Download High energy cosmic gamma rays detectors

Seminar I b - 1. letnik, II.stopnja
High energy cosmic gamma
rays detectors
Author:Urša Skerbiš
Supervisor: prof. dr. Peter Križan
Ljubljana, september 2014
Abstract
Earth is constantly exposed to high energy gamma rays, which came from outer
space. In this seminar an apparatus for their detection is described. We will present
a brief history overview of discovering cosmic rays, properties of the Cherenkov
radiation, the Imaging air Cherenkov technique, and a short description of the
H.E.S.S. experiment, its telescopes, mirror facets and the trigger system.
Contents
1 Introduction
2
2 Brief history overview
2
3 Cherenkov radiation
3
4 Imaging air Cherenkov technique
4
5 H.E.S.S. experiment
5.1 Telescopes . . . . . .
5.2 The mirror facets . .
5.3 Photo detectors . . .
5.3.1 Winston cone
5.4 Trigger system . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
6 Conclusion
1
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
6
7
7
9
9
10
11
Introduction
The Earth is constantly exposed to high-energy particles from the space. Cosmic rays
enter the Earth atmosphere even with energies beyond 1020 eV, their flux decreases by
≈ 20 orders of magnitude over the energy range 1011 eV − 1020 eV. Between 1012 eV
and 1015 eV nearly 90% of cosmic rays are protons or light nuclei and less than 0.1% are
gamma rays.
In gamma-ray astronomy nearly all discoveries have been made by a special kind of
Cherenkov detectors. In the seminar I will discus this type of detectors. I will first review
the history that lead to present Imaging atmospheric Cherenkov telescopes (IACT), basic
features of theory on the Cherenkov radiation and on the Cherenkov air technique, and
then proceed by discussing one of the instruments, the Hess telescope array in Namibia.
2
Brief history overview
In 1899 Oliver Heaviside calculated that the movement of an electron in a transparent
medium with the speed higher than the speed of light would be accompanied by a specific
conical emission, but his papers on this issue stayed unnoticed. Arnold Sommerfeld came
to a similar conclusions as Heaviside, about emissions of a special radiation, in his paper
in 1904.
First experimental report on a bluish glow, now known as Cherenkov radiation, is attributed to Madame Curie in the 1910. But first systematic study of the effect did not
take place until 1926-1929 by the French researcher L. Mallet [8]. He determined the
spectrum of the emitted light and found that it is continuous.
In 1932 Pavel Cherenkov became a PhD student of Sergei Vavilov. Vavilov decided that
the study of bluish luminescent emission should be Cherenkov’s topic. After few pull
backs Cherenkov could show in a simple elegant experiment the anisotropic character of
2
the emission: light was only emitted within certain angular range in the forward direction. Soon after the discovery theoreticians Igor Tamm and Ilya Frank had put forward a
theoretical explanation of that phenomena, and in 1958 these three scientist were awarded
the Nobel prize.
Figure 1: Igor Tamm, Pavel Cherenkov, Ilya Frank
A key role in the evolution of the Imaging Atmospheric Cherenkov Telescope technique came in 1948. Than Patric Blacket studied light emitted from the night sky and
aurorae. He has estimated that 0.01% of that light is emitted by elementary particles
as a Cherenkov light. In 1952 during his visit of Harvel he met Jelley and Galbraith
and learnt that they were also experimenting with Cherenkov light emission, but in the
water. Blackerr mentioned about his assumption of the Cherenkov light contribution in
the atmosphere. Soon after Jelley and Porter made a simple set up of a 25 cm diameter
parabolic signalling mirror in a dustbin and a single 2 inch photomultiplier in its focus.
They detected one pulse every two minutes. When they confirmed the light was more
intense in the blue part of spectrum, and measured its polarization, they proved they
measured Cherenkov pulses from air showers.
Galbraith and Jelly published a discovery paper in N ature in 1953, which is marked as
the beginning of the atmospheric air Cherenkov technique.
