PHOTODETECTOR, DETECTING APPARATUS, AND DETECTING SYSTEM

According to an embodiment, a photodetector includes a light converting unit, a first layer, a light detecting unit, and a second layer. The light converting unit converts radiation into light. The first layer absorbs the radiation. The light detecting unit is provided between the light converting unit and the first layer and detects light. The second layer is provided between the first layer and the light detecting unit, has a smaller average atomic weight than an average atomic weight of the first layer, and absorbs radiation scattered in the first layer and a fluorescent X-ray generated in the first layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-163012, filed on Aug. 20, 2015; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a photodetector, a detecting apparatus, and a detecting system.

BACKGROUND

There have been developed detecting apparatuses including a light detecting unit, such as a photodiode (PD), and a scintillator. The combination of the light detecting unit and the scintillator can provide a photon counting image having spatial resolution corresponding to the size of the scintillator. Also known are technologies for providing a computed tomography (CT) image by detecting X-rays.

The light detecting unit may possibly receive not only light resulting from conversion by the scintillator but also radiation scattered by, for example, Compton scattering in layers constituting a mounting board on which the light detecting unit is mounted. To suppress incidence of scattered radiation, there has been disclosed a configuration including radiation shielding members on the side opposite to a scintillator in a sensor section that converts radiation into charges.

DETAILED DESCRIPTION

According to an embodiment, a photodetector includes a light converting unit, a first layer, a light detecting unit, and a second layer. The light converting unit converts radiation into light. The first layer absorbs the radiation. The light detecting unit is provided between the light converting unit and the first layer and detects light. The second layer is provided between the first layer and the light detecting unit, has a smaller average atomic weight than an average atomic weight of the first laver, and absorbs radiation scattered in the first layer and a fluorescent X-ray generated in the first layer.

Embodiments are described below in detail with reference to the accompanying drawings.

First Embodiment

FIG. 1is a schematic diagram of an example of a detecting system1according to a first embodiment. The detecting system1is applicable to computed tomography (CT) apparatuses, for example.

The detecting system1includes a light source11, a detecting apparatus10, and a driving unit13. The light source11and the driving unit13are electrically connected to the detecting apparatus10.

The light source11and the detecting apparatus10are arranged (oppositely arranged) facing each other with a space interposed therebetween. A subject12is positioned between the detecting apparatus10and the light source11. The light source11and the detecting apparatus10are provided rotatably about the subject12while maintaining the oppositely arranged state.

The light source11emits radiation L, such as X-rays, to the detecting apparatus10facing the light source11. The radiation L emitted from the light source11passes through the subject12and enters the detecting apparatus10.

The detecting apparatus10detects light. The detecting apparatus10includes a photodetector20and a signal processing circuit22. The photodetector20is electrically connected to the signal processing circuit22. A plurality of photodetectors20in the detecting apparatus10according to the present embodiment are aligned in a direction of rotation of the detecting apparatus10(direction of the arrow Q inFIG. 1).

The photodetector20receives the radiation L on a first surface20athrough a collimator21, the radiation being emitted from the light source11and passing through the subject12. The first surface20ais a two-dimersional plane on which light is incident in the photodetector20.

The collimator21is arranged on th first surface20ae of the photodetector20to limit the angle of the radiation L incident on the photodetector20.

The photodetector20detects light. The photodetector20outputs a photocurrent (hereinafter referred to as a signal) corresponding to the detected light to the signal processing circuit22via a signal line23. The signal processing circui22collectively controls the detecting system1. The signal processing circuit22acquires the signal from the photodetector20.

The signal processing circuit22according to the present embodiment calculates the energy and the intensity of the radiation L incident on the photodetector20based on the current value of the acquired signal. The signal processing circuit22, for example, perform shaping and analog/digital (A/D) conversion on the waveform of a spectrum indicated by the signal acquired from a light detecting unit34, thereby calculating the energy and the intensity of the radiation L incident on the photodetector20.

The signal processing circuit22generates an image based on radiation information on the subject12from the energy and the intensity of the radiation L incident on the photodetectors20. The signal processing circuit22generates a CT image of the subject12, for example.

The detecting apparatus10may further include an integrated circuit (IC) and an A/D converter, for example, between the collimator21and the signal processing circuit22. In this case, the collimator21is electrically connected to the signal processing circuit22via the signal line23and the IC or the A/D converter. With the A/D converter, the detecting apparatus10can digitize the signal output from the light detecting unit34and then transmit it to the signal processing circuit22.

The driving unit13rotates the light source11and the detecting apparatus10about the subject12positioned therebetween while maintaining their oppositely arranged state. This configuration allows the detecting system1to generate a cross-sectional image of the subject12.

