JP2536234Y2 - Scintillation detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an improvement of a scintillation detector used as a preamplifier in a radiation measuring device for measuring an energy spectrum of radiation such as gamma rays.

[Conventional technology]

As shown in FIG. 9, a conventional scintillation detector of this type includes a scintillator 1 using, for example, NaI (T1) as a luminous material that emits light having an intensity corresponding to an input radiation dose, and this scintillator 1 A photomultiplier tube 2 for converting the emission intensity of the photomultiplier into a current pulse, and a current / current converter for converting an output current pulse from the photomultiplier tube 2 into a voltage pulse.
A voltage conversion circuit 3, a crest discrimination circuit 4 for continuously outputting a signal while an output voltage pulse from the current / voltage conversion circuit 3 exceeds a certain threshold, and a unit time from the crest discrimination circuit 4 Counting rate meter 5 for counting the number of output signals
And was composed of

[Problems to be solved by the invention]

However, the conventional scintillation detector has the following problems. That is, in a range where the input count rate of radiation to the scintillator 1 is sufficiently small, as shown in FIG. 10, individual output voltage pulses from the current / voltage conversion circuit 3 are discriminated by the peak height discrimination circuit 4 without any problem. However, when the input count rate increases, the eleventh
As shown in the figure, a so-called pile-up phenomenon occurs in which the next voltage pulse is input during the processing of one voltage pulse in the pulse height discrimination circuit 4, and a plurality of pulses are regarded as one pulse. The scintillation detector has a disadvantage that it is regarded as erroneous processing and is therefore erroneously measured even in the count rate meter 5. ) Could only be measured.

In FIGS. 10 and 11, τ _p is the time constant of the rise of the voltage pulse determined by the characteristics of the photomultiplier tube 2, and τ _d is the decay time constant of the voltage pulse determined by the decay characteristic of light emission in the scintillator 1. Body material is NaI (T
1) about 250 ns]. And
The rise time constant τ _p and the decay time constant τ _d are indices that are defined such that the larger the value, the longer the rise time and the decay time of the voltage pulse.
Further, the rise time constant τ _p is generally very small, ie, a fraction or less of the decay time constant τ _d . Further, V _D is a threshold (Schlesinger Scholl de level) in the pulse-height discriminator circuit 4, T _P denotes the output pulse width of the pulse height discriminator circuit 4 in the absence the pileup each.

However, inventor of this application, or relationship between the output count rate n _o by counting rate meter 5 and the input count rate n _i of the radiation to the scintillator 1 in the above is how specifically, the fact that Experimental and theoretical analysis shows that
As shown in FIG. 12, as ideally shown in phantom n _o =
Although it should be n _i , as shown by the solid line, n _o = n _i · exp (−n _i · T _P )..., and the output count rate increases as the input count rate n _i increases. n _o is gradually far been reduced from the ideal value indicated by the imaginary line, also, there is an input count rate n _i. Limit value n
_i output count rate n _o exceeds the _(P) in order to rapidly decrease,
It has been found that it is no longer possible to take simple correction measures in this range, so that the measurement itself is meaningless. Then, as a result, the measurement limit value n _{i (P)} is 1 / T _{P when} calculated from the above equation.If T _P can be reduced, the pile-up phenomenon is less likely to occur and the measurement limit value n We obtained the result that we could get a large _{i (P)} and obtain a scintillation detector that could measure up to the range of input count rate higher than before.

This invention was made based on the above-mentioned conventional circumstances and the results of the study obtained recently.
Providing a cinnylation detector that can measure up to the range of input count rate higher than before by reducing the disadvantageous phenomenon of pile-up, which was easy to occur in the past, while only making relatively simple structural improvements Is to do.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a scintillator 1 which emits light having an intensity corresponding to an input radiation dose, and a photoelectron which converts the light emission intensity of the scintillator 1 into a current pulse as shown in FIG. Multiplier tube 2, current / voltage conversion circuit 3 for converting an output current pulse from photomultiplier tube 2 to a voltage pulse, and current / voltage conversion circuit 3
Continuously while the output voltage pulses from exceeds a certain threshold V _D and the wave height discriminator circuit 4 for outputting a signal, counting rate meter for counting the number of output signals per unit time from the pulse height discriminator circuit 4 5, the decay time constant of the output voltage pulse from the current / voltage conversion circuit 4 is reduced between the current / voltage conversion circuit 3 and the peak height discrimination circuit 4 (τ _d → τ _a : τ _a <τ _d ) and an attenuation time constant conversion circuit Z for shortening the decay time is provided.
In addition, as shown in FIG.
It is characterized in that a thermistor S for constantly exhibiting constant conversion characteristics regardless of changes in the ambient temperature is provided.

