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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
[email protected]
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
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Table of Contents 1. Basic Concept of OWL -AirWatch Experiment .
. . . . . . . . . . . . . . . . .3 2. Derivation of Scaling Laws .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.1. Formula for Signals 2.2. Formula for Noise, SNR and Energy
Threshold 2.3. Formula for Photo-detectors 2.4. Angular Resolution
2.5. Covered Area and Aperture 2.6. Summary
3. Expected Performance of Current Design . . . . . . . . . . .
. . . . . . . . . . 11
3.1. Basic Principles of Detector Optimization 3.2. Delta Launch
Vehicles 3.3. AirWatch Design 3.4. Multi-OWL Design 3.5. Comparison
with Other Previous/Ongoing Experiments
4. Requirement of the Photo-Detectors . . . . . . . . . . . . .
. . . . . . . . . . . . . 25
4.1. Physical Dimensions 4.2. Signal Sensitivity 4.3. 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. Metal-channel PMT 6.2. Flat Panel PMT 6.3. Katsushi's Dream
Detector 6.4. Summary and R&D plan
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 45
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 46
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1. Basic Concept of OWL -AirWatch experiment The OWL-AirWatch is
a space-based, next generation fluorescence detector to study ultra
high-energy 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.
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.
Owl-AirWatch Detector
Ground
kmH 500≈
Atmosphere (~10km) Cosmic Ray Shower
FOV ≈ 60o
km600≈
Cosmic Ray
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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 wide-
angle 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. 1µsec 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.
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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].
sec)//()(
10)(
)(4220)( 22220 nmkmRR
TAeVE
photonspeSξη∆××= (1)
Where A (m2) is the area of the light collector, η is the
quantum efficiency, ∆T (nsec) is the gate width, R (km) is the
distance from the signal source to the detector, ξ(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 Θ from the
zenith, and angle β 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 α, each photo-detector has a field of
view of ∆α, the physical track length inside of ∆α is ∆Ltrack, the
track length projected onto the plane perpendicular to the R-vector
is ∆Lα.
Figure.2 Various geometrical parameters between the OWL-AirWatch
and Cosmic rays.
H
Owl-AirWatch Detector
Ground
∆α
α
Cosmic Ray Shower
R
∆Ltrack
∆Lα
Θ
β
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Table.1 below summarizes the notation of parameters and their
default values in the following analysis.
Parameter Description Notation Default Value
Energy of Cosmic Ray E 1020 eV
Satellite Height from the ground H 500km
Total Field of View the detector mirror FOV 60o
Diameter of Mirror D 3m
F-stop of Mirror fstop 1.0
Distance between the Cosmic-ray Shower and the Detector R -
Light Attenuation Factor after traveling the distance R ( )Rξ
0.5 The incident angle the cosmic ray from the zenith Θ - Viewed
angle from the detector to the cosmic ray shower (measured from the
zenith) α -
Incident angle of the cosmic ray (measured from the R-vector) β
-
Field of View of Photo-detector Pixel ∆α 0.1o
Quantum Efficiency of Photo-detector η 0.2
Integration Time of Flash ADC ∆T 1µsec
Table.1 Notation of parameters and their default values
There are several simple geometrical relations such as,
( ) αββπα αα ∆=∆=−=∆∆=
R
L
L
L
R
H
track
,sin2cos,cos . (2)
Taking these relations and 42DA π= (where D is the diameter of
the mirror), the signal given by Equation (1) is conveniently
expressed by,
∆
⋅×=∆ sec11.0
)(3
50010
cos)(8.1122
202
µηξα TR
mD
Hkm
eV
EpeS T (3)
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Furthermore, once the shower traveling time in the FOV is
integrated over, ∆T can be taken as
∆
⋅
⋅×=
⋅∆⋅=
⋅∆⋅=
⋅∆=∆=∆
o1.0500sincos1
sec)(9.2
sincos.
sinsin
αβα
µ
βαα
βα
βθ
kmH
c
H
c
R
c
L
c
LT tracktrack
(4)
Here, c is the speed of light ( sec/3.0 µkmc = ). The track
length in ∆α, ∆Ltrack, is also given by
∆
⋅
⋅×=∆
o1.0500sincos1)(870
αβα km
HmLtrack (5)
The projected length of ∆Ltrack onto the plane perpendicular to
R-vector is simply,
∆
⋅×=∆
o1.0500cos1)(870 α
αα kmH
mL (6)
Using Equation (4) for ∆T in Equation (1), one can obtain the
following expression as well.