3
Cherenkov radiation
c0
In a transparent substance with a refractive index n, the speed of light is equal to c = ,
n
where c0 is the speed of light in vacuum. An interesting effect occurs, when a charged
particle travels with v > c. It is possible for a particle to move with velocity higher than
the speed of light in a given substance, because c is smaller than c0 , which is the upper
limit for particle velocity.
The electrical field of a particle affects the molecules behind it, but it does not affect
those in front of it. The centre of negative and positive charge part of molecules do not
coincide any more, so the group of molecules now has an electric dipole moment. During
the propagation of the particle through the medium, more and more groups get polarized
while groups, which already were polarized, lose the electric dipole moment when the
charged particle moves away.
The time dependent dipole moment along the path of charged particle causes electromagnetic radiation, known as Cherenkov radiation. The resulting electromagnetic waves
have a conical wave front at an angle ϑ with respect to the particle direction.
3
Figure 2: Cone of Cherenkov radiation β = vc .
c0
It is easy to see (Fig 2) that cos (ϑ) = nv
. Cherenkov radiation has a continuous
spectrum and it is the strongest in the interval between 100 and 600 nm. This is the area
of visible and UV light [2].
4
Imaging air Cherenkov technique
In particle physics experiments, the angle ϑ is usually measured, and the velocity of
the particle is calculated from it. One of the techniques that use Cherenkov radiation is the imaging air Cherenkov technique. Instead of measuring the velocity of the
particle, it measures the energy of high energy cosmic gamma rays. When a high
energy-gamma ray interacts in the atmosphere, it generates particles in the shower
(mostly electrons and positrons). The number reaches its maximum at about 10km above
ground. The shower dies away deeper in the atmosphere. Since these particles move with
a velocity near speed of light, they emit Cherenkov light.
The light is beamed around the direction of the incident particle and reaches the ground
over an area of about 250m in diameter. This area is known as the Cherenkov light pool.
If the primary photon has an energy round 1TeV, only about 100 photons per m2 reach
the ground. They arrive within a very short time interval of a few nanoseconds (as shown
in figure 3).
4
Figure 3: Impact photon, air shower and Cherenkov light pool [3].
A telescope located within the light pool detects the event, if its mirrors are large
enough to collect enough photons. The effective detection area of a Cherenkov telescope
is given by the size of Cherenkov light pool and is some orders of magnitude bigger than
the detection area of satellite instruments for detecting gamma rays before they interact
with the atmosphere.
Images obtained from the telescope show the tracks of the air shower. Their intensity
depends on the energy of the initial gamma ray.
Figure 4: Typical image obtained with IACT [3].
The image of the air shower in a camera is roughly elliptical (Fig. 4.). The direction,
energy and species of the primary particle can be determinated from the image. From
the shape of the image, the background illumination can be excluded. From single twodimensional image it is not possible to reconstruct the exact shape of the shower in space.
For this purpose several telescopes within one light pool are used, and allow a stereoscopic
reconstruction of the shower geometry.
5
Figure 5: Multiple telescopes for the stereoscopic reconstruction of single event [3].
5
H.E.S.S. experiment
An example of a large Cherenkov imaging telescope is the High Energy Stereoscopic
System (H.E.S.S. experiment) in Namibia. The experiment is named after Victor Franz
Hess, an Austrian-American physicist. He was born in Styria, Austria on 24 June 1883
and died on 17 December 1964 in Mount Vernon, USA.
Between years 1911 and 1912 Hess measured the radiation at altitudes up to 5.3 km. The
result showed that the level of radiation decreased up to an altitude of about 1 km, but
above that the level increased considerably. Radiation detected at 5 km high is about
twice that at sea level. His conclusion was that there was radiation coming into the
atmosphere from outer space. In 1925 Robert A. Milikan confirmed Hess’s discovery and
named this kind of radiation ”cosmic rays”. Hess was awarded Nobel prize in 1936 for
discovering the cosmic rays [7].