[Action]

In the scintillation detector having the above characteristic configuration, the scintillation detector has a relatively large decay time constant τ _d determined by the decay characteristic of light emission in the scintillator 1.
Instead of guiding the output voltage pulse from the current / voltage conversion circuit 4 to the crest valve circuit 4 as it is in the prior art, the decay time constant of the output voltage pulse from the current / voltage conversion circuit 4 is converted into an decay time constant. on converted than the tau _d on small tau _a by circuit Z, so as to introduce into the pulse-height discriminator circuit 4, as shown in FIG. 2, the voltage pulses in the pulse-height discriminator circuit 4 than the conventional short by which is adapted to attenuate at the time, as the pulse height discriminator circuit discriminated by the threshold value V _D more pulse lengths at 4, so shorter than the conventional T _P T _P 'is obtained.

Therefore, the pile-up phenomenon that the next voltage pulse is input during the processing of one voltage pulse in the pulse height discrimination circuit 4 is less likely to occur, and the radiation input count rate n to the scintillator 1 is reduced. _i and the relationship between the output count rate n _o by counting rate meter 5 is as shown in Figure 3. As a result, the measurement limit value n _i of the input count rate n _{i (P),} a much larger 1 / T _P 'is obtained than the conventional 1 / T _P, therefore, higher than the conventional input counting rate n _i output count rate n _o to the range of can now be fully measurable without greatly reduced.

The damping time constant conversion circuit Z is provided with the temperature compensating means S so that constant conversion characteristics can always be exhibited stably irrespective of the change in the ambient temperature. Even if the decay time constant of light emission changes, peak shift and pile-up at a high counting rate are reduced, so that a high counting rate output can be stably obtained.

〔Example〕

The overall schematic configuration of the scintillation detector according to the present invention is as shown in the block circuit diagram of FIG. 1, so that the description is omitted to avoid duplication, and the detailed configuration of this main part is described here. I will explain only.

Figure 4 shows the main part detailed structure of the basic embodiment, as shown in this figure, the output current pulses i from the anode of the photomultiplier 2 (t), the fixed resistor and the amplification of the resistor R _i And a voltage pulse v _i (t) converted and amplified into a voltage by the current / voltage conversion circuit 3 and output.
After passing through a decay time constant conversion circuit Z that operates so as to shorten the decay time, it is configured to be introduced into the wave height discrimination circuit 4. In this embodiment, the attenuation time constant conversion circuit Z uses a circuit generally called a pole zero canceller, and uses fixed resistors R ₁ and R ₂ , a capacitor having a capacitance C, and an amplifier having an amplification degree B. It is composed of

Now, in the above configuration, the output voltage pulse v _i from the current / voltage conversion circuit 3 (t) is the _{v i (t) = A ·} R i · i (t), whereas, the light emission in the scintillator 1 Decay time constant
if _{d, i (t) = i} 0 · exp (-t / τ d) ( where, i ₀ is a constant) from _{a, v i (t) = A} · R i · i 0 · exp (- t / τ _d ).

Therefore, the constant in the attenuation time constant conversion circuit Z is represented by R ₁ · C
= Τ _d , the output voltage pulse v ₀ (t) from the decay time constant conversion circuit Z becomes v ₀ (t) = A · B · R ₁ · i ₀ · exp (−t / τ _a ) Next, the form of rapidly attenuated little new decay time constant tau _a than the tau _d. Therefore, the wave height discrimination circuit 4
The length of the output pulse from T _p ′ becomes shorter than the conventional T _P ,
As a result, the output count rate n _o by counting rate meter 5 has come to not drop significantly to a range of substantially high input count rates than conventional n _i (number Mcps according to experiments).

It should be noted that the attenuation time constant conversion circuit Z is not limited to the above-described configuration, but may employ another configuration as shown in FIG. 5, for example. When the circuit configuration of FIG. 5 is used, it is sufficient to set τ _d = (R ₁ · R ₂ + R ₂ · R ₃ + R ₃ · R ₁ ) · C / R ₂ .

By the way, the decay time constant τ _d of light emission in the scintillator 1 shows a change as shown in FIG. 7 due to a change due to the ambient temperature. As a result, the output voltage pulse v ₀ (t) changes variously as shown in FIG. 8, and when the ambient temperature is low, the pile-up phenomenon easily occurs. When the ambient temperature is high, the pile-up phenomenon occurs. As shown in FIG. 8, the overshoot occurs and the baseline shift easily occurs, and the threshold V _D in the wave height discrimination circuit 4 is increased.
Is likely to be relatively high, so that a stable output count rate may not always be obtained.

Therefore, in order to prevent the occurrence of such an inconvenient phenomenon, a configuration as shown in FIG. 6 may be adopted. That is,
In this embodiment, a temperature compensating means is provided in the attenuation time constant conversion circuit Z so that a constant conversion characteristic can be stably exhibited regardless of a change in the ambient temperature.