While the subject12is a human body, for example, it is not limited thereto. The subject12may be an animal or a plant, or a non-living material such as an object. In other words, the detecting system1is applicable not only to detecting apparatuses that generate a tomographic image of human bodies, animals, and plants but also to various detecting apparatuses, such as security apparatuses, that perform fluoroscopy on objects, for example.

FIGS. 2A and 2Bare views for explaining an example of the photodetector20.FIG. 2Ais a view of an aligned state of the photodetectors20. The detecting apparatus10includes a plurality of photodetectors20. The photodetector20has a rectangular shape with its long side extending in a direction that intersects with the rotation direction Q, for example. The photodetectors20are aligned in substantially arc shape in the direction of rotation of the photodetector20(refer to the arrow Q inFIG. 2A). In other words, the photodetectors20are filled to form a plane (tiling) along the first surface20aserving as a light incident surface.

FIG. 2Bis a schematic diagram of the photodetector20. The photodetector20is detachably provided to the detecting apparatus10. The photodetector20includes a mounting board26and scintillators18.

The scintillator18is an example of a light converting unit. The scintillator18converts radiation, such as X-rays, into light (photons) having a longer wavelength than that of the radiation. The scintillator18is made of a scintillator material. The scintillator material emits fluorescence (scintillation lioht) upon incidence of radiation, such as X-rays. The scintillator material is appropriately selected depending on the application target of the detecting apparatus10. Examples of the scintillator material include, but are not limited to, Lu2SiO5:(Ce), LaBr3: (Ce), YAP (yttrium aluminum perovskite):Ce, Lu(Y)AP:Ce, etc.

The mounting board26includes a supporting member24and light detecting units34.

The light detecting unit34detects light. The light detecting unit34is a photomultiplier tube or an avalanche photodiode (APD), for example. The APD is a known avalanche photodiode. The light detecting unit34according to the present embodiment is driven in a Geiger mode, for example.

FIG. 3is a plan view of an example of the photodetector20. As illustrated inFIG. 3, a plurality of light detecting units34are arranged in a matrix (refer to a direction of the arrow X and a direction of the arrow Y inFIG. 3). In the photodetector20, the light detecting units34correspond to one pixel (refer to a pixel area30), and a plurality of pixel areas30are arranged in a matrix. Arrangement in a matrix means arrangement in a row direction (direction of the arrow X) and in a column direction (direction of the arrow Y). The row direction (direction of the arrow X) and the column direction (direction of the arrow Y) are directions orthogonal to each other on the first surface20aof the light detecting units34. The structure having the light detecting units34in each pixel area30can facilitate replacement of the photodetector20in repairing, for example.

InFIG. 3, the pixel areas30each include 25 (5×5) light detecting units34. The number of light detecting units34constituting the pixel area30, however, is given by way of example only and is not limited to 25. A reflective member27that reflects light may be provided between the pixel areas30.

The scintillators18are arranged on the first surface20aside of the light detecting unit34. The scintillators18are arranged at positions corresponding to the respective pixel areas30. Specifically, the scintillators18are arranged such that areas obtained by projecting the scintillators18onto the light detecting units34in the thickness direction of the scintillators18(corresponding to the thickness direction of the light detecting units34) cover the respective pixel areas30each composed of the light detecting units34.

The scintillators18may be arranged in a manner corresponding to the respective light detecting units34. Alternatively, the scintillators18may be arranged in a manner continuously covering a plurality of pixel areas30. In other words, the scintillators18may be arranged in a manner continuously covering a plurality of light detecting units34in a plane direction of the first surface20a.

The photodetector20according to the present embodiment further includes a first layer50and a second layer52. In this example, the photodetector20also includes the supporting member24. The first layer50, the second layer52, and the supporting member24will be described later in detail.

FIG. 4is a schematic illustrating an example of a sectional view of the photodetector20. InFIG. 4, the scintillators18are arranged at positions corresponding to the respective light detecting units34, for example.

In the photodetector20, the scintillators18are stacked on the mounting board26that is provided with the light detecting units34. The collimators21are arranged above the scintillators18.

In the example illustrated inFIG. 4, the collimators21are arranged at positions corresponding to respective boundaries between adjacent scintillators18in a direction orthogonal to the thickness direction of the mounting board26. In this case, the collimators21have a function to limit the incident angle to the scintillators18of the radiation L incident on the scintillators18. In other words, the collimators21have a function to suppress incidence, on the scintillators18, of radiation L having a large incident angle to the scintillators18.

The radiation L having a large incident angle to the scintillators18is more likely to generate photons in adjacent scintillators18simultaneously. To address this, the collimators21are arranged at positions corresponding to respective boundaries between adjacent scintillators18on the side of the scintillators18opposite to the light detecting unit34. This structure can suppress deterioration of the detection accuracy.

The incident angle to the scintillators18according to the present embodiment means an angle from an axial direction corresponding to the thickness direction of the scintillators18. The thickness direction of the scintillators18corresponds to a direction orthogonal to the array direction of the scintillators18and the light detecting units34and the thickness direction of the mounting board26.

