2020年2月11日星期二

New Material For Scintillation Crystal -- Lead Tungstate

When the inorganic crystal is subjected to high-energy particles or high-energy rays (such as X-rays, gamma-rays), a phosphor that emits visible light due to excitation of particles or rays, which is a scintillation crystal, and lead tungstate is a scintillation crystal.

lead tungstate crystals image

This property makes lead tungstate an indispensable part in nuclear medicine, high-energy physics, nuclear technology, space physics and petroleum exploration. Scintillation crystal is mainly used for CT machine, particle collider, detector and even laser weapons and other manufacturing, the common oxide scintillation crystals are mainly bismuth germanate, cadmium tungstate, calcium tungstate, cadmium molybdate and calcium molybdate, lead tungstate is a new scintillation crystal found in 1990s relative to other crystals.

Lead tungstate (PbWO4) monocrystalline belongs to the scheelite structure, its melting point is 1123 °C, its scintillation performance is not lower than other tungstate crystals, In 1990, some scholars used Czochralski method to lift the lead tungstate single crystal for the first time. This crystal has crystal coloring defects, and its coloring mechanism can be considered as follows: the formation of oxygen vacancies leads to the formation of color centers, including oxygen vacancies, color centers, defects and Fe2+.

A center of color complex formed by impurity defects; Fe2+ produces light absorption near 4 60nm. The annealing process is carried out in an air atmosphere, enabling the diffusion of oxygen components into the crystals to reduce the number of oxygen vacancies Vo, thus improving the performance of the crystals to some extent. It was also found that the irradiation hardness and transmittance of lead tungstate crystals in short wavelength range (330450 nm) were obviously enhanced by doping some trivalent ions so as to improve the optical properties of the crystal.

Although the lead tungstate crystals have defects of low luminous efficiency, because of its large density, small radiation length, strong radiation hardness and short attenuation time, it is the preferred scintillation crystal for Large Hadron Collider high resolution electromagnetic calorimeter.

Lead tungstate crystals also have a glorious history, in September 20, 2008, the world's largest scientific program, CERN, was formally operated by the Large Hadron Collider (LHC), it began the journey of human beings to reveal the origin of the universe and to find the source of quality. CMS (Compact Muon Selinoid) is one of the four projects of the Hadron Collider project, from May 2005 to March 2008, the Shanghai Silicate Research Institute provided nearly 5000 large size lead tungstate crystals for CMS electromagnetic energy devices. The Shanghai Silicate Institute has also won the CMS Industrial Award top prize awarded by the European Nuclear Center (CERN) in Geneva, Switzerland.

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