∆
⋅×=
⋅⋅
⋅××=∆
o1.01.0)(
3500
10sincos
)(2.34
sec)//()(
sin10)(
)(4220
2
20
2220
αηξβα
ξβ
ηα
R
m
D
H
km
eV
Epe
nmkmR
R
c
FOVA
eVEphotonsS
(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 Ω(deg2)
and integration time of ∆T (nsec), then the background noise level,
B, is given by,
)degsec//()(12.0)( 22 nmTAphotonspeB Ω∆×= η . This can be
re-expressed by,
22
1.0sec12.03)(7.1
∆
∆
×=∆ o
αµ
η TmD
peB T (8)
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The important criterion to separate signal from noise is the
Signal to Noise Ratio (SNR). Generally speaking, SNR is given by σS
. In the simplest case, σ has two contribution: Poisson
statistics of signal itself, Ssignal =σ , and Poisson statistics
of the dark sky noise, Bdark =σ . Thus,
BSdarksignal +=+=22 σσσ
BS
SSSNR
+==
σ (9)
Once the signal becomes significantly larger than the noise, SNR
simply becomes S1 . As a result, the SNR (after ∆T of integration)
can be expressed by,
∆
⋅×≈∆ sec11.0
)(103
500cos4.3 20 µ
ηξασ TReV
EmD
Hkm
SNR T (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 2σ (i.e. 4
photoelectrons), the energy threshold, Eth, is given by,
∆
⋅⋅
××≈
TRDm
kmHSNR
eVEthsec1
)(1.03
500cos1
2104.3
22
2
219 µ
ηξασ (11)
2.3 Formula for Photon Detectors The pixel size of the photon
detector, detector(mm), is given by,
∆
×=
∆⋅⋅=
o1.031)(2.5
α
α
mDf
mm
Dfd
stop
stopetector
(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,
×=
⋅⋅=
o6031)(1.3
FOVmDf
m
FOVDfD
stop
stopetector
(13)
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Lastly, the number of pixels of photon detector is simply given
by,
225
2
1.060
108.2
4#
∆
××=
∆⋅=
α
απ
o
o
FOV
FOVPixels
(14)
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, σwidth, divided by the observed
track
length, Ltotal. (i.e. total
width
L
σσθ = ).
What is σwidth? 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 ∆Lα × ∆Lα × ∆LT. Therefore a typical value of
σwidth is given by,
222,
3.2 Twidthwidth
width LLLL ∆+∆=∆∆= ασ .
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,
7.0#3.2 ×××∆=
hitsL
L
total
widthθσ
Here, the number of hits is roughly given by width
total
L
Lhits
∆=# .
By combining the above three equations, one can derive the
following formula for the angular resolution.
2
3
22252.0
∆+∆×=
total
T
L
LLrad αθσ (15)
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As the final step, Ltotal can be replaced by Θ≈
cos10km
Ltotal , where Θ is the zenith angle of the
Cosmic ray shower. Under the condition of ∆LT
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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, )(01.0 3kmAreaVolume ⋅≈ (19)
×≈
⋅×≈ −
263
33
1010.0_
)(100
)(101
km
AreafactorDutykm
kmAreaVolumeeff (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.
