JPH056155B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a radiation sensor with good contrast resolution that can be used in an X-ray diagnostic device, an X-ray inspection device for nondestructive testing, and the like.

Conventional structure and its problems Modern computer tomography devices have good contrast resolution and can distinguish between substances with slightly different radiation absorption coefficients, such as muscle and fat, and can be used to contrast large cancers, etc. It is now possible to identify them without drugs. this is,
The main reason for this is that the detection efficiency has improved due to the improvement in the SN of the radiation sensor.

However, the ability to identify small cancers, etc. is still insufficient, and the exposure dose cannot be said to be low, so there is a need to further reduce it.

From this point of view, the inventor of the present application is developing a radiation sensor using semiconductor materials that has even better detection efficiency and responsiveness.

Radiation sensors using compound semiconductors have the problem of changes in detection efficiency and response during operation, that is, the detection efficiency and response of the sensor decreases during radiation measurement, making accurate radiation detection difficult. I was faced with the fatal problem of not being able to do so.

In addition, as a conventional radiation measurement method, the pulse measurement method is not used, and the SN of radiation detection is
This is due to the fact that conventional radiation sensors cannot provide a high-speed pulse response with a short pulse width.

Purpose of the Invention In view of the above problems, the present invention provides a novel radiation sensor that uses a compound semiconductor material, has excellent stability during operation, has good radiation detection efficiency, has a low exposure dose, and is capable of high-speed pulse measurement. The purpose is to provide

Structure of the Invention The radiation sensor of the present invention uses a compound semiconductor having an effective atomic number of 30 or more for photoelectric absorption as a sensor material, and electrodes are provided on opposing crystal planes of the compound semiconductor to absorb incident radiation. It counts radiation photons as electrical pulses. In addition, the spacing between the opposing electrodes is 0.1 mm.
or more and 0.5mm or less.

DESCRIPTION OF EMBODIMENTS The radiation sensor of the present invention will be described by way of embodiments. Figures 1a and 1b show the radiation sensor of this example.
FIG. 2 is a configuration diagram of a radiation detection element array configured by arranging a plurality of radiation detection elements in a line on a base; a is a plan view; b is a top view;
is a front view.

In the figure, 1 is a semiconductor detection element crystal, 2 is its substrate, 3 is an upper electrode of the element, and 4 is a lower common electrode. By measuring the radiation dose while driving this element array along the subject, a two-dimensional image signal can be read.

In the configurations shown in FIGS. 1a and 1b, CdTe or GaAs was used for the semiconductor detection element crystal. Further, the potential difference applied between electrodes 3 and 4 was 100V.

Now, in the above configuration, it has been found that the radiation detection efficiency and the stability of response are significantly affected by the thickness of the semiconductor detection element crystal, that is, the distance between the electrodes 3 and 4.

In other words, if the distance between the electrodes is 0.5 mm or more,
The detection efficiency and response decrease with the operating time of the element, making it practically impossible to use it as a radiation sensor. However, when the electrode spacing is 0.5 mm or less, the detection efficiency and response decrease. It was found that the radiation is stable over time and that stable and accurate radiation detection is possible at all times. Note that the X-rays used at this time were 60keV X-rays.

The reason for this is thought to be as follows.

That is, as shown in FIG. 2, when radiation enters a semiconductor crystal, pairs of electrons and holes are formed within the semiconductor crystal.

Of the electrons and holes generated in this way, since the electrons have a long mean free path, the probability that they will be trapped in a crystal as thin as 0.5 mm as described above is extremely low.

However, since the mean free path of holes is extremely short compared to that of electrons, if the thickness of the crystal exceeds a certain value, specifically, if it becomes thicker than the mean free path of holes, The probability of being trapped increases.

The holes thus trapped form a space charge region within the crystal, but since the number of trapped holes increases with the operating time of the device, this space charge region also expands with the operating time of the device. Go.

