Method for the detection of ionizing radiation

Technical background

0001The invention relates to a method for scintillation counting by ionizing radiation.

0002In Radionuklidlaboren, in nuclear facilities or in general radiation protection measurements, eg for determination of radioactive contamination or the dose rate and the activity measurements are carried out regularly.

0003Also in process measurement technology, for example for measuring the level, density or mass per unit area of ​​nuclear measurement methods are widely used. As detectors especially scintillation counting tubes and ionization chambers are used.

State of the art

0004From KNOLL "Radiation detection and measurement", 2^nd edition (p.231 and 237 S) shows that ZnS is the material of choice for the detection of alpha radiation and therefor has a high luminous efficacy. By contrast, ZnS is to detect other types of radiation apparently not suitable.

0005Therefore, the already in the <patcit id="pcit0001" dnum="US5796108A"><text>US 5,796,108</text></patcit> described prior art the following:<ul><li>If only alpha radiation is measured, only ZnS is used as a scintillator. If only beta radiation is measured, a plastic scintillator but is used desired alpha and beta radiation are measured simultaneously and separately, as a "sandwich" scintillator is used. This consists of a flat plastic scintillator having thereon a layer of ZnS, the latter facing the sample. The thickness of the ZnS layer is selected so that preferably all alpha particles are stopped and thereby produce light, which is about 6 mg / cm² Layer thickness is achieved. In each embodiment, is located on the scintillator, a light-tight radiation entrance window, usually a metallised plastic film.</li></ul>

0006The scintillator may be mounted directly on the entrance window of the photomultiplier. However, this is at Oberftächen-Kontaminatiönsmonitoren not possible, because the scintillators surfaces of typically 100-200 cm² exhibit, while the inlet window preferably used photomultiplier have only about 25 mm in diameter. Therefore, here, the photons are bundled from the scintillator by means of a reflector on the photocathode.

0007The output pulses of the photomultiplier through a linear amplifier with pulse shaping times of typically 1-20 microseconds. The distinction between alpha rays on the one hand, beta / gamma / x-rays on the other hand takes place in the known process with sandwich scintillators with reference to the amplitudes, which are sorted by pulse height discriminators in the corresponding channels.

0008This method has a number of disadvantages:<ul><li>The sensitivity is low for low-energy beta radiation, as they must first penetrate the ZnS layer, where they produce no measurable by conventional methods signal before reaching the plastic scintillator with their residual energy. Obtained in this case also no apparent plateau more, ie, no stable operating point in the function of the pulse rate as a function of the high voltage.</li></ul>

0009Plastic scintillators with ZnS coating also require a special and complex manufacturing process and therefore high costs for the detector and therefore the measuring system.

0010In the <patcit id="pcit0002" dnum="US4476388A"><text>US 4,476,388</text></patcit> is proposed a method which uses two scintillators and with two photomultipliers.

0011The <patcit id="pcit0003" dnum="EP1074856A2"><text>EP 1074856 A2</text></patcit> describes a process which uses a scintillator having a multiple Szintillationscharakteristik.

Summary of the Invention

0012The object of the invention is to design a measuring system with low production costs of the detector, with the aim of increasing the sensitivity of the measuring system, especially for small beta energies, with only slight sensitivity to change in a wide temperature range from -20 to + 50 degrees C and with good long term stability.

0013This object is achieved according to the features of claim 1.

0014Surprisingly, it allows the combination of a single scintillator, the decay time of about 100 ns obtained and thus makes possible a time resolution of single-photon pulses with a single photon counter, the measurement of all types of radiation such as alpha, beta, gamma and x-radiation. As a single photon counter (photon counter) is called a detector for photons, preferably in the visible range or in the near ultraviolet or infrared. It consists of a fast photomultiplier with high internal gain, uA with a high number of dynodes (z. B. 10) is reached, a stabilized high voltage supply and a fast amplifier / discriminator with standard pulse output. A single-photon counter may use an avalanche photodiode instead of a photomultiplier, a semiconductor device with internal reinforcement, for example.