The radiation L incident on the scintillators18through the collimators21is converted into light (photons) having a longer wavelength than that of the radiation L by the scintillators18. The light then reaches the mounting board26including the light detecting units34. The light (photons) having a longer wavelength than that of the radiation L resulting from conversion by the scintillators18may be hereinafter simply referred to as light.

The light detecting units34provided on the mounting board26detect the incident light. The photodetectors20output signals based on the detected light to the signal processing circuit22via the signal lines23.

The surface of the scintillators18and the areas between adjacent scintillators18may be provided with a reflective member or a reflective layer that reflects photons. Furthermore, a portion between the scintillators18and the light detecting units34may be provided with a resin layer having a light-guide function to guide the light resulting from conversion by the scintillators18to the light detecting units34. The scintillators18and the collimators21may be arranged in contact with each other or with a predetermined gap interposed therebetween.

In the conventional technologies, the light detecting units34may possibly receive not only the light resulting from conversion by the scintillators18but also radiation scattered in the layers constituting the mounting board26and fluorescent X-rays generated by incidence of the radiation L on the layers constituting the mounting board26.

FIG. 5is a view for explaining an example of scattering of radiation in a conventional photodetector200. Part of the radiation L incident on the photodetector202may possibly reach the mounting board26without passing through the scintillator18. Furthermore, part of the radiation L incident on the photodetector200may possibly reach the mounting board26without being absorbed by the scintillator18. When scintillator18is made of a scintillator material of a lower density, the radiation L is more likely to pass through the scintillator18and reach the mounting board26, for example.

If photons of the radiation L that reach the mounting board26are scattered in any of the layers constituting the mounting board26, the photons of scattered radiation S may possibly reach the scintillator18. The photons that reach the scintillator18are converted by the scintillator18and then reach the light detecting unit34.

In this case, the light detecting unit34detects light obtained by converting, by the scintillator18, the radiation L incident on the scintillator18from the outside and also detects light obtained by converting, by the scintillator18, the photons of the radiation S from the mounting board26.

Specifically, energy E1 of photons of the radiation S generated in a direction different by 180 degrees from the direction of incidence on the mounting board26is represented by Expression (1) in megaelectron volts (MeV).

In Expression (1), E1 denotes energy of photons of the radiation S scattered in a direction opposite to the incident direction, that is, energy of back-scattered photons, and E denotes energy of the radiation L incident on the photodetector200from the outside.

FIG. 6is a graph40A illustrating an example of an energy spectrum calculated from the number of photons detected by the light detecting unit34in the conventional photodetector200.

In a case where radiation having single energy E is incident on the photodetector200, for example, the light detecting unit34detects light represented by an energy spectrum having a peak P1 and a peak P2. The peak P1 is a peak of energy of light obtained by converting, by the scintillator18, the radiation L incident on the photodetector200from the outside. The peak P2 is energy of the radiation S.

The light detecting unit34of the conventional photodetector200thus detects not only the energy at the peak P1 to be originally detected but also the energy at the peak P2 caused by the radiation S. As a result, the detection accuracy deteriorates.

As the ratio of generation of the radiation S to the light obtained by converting the radiation L by the scintillator18increases, the ratio of the number of photons that form the peak P2 to that of the peak P1 increases. As a result, the detection accuracy further deteriorates.

FIG. 7is a view for explaining generation of fluorescent X-rays in the conventional photodetector200.

Part of the radiation L incident on the photodetector200may possibly reach the mounting board26without passing through the scintillator18. Furthermore, part of the radiation L incident on the photodetector200may possibly reach the mounting board26without being absorbed by the scintillator18.

If part of photons of the radiation L that reach the mounting board26reach the layers constituting the mounting board26, fluorescent X-rays specific to the layers may possibly be generated.

Generated fluorescent X-rays F may possibly reach the scintillator18. The fluorescent X-ray F that reach the scintillator18are converted by the scintillator18and reach the light detecting unit34.

In this case, the light detecting unit34detects light obtained by converting the radiation L by the scintillator18and light obtained by converting the fluorescent X-rays F by the scintillator18.

FIG. 8is a graph40B illustrating an example of an energy spectrum of light detected by the light detecting unit34in the conventional photodetector200.

As illustrated inFIG. 8, the conventional light detecting unit34detects light represented by an energy spectrum having the peak P1 and a peak P3. The peak P1 is the same as the peak P1 described above. The peak P3 is a peak of energy of the fluorescent X-rays F.

The conventional light detecting unit34detects not only the energy at the peak P1 to be originally detected but also the energy the peak P3 caused by the fluorescent X-rays F. As a result, the detection accuracy deteriorates.