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Parameter Description Formulas
Photo-detector Pixel Dimension
∆
×=
o1.031)(2.5
αm
Dfmmd
stop
etector
Dimension of Photo-detector Total Area
×=
o6031)(1.3
FOV
m
DfmD
stop
etector
Total Number of Pixels 22
5 1.060
108.2#
∆
××=
α
o
o
FOVPixels
Track Length in ∆T (= c⋅∆T )
∆×=∆sec1
)(300µ
TmLT
Track Length in ∆α
∆
⋅×=∆
o1.0500cos1)(870 α
αα kmH
mL
Shower Max Signal at E = 1020 eV in ∆T
∆
⋅×=∆ sec11.0
)(3
50010
cos)(8.1122
202
µηξα TR
mD
Hkm
eV
EpeS T
Shower Max Signal at E = 1020 eV in ∆α
∆
⋅×=∆ o1.01.0
)(3
50010sin
cos)(2.34
2
20
αηξβα
αR
m
D
H
km
eV
EpeS
Dark Sky Noise 22
1.0sec12.03)(7.1
∆
∆
×=∆ o
αµ
η TmD
peB T
SNR in ∆T (for B
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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 Low-Earth 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 two-stage 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]
)
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Figure.3 Delta III Spacecraft Envelope, 4.0-m (13.1-ft)-dia
Fairing, Two-Stage Configuration
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Figure.4 Delta III Vehicle, Two-Stage Circular Orbit Altitude
Capability
Figure. 5 Typical Delta III LEO Mission Ground Trace
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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
FADC gate and given by sec2.3sec/300
974 µµ
α ==∆=m
m
c
LTtotal
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.
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Figure.4 Conceptual design of the baseline OWL-AirWatch
detector
3.5m
Battery/Electronics etc. etc.
Calibration Light Source
Focal Plane
Fresnel Lens
Entrance Pupil
Fresnel Lens
Deployable Light Shield
4m
4m
Shutter
Payload Attach Fitting etc.
Deployable Solar Panels etc.
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- 18 -
Table.4 Baseline Parameters of AirWatch Detector under
consideration.
Parameter Description Notation AirWatch Double Fresnel
Satellite Height H 500km
Total Field of View FOV 60o
Diameter of Entrance Pupil D 2.5m
Diameter of Fresnel Lens - 3.5m
F-stop of Mirror fstop 1.3
Quantum Efficiency η 0.2
Light Attenuation Factor ( )Rξα ⋅2cos 0.5 Integration Time ∆T
833nsec
Field of View of Photo-detector Pixel ∆α 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 ∆α (at β = 90o) ∆Lα 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 ∆α) S∆α 22pe
Night Sky Noise (in ∆T) B∆T 1.3pe
Signal to Noise Ratio at E = 1020eV (in ∆T) SNR∆T 2.4σ
Energy Threshold (for SNR = 2σ) Eth 7.1×1019eV
Angular resolution σθ 1.4o
Area Viewed by the Whole Detector Area 2.6×105km2
Effective Aperture Apertureeff 8.3×104km2str
Effective Neutrino Target Volume Volumeeff 260km3
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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.
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- 20 -
Figure.6 Concept of Multi-OWL consisting of six OWLs at 1000km
high.
Top View
Earth
Multi-OWL Detector
R=6380km
30o
H=1000km
6680km
60o
30o
Side View
3340km
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- 21 -
Figure.7 A possible stereo view arrangement of six OWLs at
1000km heigh
Earth
H=1000km
Side View
60o
Top View
R=6380km
2000km
3500km
2000km
Multi-OWL Detector
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- 22 -
Parameter Description Notation Single-OWL Low
Single-OWL High
Multi-OWLs Stereo
Multi-OWLs Mono
Number of Detectors N 1 6 Satellite Height H 500km 1000km 1000km
Total Field of View FOV 60o 60o 120o
Effective Diameter of Mirror D 2.5m 5m (× 6) F-stop of Mirror
fstop 1.3 1.0 Quantum Efficiency η 0.2 0.5 Light Attenuation Factor
( )Rξα ⋅2cos 0.5 0.05 ~ 0.5 Integration Time ∆T 833nsec 833nsec
Field of View of Photo-detector Pixel ∆α 0.11o 0.036o
Photo-detector Pixel Dimension detector 7mm 3.2mm Dimension of
Photo-detector Total Area Detector 3.3m 5m (× 6) Total Number of
Pixels #Pixels 200K 15M Track Length in ∆α (at β = 90o) ∆Lα 974m
1.95km 630m ~ 1.3km Track length in ∆T (= c⋅∆T ) ∆LT 250m 250m
Shower Max Signal at E = 1020 eV (in ∆T) S∆T 5.6pe 1.8pe 7 ~ 17pe
0.2 ~ 17pe Night Sky Noise (in ∆T) B 1.3pe 1.3pe Energy Threshold
(for SNR = 2σ) Eth 7.1×1019eV 2.8×1020eV 2~5×1019eV 2~20×1019eV
Angular resolution σθ 1.4o 3.8o 0.74o 0.74 ~ 5.6o
Area Viewed by the Whole Detector Area 2.6×105km2 1.0×106km2
5.5×106km2 2.5×107km2
Duty Factor Duty_factor 10% 10% Effective Aperture Apertureeff
8.3×104km2str 3.3×105km2str 1.8×106km2str 8.0×106km2str Effective
Neutrino Target Volume (Water eq.) Volumeeff 260km3 1040km3 5,500
km3 25,000 km3
Table.5 Baseline Parameters of proposed Single- and Multi-OWL
detector.
