JPH09118593A - Production of lutetium-containing crystal for scintillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a crystal containing lutetium having uniform scintillation characteristics by forming melt of lutetium compound and making grow a crystal from the melt while maintaining an interface between the crystal and the molten melt in flat condition.

SOLUTION: A crucible 12 is arranged within housing 10 of Czochralski crystal growing apparatus. And a mixture containing high purity Lu₂O₃, SiO₂ and CeO₂ is supplied into the crucible 12 and the mixture is melted by heating with RF induction heating coil 14. Next, a seed crystal is inserted from a hole provided in the housing 10 and is made contact with the melt. Next, while maintaining the interface between the crystal and the melt in flat condition, the crystal containing lutetium (example; lutetium oxy-ortho silicate crystal) is grown from the melt by Czochralski method. And the obtained crystal containing lutetium is used as a scintillator and scintillation detectors such as gamma-ray/X-rays detector, etc., are manufactured.

COPYRIGHT: (C)1997,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for growing lutetium-containing crystals such as lutetium oxyorthosilicate crystals, and more particularly to such a method for growing crystals for use as a scintillator. It is a thing. The present invention also provides improved scintillation detectors incorporating such crystals.

[0002]

Cerium-doped lutetium oxyorthosilicate crystals are known to be useful as scintillators for gamma and X-ray detectors. US Pat. No. 4,958,080 and US Pat. No. 5,02
No. 5,151 is Ce _2x Lu _{2 (1-x)} SiO ₅ and still 2 × 1
A single crystal scintillator formed from a melt represented by the formula 0 ^-4 <x <3 × 10 ^-2 (hereinafter, the term LSO is used as a symbol representing this formula) is described. LSO
The method proposed for growing the is the Czochralski crystal growth method, which is the outline "Crystal Growth: AT
UTORAL APPROACH) ", W. Bard
sley, D.I. T. J. Hurle, J .; B. Mul
lins (eds) North Holland Publishing Company, 1979
Year 189-215. This method is for the growth and application of "Gd ₂ SiO ₅ : Ce scintillator for gadolinium orthosilicate scintillator (G
rowh and applications of
Gd ₂ SiO ₅ : Ce scintillator
s) ", T.S. Utsu and S.H. Akiyama, Journal of Crystal Gloss 109 (1991),
385-391, and "Czochralski growth of rare earth oxyorthosilicate single crystals (C
zochralskigrowth of rare
earth oxyorthosilicate si
single crystals) ”, C.I. L. Melc
her, R.H. A. Manente, C.I. A. P
eterson and J.M. S. Schweitzer,
Journal of Crystal Gloss 128 (199
3), 1001-1005, and "A Macintosh-based system for Czochralski crystal growth (A Macintosh-based System).
For Czochralski Crystal
Growth), C.I. L. Melcher, R.M.
A. Manente, C.I. A. Peterso
n, J. et al. S. Schweitzer, M .; A.
Singelenberg and F.S. J. Bruni,
Scientific Computing and Automation, January 1994, 39-45, LSO Scintillator.

While these methods can be used to make scintillator crystals useful in certain applications, they vary between the scintillation behavior of different crystals cut from a single bowl of LSO. There is. This variation is not seen in properties other than scintillation behaviour, i.e. for all other intents and purposes the bowl appears to be uniform, but the resulting crystals are It has a gamma ray spectrum with broad or multiple peaks. However, LSO
For spectroscopic use of crystal scintillators, it is desirable that their behavior be as uniform as possible.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of making a crystal for use as a scintillator that exhibits a substantially uniform scintillation behavior throughout the crystal. Another object of the present invention is to provide an improved Czochralski crystal growth method for producing scintillator crystals.

[0005]

According to the present invention, there is provided a method for producing a lutetium-containing crystal, wherein:
It keeps the interface between the crystal and the melt from which it is drawn substantially flat as the crystal is grown. In the Czochralski growth method, the rotational speed of the crystal and its diameter are typically controllable to provide a flat interface when the crystal is drawn. The lutetium-containing crystals are preferably those that can be used as scintillators and can typically have lutetium oxyorthosilicate.

