My name is Rainer W. Novotny and I am working as a senior scientist at the 2nd Physics Institute of the Justus-Liebig-University at Giessen in Germany. I received my PhD in 1976 at the University Heidelberg and moved in 1983 to Giessen. The main part of my research career was dedicated to the field of Nuclear and Hadron Physics developing new detector concepts and systems to measure high energetic particles and photons, which are emerging from collisions, when accelerated particles are hitting nuclei and convert the kinetic energy for example in the production of new particles following Einstein's well known formula E=mc2. The relation tells us that energy (E) and mass (m) are equivalent.

In these years I gathered the expertise in a special type of detector which are called scintillators. These are materials which absorb energy as carried by photons or by stopping energetic particles and convert the deposited energy into visible or ulta-violett light in a proportional manner. If one collects the light in a quantitative way with appropriate photo sensors, one gets a set of informations: the deposited energy and the time, when a particle was stopped, since the mechanism of generating light can be very fast on the level of 1/1,000,000,000 seconds. This mechanism of scintillation is usually better known as fluorescence or phosphorescence. Minerals, when illuminated with ultraviolet light, start to shine in different impressive colors. Phosphorescence was used to equip the numbers on the face of a clock by an appropriate paint. During the day daylight is absorbed by the paint and emits the scintillation photons over a long period of hours during night time giving a chance to read the hour in the dark. Nowadays one can buy many toys which glue in the dark after being exposed to light.

The effect of fluorescence when illuminating a mineral with UV-light

(picture taken from http://de.wikipedia.org/wiki/Datei:FluoriteUV.jpg)

The present WP22 work package is a continuation of the former WP21 dealing with the development of thin fibers made of inorganic scintillating materials. As explained before, scintillators provide the possibility to identify high energetic particles and photons by the light generated due to the interaction of the incoming radiation with the electrons  in the material. Scintillators can be based on organic materials, large complex molecules which are contained in some host material or inorganic crystals. Some of the well known materials are NaI(Tl), CsI, PbWO4 or more complex mixtures abbreviated as BGO, LuAG or LYSO, respectively. There are many different internal mechanisms which lead to the generation of scintillation light. As a consequence, the emitted light can be of different color and be emitted within short time intervals of 10-9 s up to 10-3s. The composition and density of the material can have very different absorption power and sensitivity to various types of radiation. That is similar to the fact that a sheet of lead is more appropriate to protect against X-rays than a thick layer of wood containing only light elements like carbon, oxygen or nitrogen.

Schematic view of a scintillation detector

The above mentioned materials can be produced in a wide range of geometrical shapes and sizes. The inorganic crystals are grown from a melt in a very time and energy consuming technology. The raw crystals are later on cut, shaped and polished to guarantee, that the generated scintillation light can escape effectively and reach any type of used photo sensor, which converts the light intensity in an electric signal. The active volume of such a detector determines in addition the detection point. Since organic scintillator material can be produced as a liquid mixed with the active molecules and being casted in any shape, fibers have been produced similar to optical fibers used for signal transmission. These fibers can be produce with round and quadratic cross section in diameters down to 0.2mm and length up to hundred of meters in length. However, organic scintillators are composed of light elements like carbon and oxygen. Therefore, they have a low efficiency for absorbing high energetic photons or even X-rays and they are not very tolerant being operated in a strong radiation environment.

Typical shapes of organic
scintillating fibers.

Therefore, the present work package has concentrated on a new technology, to grow fibers made of inorganic materials, like LuAG or LYSO, containing very heavy elements, which fulfill all the envisaged properties such as high absorption power, bright scintillation emission and radiation hardness, for example. These fibers are pulled under very critical conditions out of a crucible containing the melt kept at a temperature up to 2000oC. Unfortunately, there are many parameters which have to be optimized to grow homogeneous fibers of equal quality, without cracks and up to a maximum length of 1m.

So far, many fibers based on both materials have been produced. In spite of remaining imperfections the achieved results are looking very promising.

Typical scintillation crystals prepared for detector assembly: BaF2, LYSO, PbWO4
Schematic view of a scintillation detector

3) The development of the inorganic fibers is a collaboration among the manufacturer FiberCryst in Lyon and collaborators at the physics institutes at the universities at Bonn (Germany), Groningen (The Netherlands) and the GSI at Darmstadt (Germany). The latter one is a joint effort in using and optimizing so called silicon photomultipliers as photo sensors. The small devices with an active surface of a few mm2 are perfectly suited to readout individually the small fibers with typically 0.5 - 1.5mm2 diameter.

4) As mentioned before, inorganic fibers would deliver all missing properties of the widely used organic fibers. The geometrical dimensions provide automatically fine granularity and position information on the level of the diameter. Bundles and arrays of these fibers could be used to measure beam profiles, track the path of ionizing particles or serve as active targets. Detectors, which are designed to measure at very high energies the energy of detected particles - commonly called calorimeters - require a large active volume to confirm that the measured particle are completely stopped. This requirement is like in a calorimeter, where we measure the correct increase of the temperature only when the added component is completely contained. At very high energies we would need a huge volume of inorganic scintillator crystals to meet that conditions, which is unrealistic since the costs would explode and the maximum size of an individual crystal to be grown is technologically limited. Therefore, so called sampling calorimeters are built. Only a small percentage of the volume is active and provides a signal. The main part is made of dead absorber material, to guarantee complete stopping. Many of these detector are using organic fibers for the readout, which could be replaced by the new fibers leading to significantly improved detector quality.

As in many cases, basic detector concepts in medium and high-energy physics have initiated application in industry and medicine. A typical example is the so called Positron Emission Tomography (PET),  which is exploiting the mechanism, that a positron - the anti-particle of the electron - when it combines with an electron in matter, annihilates by leading to a pair of photons of 511 keV energy emitted in exactly opposite direction. If a punctual source of positrons is creating many of these photon pairs emitted in different directions one can reconstruct the origin if one detects these photon pairs in coincidence. Using a fine segmented detector array, which covers completely the emission source and provides the point of impact of the photon pairs, the crossing of all reconstructed photon directions identifies the source position. In the medical application, molecules like sugar marked with positron emitting isotopes are injected into the human body and might be primarily collected in very active cells such as in a tumor. The local annihilation in these cells will mark in the tomograph the location of tumor. The accuracy for location is strongly dependent on the position information for the point of impact. Using high quality short inorganic fibers in dense packages could provide detector elements with extremely high granularity.

The present work package, which is dealing with a new technology to grow detector material to open up a wide field of applications in basic research or applied sciences or even medical application, appears to be a very special task. However, the steps to reach a practical solution and prepare all installations for an efficient mass production of something so far not available contains the whole spectrum of experimental work in science. One has to confirm and fix the goals, develop new technologies, set-ups for testing, communicate with the producer for further improvement, collaborate in particular in a small international community and make your results public in publications, conference talks or in the internet. These typical steps and task cover the experience a young student can gather when working on a Bachelor or Master of Science Thesis or later as a PhD student. Such an education process is on one hand exciting, not boring and brings young people in a position to have many opportunities in our technology oriented society.


The HadronPhysics3 project is supported by the European Union
under the 7th Framework Capacities Programme in the area of Research Infrastructures (RI).