Figure 6: Victor Franz Hess in balloon with all equipment for measuring cosmic rays [7].
6
5.1
Telescopes
In the first phase of the H.E.S.S. experiment 4 telescopes were placed in the corners of a
square with the side length of 120 m. The diagonal of the square is oriented north-south.
In the second phase a fifth telescope was added in the middle of the square. With the
additional large telescope, the sensitivity, angular resolution and energy coverage were
considerably improved.
Figure 7: Placement of Telescopes of the H.E.S.S. experiment. Four 12 m telescopes are
in the corners of square and 28 m telescope in its middle. Taken from [3].
Each of four 12 m telescopes has a reflective area of about 100 m2 , a camera field
of view of 5 deg diameter and a pixel size of 0, 16 deg. The reflector of the telescope is
segmented. Characteristic dish size d is 13 m and f = 15, 6 m, so the telescopes have
f
≈ 1, 2.
d
5.2
The mirror facets
H.E.S.S. telescopes use segmented reflectors composed of many individual mirror facets.
The facets are manufactured as spherical mirrors and arranged in a Davies-Cotton layout, in which all reflector facets have the same focal length f , which is identical to the
focal length of the telescope as a whole. The facets are arranged on a sphere with a radius of 2f . Such an arrangement was chosen because of the cost and optical error reasons.
7
Figure 8: Arrangement of the mirror facets; left is the cross section and right is the front
view [5].
Each of 380 mirror facets have a round shape, with a diameter of 60cm. Facets are
made of glass with aluminium ground coating. The material was chosen because of its
long term stability. Exact specifications are given in the table below.
Material
Aluminized optical glass, thickness ≈ 15 mm
Protection
Quartz coating for outdoor use
Diameter of mirror
600 ± 1 mm
Diameter of reflecting surface
600 ± 0, 5 mm
Mounting
glued to support unit at three points
Focal length
15, 00 ± 0, 25 m
Specular reflectivity
at least 80% between 300 and 600 nm
Point spread function
80% of light in 1 mrad diameter
Figure 9: Specification for the mirror facets [5].
Mirrors are attached to the dish structure. The purpose of the mirror support units is
to attach the mirrors firmly to the dish structure, and to enable the remote adjustment
of mirrors. It must not impose significant stress onto the mirror, and must allow for
differential thermal expansion of the mirror and its support.
8
Figure 10: Technical drawing of 12m telescope from side and back view. On right drawing
mirrors are removed in one section of the dish to view the support beams.
A mirror support unit consist of a support triangle carrying one fixed mirror support
point and two motor-driven actuators. At these three points mirror is attached by using
steel pads glued to the back of the mirror.
5.3
Photo detectors
IACTs use a matrix of photo multipliers in the focal plane of the segmented mirror in
the David-Cotton geometry to detect the light flashes of Cherenkov light generated by
ultra-relativistic charged secondary air shower particles against a large background due
to the light of the night sky.
As it was already discussed above characteristics of the Cherenkov light are a small time
dispersion, a spectrum convoluted with atmospheric absorption and a low photon field.
To trigger the camera, typically a coincidence between a number of photomultiplier tubes
(PMTs) is required, with the signal in each PMT exceeding some threshold.
Each of cameras of the H.E.S.S. experiment is made of ≈ 700 pixels 1 . They are arranged
in circles. One PMT has the viewing angle of 0, 16 deg. It follows that the whole viewing
angle is 4, 3 deg. If all of additional places for PMTs would be filled, the field of view
would increase to 5 deg.
The maximal camera diameter is 1, 4 m. Each PMT is connected to a HV card containing
a DC-DC converter and an actively stabilized base.
Because of shape and properties of PMTs, there is a lot of inactive space between them.
Photons that would hit the inactive region of the camera are redirected by light concentrators of the Winston cone type.