The shape of the solid-liquid interface is determined by the shape of the freezing isotherm. As a result, the object of the invention is achieved by controlling the shape of the freezing isotherm in the main part of the crystal growth. At the start and end of the growth, the desired conditions may differ. Temperature conditions are determined by crucible size and location in the heating environment, furnace design, insulating material used, and liquid melt flow pattern. The flow pattern of the liquid melt is
It is a function of melt viscosity, crystal diameter versus crucible diameter, and crystal rotation speed.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
The present invention will be described with reference to examples. Crystal growth is performed by a Czochralski crystal grower (CGS-5, Crystallox having a furnace as shown in FIG. 1).
x) company). The furnace has a housing 10 formed of a refractory and insulating material and having a 2 inch inner diameter iridium crucible 12 therein (see Engelhard, U.S. Pat. No. 4,444,728). ). The housing has holes for inserting iridium wires or seed crystals (not shown) within the crucible to act as seeds for the crystals. It is possible to rotate the wire during the crystal growth period.
An RF inductive heating coil 14 surrounds the housing at the level of the crucible 12. During use, the raw material, ie, preferably 99.99% pure Lu ₂ O ₃ , S
Powders of iO ₂ (dried at 1000 ° C.) and CeO ₂ are mixed in suitable proportions and pressed into pellet form (if appropriate) and loaded into the crucible.
The material is then melted and the melt is allowed to stand for a suitable period of time.
For example, aging is performed for 24 hours to stabilize the growth atmosphere. The melt is typically Ce _2x
It has a formula of Lu _{2 (1-x)} SiO ₅ , where 2 × 10 ⁻⁴
<X <6 × 10 ⁻² (a preferable range for x is 1 × 10 ^{−3 to} 2 × 10 ⁻² ). Crystals are drawn from the melt using either LSO rotating seed crystals or 1 mm iridium wire. A crystal bowl is grown in a nitrogen atmosphere, which can have a small amount of oxygen. During the growth period, the rotation rate, temperature (RF power), and crystal pull rate are controlled by a control system, which is described in more detail below. After pulling out the crystals, they are cooled to room temperature. This was carried out for 48 hours, but it was possible to cool it to room temperature in 7 hours without causing damage to the crystal.

The growth system is shown schematically in FIG. 7, which has a crucible load cell that allows automatic adjustment of the power level to maintain a specified crystal diameter during the growth process. It was possible to effectively grow a crystal bowl with the desired dimensions using DOS-based software supplied by Crystallox to control the crystal growth process. However, in order to improve real-time observation of control characteristics, access to archived data, and user-friendly conditions, LabVIEW 2.1.TM for controlling processes, including extensive software control of equipment.
It is desirable to use a Macintosh computer running 1 software (National Instruments). The basic prerequisite of this program is automatic radius control (ARC), in which case it should be possible to see all the parameters related to a particular operation and on the same screen display. Can be changed to. During the ARC mode, the measured and desired radii and% power are plotted on a strip chart-like display, which shows data for about 10 previous hours when the control cycle time is 5 minutes. . In addition, the current and average crucible weights are also plotted. Growth length, expected completion time, desired crystal shape, and other parameters are also available. Data from times earlier than those shown on the display chart can be quickly accessed by "scrolling" the display. Therefore, all relevant data for a particular growth operation can be accessed at any time during the growth process.

The Crystal Locks system is IEEE4
It uses both 88 and RS422 buses. Two Prema 6030DVMs communicate with an IEEE bus and Eurotherm 822 process controller, and two Crystallox Ms.
The SD4000 communicates via the RS422 bus (Fig. 7). The DVM measures the voltage produced by a load cell that weighs the crucible and its contents and a power pickup coil that monitors the RF generator output. The withdrawal and rotation speeds are controlled by the MSD 4000, and the Stenelco generator power output is controlled by the Eurotherm 822 process controller. The control computer is a Macintosh ci with video monitor that has access to the network.

