Angle
Angle Calibration
Calibration ofof emulsion
emulsion read-out
read-out systemsystemforfor gamma-ray
gamma-ray telescope
telescope byby testtest beam
beamNagoya University

Beam Test @SPS/CERN

Yuya Nakamura for GRAINE collaboration

Beam

2cm

1 chamber= stack 3 ﬁlms

silicon detector

Abstract

1cm

In our laboratory we use Hyper Track Selector (HTS) which is the worldʼsfastest emulsion read-out system. HTS has an optical systematic error in
angular measurement caused by diﬀerence of refractive index between
emulsion and base ﬁlm, and it makes 3% uncertainty in absolute angle. Suchkind of error is a factor which makes imaging performance of gamma-ray
telescope worse at larger angle. I conducted calibration by a beam test with
400 GeV proton beam at SPS/CERN to reduce that uncertainty to 1% or less.

5mm-thick Al platefor ﬂatness

beam proﬁle bysilicon detector

48m6m
Controlled incident angle
exactly by rotary table(Accuracy of rotary table
is within 1mrad that ismeasured by FARO arm)

400 GeV
Proton Beam

Extraction
pointwire chamber

Application of emulsion is expanding
Emulsion is 3D track detector with sub-micron accuracy. Recently applicationof emulsion is expanding into imaging ﬁelds (cosmic gamma-ray observation,muon-radiography).

Side view
beam intensity
1shot=1.2x104~1.6x104 protons
2shot every angle-25(deg)GRAINE is gamma-ray telescopeexperiment with nuclear emulsion.
A demonstration test wasperformed with a balloon-borne
emulsion telescope in 2015. The
aim is to detect Vela pulsar at
200MeV energy region. Angularresolution is expected to be
10mrad at 200MeV with MC.

Angular Resolution

GRAINE

Gamma-Ray Astro-Imager with Nuclear Emulsion→FARO arm
3D coordinate
point detector
with 30μm
accuracy.

0

exposed 9 angle for64
1chamber(-64, -55, -45, -25, 025, 45, 55, 64 degree)
exposed 2chamber

400 GeV
Proton Beam

10mrad25

Top view
200MeV

Analysis

HTS has an optical systematic error
Angle distribution

base track
grain(Ag)beam spot

μWe scan grains in emulsion and makemicro-tracks. Micro-tracks are visible
tracks, but their angle is incorrect
because they are inﬂuenced by shrink
and distortion. So we make virtual
tracks in base ﬁlm called base-track.
Base tracks are not so inﬂuenced bysuch things and their angles more
resemble real tracks.

emulsion

micro-trackbase-trackmbase ﬁlm

micro-track

determination of Angle Meanemulsionget the number ofentries at the top

HTS reads grains and identiﬁes a boundary
between emulsion and base ﬁlm, and we makebase-tracks by micro-tracks. But now a boundary is
recognized incorrectly because of diﬀerence ofrefractive index between emulsion and base ﬁlm.
It makes a systematic error of base-track angle.
Here isa boundary

Track Position distribution

cut the angle at thehalf of the entries

Resultemulsion
imaging pointbase ﬁlm
imaging point

HTS thinks..
Here is aboundary!

emulsionincorrect base track is madebecause of wrong boundary

After performing some correction I made a plot of absolute angle vs a diﬀerence
between absolute angle and HTS scanned angle, and ﬁtted by linearly
approximating. This slope of the straight line is just k that I showed.

The slope k iscorrect base track

0.0523 0.0030

Base track angle issystematically larger!

HTS scanned angle - Absolute track angle= k Absolute track angle

Vertical axis =HTS scanned angle- Absolute track angle
Horizontal axis =Absolute track angle

If we can understand k
we can correct this angle gap
for GRAINE . . .
I aim to reduce error of k within 1% because
we want to reduce angle diﬀerence causedby it to 5mrad or less.

take the average in this range

200 MeVgamma-raytanθ = -1.5, 1.5 are not used forﬁtting because scan parameters are
not ﬁxed exactly for large angle yetand angle mean is not decided well.

It means that if a particle enter
emulsion as incident angle istanθ=1.00 HTS show its base track
angle is about 1.05!

Error of the k I showed is only
ﬁtting error but it seems thatthe error is within 1%,