JP6558676B2 - Radiation detection element and radiation detection apparatus - Google Patents

Description

  The present invention relates to a radiation detecting element and a radiation detecting apparatus having high radiation resistance.

  In addition to the widespread use of radiation in various fields such as medicine, industry, and agriculture, there are harsh radiation environments such as nuclear power plants, and radiation detection is necessary to ensure safety or to prevent accidents. It is necessary to know the radiation information in the environment by the vessel.

  There are several types of radiation detectors, such as those that use the ionizing action of radiation and those that use the luminescence action of radiation. Scintillation that combines a substance that emits radiation (scintillator) with a photodetector (light sensor). Detectors are also widely used. In many cases, the emission wavelength of the scintillator is visible light, and a photomultiplier tube or a semiconductor photodiode is used for the photodetector. However, the photomultiplier tube requires a high voltage, and it is difficult to reduce the size of the photomultiplier tube. Semiconductor photodiodes are also used at high voltages to increase sensitivity.

  A typical scintillation detector currently used is a combination of a sodium iodide (NaI: Tl) scintillator doped with thallium (Tl), a photomultiplier tube and a semiconductor photodiode. NaI: Tl scintillators have high sensitivity but are deliquescent and difficult to handle and have low radiation resistance. For this reason, there is a demand for a scintillation detector that is small, light, easy to handle, and high in radiation resistance.

Also, yttrium aluminum perovskite (YAP: Ce) doped with cerium (Ce) has a sensitivity of about 40% compared to NaI: Tl, but has no deliquescence and has a radiation resistance strength of 10 ³ times. It is considered excellent as a scintillator for detectors.

  However, the emission wavelength of the YAP: Ce scintillator is in the ultraviolet region unlike other materials, and has a peak near 370-340 nm (nanometers). Therefore, an optical sensor that efficiently detects such a light emission wavelength is required (see Non-Patent Document 1).

  Further, in order to realize a radiation detector having a high radiation resistance, an optical sensor having a high radiation resistance is required together with a scintillator having a high radiation resistance.

  Zinc oxide is a so-called wide band gap semiconductor having a band gap of 3.37 eV (electron volt) at room temperature, and therefore is essentially insensitive to visible light, and sensitive to ultraviolet light in the short wavelength region from about 370 nm. For this reason, it is one of the most suitable materials for an ultraviolet sensor. Ultraviolet sensors using zinc oxide have been studied so far and are described in Patent Documents 1 to 4 and Non-Patent Document 2.

  The ultraviolet sensor system using zinc oxide can be classified into a photoconductive type (see Patent Document 1 and Patent Document 2) and a Schottky type (see Patent Document 3 and Patent Document 4). The Schottky type has low sensitivity but has a fast response time, and is therefore suitable for an ultraviolet sensor for applications requiring a high-speed response. On the other hand, the photoconductive type has high sensitivity but has a slow time response, and is therefore suitable for an ultraviolet sensor in applications where high sensitivity detection is required. The photoconductive type is in principle higher in radiation resistance and ultraviolet resistance than the Schottky type. Further, since the photoconductive type has a structure in which a pair of ohmic electrodes are arranged, it is easier to manufacture than the Schottky type that requires Schottky contact.

  Several radiation detectors using a combination of an ultraviolet light emitting scintillator and an optical sensor element have been proposed (see Patent Document 5, Patent Document 6, and Patent Document 7). In these proposals, a PIN diode (Patent Document 5), a photomultiplier tube and a photodiode (Patent Document 7), and a Geiger mode APD (avalanche photodiode) (Patent Document 6) are used as photodetectors. Yes. However, these elements are uneasy to satisfy all the conditions such as small size and robustness, low operating voltage, high sensitivity, characteristic stability, and radiation resistance.

  In addition, as described above, zinc oxide is sensitive to the emission wavelength region of YAP: Ce scintillator, but has a radiation resistance strength that is 10 times greater than that of a semiconductor sensitive to the same ultraviolet region such as gallium nitride (GaN). It is said that.

