High energy cosmic gamma rays detectors

Transcription

1 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.

2 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 Telescopes The mirror facets Photo detectors Winston cone Trigger system Conclusion 11 1 Introduction The Earth is constantly exposed to high-energy particles from the space. Cosmic rays enter the Earth atmosphere even with energies beyond ev, their flux decreases by 20 orders of magnitude over the energy range ev ev. Between ev and 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 First experimental report on a bluish glow, now known as Cherenkov radiation, is attributed to Madame Curie in the But first systematic study of the effect did not take place until 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

3 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 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 Nature in 1953, which is marked as the beginning of the atmospheric air Cherenkov technique. 3 Cherenkov radiation In a transparent substance with a refractive index n, the speed of light is equal to c = c 0 n, where c 0 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 c 0, 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

4 Figure 2: Cone of Cherenkov radiation β = v c. It is easy to see (Fig 2) that cos (ϑ) = c 0. Cherenkov radiation has a continuous nv 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 m 2 reach the ground. They arrive within a very short time interval of a few nanoseconds (as shown in figure 3). 4

5 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

6 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

7 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 m 2, 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 d 1, 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

8 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 Protection Diameter of mirror Diameter of reflecting surface Mounting Focal length Specular reflectivity Point spread function Aluminized optical glass, thickness 15 mm Quartz coating for outdoor use 600 ± 1 mm 600 ± 0, 5 mm glued to support unit at three points 15, 00 ± 0, 25 m at least 80% between 300 and 600 nm 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

9 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 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

10 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

11 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

12 References [1] R. Mirzoyan, Astroparticle Phys. 53, p.91-98, (2014). [2] Janez Strnad, Fizika 3. del, p , DMFA, Ljubljana (2009). [3] [4] A. Kohnle, J. Mattes, et. al., Nucl. Instr. and Meth. in Phys. Res. A 442, p , (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] [7] Francis Hess. [8] Mallet L C.R., Acad. Sci. Paris, 183, 274, (1926). [9] //magic.mpp.mpg.de/. 12