The region within the crystal 5 where X-rays can be detected is
Since this is a region excluding the space charge region, in other words, the temporal expansion of the space charge region means a temporal decrease in the X-ray detectable region, that is, a temporal decrease in detection efficiency.

On the other hand, if the thickness of the crystal 5 is equal to or less than the mean free path of a hole, the probability of the space charge region occurring is sufficiently low, and the detection efficiency is considered to be stable without decreasing over time. It will be done.

In this example, the thickness of the crystal is 0.5.
It is speculated that the reason for the completely different detection efficiency and stability of response across mm.

Next, in this example, the thickness of the crystal was set to 0.5
Sufficient detection efficiency and responsiveness could be ensured even when the diameter was less than mm.

The reason is as follows.

The absorption of radiation photons within a crystal involves the photoelectric effect and Compton scattering.

In Compton scattering, only a portion of the energy of radiation photons is absorbed within the crystal.

On the other hand, the absorption process of photons in the photoelectric effect is divided into absorption of generated secondary electrons and absorption of characteristic X-rays. Of these, secondary electrons can be completely absorbed if the crystal has a thickness sufficient for the range of the secondary electrons. For example, for X-rays below 200eKV, the range of secondary electrons is approximately 0.1 mm for GaAs and approximately 0.08 mm for CdTe.
Therefore, if the thickness of these crystals is 0.1 mm or more, secondary electrons can be completely absorbed.

In the energy range of X-rays used to create practical X-ray images, if the effective atomic number of the crystal used as a sensor is 30 or higher, absorption due to the photoelectric effect is sufficiently dominant over Compton absorption, so 0.1 Sufficient detection efficiency can be ensured even with crystals as thick as mm. Note that the effective atomic numbers of GaAs and CdTe are 32 and 49, respectively.

Also, regarding the characteristic X-rays in the photoelectric effect,
Although it is not as important as the absorption of secondary electrons, from the viewpoint of measurement accuracy, it is preferable to allow the sensor to absorb it as much as possible. The absorption coefficient of characteristic X-rays due to the photoelectric effect is
Depends on the atomic number of the crystal material used as a sensor,
It increases with increasing atomic number. For example, for GaAs with current number 30 or higher, the half-value layer is 0.01mm at 12eKV.
It is. Also, for CdTe, the half value layer is 0.1 at 27eKV
mm, and practical measurements are possible if the crystal thickness is 0.1 mm or more.

Moreover, by measuring radiation in a region where the photoelectric effect is dominant, the sensor can obtain a pulse output waveform with a high peak value, which is easy to distinguish from noise in the measurement circuit and has a high S/N ratio. Can perform high-quality measurements.

Figure 4 shows the crystal thickness of CdTe and GaAs and 60keVX
It is a figure which shows the relationship of the absorption rate (%) of a line. The X-rays used for diagnosis are around 60keV. At a thickness of 0.5 mm, CdTe has an effective atomic number for photoelectric absorption.
49, which has an absorption efficiency of about 80% and is sufficiently large. 0.1
Even if the thickness is about 30 mm, it can be practically used. GaAs
has an effective atomic number of 31 and has an absorption efficiency of about 30% at a thickness of 0.5 mm, which is lower than CdTe, but can be used practically.

A crystal with an effective atomic number smaller than 30 has a low X-ray absorption efficiency, in other words, a low X-ray detection efficiency and cannot be put to practical use.

In addition, in this example where the crystal thickness was 0.5 mm,
High-speed pulse measurement is possible.

This point will be explained in detail using FIG. 2. Electron/hole pairs generated in the semiconductor by radiation quanta move to the anode and the hole to the cathode in accordance with the electric field applied to the semiconductor. This causes a pulsed current to flow externally. It is known that the time width t of this pulse signal is expressed by the first equation.

t=d ² /μ・Va ... (a) where t: Pulse time width d: Thickness of semiconductor detection layer μ: Mobility of electrons or holes Va: Applied voltage to semiconductor detection layer Above formula According to , the pulse duration is proportional to the square of the thickness of the semiconductor layer, so if the thickness is reduced to 1/10, the pulse width will be 1/100, for example.