JP 2006-278487 A Japanese Patent Laid-Open No. 3-241777 JP 2007-201393 A JP 2012-146705 A JP 2008-24739 A JP 2010-122166 A JP 2012-149223 A

“YAP: Ce (Yttrium Aluminum Perovskit, Ce + doped) crystals”, [online], PROTEUS Inc., [December 16, 2013 search], Internet <URL: http://www.apace-science.com /proteus/yap.htm> Jian Zhong and Yicheng Lu, ed. By C. W. Litton et al., “Zinc Oxide Materials for Electronic and Optoelectronic Device Applications (Chapter 11) ZnO-Based Ultraviolet Detectors”, John Wiley & Sons Ltd., 2011, pp.285-329

In view of the above background, there is a need for a highly sensitive radiation detector that is small, light, easy to handle, and high in radiation resistance.
An object of the present invention is to provide a radiation detection apparatus that is small and light, easy to handle, and high in radiation resistance.

In order to achieve the above object, the present invention is a combination of a photoconductive ultraviolet detecting element having a pair of electrodes formed on the surface of a zinc oxide-based material at a predetermined interval and a luminescent substance that emits ultraviolet rays by radiation. There are provided a radiation detection element comprising: a radiation detection element in which a luminescent substance that emits ultraviolet rays by radiation is cerium-doped yttrium aluminum perovskite; and a radiation detection apparatus including such a radiation detection element.
The combination is preferably a combination of a substance that emits ultraviolet rays in a region where the zinc oxide-based material is sensitive to radiation (for example, YAP: Ce scintillator) and a zinc oxide-based ultraviolet sensor.

  Further, in order to detect weak light emission such as YAP: Ce scintillator, the type of the ultraviolet sensor is preferably a photoconductive type with high sensitivity. The photoconductive sensor has a radiation resistance higher than that of a Schottky-type or pn-junction type photodiode because its photocurrent flow direction is perpendicular to the incident direction of light or radiation. A Schottky-type or pn junction type photodiode cannot be used if conduction occurs even at one location on a wide metal-semiconductor contact surface or pn junction surface. The effect is particularly large when used under a high reverse bias.

  In this way, by combining a scintillator such as YAP: Ce with high radiation resistance and a photoconductive ultraviolet sensor that is structurally considered to have high radiation resistance using a zinc oxide-based material with high radiation resistance. Therefore, it is possible to provide a radiation detector that is small, light, simple in structure, easy to handle and high in radiation resistance.

  Further, the present invention can be realized in any other form such as a method, a system, etc., in addition to being realized as an element or an apparatus.

  According to the present invention, a radiation detector having a small size and light weight, simple structure, easy handling, and high radiation resistance can be easily manufactured, and an element shape or a combination of a plurality of elements can be easily realized according to the purpose of use. It becomes. Since the zinc oxide-based ultraviolet sensor has no sensitivity to visible light, it can be used in a bright room without a visible light absorption filter. In addition, the photoconductive ultraviolet detection element is less affected by external disturbance because it has a higher current level than a pn junction or Schottky photodiode. Therefore, it is not necessary to amplify the signal immediately at the detection element portion, and the electronic circuit can be simplified.

It is typical sectional drawing which shows the structure of one Embodiment of the radiation detection element by this invention. It is a figure which shows another structural example of a radiation detection element. It is a figure which shows another structural example of a radiation detection element. It is a figure which shows the structure of embodiment of a radiation detection apparatus provided with the radiation detection element shown in FIG. It is a figure which shows the structure of another embodiment of a radiation detection apparatus. It is a figure which shows the structure outline of the specific Example of the radiation detection element of this invention. It is a figure which shows the emission spectrum by X-ray irradiation of the YAP: Ce scintillator used in the Example of FIG. FIG. 5 is a diagram showing the relationship between the X-ray tube input power (X-ray intensity) and the emission intensity of the YAP: Ce scintillator in the YAP: Ce scintillator used in the example of FIG. 4. It is a figure which shows the spectral characteristic of the photocurrent of the ultraviolet sensor used in the Example of FIG. It is a figure which shows the relationship between a photocurrent and irradiation X-ray intensity which were obtained in the Example of FIG. It is a figure which superimposes and shows the emission spectrum of the YAP: Ce scintillator used in the Example of FIG. 4, and the photocurrent spectrum of an ultraviolet sensor.