0015By the single-photon counter downstream evaluation circuits the individual types of radiation can be distinguished from each other and separated it and measured simultaneously or also individually or together.

0016The single-photon measurement is combined with another method, namely, such that the single-photon measurement is used for the measurement of beta and gamma radiation, alpha radiation, however, is measured in a conventional manner with a slow amplifier-discriminator.

0017A scintillator with the above properties can be produced relatively easily either by sedimentation with a suitable solution or by coating with a spray gun, which significantly reduces the production costs of the detector and thus the measuring system.

0018Advantageous developments of the technical teaching of claim 1 mainly concern the evaluation. By suitable configurations of the evaluation circuit exclusively for basic evaluation as part of the single-photon measurement or additional circuit components, the measuring system of the invention for measurements in a wide range of applications can interpret; so are particularly individual measurements of a given type of radiation, but also simultaneous measurements of multiple types of radiation, such as for use in radiation protection, with radiometric measurement methods, or for use in dosimetry, realized.

0019The inventive correlation circuit in the subsequent evaluation circuit this output pulses are analyzed in order to get a good signal to noise ratio, or to separate different types of radiation from each other.

0020Such a correlation circuit detects from the time sequence of the standard pulses of the single-photon counter typical, originating from the interaction of ionizing radiation with the scintillator sequences may be severed by the both the inevitable noise pulses of the interest ionizing radiation events, and the ionizing radiation of its kind by (alpha radiation on one hand and beta / gamma / x-ray on the other) can be identified. A purpose designed correlation circuit is designed such that a pulse of the Einzelphotonänzählers a gate for a predetermined time gate opens, are counted within the pulses may be more. 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 be made as to whether the goods arrive during the gate time pulses are a result of an ionizing radiation event of this kind.

0021A further development of such a correlation circuit allowed by specifying two typical values ​​for the ionizing radiation events then also distinguish these types of radiation based on the number N of arrivals during the gate window pulses.

0022This type of correlation measurement of the output pulses of the single-photon counter will hereinafter be referred to briefly as "burst recognition".

0023In the burst detection Single photon events can contribute due to the thermal electron emission from the photocathode due to its statistical distribution only random coincidences. Therefore, can be neglected in the zero effect of this contribution to the count rate, although at higher temperatures increases the thermal electron emission.

0024Since the operating point, that is, the high voltage of the photomultiplier, is chosen such that it is in the single-photon plateau, is also obtained for the burst pulse rate independent of the energy of ionizing radiation a good plateau.

0025By the single photon counter subsequent burst detection is both a single measurement (specification of typical N-value of the radiation to be detected), as well as a simultaneous or parallel measurement of several kinds of radiations is possible, in which from the number of the counted in the gate window pulses on the type of the detected radiation event is closed.

0026According to the invention, a separate channel is provided, which works for the detection of alpha particles according to the known method with a charge-sensitive preamplifier followed by a pulse shaping of about 1 s and an integral discriminator, and is hereinafter referred to briefly as "alpha channel".

0027With such alpha channel independent of time counts are mutually possible; should during the measurement of ionizing events are registered simultaneously in the alpha channel alpha pulses or pulses from cosmic radiation which generate due to the very intense interaction with the scintillator large flashes of light and undesired afterglow (phosphorescence) and thus single-photon signals to the burst detection by the burst recognition responsive, so is disabled by a signal generated by the alpha channel veto signal burst detection to avoid artifacts including interference from the afterglow of the scintillator.

0028Further advantageous embodiments are further subclaims.

Brief Description of Drawings

0029Embodiments of devices for carrying out the method according to the invention are explained below with reference to drawings, in which:<dl id="dl0001" compact="compact"><dt>Fig.1:</dt><dd>A block diagram of an arrangement for performing a method for detecting ionizing radiation,</dd><dt>Figure 2A-C:</dt><dd>in accordance with the arrangement <figref idrefs="f0001">figure 1</figref> measured 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 between types of radiation by means of burst recognition,</dd><dt>Figure 5:</dt><dd>a second block diagram of an arrangement for distinguishing between types of radiation by means of 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

0030<figref idrefs="f0001">figure 1</figref> shows an arrangement for carrying out a detection method in its basic embodiment.

