DE102013209630B4 - Method and device for measuring airborne alpha and beta radiation of artificial origin - Google Patents

Abstract

Method for measuring airborne alpha and beta radiation of artificial origin by means of a detector (10) in which radiation contributions of natural origin are at least partially compensated by means of an alpha-beta pseudo-coincidence difference method, comprising the steps of: Pulsing when an alpha particle hits the detector (10), generating beta pulses when a beta particle hits the detector (10), generating pseudocoincidence pulses using the alpha-beta pseudo-coincidence difference Method, - suppressing those alpha pulses and beta pulses which are associated with a pseudo-coincidence pulse, - counting the pseudo-coincidence pulses and - counting only unpressed alpha pulses and beta pulses, characterized in that - the alpha Activity Aα and the beta activity Aβ can be calculated according to the following formulas: Aα = Kα * (Rα-Roα-a-Rps) and Aβ = Kβ * (Rβ-Roβ-b * Rps), wherein Kα denotes a calibration factor for an alpha channel, Kβ denotes a beta beta calibration factor, Rα denotes a count rate of unbacked alpha pulses, Rβ denotes a count rate of unbacked beta pulses, Roα and Roβ denote zero effective count rates, a and b denote compensation factors, and Rps denotes a pseudo-coincidence rate, where a is chosen to be approximately equal to 1 and b is selected to be approximately equal to 3.

Description

The invention relates to a method and a device for measuring airborne alpha and beta radiation of artificial origin by means of a detector, for example in the form of a semiconductor detector, a scintillation detector or a gas-filled proportional counter tube.

For the measurement of artificial alpha and beta radiation on aerosols in the presence of radiation of natural origin, the application of the alpha-beta pseudo coincidence discrimination or the alpha-beta pseudo-coincidence difference method (ABPD) has proven itself. In alpha-beta pseudo-coincidence discrimination, detected events can be attributed to the presence of natural radioactivity when an alpha event and a beta event are registered within a defined period of time. A count rate determined in this way is provided with alpha and beta-specific multiplication factors and then subtracted from the total measured alpha or beta count rate, whereby a measure of the radioactivity of artificial origin can be calculated. Such a method is for example in the Auslegeschrift DE 15 64 265 B described.

Alpha Beta Pseudo-coincidence discrimination is used in conjunction with both gas flow detectors, scintillation detectors and semiconductor detectors. In semiconductor and scintillation detectors usually a single detector is used for radiation measurement, alpha and beta signals or alpha and beta pulses are obtained from so-called single-channel discriminators with appropriate thresholds. The gas-filled proportional counter tube usually three detector chambers are used, which are separated by films. Directly above the sample, usually a paper filter dusted with the aerosols of the air, is the alpha meter tube, above it the beta meter tube separated by a very thin mylar foil and above it a screen counter tube separated by a thicker partition for electronic suppression of the pulses from the cosmic radiation in the beta channel.

In the DE 10 2010 000 836 A1 the alpha-beta pseudo-coincidence discrimination has been improved especially for semiconductor detectors by additionally compensating the radiation contributions of natural origin in addition to the alpha-beta pseudo-coincidence discrimination by means of energy discrimination, whereby the radiation contributions of natural origin are more effectively compensated.

The US 3 339 070 A and the EP 0 412 194 A1 each show generic methods or devices.

The US 4 031 392 A shows a circuit for counting delayed coincidences in real time.

The invention has for its object to provide a method and apparatus for measuring airborne alpha and beta radiation of artificial origin by means of enumerated detectors available in which radiation contributions of natural origin can be compensated even more effectively than described above and thus even smaller Detection limit for artificial alpha and beta radiation is achieved.

The invention achieves this object by a method having the features of claim 1 and a device having the features of claim 5.

Preferred embodiments are subject of the dependent claims.

The method is for measuring airborne alpha and beta radiation of artificial origin by means of a detector, for example in the form of a semiconductor detector, a scintillation detector or a gas-filled proportional counter tube.

Conventionally, alpha signals or alpha pulses are generated when an alpha particle hits the detector. Accordingly, beta signals or beta pulses are generated when a beta particle hits the detector. The alpha and beta pulses may be rectangular pulses of a predetermined pulse duration. Such a pulse can also be called a standard pulse.

Furthermore, pseudo-coincidence pulses are generated by means of the per se known alpha-beta pseudo-coincidence difference method or a pseudo-coincidence circuit.

It suppresses those alpha pulses and beta pulses which have a pseudo coincidence pulse, ie for which a pseudo coincidence pulse has been generated by means of the alpha-beta pseudo-coincidence difference method or the pseudo-coincidence circuit.

