Domestic Crystals Manufacturing for Photonics Devices. Part 1 – Scintilliators
The technologies for detecting ionizing radiation are used in astrophysics, high-energy physics, in computer tomographs, for geological mineral survey, in systems used for customs screening, detection of drug substancess and explosives. Solid-state semiconductor and scintillation detectors are used to detect radiation. The principal difference between the operation of solid-state semiconductor and scintillation detectors for detecting ionizing radiation is that when scintillation material interacts with radiation, an optical signal is formed, while in the case of a semiconductor one, it immediately forms an electrical signal. (Fig.1)
The trends of recent years and the state of affairs in Russia in the field of obtaining crystals for the manufacturing of both scintillation and semiconductor detectors for detecting ionizing radiation are considered in this paper. The first part is devoted to some new scintillation materials. Such materials can be used in the technique of detecting ionizing radiation for medical diagnosis, three-dimensional positron-electron computed tomography and X-ray computer fluorography, nuclear geophysics, nondestructive testing and food quality assessment. Obviously, that the requirements for scintillators in medicine and in other fields, for example in physics, are different. First of all, for medicine, the effectiveness is of prior importance, which will ensure a reduction in the radiation dose of the patient. For physics, where detectors can contain tons of scintillator, the price of the scintillator material and its radiation strength are of importance, which is not determinative for medical applications due to the small size of the installations and small radiation doses. New scintillation materials for specific requirements of nuclear medical diagnostics are mainly considered in this article.
The world trends in the field of medical instrument making have undergone significant changes in recent years. This is caused by the need to improve the quality of diagnostics, which leads to the creation of new highly informative diagnostic devices. One of the methods for organ imaging is radiography. The quality of the image obtained by X-ray diffraction is determined by two parameters: spatial resolution and contrast sensitivity.
Currently, the sensor for converting X-rays into light using a scintillator is the most widely used technology in digital radiography. Inorganic scintillators are used to detect X-ray and gamma radiation, such as gamma rays with energy of 511 keV, since they have a higher density and atomic number, thus increasing the detection efficiency . New scintillation substances have high consumer properties, namely, high density, high light yield, short time of scintillation, which widens the range of their application.
However, often radiography is powerless to detect pathologies. For example, X-rays do not show intervertebral discs, since the X-ray is not blocked by the cartilaginous tissue. This results in the need to use other modern methods of tomography.
Tomographic layer-by-layer imaging of the internal structure of an object has a major difference from the usual shadow one. Its determining value for medical diagnostics is that it does not contain interfering shadows. In the most structurally-complicated medical images, the abundance of superimposed shadows of various organs worsens the subjective perception of small contrast details by several times.
The methods of tomographic studies include PET – positron emission tomography.
POSITRON EMISSION TOMOGRAPHY
PET is one of the most informative methods used in nuclear medicine for the diagnosis of oncological, neurological and cardiac pathologies. The principle of positron emission tomography is based on the phenomenon of registering two oppositely directed gamma rays of the same energy that arise as a result of annihilation. The annihilation process occurs when the positron emitted by the nucleus of the radionuclide (radioisotope) meets the electron in the tissues of the patient (Fig. 2).
PET is the most sensitive study of the entire organism, capable of assessing not only anatomical (similar to computed tomography), but also initial functional changes in any human organs at the cell level due to the positron-emitting isotope’s ability to penetrate into the cell of the organism. The greatest practical application of PET is found in oncology, cardiology and neurology, although it can be used in other fields of medicine. The main producers of PET scanners are currently General Electric, Siemens, Philips, Shimadzu.
The first PET scanners were equipped with NaI:Tl-based scintillation crystals. Moreover, each crystal was equipped with an individual photodetector. With the detection of bismuth-germanium crystals (BGO), most of the instruments were switched to this material, since it has a higher efficiency in detecting gamma radiation. The detecting unit acquired a more universal design, consisted of 64 elements made from BGO, and was equipped with 4 photodetectors (PMTs). The materials based on complex oxides of lutetium, lead, bismuth, tungsten, etc. were also synthesized. Later, other scintillation materials such as BaF2, YalO3:Ce, Gd2SiO5:Ce (GSO) were developed. More recently, Lu2SiO5:Ce (LSO) based crystals have been discovered .
A typical scintillator for PET is a transparent single crystal wherein the width of the forbidden band is over 5 eV. There are no energy levels in the band gap in a perfect single crystal, free from defects and impurities. However, most scintillators are doped with activating ions, which create specific energy levels in the band gap. REM ions are used as those additives. After the absorption of ionizing gamma radiation by the crystal, some of the energy is localized in the activator ions. The transition of these ions from the excited state to the quiescent state results in emission of photons usually in the region of 4 eV, which corresponds to visible blue light (blue: 485 to 500 nm or 2.55–2.48 eV).