0031A photomultiplier 30, a scintillator 10 is assigned as a component of a detector, whose structure according to the invention in detail by means of embodiments under the <figref idrefs="f0003 f0004">figures 6-11</figref> will be described. The photons generated in the scintillator 10 due to the interaction described above are registered in the downstream photomultiplier 30, amplified, and the output pulses of the photomultiplier 30 are a fast single photon amplifier 22 supplied with discriminator,

0032This in <figref idrefs="f0001">figure 1</figref> dashed circled array of photomultiplier 30 and single photon amplifier 22 is hereinafter referred to as a single photon counter 40th

0033The single photon counter 40 is an evaluation circuit 20 downstream, which can be configured in various ways, as will be explained in the following.

0034The evaluation is used for delivery of counts due to 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

0035For high voltage power supply of the photomultiplier 30 is used, a supply unit 21 which is also driven by the microprocessor unit 24th

0036From photocathode single photoelectron be resolved by incident photons corresponding to the quantum yield. These are multiplied in the dynode and generate at the anode very fast output signals having a width of typically 10 ns, as in<figref idrefs="f0001">FIG. 2C</figref> shown.

0037Although each signal from a single photoelectron originates, the Impulshöhenvertellung provides (<figref idrefs="f0001">Fig. 2B</figref>) Because of the statistical nature of the secondary-electron multiplication as a broad peak. Apart Dern shows the pulse-height distribution a steep increase at low amplitudes, resulting from photoelectric effects from dynodes, electronic noise, etc..

0038A stable operating point is obtained when the discriminator in the "valley" between the two regions is set to the designated DS position. Altering the high voltage is obtained at the output of the discriminator in the<figref idrefs="f0001">Fig. 2A</figref> illustrated function of the pulse rate. It shows a so-called. Plateau, ie an area in which the count rate varies only slightly as a function of the high voltage.

0039The high voltage is adjusted so that it lies in the plateau (operating point AP). This drift of Dynodenverstärkung, the high voltage, the electronic amplification, etc. to act only little effect on the result. Therefore, single-photon counters record next highest sensitivity by excellent long-term stability.

0040It must be mentioned that electrons are released not only by photons, but also spontaneously from the photocathode and thus an undesired zero effect, which is highly dependent on temperature triggered. The elimination of this disorder is also achieved by the invention.

0041By means of a display 24A is visualized the evaluation result.

0042In a simple embodiment, the evaluation circuit operates as Zählratenuntersetzer. Since a single radioactive event in a plurality of single photons are generated, which are registered by the single-photon counter 40 in the evaluation circuit 20, a suitable Zählratenuntersetzung is defined (eg 20), for example, each 20 output pulses (burst) of the single-photon counter 40 a Zählratenimpuls representative to assign an ionizing event. A recognition of a particular type of radiation is hereby intended.

0043<figref idrefs="f0001">2B</figref> shows a typical pulse height spectrum for a suitable Einzeiphotonenzählung photomultiplier 30th

0044The increase at low amplitudes is due to thermal electrons from the dynodes and electrical noise, the following maximum at higher amplitudes in the pulse-height distribution corresponding to individual electrons from the photocathode, which may be triggered by light quantum photoelectron or thermal electrons from the photocathode.

0045The discriminator DS is placed in the minimum between single electron maximum and noise to make the above-mentioned single-photon plateau EP (<figref idrefs="f0001">figure 2A</figref>) to obtain.

0046<figref idrefs="f0001">2C</figref> shows a typical single-photon pulse with a half-width of about 10 ns.

