
1) Hamamatsu R7600-M16 is a practical solution with reasonable specs. However the dead
space and gain non-uniformity are two major concerns.
2) Hamamatsu Flat Panel PMT significantly reduces the dead space. But gain non-uniformity
could remain as poor as R7600-M16. The thick (compared to R7600-M16) glass window is
another concern.
3) "Katsushi's Dream Detector" is an attempt to overcome the non-uniformity problem of the
above two. With Solid State photo-cathode, quantum efficiency should be dramatically
improved to 50% level as well.
Hamamatsu R7600-M16 is the heaviest (1.56gram per pixel). Since the focal plane of our baseline OWL-AirWatch detector consists of 200,000 pixels, the total weight would become 310kg.
This is still of order 5% of the total allowed weight of the spacecraft. Further weight reduction is
expected for the Flat Panel. Thus the weight of photon detectors is not an issue.
There are several peripheral R&D efforts to be pursued in addition to the further development of
photon detectors themselves.
1) In case of R7600-M16, the light collector must be carefully designed to minimize the dead
space, while signal loss and cross talk are minimized.
2) Mechanical structure of the support frame requires careful study. The concept of using a flat
panel of ~70cm square was presented, but it is not engineered. Space-qualified engineering
design requires more expert thought.
3) The power consumption by the HV power supply is not analyzed yet. The idea exists to
operate PMTs under lower HV until a self-trigger activates the HV and readout system. Under
this scenario, the last dynode signal would be read out by a high-again preamplifier to provide
a self trigger signal.
In summary, developing the ideal the photon detector for OWL-AirWatch is indeed a challenging
project. However, thanks to recent technological advancement, there is a reasonable existing
solution and we are confident that eventually we will get much better solution one way or another.
- 43 -
Parameter Description
Notation
Pixel Size
Number of Pixels per Unit
Physical Dimension of One unit
Window Thickness
Window Transparency at 350nm
Weight per Pixel
Dead Space
Quantum Efficiency
Cathode Non-uniformity
Anode Non-uniformity
Photo-electron Collection Efficiency
Cross Talk between Pixels
Excess Noise Factor
Intrinsic Gain
Dynamic Range
Equivalent Noise Charge
Transit Time Spread for Single PE
Rise Time and Fall Time
Pulse Width
Readout Speed per Detector Unit
Dark Current per Pixel
Dark Pulse Rate per Pixel
After Pulse
Life Time
Power Consumption of HV per Pixel
Cost per Pixel
detector
#Pixels
Dunit
W
-

Col
ENF
G
ENC
TTS
Idark
-
Specifications
Minimum
Ideal
~ 6mm
16
>64
> 2.5cm
> 5cm
<1mm
> 80%
> 90%
< 1.5gram
< 1gram
< 20%
< 10%
> 20%
> 50%
< 20%
< 10%
< 50%
< 20%
> 80%
> 90%
< 5%
< 2%
< 1.1
1.0
> 3,000
> 105
~1,000
~10,000
< 1000e
< 300e< 1nsec
< 0.5nsec
< 5nsec
< 2nsec
< 10nsec
< 5nsec
<1sec
< 1nA
< 10kHz
< 1%
10 Years
< 500W
< 200W
< $50
< $20
Hamamatsu
R7600-M16
4.0mm
16
2.57cm
0.8mm
90%
1.56gram
54%
20%
20%
50%
80%
1%
1.1
~106
10,000
1000e0.3nsec
1.0nsec
1.5nsec
<1sec
0.2nA
<100Hz
< 1%
>10 Years
~200W
~$40
Hamamatsu
Flat Panel
5.6mm
64
5.05cm
2.8mm
90%
1.09gram
10%
20%
20%
50%
80%
1%
1.1
~106
10,000
1000e0.3nsec
1.0nsec
1.5nsec
<1sec
0.2nA
<100Hz
< 1%
>10 Years
~200W
?
Table.10 Comparison of the OWL-AirWatch photo-detector candidates.
- 44 -
Katsushi's
Dream
3.0mm
256
5.05cm
0.8mm
90%
0.25gram
10%
50%
10%
10%
90%
1%
1.0
~105
106
300e0.5nsec
2nsec
5nsec
<1sec
0.1nA
<100Hz
< 1%
>10 Years
<100W
??