As described with reference toFIGS. 5 to 8, the conventional light detecting unit34may possibly detect not only the energy at the peak P1 to be originally detected but also the energy at the peak P2 caused by the scattered radiation S and the energy at the peak P3 caused by the fluorescent X-rays F. As a result, the detection accuracy of the conventional photodetector200deteriorates.

To address this, the photodetector20according to the present embodiment includes the first layer50and the second layer52.

FIG. 9is a schematic diagram of an example of a section of the photodetector20according to the present embodiment. The first layer50is provided on the side of the light detecting unit34opposite to the scintillator18in the thickness direction with the second layer52interposed between the first layer50and the light detecting unit34. In other words, the light detecting unit34is provided between the scintillator18and the first layer50. The second layer52is provided between the light detecting unit34and the first layer50.

In other words, the photodetector20according to the present embodiment has a structure obtained by stacking the first layer50, the second layer52, the light detecting unit34, and the scintillator18on the supporting member24in this order.

A multilayer stack obtained by stacking the supporting member24, the first layer50, the second layer52, and the light detecting unit34in this order may be referred to as the mounting board26. The mounting board26does not necessarily include the supporting member24.

The first layer50absorbs the radiation L. The first layer50has a larger average atomic weight than that of the second layer52.

The first layer50may absorb at least part of the radiation L. The first layer50may preferably absorb or transmit 50% or more of the incident radiation L and more preferably absorb 90% or more of the incident radiation L.

At least part of the radiation L incident on the first layer50is absorbed by the first layer50. As a result, the first layer50suppresses generation of the radiation S scattered in the first layer50and generation of fluorescent X-rays specific to the first layer50.

The average atomic weight of the first layer50and the second layer52according to the present embodiment means an average atomic weight of elements other than impurities included in the first layer50and the second layer52. The impurities mean elements of equal to or less than 5% by weight to a total quantity of materials constituting the first layer50and the second layer52of 100% by weight.

The first layer50preferably includes elements having a larger atomic number. Elements having a larger atomic number mean elements the atomic number of which is larger than the largest atomic number of elements included in the second layer52.

The first layer50may include the same element as the element having the largest atomic number out of the elements included in the second layer52. In this case, the content rate of the element having the largest atomic number included in the first layer50is higher than that of the second layer52.

The first layer50preferably includes at least one kind of element selected from Ag, Cu, Fe, and Mo, for example. To suppress the radiation S, the first layer50especially preferably includes at least Ag out of these elements.

The first layer50may be made of one kind of element or a compound or a mixture of a plurality of kinds of elements.

The thickness of the first layer50simply needs to satisfy the functions and the requirements described above and is not limited. In other words, the first layer50simply needs to have a thickness with which the first layer50can maintain the function to absorb at least part of the radiation L. The first layer50has a larger weight than that of the second layer52because it has a larger average atomic weight than that of the second layer52. The thickness of the first layer50is appropriately adjusted depending on the weight and the size of the photodetector20required in mounting the photodetector20on various apparatuses or devices.

The thickness direction according to the present embodiment corresponds to a stacking direction of the first layer50, the second layer52, the light detecting unit34, and the scintillator18.

As described above, the first layer50is positioned on the side of the second layer52opposite to the light detecting unit34in the thickness direction of the photodetector20. The position of the first layer50in the plane direction orthogonal to the thickness direction preferably corresponds to the position of the light detecting unit34.

FIG. 10is a plan view schematically illustrating an example of the photodetector20viewed from the light detecting unit34. As illustrated inFIG. 10, the first layer50is preferably arranged such that a first projection area A obtained by projecting the first layer50onto the light detecting unit34covers at least the light detecting unit34. The first layer50preferably has such a size in the plane direction and is arranged at such a position in the photodetector20that the first projection area A covers the light detecting unit34.

Referring back toFIG. 9, the second layer52is provided between the first layer50and the light detecting unit34as described above.

The second layer32has a smaller average atomic weight than that of the first layer50. The second layer52transmits at least part of the incident radiation L. Specifically, the second layer52transmits at least part of photons of the radiation L. The second layer52preferably transmits all the incident radiation L. In other words, the second layer52preferably transmits all the photons of the radiation L in the thickness direction.

The second layer52absorbs the radiation S scattered in the first layer50and the fluorescent X-rays F generated in the first layer50by the radiation L incident or the first layer50.

In other words, the second layer52absorbs the radiation S and the fluorescent X-rays F output from the first layer50and incident on the second layer52. As a result, the second layer52can prevent the radiation S and the fluorescent X-rays F from reaching the light detecting unit34.

The second layer52may absorb at least part of the radiation S and the fluorescent X-rays F. The second layer52may preferably absorb 50% or more of the radiation S and the fluorescent X-rays F output from the first layer50and incident on the second layer52and more preferably absorb 90% or more of the radiation S and the fluorescent X-rays F.