In this embodiment, the details are as follows.

The effective atomic numbers of CdTe and GaAs are 49 and 32, respectively. The hole mobility of CdTe is μh=
80cm ² /V・sec, or for GaAs, μh=400cm ² /
It is Vsec. At an applied voltage of 100 V, the relationship between crystal thickness and pulse width is plotted as shown in Figure 3. CdTe is about 1 μsec for 1 mm thickness, and GaAs is about 1 μsec for 1 mm thickness.
0.25μsec, 0.5mm thickness for CdTe, 0.3μsec,
For GaAs, it is 0.06μsec. In this way, 1mm thickness and 0.5
With a thickness of mm, the pulse width will differ several times.

In particular, when the crystal thickness is 0.5 mm or less, the pulse width is 0.2 μsec or less, and the time resolution of the pulse can be improved to almost the limit that can be processed with modern practical circuit technology.

Now, the pulse integration time for one pixel is
If it is 100msec, the number of pulses with a pulse width of 0.1μsec is
100msec ÷ 0.1μsec = 10 You can get about ⁶ pieces.
For radiated pulses, the error due to statistical fluctuations is
√10 ⁶ = 10 ³ , and the measurement error at that pixel is
It will be 0.1%. It is said that the difference in X-ray absorption coefficients of biological soft tissue constituents such as muscle, fat, and water is less than a few percent, and in order to detect this difference, a pixel measurement accuracy of 0.1% is said to be necessary. Therefore, it can be said that the error factor due to the statistical fluctuation of the incident X-rays described above is at a level that maintains this accuracy.

Figures 5a and 5b show another specific example of constructing a radiation detection element array using the radiation sensor of the present invention, in which a common electrode is attached to one side of an elongated crystal and this is attached to a substrate. This is an example in which a plurality of electrodes are further attached to the top. This embodiment has a feature that it can be manufactured more easily than the embodiment shown in FIG.

The radiation detection element array constructed using the radiation sensor of the present invention may have various other configurations.

Effects of the Invention To summarize the above, by using a semiconductor crystal with an effective atomic number of 30 or more as a radiation sensor and making the crystal thickness 0.5 mm or less, the response is stabilized over time and the detection efficiency is improved. A high radiation sensor can be obtained.

For example, when using a radiation detection element array made of CdTe with a thickness of 0.5 mm, the absorption efficiency of 60 keV X-rays reaches 80%, which means that it has a detection efficiency and response close to the theoretical limit, which is 0.1 to 1 mR. X-ray images with good contrast can be obtained.
This corresponds to 100 times the sensitivity of silver halide X-ray photography.

Furthermore, since high-speed pulse measurement is possible, the sensor of the present invention has very good contrast resolution and can distinguish minute differences in living tissues.

[Brief explanation of the drawing]

Figures 1a and b are a plan view and a front view of a radiation detection element array constructed using the radiation sensor of the present invention, respectively, Figure 2 is an explanatory diagram of the principle of radiation detection, and Figure 3 is an illustration of the element thickness. FIG. 4 is a diagram showing the relationship between the element thickness and the absorption rate of 60 keV X-rays, and FIGS. FIG. 6 is a plan view and a front view of another radiation detection element array. 1... Semiconductor element crystal, 3... Electrode, 4... Other electrode.

Claims (1)

[Claims] 1. A radiation sensor that counts individual radiation photons as electric pulses and includes a compound semiconductor and electrodes formed on opposing surfaces of the compound semiconductor, wherein the effective atomic number of the compound semiconductor for photoelectric absorption is 30. A radiation sensor according to the above, and wherein the distance between the opposing electrodes is 0.1 mm or more and 0.5 mm or less.