The Eurotherm 822 digital-to-analog converter has 10-bit resolution in the normal power operating region of about 70% of maximum power. This resolution appears to be sufficient when compared to the observed reproducibility and stability of the Stenelco generator. The DVM resolution is 1 Vd. c. And 10Va. For the power pickup coil. c. It is.

The control is standard proportional integral derivative (PID)
It works through a feedback loop. The parameter controlled is the RF generator power. The power P (t) at time t is determined by the following equation.

[0012]

(Equation 1)

Note that P (0) is the power at time 0 when the automatic control is initialized, and is PG, IG, and DG.
Are proportional, integral and derivative gains respectively, and err
Is the instantaneous error in the crystal radius. Note that in current software the derivative gain is always zero.

Since Czochralski growth is not a stationary process, trends in power are observed over long periods of time. It has been found that calculating this trend (base velocity) and extrapolating in time is helpful in maintaining precise control. Therefore, equation (1) is modified to add a base velocity trend to the calculated power even for zero error conditions. This error is defined as the difference between the desired crystal radius and the measured crystal radius. The measured crystal radius is calculated from the following formula.

[0015]

(Equation 2)

DW / dt is the first derivative of the crystal weight, Pr is the drawing speed, R is the crucible radius, and ds and dl are the crystal and melt densities, respectively. The desired crystal radius is derived from an algorithm that calculates a smooth monotonic increase in crystal radius from the initial seed diameter to the full diameter, and a tail-off or decrease to zero radius at the end of the growth process. The second term in the denominator of equation (2) arises from the molten drop in the crucible.

The Crystallox traction mechanism uses a microstepper motor and is controlled by a drive which allows incremental changes in traction or withdrawal speed with an accuracy of 10,000 times. Utilizing this feature,
Software controls the crystal pull rate to maintain a constant and absolute growth rate throughout the process. In order to compensate for the change in melt drop rate as the crystal radius changes, the withdrawal rate is reduced during the seed diameter to full diameter growth period.

Ideally, the crystal radius is calculated from the value of dW / dt using a moving window of weight reading as a function of time. However, in current systems, the crucible experiences a levitation or levitation effect from the induction heating coil, which changes when the generator power is adjusted according to equation (1). Therefore, the weight and time values must be collected on a time basis during periods of no power adjustment. 5 in the current application
The time base of minutes is used for weight readings taken at 4 second intervals. This is the same technique used by the Crystal Rocks software, which produces very relevant results.

As mentioned above, due to the levitation effect of the crucible, it is not possible to extend the weight reading beyond the selected time base. As a result, dW / dt
(And the value of the crystal radius calculated from it) may be noisy. Most of the noise in this signal can be removed using the following types of digital filters.

[0020]

(Equation 3)

Note that FPi is the value of the filtered parameter at time i, Ri is the reading at time i, and f is the digital filter. The value of f is between 0 and 1. As you can see by inspecting equation (3),
If the filter is 0, no information will pass,
A value of 1 is equivalent to the unfiltered signal. This equation produces an exponentially weighted average of the measured parameters. It has been found ideal to eliminate Gaussian noise. From this calculated value it is possible to easily modify the software to use this type of digital filtering in many places so that the displayed values of crystal weight and radius are considerably smoothed. is there. However, in order to ensure a quick response of the control system, unfiltered values should be used exclusively in the control algorithm.

LSO has a density of 7.41 g / cm ³ and a melting point of about 2,100 ° C. The following data is from seed crystals obtained from the previous growth process or crystals grown from a iridium wire in a nitrogen atmosphere with 0.3% oxygen.

