地上ガンマ線望遠鏡 CTA のための高効率な集光装置の試作
奥村 曉
名古屋大学 宇宙地球環境研究所
Dang Viet Tan A、林田将明 B、Jim Hinton C、片桐秀明 A、
野田浩司 B、手嶋政廣 B,D、山本常夏 E、吉田龍生 A
茨城大理 A、東大宇宙線研 B、マックスプランク核物理 C、マックスプランク物理 D、甲南大理工 E
2018 年 3 月 25 日

日本物理学会

oxon@mac.com

Cherenkov Telescope Array (CTA)
cherenkov
telescope
array

Image Credit: G. Pérez, IAC, SMM
2

Cherenkov Telescope Array (CTA)
Large-Sized Tel. (LST)
20 GeV

Very-high-energy
Gamma Ray

3 TeV

Medium (MST)
80 GeV

50 TeV

Air Shower

Small (SST)
1 TeV

300 TeV

Cherenkov Photons∝ Energy

R ~150 m
K. Bernlöhr
3

LST Status
1st Large-Sized Telescope under construction at
Roque de los Muchachos Observatory, La Palma, Spain
As of Feb 17, 2018 (~2400 m a.s.l)
Mirror Support Structure
without Mirrors

Dummy Camera Mass

1st LST is underconstruction for theCTA Northern Array at
La Palma, Spain
Camera, mirrors, andauxiliary systems are in
the final development
and testing phase
LST First light isscheduled in 2018

Credit: Thomas Schweizer
4

The More Photons, the Lower Threshold & Higher Sensitivity
Cherenkov
Photons- 300–600 nm- 5–20 ns- R ~150 m

Aleksić et al. / Journal of High Energ

Crab Nebula Spectrum

Night-Sky
Background> 400 nm

E2 dN/dE (TeV cm−2 s−1)

StrayPhotons

34

MAGIC (2015)

GroundO(104 m2)
Space
Eﬀ. Area = O(1 m2)
CTA Large-Sized Telescopes

E (GeV)
Fig. 2. Spectral energy distribution of the Crab Nebula from 100 MeV to ∼30 TeV−2 γ -ray−3
obtained by Fermi-LAT and MAGIC, together with the fit results from otherexperiments. The black arrow indicates the systematic uncertainty on the energy scale,whereas the shaded area indicates the systematic uncertainty on the flux normalization and the photon index. The solid red line is the log-parabola fit to the MAGIC
data alone (the same as in Fig. 1). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

Typically dN/dE ∝ E – ENight-sky background is dominant for lower-energy gamma rays

Larger mirror area or higher photodetector
eﬃciency
the trigger
Crab Nebula
emission islowers
a log-parabolicshape, we energy
estimate theposition of the IC peak by a log-parabola fit to the data. In all fits
threshold → higher sensitivity
described below we take the correlations between MAGIC spectral points into account and consider only statistical errors unless5Winston Cones
http://www.mpi-hd.mpg.de/hfm/HESS

Event Ex.

H.E.S.S. II Camera

Winston Cones

Cherenkov camera = Winston cones + Photomultiplier Tubes (PMTs) + Frontend Elec. + Back-end Elec.
Hexagonal Winston cones cover the whole camera to maximize the collection
eﬃciency and to reduce BG photons
6ROBASTOkumura+ (2015) Astropart. Phys.

ROOT-based simulator for ray tracing (ROBAST)

https://robast.github.io

Developed for cosmic-ray and gamma-ray telescopes with use of the ROOT Geom library
Open source

https://github.com/ROBAST/ROBAST
7

Winston Cones
Photons Reflectedby Mirrors (< θang/2)
Optical Axis ofthe Telescope

Light Collector

Stray Light(> θang/2)

Collection Efficiency (%)

Okumura (2012) Astropart. Phys.