COMMERCIAL SCINTILLATORS PROPERTIES
The currently existing material with characteristics best suitable for practical use is usually selected.
Another way is to create a new scintillation material that fully meets the requirements to the most extent. Most of the world leaders in the research and production of scintillation crystals are currently moving in this direction. Tables 1 to 3 show the characteristics of some commonly available scintillator materials .
BGO (bismuth germanate) has a high density and atomic number, which makes it possible to effectively detect gamma radiation. (Fig.3). It also offers high strength and is non-hygroscopic, which makes it relatively easy to manufacture a detector. Single crystals of BGO are grown in Russia at The Institute of Semiconductor Physics of SB RAS. Cadmium tungstate crystals and gadolinium silicate (GSO) are also good candidates, however they are easily broken due to the presence of cleavage in these crystals, which makes the detector more difficult to manufacture. The table shows the scintillation and optical properties of some scintillators listed in the order of afterglow time increasing.
BaF2 has the minimum afterglow time of 0.8 ns. However, the radiation is weak and located in the far UV region of the spectrum at 220 nm, which requires photodetectors with more expensive quartz windows. It also has a long secondary component of the afterglow spectrum equal to 600 ns.
Cesium fluoride (CsF) has a very short afterglow time of 4 ns, but its intensity is so weak that this scintillator is practically not used.
The main efforts of specialists are aimed at finding such material formulations that consistently provide an increase in the scintillators performance of PET scanners.
MOST ADVANCED PET SCANNER SCINTILLATORS
When crystals based on Lu2SiO5:Ce (LSO or LCO), designated for the most advanced PET scanners , have been discovered, it occurred that Lu2SiO5:Ce has a number of advantages. Firstly, the best combination of afterglow time in 40 ns, and high radiation intensity. Secondly, it does not have secondary (slow) components of afterglow.
It should be noted that research of influence of various alloying additives in scintillation crystals, which affect the afterglow time and light yield has recently intensified significantly.
For this purpose, the results of investigations of scintillation detector based on LaBr3:Ce crystal and photomultiplier tube FEU(PMT)-184 are presented . Promising results on joint doping of cerium and other components (Li, Mg, Ca, etc.) have been obtained.
The Russian scientists Yu.D.Zavartsev, A.I.Zagumenny, S.A.Kutovoy have carried out the systematization of information on the commercialization of research results in the development of scintillation materials based on cerium-doped lutetium orthosilicate [6, 7]. The authors state that the research of scintillation materials based on cerium-doped lutetium orthosilicate carried out since 1992 were promising and have been developed in many foreign universities and leading companies: Philips, Siemens, Hitachi, Saint-Gobain, etc.
In 1992, the same authors grown Ce:LSO crystals and provided them for research by Russian scientists working in the field of high energy physics at the Physical Institute of the Russian Academy of Sciences and Moscow University . These samples were used for experiments at CERN (Switzerland) and Germany; the results of the studies were presented at conferences.
In 1994–1997, Yu.D.Zavartsev, A.I.Zagumenny, S.A.Kutovoy completed the first world’s studies of the properties of mixed lutetium-gadolinium scintillating crystals Ce:Lu2-xGdxSiO5 (LGSO). Mixed crystals of garnets have been the subject of research and publications of the authors. A similar methodology has been used by the authors to change the chemical composition of the lutetium-based scintillation substances.
Earlier, before the study of Ce:Lu2-xGdxSiO5 crystals, it was believed that the solid solution will show deterioration in the energy resolution of the scintillation substances in comparison with monocationic compounds. However, studies have shown that partial replacement of lutetium ions with gadolinium ions does not lead to degradation of the most important scintillation parameters.
PERIODIZATION OF PRODUCTION DEVELOPMENT
During 1991–2016, the compositions have been developed and lutetium oxide-based crystals commercial production has been set up, which historically can be divided into three generations (Table 4.)
During 2010–2014, Hitachi began to growth crystals Ce:Lu2-xGdxSiO5 (LGSO) for tomographs manufactured in Japan. The parameters of LGSO-based scanners are second to LSO, LYSO and LFS (Lutetium Fine Silicate) crystals, so since 2016 Philips uses pixels only from the LFS crystals grown in China for its scanners.
The technology of growing large MLS crystals is organized in Canada, the main suppliers of large LFS crystals are the USA and China. Depending on the LFS composition and the size of the crucible, the diameter of the large crystal can reach the diameter of 65 to 95 mm. Although the high world demand of crystals, the manufacture such materials is absent in Russia.
SCINTILLATION SINGLE CRYSTALS MANUFACTURING TECHNOLOGIES
The main method of growing single crystals of complex inorganic scintillation materials is the normal method of directional solidification in its different modifications: methods by Czochralski, Kyropoulos, Nacken, Stockbarger, Bridgman, Stepanov.
