Method for the detection of ionizing radiation

The invention relates to a method and an apparatus for scintillation counting by ionizing radiation.

In Radionuklidlaboren, in nuclear facilities or in general Radiation measurements are regularly eg for determination of radioactive Contamination or the dose rate and the activity measurement performed.

Also in process measurement technology, for example for measuring the level, the Density or basis weight are widespread nuclear measurement methods widely. The detectors are especially scintillation counting tubes and Ionization chambers used.

State of the art

From KNOLL "Radiation detection and measurement", 2^nd edition (p.231 and 237 S) it can be seen that ZnS the material of choice for the detection of is of alpha-radiation, and for this purpose has a high luminous efficiency. On the other hand ZnS is to detect other types of radiation apparently not suitable.

Therefore, the prior art already described in US 5,796,108 following:

If only alpha radiation is measured, only ZnS is used as a scintillator. If only beta radiation is measured, a plastic scintillator is used. However, if alpha and beta radiation is measured simultaneously and separately be, a "sandwich" scintillator is used. This is from a flat plastic scintillator with a layer applied thereto , The latter of the sample faces of ZnS. The thickness of the ZnS layer is chosen that preferably all alpha particles are stopped while light produce what at about 6 mg / cm² Layer thickness is achieved. In each embodiment, located on the scintillator, a light-tight radiation entrance window, usually a metallised plastic film.

The scintillator can directly on the entrance window of the photomultiplier be mounted. However, this is at surface contamination monitors not possible since the scintillators surfaces of typically 100-200 cm² exhibit, whereas the inlet window preferably used photomultiplier only about 25 mm in diameter. Therefore, here are the photons from the scintillator bundled by means of a reflector on the photocathode.

The output pulses of the photomultiplier through a linear amplifier with pulse shaping times of typically 1-20 microseconds. The distinction of alpha radiation on the one hand, beta / gamma / x-ray on the other hand takes place in the known process with sandwich scintillators at hand Amplitudes by pulse height discriminators corresponding to the Channels are sorted.

This method has a number of disadvantages:

The sensitivity is low for low-energy beta radiation, as these must first penetrate the ZnS layer and there is no conventional Methods produce measurable signal before it with their residual energy reach the plastic scintillator. Obtained in this case also no apparent plateau more, ie, no stable operating point in the dependence the pulse rate as a function of high voltage.

Plastic scintillators with ZnS coating also require a special and complex manufacturing process and therefore high costs of Detector and therefore the measuring system.

Summary of the Invention

Object of the invention is to design a measurement system with low Production costs of the detector, with the aim of increasing the sensitivity the measuring system especially for small beta energies, with only minor Sensitivity to change in a wide temperature range of -20 to + 50 degrees C and with good long term stability.

This object is achieved according to the features of claim 1.

Surprisingly, the combination of a single scintillator, the decay time of about 100 ns and thus obtained a temporal Resolution of the single-photon pulses possible with a single photon counter the measurement of all types of radiation such as alpha, beta, gamma and X-ray radiation. referred to as single-photon counters (photon counter) to a detector for photons, preferably in the visible range or in the near ultraviolet or infrared. It consists of a quick Photomultiplier with high internal gain, uA at a high Number of dynodes (z. B. 10) is reached, a stabilized high voltage power supply and a fast amplifier / discriminator with standard pulse output. A single-photon counter, instead of a photomultiplier Also, a semiconductor device with internal gain such as avalanche Photodiode use.

By the single-photon counter downstream evaluation circuits the individual types of radiation and distinguished thus separated and measured simultaneously or also individually or together will.

It is also possible for the inventive method with conventional to combine, for example, the method of the invention for the measurement of to use beta and gamma radiation, whereas alpha radiation in conventional Way with a slow amplifier-discriminator to measure.

A scintillator with the above characteristics can relatively easily either by Sedimentation with a suitable solution or by coating with a Airbrush be prepared, what and the manufacturing cost of the detector thus the measuring system substantially reduced.

Advantageous developments of the technical teaching of claim 1 relate primarily to the evaluation circuit. By suitable configurations the evaluation only for basic evaluation under the single-photon measurement or additional circuit components allows the measurement system according to the invention for measurements in interpret the scope of a wide range of applications; so are particularly Single measurements of a given type of radiation, but also simultaneous measurements of multiple types of radiation, such as for use in Radiation Protection in radiometric measurement method or for use in dosimetry, realized.

