BULK SINGLE CRYSTAL DOPED ZINC OXIDE (ZnO) SCINTILLATOR

BACKGROUND OF THE INVENTION

This invention corresponds to detection of high energy particles and radiation, called scintillation, and more specifically, a fast, bright inorganic scintillator based on wide, direct band gap ZnO based bulk single crystal. A scintillator material converts high energy particles (proton, electron, neutron, a - particle etc) or high energy photons (X-ray, 7 ray) onto ultraviolet (UV)/visible photons which are easily detectable with a conventional photomultiplier tube, semiconductor detectors etc. For over a century, inorganic materials have been employed for use in detecting these high energy sources. For a perfect scintillator, the crystal should possess the following properties: high density and atomic number, high light output, short decay time, easily detectable wavelength emission, mechanical ruggedness, radiation hardness and low cost. Tremendous work has been devoted to develop these scintillators and a review of these materials is given by S.E.Derenzo, MJ. Weber, E.B.Courchesne and M.K.Klintenberg "The quest for the ideal inorganic scintillator" Nuclear Instruments and methods in physics research A Vol. 505 (2003) 111; J.L. Humm, A. Rosenfeld, A.D.Guerra "From PET detectors to PET scanners" European Journal of Nuclear Medicine and Molecular Imaging Vol.30 (2003) 1574; and W.W.Moses "Current trends in scintillator detectors and materials" Nuclear Instruments and methods in physics research A Vol. 487 (2002) 123. A summary of the properties of the scintillator crystals used is tabulated in table 1.

The current commercial scintillator crystals do not have all the properties desired. This has led to discovery of semiconductor-based scintillators. Direct band gap semiconductor scintillators are a class of material that have been identified as possible future scintillators due to very fast response decay times as well as being reasonably luminous. It has been reported that Ga doped ZnO powder can be used to make an extremely fast scintillator (W. Lehman "Edge emission of n-type conducting ZnO and CdS," Solid State Electronics, Vol. 9 (1966) 1107) with relatively very high light yield, and it has been successfully employed in detecting alpha particles (P.A.Hausladen, J.S.Neal, J.T.Mihalczo - "An alpha particle detector for a portable neutron generator for the nuclear materials identification system (NMIS)" Nuclear Instruments and Methods in

Physics Research B Vol. 241 (2005) 835). It is believed that single crystals of doped ZnO will result in even better optical emission properties. Indium (In)- doped ZnO single crystals show a decay time of 0.65 nanoseconds (ns) compared to plastic scintillator with decay time of 1.0 ns (P.J.Simpson, R. Tjissem, A.W.Hunt, K.G.Lynn, V.Munne "Superfast timing performance from ZnO scintillators" Nuclear Instruments and Methods in Physics Research A 505 (2003) 82). Growth of In doped ZnO single crystals by hydrothermal technique has been reported (A. Yoshikawa, Y.Kagamitani, D.Ehrentraut, H.Ogino, M.Nikl, T.Fukuda, I.Niikura, K.Maeda "Growth and scintillation properties of ZNO for ultra fast semiconducting scintillators" Presented at the 16 ^th American Conference on Crystal Growth July 2005 Big Sky, MT) in the abstract of 16 ^th American Conference of Crystal Growth. Furthermore, an investigation on ZnO based scintillators has been presented (L. Boatner "Investigation of ZnO based scintillators" Presented at Nuclear Science Symposium and Medical Imaging Conference. Puerto Rico, October 23-29, 2005).

It should be noted that the applicant's claims detail in situ doping in bulk form, so dopant uniformity is relatively constant through the thickness of the substrate, especially when compared to implantation or diffusion techniques. It is of interest that hydrothermal bulk growth techniques typically include an alkaline mineralizer containing one or a combination of the acceptor dopants: Li, Na, or K. These impurities incorporate into the lattice and greatly affect the scintillating properties.

SUMMARY OF THE INVENTION

The main objective of this invention is to have a very fast and bright scintillator, by incorporating Group III and/or lanthanide elements into bulk ZnO single crystals with a concentration ranging from 0.0001 to 10 mol. %. The doped scintillator crystal converts ionization energy into fast scintillation light with very high efficiency at room temperature. The decay time of the scintillator is 100ns or shorter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram of the modified Bridgman growth apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is a superior inorganic semiconductor scintillator based on doped ZnO bulk single crystal. The scintillator crystals are doped by any combination of group III elements and lanthanide series elements. Doping with different combination between group III elements and lanthanide elements provide very fast, highly bright scintillation mechanism. This doped ZnO single crystal scintillator works on the basis of radiative recombination of electron-pair created due to high energy particles and radiation. Doping of ZnO introduces a degenerate donor band overlapping the bottom of the conduction band and increases the conductivity of the material ionization holes due to high energy radiation and high energy particles resulting in broad near band- edge emission.

The doped ZnO crystals are grown by high pressure melt growth technique. The crystal growth apparatus, seen in Fig.l, utilizes a modified Bridgman growth technique including a pressure vessel that contains pressurized oxygen (1). The apparatus also includes a cooling unit (2) that is situated in the pressure vessel. The cooling unit receives a coolant flow from outside of the vessel (3) and has cooled surfaces that define an enclosure, which receives the ZnO with proper dopant concentration (0.0001 to 10 mol.%).

The apparatus further includes an inductive heating element (4) situated in the vessel, which is coupled to receive rf power externally to the vessel (5). The element heats the interior portion of the doped ZnO to form a molten interior portion contained by a relatively cool, exterior solid-phase portion of the doped

ZnO that is closer relative to the molten interior, to the cooled surfaces of the cooling unit. Due to the pressure exerted by the gas contained in the vessel, the liquid interior of the doped ZnO becomes congruently melting to prevent its decomposition. The cooling unit is then lowered (6) through the element to produce crystal nucleation at the base of the cooling unit and preferential crystal growth through the distance traveled.

In addition to rf power, the heating element receives a coolant flow (7) from a feed through that extends through a wall of the pressure vessel. In proximity to the vessel wall, the feed through has two coaxial conductors (8) to improve the electric power transfer to the heating element and to reduce heating of the external surfaces of the vessel. The two conductors of the feed through are cylindrical in shape, and define two channels for channeling a coolant flow to and from, respectively, the heating element.

Example 1

A precursor which will yield 0.0001 mol% Ga is added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 2

A precursor which will yield 1 mol% Ga is added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 3

A precursor which will yield 5 mol% Ga is added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 4

A precursor which will yield 0.0001 mol% Gd is added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 5

A precursor which will yield 5 mol% Gd is added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 6

Precursors which will yield 0.1 mol% Ga and 1 mol% In are added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 7 Precursors which will yield 1% mol% Gd and 1 Mol% Lu are added to the

ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 8

Precursors which will yield 1 mol% hi and 1 mol % Gd are added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 9

Precursors which will yield 5 mol. % Ce and 3 mol% Tl are added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

Example 10

Precursors which will yield 4 mol.% In , 1 mol% Ce , 2 mol% Lu, and 5 mol. % Ga are added to the ZnO precursor before crystal growth in the cooling unit (2). The charge is melted and crystals are directionally solidified as described. The resulting crystals are processed into polished substrates.

To one skilled in the art, it should be immediately obvious that there exist innumerable combinations that can be utilized to achieve the net positive addition of dopants to a ZnO single crystal using a variety of dopant impurities. The specified embodiments serve as descriptions of possibilities but do not limit the scope of the invention.