JPS6348489A - Radiation detector - Google Patents

Abstract

PURPOSE:To greatly improve the S/N and contrast of a radiation detector which outputs an electric signal corresponding to the intensity of incident radiation by increasing the contribution of low-energy radiation photons to the output of a photoelectric converting element relatively. CONSTITUTION:A photodiode 1 as a photodetector and a scintillator are laminated across an optical adhesive layer as the basic constitution of an X-ray detector. The scintillator 2 is constituted by arranging a 1st scintillator 2A on an X-ray incidence end side and a 2nd scintillator 2B on a projection end side for scintillation light, and both scintillators 2A and 2B are jointed by the optical adhesive layer 3 into laminate structure. Scintillation characteristics are made difference between the incidence and projection end sides of those scintillators and the ratio of a signal based upon light generated by the scintillation of low-energy radiation to the output signal of the photoelectric transducer 1 is increased.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a radiation detector that outputs an electrical signal according to the intensity of incident radiation, and in particular to a scintillator structure that performs X-ray to light conversion. Regarding the improvement of

(Prior Art) As an example of this type of radiation detector, a solid-state X-ray detector that combines a photodiode and a scintillator is conventionally known. Its typical configuration is T, Newto
Radiology of the 5kull by n
and brain vol. 5 J, a photodiode and a scintillator are laminated via an optical adhesive layer.

In an X-ray detector with such a configuration, X-rays are detected by converting the X-rays incident on the scintillator into light, and outputting an electrical signal according to the amount of detected light using the photodiode. There is.

[Problem that the invention seeks to solve]

The XvA detector having the above configuration has the following problems.

The output charge of the photodiode is largely dominated by high-energy components of the incident X-rays, resulting in a loss of signal/noise ratio.

This will be explained theoretically.

Assume that the distribution of the incident X-ray spectrum in one sample is a function P (E) as shown in FIG. The number N of incident X-ray photons at this time is as follows.

If this measurement is repeated several times by counting one X-ray photon at a time, a normal distribution with a center value N and a variance N is obtained. That is, the signal/noise ratio for this measurement is
N/, JfTr=/y, which is the physical limit of the signal/noise ratio.

Now, in a system that measures the output charge of a photodiode, if one photon generates q (E) coulombs as a detector output, the detector output Q is as follows. Do this multiple times and set the Variance to ■
Then, . The signal/noise ratio in this case is S/N=Q/V/7.

Here, Cauchy-5chwartg for the integral
Due to the inequality: Therefore, in systems that measure charge, the signal-to-noise ratio is always worse than the physical limit.

In other words, the equality holds true in the above inequality because, from the theorem, (P(E))"・Q(E)/(q(E))"=con
Only when st, that is, only when q(E)-const.

However, q (E) is generally approximately the X-ray energy E
is proportional to. This is because the total energy of scintillation light generated by one X-ray photon is proportional to the energy of the X-ray photon.

■As the resolution increases, the detector aperture width tends to become narrower, but in this case ■° the situation becomes even worse.

That is, the average depth at which scintillation light is generated (the depth from the incident end of the scintillator) d is a function of the energy of the X-ray photon d (E), and if E is small, light is emitted at a shallow position.

In the scintillator 10 shown in FIG. 4, when E is small, scintillation light generated at a shallow position determined by the function d (E) reaches the photodiode 11 through multiple reflections, but is absorbed in the process. Therefore, low-energy X-ray photons have poor light transmission efficiency, resulting in an increased loss of S/N ratio.

In other words, it can be seen that q(E) is not approximately proportional to E, but is more strongly dependent on E.

■There is a loss not only in the S/N ratio but also in contrast.

That is, it is said that the soft tissue of a living body has good contrast with low-energy X-rays. However, the photodiode output is limited by the fact that the amount of scintillation light is proportional to the X-ray photon energy, and that the scintillation position is deep for high-energy X-ray photons.
The wire has better light transmission efficiency to the photodiode,
Due to these two reasons, it is almost dominated by high-energy X-rays. Therefore, the reconstructed image obtained using this data could not be said to have good soft tissue contrast.

Therefore, an object of the present invention is to solve the above-mentioned conventional problems, relatively increase the contribution of low-energy X-ray photons to the photodiode output, and improve the S/N contrast. The objective is to provide a radiation detector that can.

[Structure of the invention]

(Means for Solving the Problems) The present invention is a scintillator that emits scintillation light according to the intensity of incident radiation, and is configured to have different scintillation characteristics on the radiation input end side and the scintillation light output side. Of the output signals of the photoelectric conversion element that converts the output light from this scintillator into electrical signals,
A radiation detector is constructed by increasing the proportion of signals generated by light emission at the incident end side, which is the scintillation position of low-energy radiation.

(Function) Focusing on the fact that low-energy radiation causes scintillation at a position shallower than the radiation input end, the scintillation characteristics are made different between the input end and the output end of the scintillator, thereby reducing the output signal of the photoelectric conversion element. , the proportion of the signal due to light generated by scintillation of low-energy radiation can be increased.

Therefore, the soft part Mi that reflects low-energy radiation well
The contrast of the fabric is improved, and furthermore, (1 (E)), which depends on radiation photon energy, can be made more constant and equalized, thereby bringing the S/N ratio closer to the physical limit.