The second layer52simply needs to be made of a material that satisfies the functions and the requirements described above. The second layer52, for example, preferably includes elements having a smaller atomic number than that of the first layer50. Elements having a smaller atomic number mean elements the atomic number of which is smaller than the largest atomic number of elements included in the first layer50.

The second layer52preferably includes at least one kind of element selected from Si, Al, and Mg, for example. To reduce the energy of the fluorescent X-rays F and facilitate the production, the second layer52especially preferably includes at least Si out of these elements.

The second layer52may be made of one kind of element or a compound or a mixture of a plurality of kinds of elements.

The second layer52needs to have a smaller average atomic weight than that of the first layer50. The second layer52is especially preferably made of an element the atomic number of which is smaller than the smallest atomic number of an element in materials constituting the first layer50.

The thickness of the second layer52simply needs to satisfy the functions and the requ plan view irements described above is not limited. Specifically, the second layer52needs to have a thickness of equal to or larger than a thickness with which the second layer52can absorb the fluorescent X-rays F generated in the first layer50. The second layer52also needs to have a thickness with which the second layer52transmits the radiation L incident on the surface thereof on the light dtecting unit34side to the first layer50.

In other words, the second layer52needs to have a thickness with which the second layer52can transmit the radiation L incident thereon without scattering it in the second layer52.

The thickness of the second layer52is adjusted to a thickness that satisfies the conditions described above depending on the materials of the second layer52and the materials of the first layer50.

In a case where the second layer52is made of Si, and the first later50is made of Ag, for example, the second layer52preferably has a thickness of equal to or larger than 0.5 mm and equal to or smaller than 2 mm. The second layer52having the thickness described above can effectively suppress generation of the scattered radiation S in the second layer52. Specifically, the second layer52made of Si and having a thickness falling within the range can provide an advantageous effect of suppressing generation of the radiation S substantially equivalent to that provided when the second layer52is made of Mo.

As described above, the second layer52is positioned between the light detecting unit34and the first layer50in the thickness direction of the photodetector20. The position of the second layer52in the plane direction orthogonal to the thickness direction preferably corresponds to the position of the light detecting unit34.

Specifically, as illustrated inFIG. 10, the second layer52is preferably arranged such that a second projection area B obtained by projecting the second layer52onto the light detecting unit34covers at least the light detecting unit34. The second layer52preferably has such a size in the plane direction and is arranged at such a position in the photodetector20that the second projection area B covers the light detecting unit34.

Referring back toFIG. 9, one of the first layer50and the second layer52may be electrically conductive. In this case, one of the first layer50and the second layer52is made of an electrically conductive material that can provide the functions and the requirements described above.

In a case where one of the first layer50and the second layer52is electrically conductive, the electrically conductive layer out of the first layer50and the second layer52may function as a wiring layer or a ground layer. The ground layer has a reference potential. The reference potential may be referred to as a ground potential.

The supporting member24is provided on the side of the first layer50opposite to the second layer52in the thickness direction.

The supporting member24supports the first layer50, the second layer52, and the light detecting unit34in the mounting board26. The photodetector20(mounting board6) does not necessarily include the supporting member24.

With the supporting member24, the photodetector20can secure the strength of the entire mounting board26.

A material of the supporting member24is not limited. The supporting member24, for example, may have an average atomic weight of any one of equal to or smaller than that of the second layer52, larger than that of the second layer52and smaller than that of the first layer50, and equal to or larger than that of the first layer50.

In a case where the supporting member24has an average atomic weight of equal to or smaller than that of the second layer52, the weight of the mounting board26(and the photodetector20) can be reduced.

In a case where the supporting member24has an average atomic weight of larger than that of the first layer50, the average atomic weight increases in order of the second layer52, the first layer50, and the supporting member24. In this case, the photodetector20can suppress generation of fluorescent X-rays F having higher light energy.

The thickness of the supporting member24is not limited. The thickness of the supporting member24is appropriately adjusted depending on the material of the supporting member24and on the weight and the size of the photodetector20required in mounting the photodetector20on various apparatuses or devices.

At least one of the thicknesses of the first layer50and the supporting member24may be adjusted so as to function as a shielding layer that shields, from the radiation L, the signal processing circuit22(refer toFIGS. 1 and 4) arranged on the side of the mounting board26opposite to the scintillator18.

The supporting member24, the first layer50, and the second layer52preferably have a uniform thickness in the plane direction (direction orthogonal to the thickness direction) in an area other than portions like via holes formed in production of the photodetector20. The uniformity in the thickness makes the generation rate of the radiation S and the fluorescent X-rays F and the transmittance and the absorbance of the radiation L uniform in the plane direction in the supporting member24, the first layer50, and the second layer52. As a result, the signal processing circuit22can reduce the load in correcting the waveform of the spectrum indicated by the signals acquired from the light detecting units34. in view of the uniformity, the sectional area of the via holes formed in the layers constituting the mounting board26(the supporting member24, the first layer50, and the second layer52) in the plane direction is preferably made as small as possible.