The rate of the output pulses of the single-photon counter can directly Degree be used for the intensity of ionizing radiation. It is however, also possible by means of a correlation circuit in the subsequent Evaluation to analyze these output pulses in order a good signal to noise ratio to obtain, or to different types of radiation inseparable.

Such a correlation circuit detects from the time sequence of the Standard pulses of the single-photon counter typical, from the interaction of ionizing radiation to the scintillator derived sequences by targeting both the unavoidable noise pulses of the interest be separated ionizing radiation events, and the ionizing Radiation by its nature (alpha radiation on one hand and beta / gamma / x-ray on the other hand) can be identified. A purpose designed correlation circuit may be designed such that a pulse the single-photon counter a gate for a predetermined time gate opens, are counted within the possibly further impetus. Depending on the number N of further pulses and the gate length by comparison with a predetermined type of radiation for a typical value of a Decision to be made whether the arrivals during the gate time are pulses result of ionizing radiation event of this kind.

A further development of such a correlation circuit allowed by the Specification of two typical values ​​for the ionizing radiation events then a distinction between these types of radiation on the basis the number N of arrivals during the gate window pulses.

This type of correlation measurement of the output pulses of the single-photon counter briefly referred to as "burst recognition" below.

In the burst recognition single photon events can due to the thermal electron emission from the photocathode due to its statistical Distribution contribute only accidental coincidences. Therefore, in Range of zero effect to this article neglected to count rate although at higher temperatures, the thermal electron emission increases.

Since the operating point, that is, the high voltage of the photomultiplier, so is chosen such that it is in the single-photon plateau, obtained also for the burst pulse rate independent of the energy of the ionizing Radiation a good plateau.

By being the single-photon counter subsequent burst recognition both a single measurement (specifying the typical N-value of the detected Radiation), as well as a simultaneous or parallel measurement of several Types of radiations is possible, in which from the number of the gate window pulses counted on the nature of the detected radiation event is closed.

For simultaneous measurement of various types of radiation, the evaluation circuit exclusively with the single-photon measurement in conjunction work with the burst recognition, but also according to an embodiment the measuring system according to the invention a separate channel provided of the detection of alpha particles according to the known method with a charge sensitive preamplifier with a subsequent pulse shaping works of about 1 s and an integral discriminator, and the hereafter is referred to as "alpha channel".

With such an alpha channel are mutually independent of time counts possible; should during the measurement of ionizing events by the burst recognition simultaneously in the alpha channel alpha pulses or pulses be registered from the cosmic radiation, which due to the very intensive interaction with the scintillator large flashes of light and unwanted Afterglow (phosphorescence) and thus single-photon signals produce, on which also addresses the burst recognition, so is a the alpha channel generated veto signal burst detection disabled to Artifacts including interference from the afterglow of the scintillator to avoid.

Further advantageous embodiments are further subclaims.

Brief Description of Drawings

Embodiments of devices for carrying out the invention Method are below with reference to drawings, show it:<dl tsize="11" compact="compact"><dt>Fig.1:</dt><dd>A block diagram of a preferred arrangement for carrying out of the inventive method,</dd><dt>Figure 2A-C:</dt><dd>measured in the arrangement of Figure 1 count rates and their amplitudes,</dd><dt>Figure 3:</dt><dd>a block diagram of an arrangement for measuring a radiation type by burst detection,</dd><dt>Figure 4:</dt><dd>a first block diagram of an arrangement for distinguishing of radiations by burst detection,</dd><dt>Figure 5:</dt><dd>a second block diagram of an arrangement for distinguishing of types of radiation means burst recognition and alpha channel,</dd><dt>Figure 6:</dt><dd>a representation of a first embodiment of the detector,</dd><dt>Figure 7:</dt><dd>a representation of a second embodiment of the detector,</dd><dt>Figure 8:</dt><dd>an illustration of a third embodiment of the detector,</dd><dt>Figure 9:</dt><dd>an illustration of a fourth embodiment of the detector,</dd><dt>Figure 10:</dt><dd>an illustration of a fifth embodiment of the detector, and</dd><dt>Figure 11:</dt><dd>an illustration of a sixth embodiment of the detector.</dd></dl>

Description of Embodiments

Figure 1 shows an arrangement for carrying out the invention Process in its basic configuration.