Circular

100

Ideal 2D
Hexagonal

80
60
40

Ideal 2D Winston Cone
Axisymmetric Winston Cone

20

Hexagonal Winston Cone

Dead Area
PMT00510152025 30
35 40
Incident Angle θ (deg)Invented by Winston (1970) a.k.a. Compound Parabolic Concentrators (CPCs)
In 2D space, Winston cones ideally work (100% for θ < cutoﬀ, 0% for θ > cutoﬀ )Not ideal for PMT arrays8Bézier-Curve Profile

Collection Efficiency (%)(a)

Okumura (2012) Astropart.
R = 1.0, n Phys.
= 1.0

θ cross

100(a)

ρ1(b)

(c)
P2
(1, 1)

P3 Eﬀ.
Higher Col.
(1, 1)
Sharper Cutoﬀ

60

Q2

Inclined Parabola

P0(0, 0)

R0Q020rP0(0, 0)00Bézier Curve

P1Bézier(P1rQuadratic, P1z)
r5Cubic Bézier
1015mm

ρ2z

B(t)

mm

Q0P1(P1r, P1z)

40

P2(P2r, P2z)
Q1
WinstonConeρ 1 = 20z

R1ρ 1 = 30

B(t)

F
ρ280

Q1L
θmax

θ max

20

2530
35
Incident Angle θ (deg)(c)

R = 0.9, n = 1.5

Efficiency (%)

Inclined parabola (conventional Winston cones) is not ideal for hexagonal pixels
100

θ crossθ max

Use of a Bézier curve profile → higher collection eﬃciency & sharper cutoﬀ809UV-Enhanced ESR
Okumura+ (2017) JINST

UV-Enhanced

Reflectance (%)

102

Reflectance (%)

Aguilar+ (2015) J.A.
Astropart.
Aguilar et Phys.
al. / Astroparticle Physics 60 (2015) 32–40
100 100
98 90
96 80
94 70
92 60
90 50
88 40

Normal ESR 20 °UV Enhanced 20°

86 30

Normal ESR 30 °UV Enhanced 30°

Normal ESR 40 °UV Enhanced 40°

84 20

Normal ESR 50 °UV Enhanced 50°

Normal ESR 60 °UV Enhanced 60°

82 10

Simulated Al Coating + 2 Layers

Fig. 9. Simulated reflectivity as a function of wavelength, k, for a R-enhanced
Aluminum coating from Thin Film Physics. Three different incident angles areshown.

NormalUV Enhanced 70°

Fig. 11. Prototype light concentrator manufactured with the injection moldi

80technique
0 and coated in two halves separately.
300 300350
500 650 550
350 400 400
450 500450550 600
700 750 600
800
Wavelength
Wavelength(nm)(nm)
4. Optical measurements of prototypes

54 additional layers (Ta2O5 & SiO2) on 3M Vikuiti
Enhanced Specular Reflector
A very important element in the design of the light concentrato
is the manufacturing of the substrate since this can severely affe
(ESR, ~1000-layer polyester film)

the properties and reflectivity of the coating. In our design we us
the standard plastic injection molding technique. Being an indu
trial large scale production technique, each light concentrator sub
strate can cost a few tenths of euro. The requirement in terms
10~93% to almost 100% (w/ ~1% err.) in λ = 300–600 nm and θ = 20°–70°

PMT Characteristics
Anode Sensitivity

Cathode Sensitivity

10.8q = 22 mmExit Aperture

Relative Anode Sensitivity

Averaged Relative SensitivityOkumura+ (2017) JINST

0.6

1.3

Most Sensitive at ±60°

1.2
1.11Y0.4

Y0.9

Dy10.80.2

Dy2
Side View

0R11920

ESR<20<10Front View

01020
Light Source Position Y (mm)

0.7

50 mm

−90−60

Plastic Cone
−300306090
Angle of Incidence (deg)CTA LSTs and MSTs will use Hamamatsu R11920-100-20 and R12992-100-05
Spherical matte-surface photocathode
Positional and angular dependence must be taken into account in the design11Angular Distribution of Incident Photons
Number of Photons