7. Conclusion
It was demonstrated that the OWL-AirWatch is a serious, realistic experiment to study ultra highenergy cosmic rays and neutrinos with at least ten times higher statistics than the Auger or TA
project. It is not an imaginary experiment. To evaluate its feasibility, several useful scaling laws
were derived first from the existing experiments such as HiRes at Utah. Based on these laws, the
baseline detector concepts and all the important parameters were presented.
Our current baseline detector satisfies the following three goals:
1) Effective Aperture (after the correction of duty factor) will be an order of 80,000km2str, at
least ten times larger than the Auger or the Telescope Array.
2) Energy threshold will be well below 1020eV.
3) Angular resolution will be of order 1o.
It requires ~200k pixels of advanced photo-detectors with single-photon counting capability.
Thanks to recent developments in industries, there is at least one existing detector candidate,
Metal Channel Plate PMT R7600-M16, and even more attractive ones are promising.
Although there are still many technical challenges to be solved, there is no fundamental obstacle
so far. Our future seems bright.
Acknowledgement
My special thanks go to Yoshi Takahashi who has been providing me all the necessary
information to consider the detector optimization described in this document. Discussions with
him in many occasions always keep me focus on important physics and other issues. I would like
to thank Dave Cline and Gene Loh for bringing me into this exciting field of ultra high-energy
cosmic rays in early 98. I would also like to thank David Lamb for optics design, Osvaldo
Catalano for trigger and electronics design. Their contributions were essential for photon detector
optimization. Lastly I am grateful to Yuji Yoshizawa for providing me the most updated
technical information of various photon detectors under development at Hamamatsu.
- 45 -
References
[Arisaka.1] Katsushi Arisaka,
"Advances in Vacuum Photon Detectors for High Energy Experiments".
Talk presented at SCIFI 97: Conference on scintillating fiber detectors.
Notre Dame, Indiana, November 1997.
Published in AIP Conference Proceedings 450.
[Arisaka.2] Katsushi Arisaka,
"New Trends in Vacuum Based Photon Detectors".
Talk presented at Second conference on new developments in photon detection,
BEAUNE 99, Beaune, France, June 21-15, 1999.
To be published to N.I.M.
[Catalano]
Osvaldo Catalano,
"AirWatch from Space" Progress Report, Sep 24, 1999
[Cushman]
Priscilla Cushman,
"Status of the CMS hadron calorimeter HPD readout system".
Talk presented at Second conference on new developments in photon detection,
BEAUNE 99, Beaune, France, June 21-15, 1999.
To be published to N.I.M.
[Delta III]
The Boeing Company,
"Delta III payload Planners Guide, MDC 99H0068"
http://boeing.com/defensespace/space/delta/deltahome.htm
[HiRes]
Univ. of Utah et. al.,
"Proposal to Construct a High Resolution Eye (HiRes) Detector",
Submitted to NSF, 1992
[H6568]
Hamamatsu Photonics. Co.,
"Multi Anode Photomultiplier Tube Assembly H6568, H6568-10",
Preliminary Data, March. 1999
[Kimura]
Suenori Kimura,
"Analysis of Light Guide for R5900-M16/64",
Hamamatsu Internal Report, May 15, 1998
[Lamb]
David J. Lamb, "Current Status of AirWatch-OWL Optics",
Talk presented at OWL-AirWatch technical meeting at Palermo, Italy,
Dec 13, 1999
- 46 -
[Red Book]
John. Krizmanic etc. editors,
"Workshop on Observing Giant Cosmic Ray Air Showers from >1020 eV
Participles from Space",
AIP Conference Proceedings 433, College Park, Maryland 1997
[Takahashi] Yoshiyuki Takahashi,
"Great Science Observatories in the Space Station Era and OWL efforts in Japan".
Talk presented at "Workshop on Observing Giant Cosmic Ray Air Showers from
>1020 eV Participles from Space",
AIP Conference Proceedings 433, College Park, Maryland 1997
[Yoshizawa] Yuji Yoshizawa, "Flat Panel PMT",
Talk presented at 7th International Conference on Instrumentation for Colliding
Beam Physics, 15-19 November 1999 at Hamamatsu, Shizuoka, Japan
- 47 -