A photomultiplier 30, a scintillator 10 as a component of a detector assigned whose inventive structure in detail with reference is described in detail by embodiments with the figures 6-11.

In the scintillator 10 due to the interaction described in the opening Photons generated are in the downstream photomultiplier 30 registered, amplified, and the output pulses of the photomultiplier 30 be a fast single-photon amplifier 22 with discriminator supplied.

This dashed line in Figure 1 Edged array of photomultiplier 30 and single photon amplifier 22 is hereinafter referred to as a single photon counter 40 referred to.

The single-photon counter 40 is an evaluation circuit 20 downstream, which can be configured in various ways, as hereinafter will be explained.

The evaluation is used for delivery of counts due to a its nature identified ionizing radiation event to a Microprocessor unit 24. The evaluation and implementation of the standardized Output pulses of the single-photon counter 40 in such counts for the microprocessor unit is the object of evaluation 20th

For high-voltage supply of the photomultiplier 30 is a supply unit 21, which also driven by the microprocessor unit 24 becomes.

From the photocathode in accordance with incident photons the quantum yield of single photoelectrons triggered. These are in multiplies the dynode and generate at the anode very fast Output signals having a typical width of 10 ns, as shown in Fig. 2C.

Although each signal from a single photoelectron originates, raises the pulse height distribution (Fig. 2B) because of the statistical nature of the secondary-electron multiplication as a broad peak. Furthermore shows the pulse height distribution of a steep increase at low amplitudes, by photoelectric effects from dynodes, electronic noise, etc. originates.

A stable operating point is obtained when the discriminator the "valley" between the two regions on the designated DS position is set. Altering the high voltage is obtained on The output of the discriminator function of the pulse rate as shown in Fig. 2A. It shows a so-called. Plateau, ie an area in which the count rate as a function of the high voltage changes only slightly.

The high voltage is set so that it lies in the plateau (operating point AP). This drift of Dynodenverstärkung, the high voltage to operate, of electronic amplification, etc. but little effect on the result. Therefore, single-photon counters record next highest sensitivity excellent long-term stability.

It must be mentioned that not only electrons by photons, but also be triggered spontaneously from the photocathode, thus an undesirable zero effect which is highly temperature dependent, trigger. The elimination of this disorder is also achieved by the invention.

By means of a display 24A is visualized the evaluation result.

In a simple first preferred embodiment, the evaluation circuit works as Zählratenuntersetzer. Since a single radioactive event in a plurality of single photons are generated by the single-photon counter 40 are registered, is a in the evaluation circuit 20 set appropriate Zählratenuntersetzung (eg 20), for example, each 20 output pulses (burst) of the single-photon counter 40 a Zählratenimpuls representative assign for an ionizing event to can. A recognition of a particular type of radiation is hereby intended.

Figure 2B shows a typical pulse height spectrum of for single photon counting suitable photomultiplier 30th

The increase at low amplitudes is due to thermal electrons from Dynodes and electrical noise ago, the subsequent maximum at higher amplitudes in the pulse-height distribution corresponding to single electrons from the photocathode, the triggered by light quantum photoelectron may be or thermal electrons from the photocathode.

The discriminator DS is in the minimum between single electron maximum and noise down to allow the above-mentioned single-photon plateau EP (Figure 2A) to obtain.

Figure 2C shows a typical single-photon pulse with a FWHM of about 10 ns.

The following are now further embodiments of the apparatus for implementing further advantageous variants of the inventive method describes essentially a developing and completing the Evaluation circuit 20 represent.

In the embodiment of the device according to Figure 3 includes the Evaluation circuit 20, a correlation circuit with which the above-described Burst detection with adjustable gate duration TG and preselectable Pulse number N1 can be performed.

The operation of this circuit is such that the first discriminated Output (standard pulse) of the single-photon counter 40 a gate pulse for a certain time TG (gate length) opens, preferably 2-30 microseconds.