Finally, only non-suppressed alpha pulses and beta pulses are counted and the count rates generated in this way are evaluated to determine the alpha and beta radiation activity of artificial origin. Alpha activity Aα and beta activity Aβ are calculated according to the following formulas: Aα = Kα · (Rα - Roα - a · Rps) and Aβ = Kβ · (Rβ-Roβ-b · Rps), where Kα denotes a calibration factor for an alpha channel, Kβ denotes a beta channel calibration factor, Rα denotes a count rate of unbacked alpha pulses, Rβ denotes a count rate of unbacked beta pulses, Roα and Roβ denote zero effective count rates, a and b denote compensation factors and Rps denotes a pseudo-coincidence rate, where a is chosen to be approximately equal to 1 and b is selected to be approximately equal to 3.

According to the invention, therefore, only such pulses are counted d. H. forwarded to a counter to which no pseudo coincidence pulse is assigned, whereas in the prior art, all pulses are counted, the pseudokoinzidenten events are subtracted. In other words, according to the invention, the alpha and / or beta pulses generated by means of an amplifier are only registered by a counting unit if there is no pseudocoincidence pulse from the natural radiation.

In the alpha-beta pseudo-coincidence method, it is possible to check whether a beta particle and an alpha particle hit the detector one after the other within a predetermined period of time, in which case the alpha particle and the beta particle are the natural radiation attributed and thus an associated pseudo coincidence pulse is generated.

The predetermined period of time may be in the range of 100 μs to 400 μs, preferably 200 μs to 300 μs.

In addition to the non-suppressed alpha pulses and beta pulses, the pseudo-coincidence pulses are also counted and taken into account or evaluated in the calculation of the radiation intensity. The radiation contributions of natural origin can be radon and thoron radiation contributions.

The device is used for measuring airborne alpha and beta radiation of artificial origin and is designed to carry out the above-mentioned method.

The device comprises a conventional alpha and beta radiation detector, for example in the form of a (SI) semiconductor detector, a scintillation detector or a gas-filled proportional counter tube.

The device further includes a conventional electrical amplifier configured to generate alpha pulses when an alpha particle hits the detector and to generate beta pulses when a beta particle hits the detector.

A pseudo-coincidence unit of the device is adapted to conventionally generate pseudo coincidence pulses by means of a per se known alpha-beta pseudo-coincidence difference method. In that regard, reference is also made to the relevant specialist literature.

The device further comprises a veto-logic unit adapted to suppress alpha-pulses and beta-pulses to which a pseudo coincidence pulse belongs.

The apparatus further comprises a counting unit configured to count only alpha pulses and beta pulses not suppressed by the vetology unit.

The veto-logic unit may be connected between the amplifier and the counting unit in such a way that it passes through the alpha and beta pulses generated by the amplifier only to the counting unit, if the alpha and beta pulses do not have a pseudo coincidence pulse. In other words, suppression is performed by not forwarding or not passing the pulses to the counting unit.

The veto logic unit may include a first logic gate and a second logic gate each having a first and a second input, the respective first input of the logic gates being connected to an output of the pseudo-coincidence unit at which the pseudo-coincidence pulses are output.

The amplifier may include an alpha channel for outputting the alpha pulses and a beta channel for outputting the beta pulses.

The pseudo-coincidence unit may include a mono-flop (also referred to as a monostable multivibrator) connected to the alpha channel and extending a pulse duration of an alpha pulse by a predetermined amount of time.

The pseudo-coincidence unit may further include a first delay connected to the beta channel and configured to delay a beta pulse for a predetermined amount of time.

The pseudo-coincidence unit may further comprise a logic gate, wherein a first input of the logic gate is connected to an output of the delay element and a second input of the logic gate is connected to an output of the monoflop, wherein at an output of the logic gate. Gates the pseudo-coincidence pulses are output.

The device may comprise a pseudo-coincidence circuit. The pseudo-coincidence circuit may include the pseudo-coincidence unit and the veto-logic unit. The pseudo-coincidence circuit may further include a second delay connected to the alpha channel, an output of the second delay connected to the second input of the second logic gate of the vetology unit and an output of the first delay of the pseudo coincidence unit to the second input of the second delay first logic gate of the vetology unit is connected.

The vetology unit may have a first mode of operation in which it is adapted to suppress alpha pulses and beta pulses, which are associated with a pseudo coincidence pulse, and a second mode of operation in which it is adapted, alpha pulses and beta Do not suppress pulses to which a pseudo-coincidence pulse belongs.

The invention will be described in detail below with reference to the drawing. This shows schematically:

1 a block diagram of an apparatus for measuring airborne alpha and beta radiation of artificial origin with a vetologic and a 3-fold gas-filled proportional counter.

1 schematically shows an apparatus for measuring airborne alpha and beta radiation of artificial origin.

In the 1 The device shown can be used for example for solid filter systems or for filter belt measuring systems in which air carrying radioactive particles as emitters of artificial and natural origin, through a filter or a filter belt 70 is passed, whereby the particles at least partially in the filter or the filter belt 70 remain. A detector 10 the device is above the filter or the filter band 70 arranged and with the radiation of the particles of the filter or filter belt 70 applied.