-
- 23 -
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.
-
- 24 -
Experiments Method Covered Area
Duty Factor
Effective Aperture
Effective Neutrino Volume
Energy Thre-shold
Angular Reso-lution
Cost Start Year
Notation Area - Apertureeff Volumeeff Eth σθ - -
Unit 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
Auger (hybrid) Hybrid 3,000 10 700 0.3 ~1019 ~0.5o ~50
2004
Telescope Array Fluorescence 21,000 10 6,000 21.0 ~1019 ~0.5o
~80 ~2005
ICECUBE Cherenkov - 100 - 1.0 ~1012 ~1o ~80 ~2005
OWL (×1019)
Single, Low Fluorescence 260,000 10 83,000 260.0 ~7 ~1.4o
~2006
Single, High Fluorescence 1,040,000 10 330,000 1,040.0 ~28 ~3.8o
~200
~2008
Multi, Stereo Fluorescence 5,500,000 10 1,800,000 5,500.0 2~5
~0.7o ?
Multi, Mono Fluorescence 25,000,000 10 8,000,000 25,000.0 2~20
0.7~6o ~1000
?
Table.6 Comparison between various past, ongoing and future
experiments
-
- 25 -
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 σθ = 1o, from
Equation (16), the requirement on the field of view of each pixel
becomes,
⋅
Θ×≤∆
o
o
1500
coscos11.0 θ
σααH
km (21)
By inserting this into Equation (10), one can obtain the formula
for the optimal pixel size.
⋅
Θ×≤
o1500
31coscos
)(0.6 θσα
H
km
m
Dfmmd
stop
etector (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 ∆Lα =1km
sampling rate is desirable, and it actually gives the similar
requirement as Equation (19), shown below.
∆
⋅×≤∆
km
L
H
km
1500
cos11.0 αα o (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.
-
- 26 -
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.
.pepe
pe SS
SSSNR ===
σ (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 γη SCS olpe ⋅⋅= , where η is
the quantum efficiency, Col is the collection efficiency for
photoelectrons and Sγ 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 σ2. (i.e . σoutput2 = ENF⋅σintput2). In
case of photon detectors, the ENF is given by the formula below,
where δn stands for the multiplication factor of the n 'th dynode.
As shown later, for typical PMTs, δn 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.
n
ENFδδδδδδ L
L21211
1111
⋅++
⋅++= (25)
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.
-
- 27 -
GENC
BCENF
SCENF
ENC
oldark
olsignal
ENCdarksignal
=
⋅⋅⋅=
⋅⋅⋅=
++=
σ
ησ
ησ
σσσσ
γ
γ
2222
(26)
Here Bγ is the incident photons caused by the dark sky noise. By
substituting (26) into (24), SNR becomes,
2
222
)()( GENCBSCENF
SC
SSNR
ol
ol
ENCdarksignal
pe
++⋅⋅
⋅⋅=
++=
γγ
γ
η
η
σσσ (27)
In physics experiments, the energy resolution is commonly used
instead of the SNR, and it is given by,
γ
γγ
η
ησSC
GENCBSCENF
SNRE ol
ol
⋅⋅
++⋅⋅==
2)()(1 (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
(
-
- 28 -
At this point, to achieve superior sensitivity, the following
becomes very important.