0047Below erfindungsgernäße embodiments of the apparatus for implementing the method according to the invention will now be described, which are essentially a developing and completing the evaluation 20th

0048When Ausfuhrungsbeispiet the inventive apparatus <figref idrefs="f0002">figure 3</figref> includes the evaluation circuit 20 a Korreladonsschaltung, with the initially explained burst recognition can be performed with an adjustable gate duration TG and pre-selectable pulse number N1.

0049The operation of this circuit is such that the first discriminated output signal (standard pulse) of the single-photon counter 40 a gate pulse for a particular Displays TG (gate length) opens, preferably 2-30 microseconds.

0050The 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, this is taken as evidence of a typical value for N1 ionizing event, that is, then a count is determined by the correlation circuit supplied to the microprocessor electronics 24th

0051This variant is particularly suitable for measuring a predetermined by the selection of TG and N1 meet its kind ionizing event, either alpha radiation or beta / gamma / x-ray radiation. The number of the microprocessor unit 24 which counts initiated the evaluation circuit 20 thus represents the intensity of this selected radiation.

0052The circuit can be designed dynamically insofar as that already upon reaching the predetermined number N1 (N = N1), the gate window is closed (T &lt;TG) and the burst recognition thus is ready to register a new ionizing event again.

0053<figref idrefs="f0002">figure 4</figref> shows a first block diagram of a device in which the evaluation circuit 20 is configured such that at the same time (ie parallel) Beta-Gamma-ray radiation on the one hand and alpha particles can be on the other hand be measured only by a burst detection, namely such that the pulse number N1, the for evaluating an ionizing event as beta particles is "necessary", for example, is between 1 to 20 and a second pulse number N2, for example, set between 5 and 50 for evaluating an ionizing event as alpha particles, where N2 must be selected to be greater than N1.

0054In the rating in the predetermined gate window fallen to the gate width TG N pulses in the evaluation circuit 20 three distinguishable cases are possible with this concept: <ol><li>a) N &lt;N1 (&lt;N2):<ul><li>The registered event should be seen as neither alpha nor beta / gamma / x-ray radiation.</li></ul></li><li>b) N1 ≤ N &lt;N2:<ul><li>This is a beta / gamma / x-ray signal.</li></ul></li><li>c) N2 ≤ N:<ul><li>It is an alpha signal.</li></ul></li></ol>

0055The evaluation circuit 20 and the Alpha channel 50 are to the effect designed with this configuration is that they enable the separate collection of such radiation events and) or c) drive at satisfying the condition b a separate "Alpha Counter" 24A and "beta counter" 24B in Mikroprozessorauswerteeinheit 24th

0056In <figref idrefs="f0002">figure 5</figref> shows a second block diagram of an apparatus whose essential aspect is that the burst recognition 20 exclusively used in the evaluation for the detection of beta particles, whereas an "alpha channel" is parallel 50 operated in a conventional circuit manner solely for alpha detection. For the latter a charge sensitive preamplifier 25 are provided with a subsequent pulse shaping of about 1-20 microseconds and two integral discriminators 26 and 27th The preamplifier receives the output signal of the photomultiplier 30, which so far has a dual function in this variant, namely as a signal provider for the alpha channel and as part of the single-photon counter for controlling the evaluation circuit 20, here with a fast pulse amplifier 28 with a double pulse resolution of 30 to 40 ns is designed with a quick Integralkomparator.

0057With this arrangement, it is thus also possible, beta pulses and alpha particles separately by selecting the evaluation method (burst recognition or conventional alpha channel), or to measure simultaneously / parallel (burst recognition and alpha channel), since the output pulses of the photomultiplier 30 both to burst recognition as well as Alpha channel 50 evaluated.

0058It should be noted that due to the very intense interaction of 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) from the burst detection as ionizing events would be recognized.

0059To prevent this, when registration of alpha pulses or pulses from cosmic radiation is a simultaneous burst recognition for a certain period, for example 1 to 10 microseconds, disabled.

0060This is achieved in that the opening of the gate to the burst detection for the duration of the alpha particle detection is prohibited ( "veto signal").