The best mode for carrying out the present invention will be described below.
FIG. 1 is a schematic cross-sectional view showing a configuration of an embodiment of a radiation detection element according to the present invention. The radiation detection element 1 includes a scintillator 11 that is a light-emitting substance that emits ultraviolet rays by radiation, and an ultraviolet sensor 13 that is a photoconductive ultraviolet detection element that receives the light emission to generate a photocurrent. The scintillator 11 is, for example, cerium-doped yttrium / aluminum / perovskite (YAP: Ce), and the emission wavelength is in the ultraviolet region. The ultraviolet sensor 13 uses a zinc oxide single crystal or a thin film.

When the scintillator 11 is composed of YAP: Ce, there is almost no self-absorption of light emission by the scintillator 11, so that a thickness that sufficiently absorbs radiation can be used.
Further, a reflection film 12 is preferably attached to the radiation incident side surface of YAP: Ce as a reflection means for reflecting the ultraviolet light emitted inside and directing it toward the ultraviolet sensor.

  The ultraviolet sensor 13 is manufactured by forming electrodes 15 having a predetermined interval on the surface of a zinc oxide single crystal substrate 14 (which may be a zinc oxide single crystal thin film, but will be described here as a substrate). One of the electrodes is a + and the other is a-terminal. The shape of the electrode is arbitrary, and a shape capable of efficiently receiving YAP: Ce light emission is preferable.

Since the size of the distance between the electrodes is closely related to the characteristics of the sensor, it is necessary to fully examine it according to the purpose of use. The gain (G) of the photoconductive element is obtained by G = τμV / l ² . Here, G is the gain of the photoconductive element, τ is the carrier lifetime, V is the voltage applied between the electrodes, and l is the distance between the electrodes.
If the distance between the electrodes is small, the obtained G will be large. However, if the distance is smaller than 0.01 mm, the electric field strength will increase, and migration of the electrode material may occur, and dark current will also increase. Is better. Moreover, since G obtained when the space | interval between electrodes is large becomes small, it is good that it is 1.0 mm or less at most.

  The electrode material is preferably in ohmic contact with the zinc oxide-based material. For the n-type zinc oxide-based material, Al, ITO (indium tin oxide), In, Ga, Zn, or an alloy thereof may be used. The electrode is attached by vacuum deposition, electron beam deposition, sputtering, or the like.

The zinc oxide single crystal used for the zinc oxide single crystal substrate 14 is desirably a high quality with few defects and high mobility. In particular, the resistivity is preferably high in order to reduce the dark current. The dark resistivity is preferably 10 ⁴ Ω · cm or more, more preferably 10 ⁸ Ω · cm or more for crystals and 10 ⁶ Ω · cm or more for thin films. Since zinc oxide is usually n-type, resistivity can be increased by doping group I elements such as lithium (Li) and copper (Cu) and group V elements such as nitrogen (N) and phosphorus (P) as acceptor impurities. Can be adjusted. In particular, N has an atomic radius close to that of oxygen (O) in zinc oxide, and is considered suitable for doping.

High quality zinc oxide single crystals with few defects and high mobility can be grown by methods such as hydrothermal synthesis. Acceptor impurities can also be doped during growth for resistivity control.
The single crystal is polished after cutting and provided as an ultraviolet sensor substrate. When a zinc oxide thin film is used, it may be epitaxially grown on this single crystal substrate, or may be grown on a sapphire substrate or a quartz substrate. However, epitaxially grown single crystal thin films have the highest quality and are desirable for sensors.

The surface of the ultraviolet sensor 13 is preferably subjected to stabilization treatment, an antireflection film or a surface protective film 16 in order to stabilize the characteristics.
The radiation detection element 1 is assembled as an arrangement in which the YAP: Ce scintillator 11 faces each other so that light emitted from the YAP: Ce scintillator 11 enters the ultraviolet sensor 13. In this case, a space may be provided between the scintillator 11 and the ultraviolet sensor 13 as in the example of FIG. Further, as in another example shown in FIG. 2A, the scintillator 11 and the ultraviolet sensor 13 may be brought into close contact with each other.