The device has a gas-filled detector 10 which consists of three segments, namely an alpha detector 11 , a beta detector 12 and an optional screen counter 13 ,

The detector 10 Depending on the detector type (in this case a 3-fold gas-filled proportional counter tube), an amplifier operating as a single-channel or integral discriminator is connected downstream 20 with three separate channels 21 to 23 with different discriminator thresholds. The amplifier 20 generated at the output of the first channel 21 a pulse when a peak value of one through the alpha detector 11 generated analog signal exceeds an alpha discriminator threshold, and generated at the output of the second channel 22 a pulse when the peak of the beta detector 12 generated analog signal exceeds a beta discriminator threshold. The channel 21 Forms an alpha channel to output alpha pulses once an alpha particle on the alpha detector 11 meets, the channel 22 Forms a beta channel to output beta pulses as soon as a beta particle hits the beta detector 12 meets, and the channel 23 forms an optional, not essential to the invention screen channel.

The screen channel 23 or the associated screen counter is used to reduce the Betanulleffektes and is an optional detector that is almost always used in gas-filled detectors, but only occasionally in semiconductor detectors. It is usually absent from the scintillation detector. The screen count pulses are disregarded in the alpha-beta pseudo-coincidence difference procedure described below.

All from the amplifier 20 pulses generated are rectangular standard pulses, which have a normalized pulse duration of about 1 microseconds.

The device further comprises a pseudo-coincidence unit 30 which produces pseudo-coincidence pulses conventionally derived from natural activity. In that regard, reference is also made to the relevant specialist literature.

A vetology unit 40 the device is between the amplifiers 20 and a counting and evaluation unit in the form of a microprocessor 50 looped. The vetology unit 40 is trained by the amplifier 20 to suppress generated alpha pulses and beta pulses for or based on which the pseudo-coincidence unit 30 has generated a pseudo coincidence pulse. The vetology unit 40 Suppresses such alpha pulses and beta pulses by not sending them to the microprocessor 50 forwards. The microprocessor 50 Consequently, only such pulses counts as the vetologic unit 40 happen.

The microprocessor 50 counts, during a predetermined time interval, which may have a duration in the range of a few seconds to several hours, a number of un-suppressed pulses per channel 21 to 23 and a number of pseudo coincidence pulses and determines therefrom an associated count rate and associated radiation activity.

The pseudo-coincidence unit 30 and the vetology unit 40 are in a pseudo-coincidence circuit 60 integrated, the pseudo-coincidence circuit 60 further a delay element 61 that with the alpha channel 21 connected is. The delay element 61 For example, it may have a delay time between 0 μs and 200 μs.

The pseudo-coincidence unit 30 has a mono-flop 31 on, that with the alpha channel 21 is connected and generated from an alpha pulse with a duration of 1 microseconds a pulse with a duration of about 200 microseconds. The pseudo-coincidence unit 30 also has a delay element 32 on, for example, with a delay time of about 200 μs, with the beta channel 22 connected is. The pseudo-coincidence unit 30 further includes a logic gate in the form of an AND gate 33 with negated output on, with a first input of the AND gate 33 with an output of the delay element 32 is connected and a second input of the AND gate 33 with an output of the mono-flop 31 is connected, wherein at an output of the AND gate 33 the pseudo-coincidence pulses are output.

The vetology unit 40 has a first logic gate in the form of an AND gate 41 and a second logic gate in the form of an AND gate 42 on, with the AND gates 41 and 42 each having a first and a second input. The first input of the AND gates 41 and 42 is with the output of the pseudo-coincidence unit 30 at which the pseudo-coincidence pulses are output so that the AND gates 41 and 42 Pass pulses only if at the output of the AND gate 33 a logical one is pending, ie currently no pseudo-coincidence pulse from the pseudo-coincidence unit 30 is issued.

An output of the second delay element 61 is connected to the second input of the AND gate 42 connected and an output of the delay element 32 is connected to the second input of the AND gate 41 connected.

The alpha and beta pulses generate in the pseudo-coincidence unit or pseudo-coincidence logic, respectively 30 in the presence of natural radiation, the pseudo coincidence pulses as known in the art. The pseudo-coincidence pulses, along with the alpha and beta pulses, are removed from the amplifier 20 the vetology unit 40 supplied, so that only alpha and beta pulses to the microprocessor 50 when no pseudo-coincidence pulse has been generated for these events (veto).

It is understood that between detector 10 and microprocessor 50 other, conventional, unillustrated units can be looped or provided, for example, additional amplifiers, discriminators, logic circuits, etc.

The invention will be clarified again in contrast to the prior art.