The following describes an action when the radiation L is incident on the photodetector20.

FIG. 11is a view for explaining an example of an action when the radiation L is incident on the photodetector20. The radiation L is incident on the photodetector20from the scintillator18side.

The radiation L incident on the scintillator18is converted into light and reaches the light detecting unit34. The radiation L not converted into light by the scintillator18passes through the light detecting unit34and the second layer52and then reaches the first layer50. Part of the radiation L incident on the photodetector20may possibly reach the first layer50without passing through the scintillator18.

The first layer50absorbs at least part of the radiation L incident thereon. This mechanism suppresses scattering, in the first layer50, of photons of the radiation L that have reached the first layer50. As a result, the first layer50can suppress generation of the scattered radiation S.

As described above, the first layer50has a larger average atomic weight than that of the second layer52. As the atomic number of elements constituting the first layer50is larger, the absorbance of the incident radiation L is higher. By contrast, as the atomic number of elements constituting the first layer50is larger, the energy of the fluorescent X-rays F is larger.

Therefore, when the radiation L is incident on the first layer50, fluorescent X-rays specific to the first layer50are generated in the first layer50. The fluorescent X-rays F generated in the first layer50reach the second layer52.

As a result, the second layer52receives the radiation S scattered in the first layer50and the fluorescent X-rays F generated in the first layer50by the radiation L not absorbed by the first layer50.

The second layer52absorbs the radiation S and the fluorescent X-rays F. In other words, the radiation S and the fluorescent X-rays F traveling from the first layer50to the second layer52are absorbed by the second layer52. This mechanism can suppress incidence of the radiation S scattered in the first layer50and the fluorescent X-rays F generated in the first layer50on the light detecting unit34besides the light resulting from conversion by the scintillator18.

As a result, the signal processing circuit22receives signals output based on light accurately detected by the light detecting units34. The energy of the radiation L lost by the scintillator18is proportional to the number of photons of the light resulting from conversion by the scintillator18. The signal processing circuit22calculates the number of photons of the light resulting from conversion by the scintillator18using the signals received from the light detecting unit34. The signal processing circuit22thus can calculate backward the energy of the radiation L incident on the scintillator18.

Signals obtained by amplifying signal electrons in an avalanche manner by, for example, an APD are known to have statistical fluctuation. Furthermore, a peak of an energy spectrum detected by an APD is known to have a width even in a case where X-rays having single energy are output to the light detecting unit34(APD). To address this, the signal processing circuit22preferably calculates the energy of the radiation L incident on the photodetector20by performing a known analysis method, such as fitting, on the energy spectrum obtained from the signals received from the light detecting unit34.

As described above, the photodetector20according to the present embodiment includes the scintillator18, the second layer52, the light detecting unit34, and the first layer50. The scintillator18converts the radiation L into light having a longer wavelength than that of the radiation L. The first layer50absorbs the radiation L. The light detecting unit34is provided between the scintillator18and the first layer50and detects light. The second layer52is provided between the first layer50and the light detecting unit34. The second layer52has a smaller average atomic weight than that of the first layer50and transmits the radiation L. The second layer52absorbs the radiation S scattered in the first layer50and the fluorescent X-rays F generated in the first layer50by the radiation L incident on the first layer50.

As such, the photodetector20according to the present embodiment includes the first layer50and the second layer52. The first layer50absorbs at least part of the radiation L. The second layer52absorbs the radiation S scattered in the first layer50and the fluorescent X-rays F. The second layer52is provided between the light detecting unit34and the first layer50. This configuration allows the photodetector20to absorb at least part of the radiation L with the first layer50and absorb the radiation S and the fluorescent X-rays F with the second layer52. As a result, the photodetector20according to the present embodiment can suppress incidence of the radiation S scattered in the first layer50and the fluorescent X-rays F on the light detecting unit34.

Consequently, the photodetector20according to the present embodiment can improve the accuracy in detecting the radiation L.

Production Method

The following describes an example of a method for producing the photodetector20according to the present embodiment.

FIGS. 12A to 12Eare views for explaining an example of the method for producing the photodetector20. The production method illustrated inFIGS. 12A to 12Eis given by way of example only, and the method for producing the photodetector20is not limited thereto.

The supporting member24, the first layer50, and the second layer52are prepared first (refer toFIGS. 12A to 12C).

For example, a plate-like supporting member24is prepared and via holes42are formed on the supporting member24(refer toFIG. 12A). The via holes42are filled with an electrically conductive material to serve as through electrodes47. The formation of the via holes42and the filling thereof with the electrically conductive material are performed by a known method. Signal lines23are printed on the supporting member24(refer toFIG. 12D). The signal lines23transmit signals from the light detecting units34to the signal processing circuit22. The signal lines23are printed by a known method.