3) Quantum Efficiency (η) 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 ~1µsec 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.
-
- 29 -
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
WkW µ500
1021.01
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 pixelpAC
/32.0sec/102106.1 619 =××× − .
Assuming Gain of 106, the anode dark current is given by,
×=
610/320
GpixelnAIdark
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,
×=
6102.0_
10/2
GfactorDuty
years
TpixelCoulombQ .
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.
-
- 30 -
Specifications Parameter Description Notation
Minimum Ideal
Pixel Size detector ~ 6mm
Total Number of Pixels #Pixels ~ 2 × 105
Total Photo-Cathode Area Acathode ~ 9m2
Physical Dimension of One unit Dunit > 2.5cm > 5cm
Window Transparency at 350nm - > 80% > 90%
Weight per Pixel W < 1.5gram < 1gram
Dead Space - < 20% < 10%
Quantum Efficiency η > 20% > 50% Cathode Non-uniformity -
< 20% < 10%
Anode Non-uniformity - < 50% < 20%
Photo-electron Collection Efficiency Col > 80% > 90%
Cross Talk between Pixels - < 5% < 2%
Excess Noise Factor ENF < 1.1 1.0
Intrinsic Gain G > 3,000 > 105
Dynamic Range - ~1,000 ~10,000
Equivalent Noise Charge ENC < 1000e- < 300e-
Transit Time Spread for Single Photo-electron TTS < 1nsec
< 0.5nsec
Rise Time and Fall Time - < 5nsec < 2nsec
Pulse Width - < 10nsec < 5nsec
Readout Speed per Detector Unit (16~64ch) - 10 Years
Power Consumption of HV Power Supply per Pixel - < 500µW <
200µW Cost per Pixel - < $50 < $20
Table.7 Specifications for the OWL-AirWatch photo-detectors.
-
- 31 -
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).
-
- 32 -
Pixe
l Siz
e
#Pix
el p
er
Uni
t
Dim
ensio
n of
one
Uni
t
Wig
ht p
er
Uni
t
Dea
d Sp
ace
Qua
ntum
Ef
ficie
ncy
at
300n
m
Phot
oele
ctro
n C
olle
ctio
n Ef
ficie
ncy
Mor
tific
atio
n fa
ctor
(δ)
Exce
ss N
oise
Fa
ctor
Intri
nsic
Gai
n
Ano
de N
on-
unifo
rmity
Notation detector - - W - η Col δ ENF G - Specs (Minimum) ~6
>16 >2.5 - 20 >80 >5 3000 64 >5 - 50 >90 >10
1.0 >105
-
- 33 -
Signal to Noise Ratio (S = #Incident Photons)
Tran
sit T
ime
Spre
ad
Rise
/Fal
l Ti
me
Rea
dout
Ti
me
per
Uni
t
Cos
t per
P
ixel
Fina
l Gra
de
Remarks
Notation SNR TTS - - - - Specs (Minimum) 0.6S
-
- 34 -
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 multi-pixel 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.
Figure. 8 Front view of Hamamatsu R7600-M16.
25.7mm
17.5mm
4.1mm
4.1mm
4mm
0.5mm
Photo Cathode
22mm
Anode Pixel
-
- 35 -
Figure. 9 Mechanical structure and Catalog Specifications of
Hamamatsu R7600-M16
-
- 36 -
Figure. 10 Quantum Efficiency, Gain and Time Response of
Hamamatsu R7600-M16
-
- 37 -
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%.
-
- 38 -
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
photo-cathode area is given by 39.07.257.25
1644 =×
××mmmm
mmmm . In reality, 0.5mm of the dead space between pixels is not
dead but gives signal into adjacent pixels. By taking this into
account, the
more practical effective area is given by 46.07.255.17
2
=
mmmm , still less than a half.