The incident within the gate duration TG standard pulses are counted. If the number N of the pulses arrived standard at least equal to the predetermined Pulse number N1, so this is called detection of the value counted N1 typical ionizing event, ie, then the Correlation circuit a count to the microprocessor electronics 24 passed.

This variant is particularly suitable for measuring one of the choice TG and N1 definable in terms of its type ionizing event, So either alpha radiation or beta / gamma / x-ray radiation. The Number of the microprocessor unit 24 which counts initiated the evaluation 20 consequently represents the intensity of the selected Radiation.

The circuit may be dynamic in the sense that already N1 (N = N1), the gate window is closed when it reaches the predetermined number (T &lt;TG) and the burst recognition thus again for registration is a new ionizing event ready.

Figure 4 shows a first block diagram of an apparatus, in which the evaluation circuit 20 is designed such that at the same time (ie parallel) Beta-Gamma-ray radiation on the one hand and alpha particles on the other hand can be measured only by a burst recognition, namely such that the pulse number N1 for the evaluation of ionizing Event as beta particles is "necessary", for example, is selected between 1 and 20 and a second pulse number N2, for example, between 5 and 50 for evaluating an ionizing event as Alpha particles is provided, it must be said N2 selected greater than N1.

In the rating given to the gate window with the gate width TG Fallen N pulses in the evaluation circuit 20 are in this concept three distinguishable cases possible: <sl><li>a) N &lt;N1 (&lt;N2):<sl><li>The registered event is neither an alpha nor beta / gamma / X-radiation to evaluate.</li></sl></li><li>b) N1 ≤ N &lt;N2:<sl><li>This is a beta / gamma / x-ray signal.</li></sl></li><li>c) N2 ≤ N:<sl><li>It is an alpha signal.</li></sl></li></sl>

The evaluation circuit 20 and the Alpha channel 50 are in this embodiment the effect designed to such separate collection Radiation events and allow at satisfy the condition b) or c) a separate "Alpha Counter" 24A and "beta counter" 24B in Mikroprozessorauswerteeinheit 24 drive.

5 shows a second block diagram of a device is shown, which essential aspect is that the burst detection in the Evaluation circuit 20 exclusively used for the detection of beta particles, whereas parallel an "alpha channel" 50 in a conventional way circuit is operated solely for alpha detection. For the latter are a charge sensitive Preamplifier 25 with a subsequent pulse shaping 26 and 27 are provided by approximately 1-20 microseconds and two integral discriminators. Of the Preamplifier receives the output signal of the photomultiplier 30, which at this variant so far has a dual function, namely as a signal supplier for the alpha channel and as part of the single-photon counter for driving the evaluation circuit 20, here with a fast pulse amplifier 28 with a double pulse resolution of 30 to 40 ns with a fast Integralkomparator is designed.

With this arrangement, it is thus also possible, beta and alpha particles pulses separately by selecting the evaluation method (burst recognition or conventional alpha channel), or also at the same time / parallel measure (burst recognition and alpha channel), since the output pulses of Photomultiplier 30 for both burst recognition as well as in the alpha channel 50 be evaluated.

It should be noted that due to the very intense interaction alpha pulses or pulses from cosmic radiation with the scintillator large flashes of light, and thus a significant afterglow (phosphorescence) and thus single-photon signals are generated which, without an additional Measure (also) by the burst recognition as ionizing events would be recognized.

In order to prevent this, in registration of alpha pulses or pulses from cosmic radiation simultaneous burst recognition for deactivated certain period of time, for example 1 to 10 microseconds.

This is achieved in that the opening of the gate to the burst detection is suppressed during the duration of the alpha particle detection ( "veto signal").

Advantageously, for this operation two discriminator D1 for blocking the burst recognition through the veto signal and D2 to the beginning of the evaluation in the Alpha channel D2, said D2 is greater than D1 selected so that even at the beginning of a recognizable Alphateilchennachweises (Reaching the first discriminator D1) inhibited by the veto signal a gate pulse for the start of burst detection is the count of the output pulses for the "Alpha" scoring but begins at the second discriminator D2 exceeded.