Further, as another example shown in FIG. 2B, a condensing unit such as a lens 17 for effectively collecting ultraviolet rays generated in the scintillator 11 may be provided. Even if the shape of YAP is changed to a convex shape instead of a plate shape, the same effect can be obtained. Condensing light like this can increase the detection sensitivity of radiation. In the case of close contact, a sensor light-receiving portion having a size corresponding to the YAP light-emitting area is prepared to increase efficiency.
The power source 18 is for applying a driving voltage to the electrode 15. The ammeter 19 is for measuring the photocurrent generated when the zinc oxide single crystal substrate 14 receives ultraviolet rays.

FIG. 3A shows a configuration of an embodiment of a radiation detection apparatus using the radiation detection element 1 described above.
The radiation detection apparatus 30 shown in FIG. 3A measures the radiation dose by inputting the ultraviolet detection result (measurement value of the ammeter 19) by the ultraviolet sensor 13 in the radiation detection element 1 to the measurement control circuit 31 and processing it. It has a function to do. The measurement result can be output and displayed in an arbitrary format by the display 32.
It is not necessary for one radiation detection device 30 to have one radiation detection element 1, and as shown in FIG. 3B, a plurality of radiation detection elements 1 are connected and used as a system to obtain environmental information. Reliability can be further increased.

〔Example〕
FIG. 4 shows a schematic structure of a specific embodiment of the above-described radiation detection element of the present invention. Although the present invention is not limited to such a structure and manufacturing method, the specific embodiment will be described in detail below with reference to FIG. 4 and with reference to the data of FIGS. The same reference numerals as those in FIG. 1 are used for components corresponding to those shown in FIG.
Reference numeral 11 denotes a YAP: Ce scintillator having a size of 10 mm × 10 mm × 0.5 mm. In this embodiment, a reflective film is not attached for the sake of experimentation, but it is desirable to have a reflective film attached. In this embodiment, Cu Kα-X rays were used as the radiation source.

  FIG. 5 shows an emission spectrum of the YAP: Ce scintillator 11 used in this example by X-ray irradiation. FIG. 6 shows the relationship between the X-ray tube input power (X-ray intensity) and the emission intensity of the YAP: Ce scintillator 11. Measurement on the shorter wavelength side than 350 nm (nanometer) was not possible due to limitations of the measuring instrument.

According to the graph of FIG. 5, the peak wavelength of the emission spectrum is about 370 nm, which is in the ultraviolet region. The voltage applied to the X-ray tube was made constant at 40 kV, and the X-ray intensity was changed by changing the current value by changing the X-ray tube input power. It is known that X-ray intensity is proportional to input power (current).
According to the graph of FIG. 6, the emission peak intensity of the YAP: Ce scintillator 11 is proportional to the X-ray tube input. The transmittance of YAP: Ce was about 90% at a wavelength near 360 nm.

The ultraviolet sensor 13 using a zinc oxide single crystal was produced as follows. A zinc oxide single crystal grown by a hydrothermal synthesis method was used. In order to increase the dark resistivity, nitrogen (N) is doped during growth. The dark resistivity was approximately 10 ⁹ Ω · cm. On the Zn surface of the c-plane zinc oxide single crystal substrate 14 having a size of 5 mm × 5 mm × 0.5 mm which has been mirror-polished, two aluminum (Al) electrodes 15 having a size of 1 mm × 1 mm are provided with an interval of 1 mm. Vapor deposited. A lead wire is bonded to each electrode, and the ultraviolet sensor 13 is completed. No surface treatment or anti-reflection coating is applied.
Since the current-voltage (IV) characteristics between the electrodes 15 were linear, it can be interpreted that the Al electrode and the zinc oxide single crystal substrate 13 are in ohmic contact.

  FIG. 7 shows the spectral characteristics of the photocurrent of the ultraviolet sensor 13 using a zinc oxide single crystal measured by an ammeter 19 when a DC power supply of 1.5 V is connected as the power source 18. From this graph, it can be seen that the ultraviolet sensor 13 made of zinc oxide single crystal is sensitive to ultraviolet light having a wavelength shorter than about 370 nm, but not sensitive to visible light.

  As shown in FIG. 4, an ultraviolet sensor 13 using a YAP: Ce scintillator 11 and a zinc oxide single crystal was arranged to produce a radiation detection element. As shown in the figure, the structure is extremely simple and easy. The gap between the scintillator 11 and the ultraviolet sensor 13 is 1 mm, which is only a normal air layer. In addition, a light shield 21 is provided for preventing ultraviolet rays from the outside other than the ultraviolet rays generated by the scintillator 11 from entering the ultraviolet sensor 13.