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.
Figure. 12 R7600-M16 PMT with light collector to avoid dead
area.
2.8cm UV filter
R7600-M16 PMT
Light Guide
2.57cm
2cm
Entrance Surface of Light Guide
7mm
2cm
Surface of R7600-M16
2.57cm
4mm
-
- 39 -
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 Sub-Panels of 48 x 48 pixels that
correspond to one unit of Trigger/electronics design by the Italian
group [Catalano]. Figure. 13 A possible layout of flat panels to
cover the curved focal plane.
Sub-Panel (48x48 pixel)
Super-Panel (96x96 pixel)
67.2cm
3.36m
3.5m
-
- 40 -
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.
Figure. 14 A picture of the Flat Panel PMT and its
structure.
Tip off tube
Window (2.8 mm t)
Bialkali Photocathode
Metal Channel Dynode (10-stage)
Anode
Pixel
-
- 41 -
As shown here, the outer dimension is 50.5mm square with
effective area of 49.7mm square,
yielding the effective area of 97.05.507.49 2 =
mm
mm, remarkable improvement over R7600! If this is
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 Description/Value Unit
Spectral Response 300 to 600 nm Photo-cathode Material Bialkali
-
Material Borosilicate glass - Window
Thickness 2.8 mm Structure Metal channel Dynode -
Dynode Number of Stages 10 (12) -
Supply Voltage 1000 V Gain 106 - Number of Pixels 8 x 8 - Pixel
Size and Pitch 5.6 x 5.6 / 6.0 mm Effective Area 49.7 x 49.7 mm
Dimensional Outline 50.5 x 50.5 x 12.4 mm Weight 70 gram
Table. 9 Specifications of the Flat Panel by Hamamatsu.
-
- 42 -
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.
Figure. 15 Conceptual cross sectional view of Katsushi's Dream
Detector.
Ceramic Case
Glass Window t
50.5mm•
InGaN Photo Cathode
APD Array
Readout Electronics
47.6mm•
(16 x 16 = 256 Pixel)
HV LV Optical Fiber for Signal Readout
-
- 43 -
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 base-line 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.
-
- 44 -
Specifications Parameter Description Notation
Minimum Ideal Hamamatsu R7600-M16
Hamamatsu Flat Panel
Katsushi's Dream
Pixel Size detector ~ 6mm 4.0mm 5.6mm 3.0mm Number of Pixels per
Unit #Pixels 16 >64 16 64 256 Physical Dimension of One unit
Dunit > 2.5cm > 5cm 2.57cm 5.05cm 5.05cm Window Thickness -
80% > 90% 90% 90% 90% Weight per Pixel W < 1.5gram < 1gram
1.56gram 1.09gram 0.25gram Dead Space - < 20% < 10% 54% 10%
10% Quantum Efficiency η > 20% > 50% 20% 20% 50% Cathode
Non-uniformity - < 20% < 10% 20% 20% 10% Anode Non-uniformity
- < 50% < 20% 50% 50% 10% Photo-electron Collection
Efficiency Col > 80% > 90% 80% 80% 90% Cross Talk between
Pixels - < 5% < 2% 1% 1% 1% Excess Noise Factor ENF < 1.1
1.0 1.1 1.1 1.0 Intrinsic Gain G > 3,000 > 105 ~106 ~106
~105
Dynamic Range - ~1,000 ~10,000 10,000 10,000 106
Equivalent Noise Charge ENC < 1000e- < 300e- 1000e- 1000e-
300e- Transit Time Spread for Single PE TTS < 1nsec < 0.5nsec
0.3nsec 0.3nsec 0.5nsec Rise Time and Fall Time - < 5nsec <
2nsec 1.0nsec 1.0nsec 2nsec Pulse Width - < 10nsec < 5nsec
1.5nsec 1.5nsec 5nsec Readout Speed per Detector Unit -
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7. Conclusion It was demonstrated that the OWL-AirWatch is a
serious, realistic experiment to study ultra high-energy 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.
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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
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[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