While the illustrated Figures 1 to 5 evaluation, the inventive 30 have the output pulses of the photomultiplier for the subject, deal the following figures 6 to 11 with the generation of the ionizing event evoked flashes of light in the scintillator and the Coupling into the entrance window of the photomultiplier.

In all cases, according to the invention as a radiation-sensitive detector a transparent scintillator 12 with a decay time of at least 100 ns used, preferably ZnS and on a thin layer an opaque and very thin metallized plastic film or a light transparent carrier plate preferably made of Plexiglas or Glass applied. Because this key feature of the invention, if no further plastic scintillator longer needed.

The layer thickness of the scintillator material 12 is selected so that the alpha all interest radionuclides are stopped (mass coverage greater than 6 mg / cm²) And the self-absorption of light in the Scintillator material still can be neglected (mass coverage under 100 mg / cm²).

The simplest embodiment is shown in Figure 6:

On a thin light guide 14, which with the photocathode of the photomultiplier 30B 30 is connected, the scintillator layer 12 is applied. The entire assembly is as mechanically and with a very thin film 11 Entrance window for the radiation light-tight. This after an ionizing Event resultant electrical output signal of the 30B anode coupled out, and, as described above, the single-photon amplifier 22 and possibly supplied to the alpha channel 20A.

Alternatively, (7) the collection of the light pulses of the scintillator 12 are carried out via a reflector 15, of the Light on the anode 30A of the photomultiplier 30 with a flat or spherical Photocathode bundles. Support plate 13 and scintillator 12 are Also here below a light-tight film 11th

In the embodiment of the figure 8, the scintillator 12 is directly on the photomultiplier 30 facing side of the light-tight film 11 applied; here no translucent backing more is needed.

Figure 9 shows the embodiment of the scintillator as a rod detector in which on the outer wall of a cylindrical shaped light guide 14, the scintillator layer 12 is applied, which is an end to the photomultiplier 30 and the other end side is connected to a mirror 16th The whole Arrangement with one of the devices mentioned in the introduction light-tight in a pipe 17 with a thin wall built.

A similar arrangement is shown in Figure 10, which detector as Dosimeter is designed for gamma radiation. To the dose rate energy independent to measure, is an additional energy filter 18 to the Detector around attached. Here, the light-tight tube 17 is selected to be very thin, thus the dose rate of small gamma energy nor measured can be.

Instead of a rod can also thin optical light guide with a scintillator 12 coated and optionally bundled, whereby a particularly high sensitivity is achieved. Figure 11 shows such an Embodiment in cross section.

If the surface of the scintillator substantially larger than the surface of the cathode in single photon counting photomultiplier used, as can the use of several single-photon counter can be advantageous. This is achieved a higher sensitivity and, given a suitable positioning of the single photon counter, a better homogeneity of the spatial sensitivity. For the Further processing of the output pulses of the single-photon counter there various possibilities. Thus, each of the single-photon counter with a separate evaluation be provided. It is also advantageous to work with only one evaluation circuit, wherein the output pulses of individual single photon counter in parallel on the input stage of the evaluation circuit are switched.

In the latter case, the gate described above is one of opened one of the single-photon counter next pulse and during the opening time the pulses of all single-photon counter are counted.

List of reference numerals

<dl tsize="5" compact="compact"><dt>10</dt><dd>scintillator</dd><dt>12</dt><dd>scintillator</dd><dt>13</dt><dd>support plate</dd><dt>14</dt><dd>optical fiber</dd><dt>11</dt><dd>film</dd><dt>15</dt><dd>reflector</dd><dt>16</dt><dd>mirror</dd><dt>17</dt><dd>pipe</dd><dt>18</dt><dd>energy filter</dd><dt>20</dt><dd>evaluation</dd><dt>21</dt><dd>supply unit</dd><dt>22</dt><dd>Amplifier with discriminator</dd><dt>24</dt><dd>microprocessor unit</dd><dt>25</dt><dd>preamp</dd><dt>26,27</dt><dd>integral discriminators</dd><dt>28</dt><dd>Boosters</dd><dt>30</dt><dd>photomultiplier</dd><dt>30A</dt><dd>photocathode</dd><dt>30B</dt><dd>anode</dd><dt>40</dt><dd>Single Photon Counting</dd><dt>50</dt><dd>Alpha channel</dd></dl>