Okumura+ (2017) JINST
35×103
Field Angle = 0.00 deg

30

Field Angle = 1.00 deg
Field Angle = 1.50 deg
Field Angle = 2.15 deg

2520

Most Important

151050051015202530Angle of Incidence at the Focal Plane (deg)Also need to consider the angular distribution of incident photons to maximize
the collection eﬃciency
12

Prototype Production and Evaluation
Okumura+ (2017) JINST

φisBeam Ax

with 25-mm Mask

θwith 50-mm Cone
13

Results (465 nm)
φ = 30°Relative Anode Sensitivity Q (%)

Okumura+ (2017) JINST100100806040

Cone 1

Cone 2

Cone 3

Cone 480
Number of Photons

Relative Anode Sensitivity Q (%)φ = 0°

6035×103
Field Angle = 0.00 deg

30

Field Angle = 1.00 deg
Field Angle = 1.50 deg
Field Angle = 2.15 deg
Cone1Cone 2Cone 3

Cone 4

Cone 5

Cone 65Cone 7

Typ. Stat. Err.

00525204015

Cone 6

Cone 510200

Typ. Stat. Err.

Cone 7−30−20−100102030
Angle of Incidence (deg)200−30−201015202530
Angle of Incidence at the Focal Plane (deg)−100102030
Angle of Incidence (deg)ALC (✓)
1Define rel. anode sensitivity (~ col. eﬀ.) because col. eﬀ. Q = Amask ⇥ 3.7515 ⇥ cos ✓cannot be defined due to the angular and positional dependencies
Reaches 100% and higher in the most important angles

14Measurement v.s. Simulation (465 nm)
Rel. Anode Sensitivity Q or Col. Eff. ε (%)

Okumura+ (2017) JINST

Simulated
Rel. Anode Sens.

100
Simulated
Col. Eﬀ.

ROBAST simulations‣Rel. anode sensitivity80‣

Collection eﬃciency

60

Measured andsimulated values are
consistent within ~5%(points) for 465 nm

φ = 0 deg (Measured Q)
φ = 10 deg (Measured Q)
φ = 20 deg (Measured Q)
φ = 30 deg (Measured Q)40

φ = 0 deg (Simulated Q)
φ = 10 deg (Simulated Q)
φ = 20 deg (Simulated Q)20

φ = 30 deg (Simulated Q)
φ = 0 deg (Simulated ε )0051015

Rel. anode sensitivity
has a bump higher
than 100% due to the
angular dependence of
the PMT sensitivity

20253035Angle of Incidence (deg)15365 nm and 310 nm
Okumura+ (2017) JINST

310 nm100

Rel. Anode Sensitivity Q or Col. Eff. ε (%)

Rel. Anode Sensitivity Q or Col. Eff. ε (%)

365 nm100

80
φ = 0 deg (Measured Q)

60

φ = 10 deg (Measured Q)
φ = 20 deg (Measured Q)
φ = 30 deg (Measured Q)40

φ = 0 deg (Simulated Q)
φ = 10 deg (Simulated Q)
φ = 20 deg (Simulated Q)20

φ = 30 deg (Simulated Q)80
φ = 0 deg (Measured Q)

60

φ = 10 deg (Measured Q)
φ = 20 deg (Measured Q)
φ = 30 deg (Measured Q)

40

20φ = 0 deg (Simulated ε )005101520253035
Angle of Incidence (deg)005101520253035
Angle of Incidence (deg)Performance at 365 and 310 nm is worse than that of 465 nm presumably because
lower reflectance (~93–100%) of UV-enhanced ESR
Not very consistent with a simulation (PMT angular dependency is diﬀerent?)
But still better by 5–10 points than Al coating cones
16Conclusion
Developed new techniques to improve conventional
Winston cones‣ROBAST: New ROOT library for ray-tracing simulations‣Use of Bézier-curve profile‣

UV-enhanced ESR (92–100% reflectance for 300–600 nm and 20–70°)‣Take into account the PMT characteristics in simulation anddesign

Measured performance at 465, 365, and 310 nm‣

Rel. anode sensitivity of ~95% to ~105% for the most importantangles‣

Better by 5–10 points than Al-coating cones
17