A plate-like second layer52is prepared, and via holes44are formed at positions corresponding to the respective pixel areas30on the second layer52(refer toFIG. 12C). The via holes44are filled with an electrically conductive material to serve as through electrodes46. The formation of the via holes44and the filling thereof with the electrically conductive material are performed by a known method.

A plate-like first layer50is prepared, and at least one of via holes43and via holes43′ are formed on the first layer50to electrically connect the electrically conductive material in the via holes42of the supporting member24to the electrically conductive material in the via holes44of the second layer52(refer toFIG. 12B). Specifically, to form wirings on the surface of the first layer50, via holes, out of the via holes43and the via holes43′, that need to be formed through the first layer50are formed for the electrical connection. The via holes43are filled with an electrically conductive material to serve as through electrodes45. The formation of the via holes43and the filling thereof with the electrically conductive material are performed by a known method.

The supporting member24, the first layer50, and the second layer52are stacked in this order. The positions of the supporting member24, the first layer50, and the second layer52are adjusted such that the through electrodes47,45, and46are electrically connected in this order between layers that are adjacent in the thickness direction (refer toFIG. 12D). The supporting member24is arranged with its surface provided with the signal lines23facing the side opposite to the first layer50.

The multilayer stack obtained by stacking the supporting member24, the first layer50, and the second layer52in this order is pressurized and burnt. Subsequently, the light detecting units34are formed on the respective pixel areas30on the second layer52. The light detecting units34are formed by a known method.

Subsequently, the scintillator18is arranged on the light detecting units34, thereby producing the photodetector20(refer toFIG. 12E).

While the layers constituting the photodetector20(the supporting member24, the first layer50, and the second layer52) are pressurized and burnt in the example illustrated inFIGS. 12A to 12E, the method for producing the photodetector20is not limited thereto. The photodetector20may be produced by bonding the layers with adhesive layers interposed therebetween or applying or vapor-depositing the materials of the layers, for example. Th photodetector20may be produced by a combination of at least two of pressure-burning, vapor deposition, and application. The photodetector20may be produced by performing resistant burning or plating after pressure-burning.

The photodetector20produced by the production method illustrated inFIGS. 12A to 12Ehas the through electrodes46,45, and47as illustrated inFIG. 12E. The through electrodes46, the through electrodes45, and the through electrodes47are used to electrically connect the light detecting units34of the photodetecton20to the signal processing circuit22(not illustrated inFIGS. 12A to 12E). The light detecting units34are electrically connected to the signal processing circuit22via the through electrodes46,45, and47and the signal lines23formed on the surface of the supporting member24on the side opposite to the first layer50.

The signals output from the light detecting units34can be retrieved from the back surface of the photodetector20(surface on the side opposite to the scintillator18) via the through electrodes46,45, and47. This configuration enables the light detecting units34to be densely mounted on the second layer52.

In a case where the first layer50is electrically conductive, setting the electric potential of the first layer50to the reference potential (ground potential) can electrically separate the signal lines23and the through electrodes46,45, and47from the other portions on the mounting board26. Furthermore, setting the electric potential of the first layer50to the reference potential can increase the strength of the photodetector20against noise.

Second Embodiment

A second embodiment describes the photodetector20further including a ground (GND) layer and a wiring layer.

FIG. 13is a schematic diagram of an example of a photodetector20A according to the present embodiment.

The photodetector20A includes the scintillator18on a mounting board26A. The mounting board26A has a structure obtained by stacking the supporting member24, a ground layer60, a wiring layer62, the first layer50, the second layer52, and the light detecting unit34in this order.

The supporting member24, the first layer50, the second layer52, the light detecting unit34, and the scintillator16are identical with those according to the first embodiment.

The first layer50according to the present embodiment is electrically conductive.

The wiring layer62is provided between the first layer50and the supporting member24. In other words, the first layer50and the wiring layer62are separated to serve as different layers in the photodetector20A.

The wiring layer62is electrically connected to the light detecting unit34via the through electrodes46. The wiring layer62is also electrically connected to the signal lines23via the through electrodes47. The through electrodes46are provided through the first layer50and the second layer52to electrically connect the light detecting unit34to the wiring layer62. The through electrodes47are provided through the ground layer60and the supporting member24to electrically connect the wiring layer62to the signal lines23.

At least part of the wiring layer62is electrically conductive, and the material thereof is not limited. Signals output from the light detecting unit34are transmitted to the signal processing circuit22(refer toFIG. 1) via the through electrodes46, the wiring layer62, the through electrodes47, and the signal lines23.

The ground layer60has the reference potential and is provided between the wiring layer62and the supporting member24. The ground layer60is arranged so as not to be in electrically contact with (electrically connected to) the through electrodes46, the wiring layer62, the through electrodes47, and the signal lines23.