0061Advantageously, two discriminator thresholds are for this operation D1 provided for blocking the burst recognition through the veto signal and D2 at the beginning of the evaluation in the Alpha channel D2, where D2 is greater than selected D1, so that even with a recognizable beginning of a Alphateilchennachweises (reaching the first discriminator D1) by the veto signal a gate pulse for the start of the burst recognition is prevented, the counting of the output pulses for the "Alpha" scoring but starts only when the second discriminator D2 exceeded.

0062While explained <figref idrefs="f0001 f0002">Figures 1 to 5</figref> have the inventive evaluation of the output pulses of the photomultiplier 30 to object to employ the following <figref idrefs="f0003 f0004">Figures 6 to 11</figref> with the generation of the evoked by the ionizing event flashes of light in the scintillator and the coupling into the entrance window of the photomultiplier.

0063In all cases, a transparent scintillator according to the invention as a radiation-sensitive detector 12 with a decay time of at least 100 used ns, preferably ZnS and preferably applied as a thin layer on an opaque and very thin metallized plastic film or a light transparent carrier plate made of Plexiglas or glass. Due to this essential feature of the invention, no further plastic scintillator longer needed.

0064The layer thickness of the scintillator 12 is selected so that the alpha particles of all radionuclides of interest are stopped (mass density greater than 6 mg / cm²) And the self-absorption of light in the scintillator material can still be neglected (mass density of less than 100 mg / cm²).

The simplest embodiment is shown in Figure 6:

0065On a thin light guide 14, which is connected with the photocathode 30 of the photomultiplier 30B, the scintillator layer 12 is applied. The entire assembly is closed light-tight and mechanically with a very thin film 11 as an entry window for the radiation. The occurring after an ionizing event electrical output signal is coupled from the anode 30B, and, as described above, the single-photon amplifier 22 and optionally supplied to the alpha channel 20A.

0066Alternatively, (<figref idrefs="f0003">figure 7</figref>), The collection of the light pulses of the scintillator layer 12 can be carried out via a reflector 15 which concentrates the light onto the anode 30A of the photomultiplier 30 with a flat or spherical photocathode. Support plate 13 and scintillator 12 are also under a light-tight film 11th

0067In the embodiment of the <figref idrefs="f0003">figure 8</figref> the scintillator layer 12 is applied directly to the the photomultiplier 30 side facing the light-tight film 11; here no translucent backing more is needed.

0068<figref idrefs="f0004">figure 9</figref> shows the embodiment of the scintillator as a rod detector in which the outer wall of a cylindrical shaped light guide 14, the scintillator layer 12 is applied, which is connected to a mirror 16 with an end face of the photomultiplier 30 and the other end face. The entire assembly is installed together with one of the above-described light-tight devices in a pipe 17 with a thin wall.

0069A similar arrangement is shown in <figref idrefs="f0004">Figure 10</figref>, Which detector is designed as a dosimeter for gamma radiation. In order to measure the dose rate energy independent, an additional energy filter 18 is mounted around the detector. Here, the light-tight tube 17 is very thin chosen thus the dose rate of small gamma energies can still be measured.

0070Instead of a rod and thin optical light guide can be coated with a scintillator layer 12 and possibly package, which has a particularly high sensitivity is achieved. <figref idrefs="f0004">figure 11</figref> shows such an embodiment in cross section.

0071If the surface of the scintillator substantially greater than the cathode surface of the photomultiplier used in the single-photon counter, it may be advantageous to use multiple single photon counter. This achieves a higher sensitivity and, at an appropriate position to the single-photon counter, a better homogeneity of the spatial sensitivity. For further processing of the output pulses of the single-photon counter there are several possibilities. So may be provided with a separate evaluation of each of the single-photon counter. It is also advantageous to work with only one evaluation circuit, wherein the output pulses of the individual single-photon counters are connected in parallel to the input stage of the evaluation circuit.

0072In the latter case, the above-described gate is opened by a coming of a single photon counter pulse and the incoming pulses during opening hours of all single-photon counters are counted.

List of reference numerals

0073<dl id="dl0002" 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>