Bowl Mol% Ce seed Growth rate Diameter Rotational speed Interface LSO10 0.5 Ir wire 0.5 mm / hr 20 mm 20 rpm Convex LSO15 2.5 Ir wire 1.0 mm / hr 20 mm 20 rpm Convex LSO17 2.45 Ir wire 1.0 mm / hr 20 mm 65 rpm Truncated shows a gamma-ray spectrum from the conical LSO27 0.25 LSO seed 0.5mm / hr 26mm 59rpm flat LSO28 1.0 LSO seed 2.0mm / hr 26mm 59rpm of the flat bowl against each of these shapes and ¹³⁷ Cs in Figure 2-6 is there. From the table above, it is understood that it is possible to obtain the desired effect by adjusting the bowl diameter and rotation speed.
The actual diameter and speed required depends on the size and geometry of the crucible used (2 inches in this example).

2-4 show spectra from crystals that were not grown at a flat interface, the gamma ray spectra being large despite the radiation or irradiation source being the same in each case. Shows fluctuations. The spectrum shown therein is composed of the sum of multiple spectra at different light outputs and is due to the non-uniformity in the LSO crystal.

FIGS. 5 and 6 show spectra for crystals grown at a flat interface and show substantially similar spectra despite different growth conditions and compositions.

FIG. 8 shows an LSO manufactured by the method described above.
Figure 2 shows a schematic of an LSO scintillation detector that can be used. In FIG. 8, a single crystal LSO scintillator 110 is a gamma ray detector housing 112.
It is shown stored inside. One of the scintillators
One surface 114 is located in optical contact with the photosensitive surface of photomultiplier tube 116. On the other hand, it is possible to couple the light pulse to the photomultiplier tube via a light guide or fiber, lens, mirror, etc. The photomultiplier tube can be replaced by any suitable photodetector such as a photodiode, microchannel plate, or the like. The other surface 118 of the scintillator is preferably a reflective material, such as Teflon tape, magnesium oxide powder, to direct as many of each light flash as possible to the photomultiplier tube.
Surround or cover with aluminum foil or titanium dioxide paint. When the radiation, that is, the irradiation, is incident, LS
Light pulses emanating from the O-crystal are intercepted by the photomultiplier tube, either directly or by reflection from the surface 118, and the photomultiplier tube generates an electrical pulse or signal in response to the light pulse. These electrical output pulses are typically first amplified and then optionally processed, for example in a pulse height amplifier, to obtain the parameter of interest for the detected radiation. The photomultiplier tube is also connected to a high voltage power supply, as shown in FIG. With the exception of the LSO scintillator, all of the components and compositions described in connection with FIG. 8 can be conventional and therefore their detailed description is omitted.

The LSO scintillator detector of FIG. 8 is particularly effective as a radiation detector in a logging or logging environment for boreholes, eg, oil exploration. In this case, the detector forms part of a logging system which may be of the type shown in FIG.

FIG. 9 is a logging sonde for sensing gamma radiation obtained by bombarding the formation with high energy neutrons and for sensing the energy of the radiation for subsequent spectral analysis. 211 is shown. The sonde 211
Suspended in perforations 213 by armored multi-conductor cables 215. Perforations 213 traverse formation 217 and are filled with fluid 219 and are either open as shown or provided with a casing. The sonde 211 described below is U.S. Pat. No. 4,317,9.
It is possible to configure based on No. 93. Sonde 2
The 11 is moved in the perforation 213 by paying out and rewinding the cable 215 via the pulley 220 and the depth gauge 222 by a winch forming part of the surface apparatus 224. Normally, the logging measurement is actually performed while raising the sonde 211 along the perforation 213, but in some circumstances, the logging measurement may be performed when the sonde is lowered, or both.

The sonde 211 has a pulsed neutron source 226 for bombarding the formation 217 with fast neutrons to generate primary radiation as it rises along the perforations 213, whereupon And a radiation detector 228 for detecting secondary (gamma) radiation induced in the perforations 213 and the formation 217. Neutron source 22
6 is preferably a pulse accelerator of the type described in U.S. Pat. No. 3,461,291 and U.S. Pat. No. 3,546,512. This type of source is particularly suitable for controlling the duration and repetition rate to generate discrete bursts of high energy or fast neutrons, eg 14 MeV.