To configure this temperature compensation means, in this embodiment,
By using a thermistor S instead of a fixed resistor as the resistor R ₁ in the decay time constant conversion circuit Z, R ₁ · C makes a change equivalent to the change in the decay time constant τ _d of light emission in the scintillator 1 due to the ambient temperature. It is like that.

In this embodiment shows those configurations which gave versus temperature change characteristics to the resistance R ₁ by the thermistor S, be provided with the C to versus temperature change characteristics of a capacitor or,
The combination R ₁ · C of the resistor and the capacitor may have a temperature change characteristic. For example, when the circuit configuration of FIG. 5 is used as the attenuation time constant conversion circuit Z, τ _d = (R ₁ · R ₂ + R ₂ · R ₃ + R ₂ · R ₃ ) · C / R ₂ , it is sufficient to have the same versus temperature variation characteristics and tau _d to C of the capacitor.

[Effect of the invention]

As described above, in the scintillation detector of the present invention, the current / voltage conversion circuit for converting the output current pulse from the photomultiplier tube for converting the light emission intensity of the scintillator into a current pulse into a voltage pulse, and the current / voltage conversion circuit
Attenuation for shortening the decay time of the output voltage pulse from the current / voltage conversion circuit, between the output voltage pulse from the current / voltage conversion circuit and the wave height discrimination circuit that continuously outputs a signal while the output voltage pulse from the voltage conversion circuit exceeds a certain threshold. A time constant conversion circuit is provided, and the attenuating time constant conversion circuit is provided with a thermistor for constantly exhibiting constant conversion characteristics regardless of changes in the ambient temperature. Pulse lengths above the threshold can be made shorter than before.

Therefore, the pile-up phenomenon that the next voltage pulse is input during the processing of one voltage pulse in the pulse height discrimination circuit is less likely to occur, and as a result, the output count rate does not significantly decrease, and It is now possible to measure up to a higher input count rate range.

In addition, the scintillation detector of the present invention is provided with a thermistor for constantly exhibiting a constant conversion characteristic regardless of a change in the ambient temperature in the attenuation time constant conversion circuit. The temperature compensation can be performed in the following manner.There is no need to provide a temperature compensation circuit separately from the decay time constant conversion circuit, the circuit configuration is simplified, and the temperature compensation can be performed at a position closer to the detector. Can do enough.

Therefore, according to the present invention, even if the decay time constant of light emission of the scintillator changes, peak shift and pile-up at a high counting rate are reduced, so that a high counting rate output can be stably obtained. .

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing a basic configuration of a scintillation detector according to an embodiment of the present invention, and FIGS. 2 and 3 are graphs for explaining the operation and effect. . 4 to 8 are for explaining a specific embodiment of the scintillation detector according to the present invention. FIG. 4 is a block circuit diagram showing a main part of the basic embodiment, and FIG.
FIG. 6 is a block circuit diagram showing this modification, FIG. 6 is a block circuit diagram showing a main part of still another embodiment, and FIGS. 7 and 8 are graphs for explaining the operation. I have. 9 to 12 are for explaining the technical background of the present invention, and FIG. 9 is a block circuit diagram showing the overall schematic configuration of a scintillation detector having a conventional configuration. 10 to 12 show graphs for explaining the problems of the prior art. DESCRIPTION OF SYMBOLS 1 ... Scintillator, 2 ... Photomultiplier tube, 3 ... Current / voltage conversion circuit, 4 ... Peak height discrimination circuit, 5 ... Count rate meter, Z ... Decay time constant conversion circuit, S ... Thermistor.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takeshi Aoyama 2 Higashi-cho, Kichijoin-miya, Minami-ku, Kyoto Inside Horiba, Ltd. (56) References JP-A-59-122987 (JP, A) JP-A-55- 50178 (JP, A) JP-A-49-29047 (JP, A) JP-A-55-44219 (JP, U)

Claims (1)

(57) [Scope of request for utility model registration] 1. A scintillator for emitting light having an intensity corresponding to an input radiation dose, a photomultiplier for converting the light emission intensity of the scintillator into a current pulse, and a current pulse output from the photomultiplier for applying a voltage to the photomultiplier. A current / voltage conversion circuit for converting to a pulse, a wave height discrimination circuit for continuously outputting a signal while an output voltage pulse from the current / voltage conversion circuit exceeds a certain threshold, and a unit from the wave height discrimination circuit A scintillation detector comprising a count rate meter for counting the number of output signals per time, wherein a decay of an output voltage pulse from the current / voltage conversion circuit is provided between the current / voltage conversion circuit and the peak-height discrimination circuit. A decay time constant conversion circuit for shortening the time is provided, and the decay time constant conversion circuit includes:
A scintillation detector provided with a thermistor for constantly exhibiting constant conversion characteristics regardless of changes in the ambient temperature.