The ground layer60is electrically connected to the electrically conductive first layer50. The ground layer60and the first layer50have the same potential (that is, the reference potential). The wiring layer62is sandwiched in the thickness direction between the first layer50and the ground layer60having the reference potential.

Therefore, the first layer50of the photodetector20A has a function of a noise guard for the wiring layer62besides the functions described in the first embodiment. As described above, the wiring layer62is sandwiched between the first layer50and the ground layer60, and the first layer50and the ground layer60have the reference potential. This configuration can increase the strength of the photodetector20A against noise.

As described above, the photodetector20A according to the present embodiment includes the scintillator18, the light detecting unit34, the second layer52, the electrically conductive first layer50, the wiring layer62, and the ground layer60. The ground layer60is not electrically connected to the light detecting unit34and is electrically connected to the electrically conductive first layer50. The ground layer60and the first layer50have the same potential. The wiring layer62is provided between the ground layer60and the first layer50and is electrically connected to the light detecting unit34.

Consequently, the photodetector20A according to the present embodiment can not only provide the advantageous effects of the photodetector20according to the first embodiment but also further improve the detection accuracy.

Third Embodiment

While the first layer50according to the embodiments described above is a single layer, it may be a multilayer stack made of a plurality of layers.

FIG. 14is a schematic diagram of an example of a photodetector20B according to a third embodiment.

The photodetector20B includes the scintillator18on a mounting board26B. The mounting board26B has a structure obtained by stacking the supporting member24, a first layer51, the second layer52, and the light detecting unit34in this order.

The supporting member24, the second layer52, the light detecting unit34, and the scintillator18are identical with those according to the first embodiment.

The first layer51according to the present embodiment is identical with the first layer50according to the first embodiment except that it is a multilayer stack made of a plurality of layers. In other words, the material and the functions of the first layer51according to the present embodiment are the same as those of the first layer50according to the first embodiment. The first layer51is electrically conductive.

The first layer51according to the present embodiment has a structure obtained by stacking a plurality of first layers51A to51F in the thickness direction.

Specifically, the present embodiment includes, between the second layer52and the supporting member24, a fourth layer54A, a fourth layer54B, a fourth layer54C, a fourth layer54D, a fourth layer54E, and a fourth layer54F stacked in this order from the second layer52to the supporting member24.

The fourth layers54(fourth layers54A to54F) include areas made of the same material as the material of the first layer50according to the first embodiment (that is, areas that satisfy the same functions and requirements as those of the first layer50). These areas correspond to the first layers51A to51F.

In the example illustrated inFIG. 14, a part of the fourth layer54A corresponds to the first layers51A and51B. A part of the fourth layer54B corresponds to the first layer51C. A part of the fourth layer54C corresponds to the first layer51D. A part of the fourth layer54D corresponds to the first layer51E. A part of the fourth layer54E corresponds to the first layer51F. The fourth layer54F includes no first layer51.

The areas other than the first layers51A to51F in the fourth layers54(fourth layers54A to54F) simply need to be made of an insulating material having a smaller average atomic weight than that of the first layer51, for example.

Similarly to the first layer50according to the first embodiment, the first layer51is preferably arranged such that a first projection area obtained by projecting the first layer51onto the light detecting unit34covers at least the light detecting unit34.

FIG. 15is a plan view schematically illustrating an example of the photodetector20B viewed from the light detecting unit34. As illustrated inFIG. 15, a first projection area C obtained by projecting the first layer51(first layers51A to51F) onto the light detecting unit34preferably covers at least the light detecting unit34. The first layers51A to51F constituting the first layer51preferably have such sizes in the plane direction and are arranged at such positions that the first projection area C of the first layer51covers the light detecting unit34.

Referring back toFIG. 14, the first layer51(first layers51A to51F) according to the present embodiment is electrically conductive. Specifically, in the example illustrated inFIG. 14, the first layer51A is electrically connected to the light detecting unit34via through electrodes49. The first layer51A is also electrically connected to the signal lines23via the through electrodes49, the first layer51C, and the first layer51E. The first layer51B is electrically connected to the signal lines23via the through electrodes49, the first layer51D, and the first layer51F.

The sums of the thicknesses of the first layer51at respective positions along the plane direction are preferably equal at any position of the first layer51along the plane direction. In other words, as illustrated inFIG. 14, the sums of the thicknesses of the first layers51A to51F at respective positions in a direction (plane direction) orthogonal to the thickness direction of the photodetector20B are preferably equal to one another.

The thicknesses of the first layers51A to51F may be equal to or different from one another. The positions and the ranges of the first layers51A to51F are not limited to those illustrated inFIG. 14.

As described above, the first layer51of the photodetector20B according to the present embodiment is a multilayer stack made of a plurality of lavers (first layers51A to51F).

Also in this case, the photodetector20B can provide the same advantageous effects as those of the first embodiment.