5.3.1
Winston cone
Most modern Cherenkov telescopes use non-imaging light concentrators in front of PMTs.
The concentrators have two purposes. First they avoid dead areas due to insensitive areas
at the outer edges of the PMT cathodes and due to the support structure of PMTs, and
second they limit the solid angle viewed by the PMT and reduce noise due to the stray
light from the ground, shining past the reflector, or from the sky if the telescope observes
at low elevations.
In the H.E.S.S. telescope Winston cones are hexagonal, 21.5 mm wide and 53 mm long.
1
A pixel is defined as the PMT and the accompanying electronics.
9
Figure 11: One of the Winston cone light collectors frame [5].
They are made of plastic, composed of two halfs. Their inner side is aluminized and
covered with a thin quartz coating.They are locked in a carrier plate, which precisely
defines their location [5].
5.4
Trigger system
The triggering of Cherenkov telescopes makes use of a very short duration of the Cherenkov
light signal from air showers. A typical requirement for triggering the readout of a telescope is that a minimum number of pixels exhibit a signal larger than a given threshold
within a short time window, to reduce random triggers from the night sky background.
Following a trigger, signals are digitised and read out, resulting in a dead-time ranging
from a few 10µs to a few 10 ms, depending on the design of the data acquisition system.
In a system of telescopes, such as H.E.S.S. , it is required that two telescopes are both
triggered within a short time window. That significant reduces the rate of the background
events. Since hadronic showers, which are most of background events, have a more inhomogeneous light pool than gamma rays, the coincidence requirement disfavours first
ones.
The trigger of the H.E.S.S. cameras is made from a multiplicity trigger within overlap
sectors, each containing 64 pixels. A camera trigger occurs if the signals in M pixels
within a sector exceeds the threshold of N photoelectrons. The time window is determinated with the minimum integrated charge over a programmable threshold. For a typical
PMT pulse shape the effective trigger window is 1, 3ns.
The trigger of the H.E.S.S. telescope system is a two-level system. In the first level telescopes have local independent triggers and send signals to a central trigger system (CTS).
CTS consists of a hardware part in the central control building of the whole array, and
of interface modules located in each of the cameras.
10
Figure 12: Schematics of data flow in H.E.S.S. CTS [6]
The information on all telescope triggers arrives at the central station. If a coincidence is valid (at least two telescopes are triggered), the central station distributes this
information to all telescopes and the cameras of those telescopes which participated in
this system are read out. To enable the measurement of the system dead-time, cameras provide their current readout status together with the trigger signal. To provide a
synchronisation mechanism, the CTS assigns for each event a unique, system wide event
number, which is distributed through trigger hardware to all telescopes [6].
6
Conclusion
The Imaging atmospheric Cherenkov telescopes are a very important tool in exploring
the Universe. Stereoscopic observation as in the described experiment allows the determination of the shower axis. With this and background suppression by a multi-telescope
coincidence trigger they get good angular and energy resolution, a low-energy threshold,
and reduction and control of systematic errors [4].
It is worth noting that another important system of this kind is the MAGIC experiment
on Canary Islands. This is a IACT with a single very large 17m telescope [9].
11
References
[1] R. Mirzoyan, Astroparticle Phys. 53, p.91-98, (2014).
[2] Janez Strnad, Fizika 3. del , p. 81-82, DMFA, Ljubljana (2009).
[3] http://www.mpi-hd.mpg.de/hfm/HESS/ .
[4] A. Kohnle, J. Mattes, et. al., Nucl. Instr. and Meth. in Phys. Res. A
442, p. 322-326, (2000).
[5] K.Bernlöhr, O. Carrol, et.al. , The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescope Part I. , (2003).
[6] http://arxiv.org/pdf/astro-ph/0408375.pdf .
[7] http://en.wikipedia.org/wiki/Victor Francis Hess .
[8] Mallet L C.R., Acad. Sci. Paris, 183, 274, (1926).
[9] //magic.mpp.mpg.de/ .
12