The detector 228 is of a type suitable for detecting gamma radiation and generating an electrical signal corresponding to each detected gamma ray and having an amplitude representative of the energy of the gamma ray. To this end, the detector 228 includes a photomultiplier tube (PMT) as shown in FIG.
216 has a cerium activated LSO scintillation crystal 210 optically coupled. Suitable photomultiplier tubes are manufactured by EMR Photoelectric Company of Princeton, NJ.

Limiting the detector 228 from being bombarded directly by neutrons from the source 226 is limited by such direct irradiation.
A neutron shield 234 may be provided between the source 226 and the detector 228 to avoid saturating the eight. Furthermore, especially in the case of capture gamma radiation measurements, the sonde 211 is surrounded by a boron carbide moistened sleeve 236 and the source 226 and detector 22.
It is possible to locate it in the approximate vicinity of 8. This sleeve absorbs neutrons scattered by the formation towards detector 228, while eliminating perforating fluid in the area of detector 228 and without significantly attenuating gamma radiation emitted from the formation. The net effect is
Sonde 2 near the drilling contents and detector 228
11 to reduce the potential for neutron interactions with the material, otherwise producing detectable gamma rays that constitute the unwanted disturbance of the required gamma ray measurement.

Electric power for the sonde 211 is supplied from the surface apparatus 224 via a cable 215. The sonde 211 supplies the power source 22 at an appropriate voltage and current level.
6, detector 228, and other power conditioning circuitry (not shown) for powering downhole circuitry (within the borehole). These circuits have associated circuitry that receives output pulses from amplifier 238 and PMT 216. The amplified pulse is an analog signal that can be of any conventional type, such as the single lamp (Wilkinson rundown) type.
Pulse height analyzer (PHA) with digital converter
Applied to 240. It is also possible to use other suitable analog-to-digital converters for the gamma-ray energy range to be analyzed. It is also possible to use a linear gating circuit for controlling the time part of the detector signal frame to be analyzed. It is possible to further improve performance by using additional conventional techniques such as pulse pileup rejection.

The pulse height analyzer 240 provides a number (typically in the range 256 to 8,000) for each detector pulse according to its amplitude (ie, gamma ray energy).
Assigning one of the predetermined channels and generating a signal in appropriate digital form representing the channel or amplitude of each analyzed pulse. Typically, pulse height analyzer 2
40 has a memory in which the occurrence of each channel number in the digital signal is stored to give an energy spectrum. The accumulated total is the buffer memory 24
2 (which may be omitted in some circumstances) to the remote / cable interface circuit 244 and the cable 2
15 to the surface apparatus 224.

At the surface of the earth, cable signals are received by the cable interface and signal processing circuit 246. As will be appreciated, circuits 244 and 246 may be any suitable circuit that encodes and decodes, multiplexes and demultiplexes, amplifies and otherwise processes signals for transmission and reception to surface device 224. It can be configured. Appropriate circuits are described, for example, in US Pat.
No. 12,712.

The operation of the sonde 211 is controlled by signals sent from the master programmer 248 located in the surface equipment 224 into the downhole. These signals are received by tool programmer 250, which transmits control signals to neutron source 226 and pulse height analyzer 240.

Surface device 224 processes the data received from devices in the downhole, analyzes the energy spectrum of the detected gamma radiation, and then provides information about formation 217 and any carbonization it may contain. It has various electronic circuits that extract information about the hydrogen and use all or some of this data and information to generate a perceptible recording medium such as film, paper or tape. These circuits can comprise special purpose hardware or general purpose computers appropriately programmed to perform the same tasks as such hardware. The details of such an analysis do not form part of the present invention, and although the description thereof is omitted, for example, US Pat.
064.

Although the specific embodiments of the present invention have been described in detail above, the present invention should not be limited to these specific embodiments and does not depart from the technical scope of the present invention. Of course, various modifications are possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a furnace used to produce LSO.