[Functional Evaluation of Examples]
Evaluation of the radiation detection element 1 produced on said conditions was performed. The produced radiation detection element 1 and the X-ray irradiation source are arranged as shown in FIG. 4, the X-ray is irradiated to the YAP: Ce scintillator, and the measured X-ray intensity (X The relationship between the tube input power) and the photocurrent is shown in FIG.
The X-ray tube input power was changed by applying 40 kV (kilovolts) to the X-ray tube and changing the tube current up to 45 mA in 5 mA steps. As shown in FIG. 8, as the irradiation X-ray intensity increases, the photocurrent also increases proportionally. Here, it has been confirmed that the photocurrent is not caused by the direct incidence of X-rays on zinc oxide.

  From the above, it can be seen that the radiation detection element 1 can detect radiation. When the emission spectrum of the YAP: Ce scintillator used in this example and the photocurrent spectrum of the ultraviolet sensor were superimposed, as shown in FIG. 9, it was not possible to convert all of the scintillator emission into a photocurrent. If a scintillator whose emission spectrum is shifted to the short wavelength side is used, the detection sensitivity is considered to increase.

Although the description of the embodiment is completed as described above, the specific configuration of elements and devices, the material and characteristics of each part, and the like are not limited to those specifically described in the above description.
For example, the photoconductive ultraviolet detection element can be formed using any zinc oxide-based material, and is not limited to a zinc oxide-based single crystal substrate or a zinc oxide-based single crystal thin film. The acceptor doped into the zinc oxide-based material is not limited to nitrogen.
The luminescent substance that emits ultraviolet rays by radiation is not limited to YAP: Ce. Any material can be used as long as it is a luminescent substance that emits light at a wavelength with which the photoconductive ultraviolet detection element has sensitivity when receiving radiation.
Further, it goes without saying that the configurations described as the above embodiments or modifications thereof can be applied in any combination as long as they do not contradict each other.

  According to the above radiation detection element and radiation detection apparatus, a radiation detector having a small size and light weight, simple structure, easy handling, and high radiation resistance strength can be easily manufactured. Combinations can be easily realized. The voltage used is low and the measurement control circuit is simple. Therefore, it can be used not only in a normal environment but also in an environment with strong radiation.

DESCRIPTION OF SYMBOLS 1 ... Radiation detection element, 11 ... Scintillator, 12 ... Reflective film, 13 ... Ultraviolet sensor, 14 ... Zinc oxide system single crystal substrate, 15 ... Electrode, 16 ... Surface protective film, 17 ... Lens, 18 ... Power supply, 19 ... Current Total: 21 ... Light shield, 30 ... Radiation detection device, 31 ... Measurement control circuit, 32 ... Display

Claims (8)

A radiation detection element comprising a combination of a photoconductive ultraviolet detection element having a pair of electrodes formed at a predetermined interval on the surface of a zinc oxide-based material and a luminescent substance that emits ultraviolet light by radiation,
A radiation detecting element, wherein the luminescent substance that emits ultraviolet rays by radiation is cerium-doped yttrium aluminum perovskite .   2. The radiation detection element according to claim 1, wherein the zinc oxide-based material is a zinc oxide-based single crystal substrate or a zinc oxide-based single crystal thin film. 3. The radiation detection element according to claim 1, wherein the zinc oxide-based material has a dark resistivity of 10 ⁴ Ω · cm or more.   3. The radiation detection element according to claim 1, wherein the zinc oxide material is doped with an acceptor.   2. The radiation detection element according to claim 1, wherein a predetermined interval between the pair of electrodes formed on the zinc oxide-based material is 0.01 mm to 1.0 mm.   The radiation detection element according to claim 1, wherein the photoconductive ultraviolet detection element and the luminescent material are disposed to face each other.   2. The radiation detection element according to claim 1, further comprising condensing means for condensing ultraviolet rays emitted from the luminescent substance between the luminescent substance and the photoconductive ultraviolet detection element. . The radiation detection apparatus provided with the radiation detection element as described in any one of Claims 1 thru | or 7 .

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