(Example) Hereinafter, the present invention will be explained based on illustrated embodiments.

FIG. 1 is a schematic perspective view of an X-ray detector according to the present invention.

In the figure, the basic configuration of this X-ray detector is a structure in which a photodiode 1, which is a light receiving element, and a scintillator 2 are laminated with an optical adhesive layer 3 interposed therebetween. In this embodiment, a silicon photodiode is used as the photodiode.

A unique feature of the X-ray detector according to this embodiment is the structure of the scintillator 2. The scintillator 2 shown in FIG.
The first scintillator 2A is arranged on the X-ray incident end side, and the second scintillator 2B is arranged on the scintillation light output end side, and both scintillators 2A and 2B are bonded with an optical adhesive layer 3 to form a laminated structure. There is.

Here, the first scintillator 2A is CdWoa.

The second scintillators 2B are each made of old 4GexO and t, and each has a thickness of about 1n.

Next, the operation of the X-ray detector having the above configuration will be explained.

The first one is made of the above-mentioned material. second scintillator 2A,
The scintillation characteristics of 2B are different in the following points.

That is, while the scintillation cutoff wavelength of BiGezO+z is 350n, the scintillation cutoff wavelength of CdWo is 450tm.

On the other hand, the sensitivity spectrum of the photodiode 1 is as shown in FIG. The horizontal axis of the figure is the wavelength (nm
), the vertical axis represents the response, and the peak response range of this diode is 885±50 (nm).

From the above, the emission spectrum of Cd't4oa, which is the material of the first scintillator 2A, is higher than the sensitivity spectrum of the silicon photodiode 1, compared to the emission spectrum of B1Ge:+0+z, which is the material of the second scintillator 2B. It turns out that it is closer. Furthermore, CdWo4 has higher scintillation conversion efficiency.

Here, as mentioned above, the average depth at which scintillation light is generated (depth from the incident end of the scintillator) is
Since it is a function of the energy of the line photon,
X causes scintillation with the first scintillator 2A
The rays have lower energy than the X-rays that cause scintillation in the second scintillator 2B.

Therefore, low energy X-rays are transmitted to the first scintillator 2A.
That is, since scintillation is caused by CdWo4, the light captured by the photodiode 1 will be more strongly oscillated with lower energy than when only CdWon or B1Ge30° is used.

Therefore, image data in which low-energy X-rays are well reflected, that is, image data in which the contrast of soft tissues is improved, can be collected. Furthermore, since q (E), which depends on the X-ray photon energy E, can be leveled closer to cons t, the S/N ratio can be brought closer to the physical limit.

Note that the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the gist of the present invention.

The present invention aims to relatively increase the contribution of low-energy radiation photons to the photodiode output, and for this purpose, the scintillation characteristics are made different between the incident end and the output end. This scintillation characteristic is
In addition to what is shown in the above embodiments, the generation efficiency may be changed.

That is, even if the scintillator has scintillation characteristics in which the luminous efficiency on the incident end side is higher than the luminous efficiency on the output end side, low energy X-rays are strongly oscillated and the same effect as in the above embodiment can be achieved. Can be done.

Further, in the above embodiment, the structure is one in which two types of scintillators made of different materials are stacked, but a structure in which three or more types of scintillators are stacked may also be used. Continuously or stepwise different 1
The scintillator may be composed of several scintillators.

Note that the thickness of each scintillator in a laminated structure can be set variously depending on how much the contribution of low-energy X-ray photons is increased.

〔Effect of the invention〕

As detailed above, according to the present invention, the S/N ratio can be improved by relatively increasing the contribution of low-energy radiation photons to the output of the photoelectric conversion element.

Contrast can be significantly improved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the X-ray detector according to the present invention, FIG. 2 is a characteristic diagram showing the sensitivity spectrum distribution of the photodiode of the same detector, FIG. 3 is a characteristic diagram showing the incident xbi spectrum distribution, and FIG. The figure is a schematic explanatory diagram showing the transmission of scintillation light by low-energy X-ray photons. 1...Photoelectric conversion element, 2...Scintillator. Agent: Patent Attorney: Nori Ken, Yudo, and Dai: Noriyuki Hu, Hayabusa (Tohe?He acid two)

Claims (5)

[Claims] (1) In a radiation detector including a scintillator that emits scintillation light according to the intensity of incident radiation and a photoelectric conversion element that converts the output light from the scintillator into an electrical signal, the scintillator is located on the radiation incident end side. and the scintillation light emission end side are configured to have different scintillation characteristics, and the proportion of the output signal of the photoelectric conversion element due to light emission at the input end side, which is the scintillation position of low-energy radiation, is increased. A radiation detector characterized by: (2) The scintillator has a luminous efficiency on the incident end side,
The radiation detector according to claim 1, which has scintillation characteristics higher than luminous efficiency on the emission end side. (3) The scintillator has a scintillation characteristic in which the emission spectrum at the output end side is farther from the sensitivity spectrum of the photoelectric conversion element than the emission spectrum at the input end side. The radiation detector according to item 2. (4) The scintillator is constituted by one scintillator in which the scintillation characteristics differ continuously or stepwise in the depth direction along the radiation incident direction. The radiation detector according to item 1. (5) The radiation detector according to any one of claims 1 to 3, wherein the scintillator is formed by laminating multiple types of scintillators having different scintillation characteristics.