FIG. 2a is a schematic view showing one shape of a crystal manufactured under a certain growth condition.

2b is a graph showing a gamma ray spectrum of the crystal of FIG. 2a.

FIG. 3a is a schematic diagram showing one shape of a crystal manufactured under certain growth conditions.

3b is a graph showing a gamma ray spectrum of the crystal of FIG. 3a.

FIG. 4a is a schematic diagram showing one shape of a crystal manufactured under certain growth conditions.

4b is a graph showing a gamma ray spectrum of the crystal of FIG. 4a.

FIG. 5a is a schematic diagram showing one shape of a crystal manufactured under a certain growth condition.

5b is a graph showing a gamma ray spectrum of the crystal of FIG. 5a.

FIG. 6a is a schematic diagram showing one shape of a crystal manufactured under certain growth conditions.

6b is a graph showing a gamma ray spectrum of the crystal of FIG. 6a.

FIG. 7 is a schematic diagram showing a hardware configuration for a crystal grower.

FIG. 8 is a schematic diagram showing a gamma ray detector incorporating a scintillator according to the present invention.

FIG. 9 is a schematic diagram showing a drill logging tool incorporating the detector as shown in FIG.

[Explanation of symbols]

 10: housing 12: crucible 14: RF induction heating coil 110: LSO scintillator 112: housing 114: 1 surface 116: photomultiplier tube 118: other surface 211: logging probe 213: perforation 215: cable 216: photomultiplier tube 226: Supply source 228: Detector

Front Page Continuation (72) Inventor Frank Brunei USA, California 95405, Santa Rosa, Box 2010 (72) Inventor Charles El. Melcher United States, Connecticut 06896, West Riding, Run Post Drive 49 (72) Inventor Karl A. Peterson USA, New York 10710, Yonkers, Avondale Road 8 (72) Inventor Jeffrey S. Schweizer USA, Connecticut 06877, Ridgefield, Silver Hill Road 41

Claims (11)

[Claims] 1. A lutetium-containing crystal, which comprises forming a melt of a lutetium compound and extracting the crystal from the melt while maintaining the interface between the crystal and the melt substantially flat. How to grow. 2. The crystal according to claim 1, wherein the lutetium-containing compound has lutetium oxyorthosilicate, and the crystal is extracted while maintaining the interface between the crystal and the melt substantially flat. A method characterized by the following. 3. The lutetium oxyorthosilicate melt according to claim 2, wherein the lutetium oxyorthosilicate melt is Ce _2x Lu _{2 (1-x)} SiO ₅
And the formula is 2 × 10 ⁻⁴ <x <6 × 10 ⁻² . 4. The method of claim 3, wherein x is in the range of about 1 × 10 ^{−3 to} about 2 × 10 ⁻² . 5. The method according to any one of claims 1 to 4, which is carried out by the Czochralski crystal growth method. 6. The method of claim 5, wherein the interface is maintained substantially flat by rotating the crystal and controlling the speed of rotation and the diameter of the crystal. 7. A method according to any one of claims 1 to 6, characterized in that it comprises a bowl in which the crystals are comminuted to form individual scintillator crystals. 8. A scintillator comprising a lutetium oxyorthosilicate crystal produced by the method according to any one of claims 1 to 7. 9. A scintillator according to claim 8, and a photodetector optically coupled to the scintillator for generating an electric signal in response to generation of an optical pulse by the scintillator. Gamma ray / X
Line detector. 10. The underground stratum inspection device according to claim 9,
A gamma-ray / X-ray detector carried by a sonde as described in US Pat. Device having means for generating and recording a signal representative of at least one characteristic of 11. The irradiation source of claim 10, wherein the sonde is provided with an irradiation source for irradiating the substance in the perforation with a permeable radiation, the permeable radiation being caused by an interaction with the substance. A device capable of generating a radiation having a characteristic of carrying information about a device.