One of the determining factors in choosing a crystal growth method is the purity of the initial charge, the temperature and the melting behavior of the material synthesized, as well as the presence of various phase transitions in the temperature range from melting to room temperature. The melting range for various commercial scintillators is extremely high: from 1123 °C for PbWO4 to 2150 °C for LSO. These factors largely determine the hardware technological design of the growing process. Minor errors and lack of experience lead to huge material losses, since the burnt iridium crucible of large diameter and the outflow of the melt is the loss of 20–40 kg of expensive lutetium oxide and expensive repairs or the manufacture of an iridium crucible. An important aspect is also the post-growth high-temperature treatment of grown crystals and blanks. These treatments are able to significantly affect the basic scintillation parameters of the material, as well as reduce the percentage of rejection of crystals during the machining stages of the crystals.
There is a rather powerful production base for the production of modern equipment in Russia. The production facilities are preserved in Chernogolovka at the premises of EZAN, in Bryansk – at the premises of LLC "SPO GKMP", in St. Petersburg – at the premises of LLC "Apex", in Moscow in JSC "Giredmet", etc.
JSC "Giredmet" has many years of experience in growing single crystals. In its laboratories, practical work is carried out to grow crystals using the methods of Czochralski, Stockbarger, Bridgman  (Fig.4). More detailed characteristics of the crystals obtained in JSC "Giredmet" will be considered in the second part of the article.
Taking into account the information presented in JSC "Giredmet", it is not difficult to set up and introduce growth systems in a rather short time for prospective scintillation materials.
As noted above, LGSO is one of the promising materials that are already used worldwide in PET scanners and the latest generation radiography. However in the Russian reality, the LGSO needs such an approach to its implementation in order to go from laboratory growth and research to commercial application.
The development of new LGSO-based scintillators and the improvement of existing materials can significantly contribute to improving the equipment of nuclear medical diagnostics, which will favorably affect the performance of domestic health care system.
The critical moment, representing an independent task for the Russian industrial production of PET, is to provide domestic source rare earth metal based materials, which guarantees consistent quality parameters of the device operation – thallium compounds, lanthanum, lutetium, yttrium, cerium, gadolinium, silica high purity (not less than 99.9995%) and a given modification.
Natural sources of rare earth metals (REM) are the ores of the Kola Peninsula, the Tomtor deposit (Yakutia) and the Kutessai-Aktyuzskoye deposit (Kirghizia). In different periods, Institute "Giredmet" have developed technologies for full-cycle processing the ores of these deposits, including listed high purity oxides as final products.
Currently, JSC "Giredmet" accumulates knowledge and experience both in growing single crystals and in obtaining high-purity oxides. In 2017, JSC "Giredmet" carried out research to obtain high-purity compounds of lutetium and cerium. The conditions for the synthesis of their individual oxides with a purity of not less than 99.9995% have been developed. The following materials are studied as sources of REM: carbonates of Solikamsk plant for the production of high-purity cerium oxide; Chinese oxide (concentrate) of low-quality lutetium to produce high-purity oxide, since there is no source of lutetium in the country today.
In addition to many years of specialization in research, the company has vast experience of cooperation with the country’s leading scientific and industrial enterprises as part of the joint federal and contractual projects.
Considering the aforesaid, JSC "Giredmet" offers to solve the problem of import substitution in the production of crystals consisting of high-purity rare earth oxides, such as Lu1.8Y0.2SiO5 (LYSO)/Lu2SiO5:Ce and LaBr3:CeBr3 in scientific and technological consortium cooperation: JSC "Giredmet", GPI RAS, Kurchatov Research Center, LLC "Nuclear Technologies in Medicine", Moscow Institute of Steel and Alloys, JSC "Scientific-technical company "FOMOS MATERIALS", JSC "NIITFA", ISSP RAS.
There is a need to develop the technology and process equipment for the production of small-tonnage material having a combination of excellent counter and spectrometer properties, while dramatically reducing exposure of the subject under high-energy impact. The developed technologies will ensure import substitution in full and create prerequisites for the export of new optical products, obtaining products that exceed the world level; they will improve the efficiency of the process equipment.
The second part will relate to the trend of recent years and the state of affairs in Russia in the field of producing crystals for the manufacture of semiconductor detectors for the detection of ionizing radiation.
This research was financially supported by the Russian Ministry of Education in the framework of Agreement 14.579.21.0138 named: "Development Technology of Manufacturing Submicron Powders of Rare-Earth Oxides of High Purity for Synthesis of Scintillation Crystalline Materials of Detecting Medical Systems", unique identifier of Applied research and experimental development: RFMEFI57916X0138.