In the prior art, those in the amplifier 20 amplified and discriminated alpha and beta pulses, hereinafter referred to as Rα and Rβ, fed to a pseudokoinzide unit, as described, for example, in US Pat 1 as pseudokoinzidenzeinheit 30 is shown, and counted at the same time.

Kα

Kalibrierfaktor Alphakanal

Kβ

Kalibrierfaktor Betakanal

Rα, Rβ

Bruttozählraten

Roα, Roβ

Nulleffektzählraten

a, b

Kompensationsfaktoren

Rps

Pseudokoinzidenzrate

Those in the pseudo-coincidence unit 30 registered pseudo-coincidences or pseudo-coincidence pulses, hereinafter referred to as Rps, are also counted in the prior art together with the random pseudo-coincidences, which are not shown schematically here. Now, in the prior art, the artificial alpha and beta activity on the filter is obtained 70 , which has been adjusted for natural activity, from the following relationships:
Alpha activity: Aα = Kα · (Rα-Roα-a · Rps) (Bq) Beta activity: Aβ = Kβ · (Rβ-Roβ-b · Rps) (Bq) where:

Ka

Calibration factor alpha channel

Kβ

Calibration factor beta channel

Rα, Rβ

Bruttozählraten

Roα, Roβ

Nulleffektzählraten

a, b

compensation factors

rps

Pseudo coincidence rate

The compensation factors a and b are determined such that in the absence of artificial activity the above equations are set to zero and then resolved to a and b.

Thus: a = (Rα - Roα) / Rps and b = (Rβ-Roβ) / Rps

Typical values are a ≈ 2 and b ≈ 4.

From the ISO standard ISO11929 the detection limits NG (..) for these measuring methods can be derived and one gets:

T

Messzeit bei Aktivitätsmessung (s)

To

Nulleffektmesszeit (s)

Where:

T

Measuring time during activity measurement (s)

to

Zero effect measurement time (s)

Since the zero effect count rates are relatively small (alpha: 0.15 cps, beta: 2.0 cps) and the measurement times are relatively large (typically 1800 to 3600 s), the largest contributions come from the detectors considered here Detection limit of the product between the compensation factors and the pseudo-coincidence rates. For a given detector, the pseudo-coincidences depend on the detector parameters, e.g. B. Geometry and response probabilities for alphas and betas and the ever-changing natural ambient radiation.

According to the invention, in comparison to the prior art constant pseudo-coincidence rate by means of an electronic veto generated by the pseudo-coincidences on the alpha and beta channel achieved a reduction of the alpha and Betazählrate, so that the resulting compensation factors are smaller, so the detection limits according to the above Formula are also significantly reduced.

According to the invention, the pseudo-coincidence unit known per se 30 a vetologic 40 downstream, here exemplified by two AND gates 41 and 42 , in each case one input of the AND gate 41 and 42 the alpha or beta pulses are applied through the two delay circuits 32 respectively. 61 have gone.

The timing between pseudo coincidence unit 30 , Vetology Unit 40 and the delay elements 32 and 61 is designed such that when generating a pseudo coincidence or a pseudo coincidence pulse neither the alpha pulse nor the beta pulse in the corresponding counter input of the microprocessor 50 reach. Only if there is no pseudo-coincidence or no pseudo coincidence pulse and therefore no natural radiation is the vetologic unit 40 The alpha and beta pulses of the artificial radiation in the microprocessor are activated 50 counted or registered.

The in the microprocessor 50 Registered count rates are thus reduced by hardware around the pseudo coincidence rate and instead of Rα Rα - Rps are now obtained in the alpha channel and Rβ - Rps in the beta channel instead of Rβ as count rate. For the compensation factors a and b this results a = (Rα - Roα - Rps) / Rps and b = (Rβ-Roβ-Rps) / Rps

If a pseudo coincidence unit or pseudo-coincidence circuit according to the prior art a was equal to 2 and b equal to 4, one obtains with the pseudo-coincidence unit or pseudo-coincidence circuit 30 with vetology unit 40 a value for a of about 1 and for b a value of about 3. Inserting these values into the above equation for the detection limit and forming the ratio of the detection limits of the two circuit variants, one obtains an improvement for the detection limit of the alpha activity the factor 1.7 and for the beta activity an improvement by a factor of 1.3.

The pseudo-coincidence unit 30 requires for the alpha pulse one by means of the mono-flop 31 caused gate width of approximately 200 μs, within which after a corresponding delay, a beta-pulse can trigger pseudo-coincidence. Together with the vetology unit 40 defines this gate width for the alpha channel and the dead time of this channel, while the dead time of the beta channel is in the range of a μs.

With a count rate of 1000 cps (counts per second) for artificial alpha radiation, this means a dead time loss of about 20%, which is corrected by means of the dead time correction. If one wants to be able to measure significantly higher count rates in the alpha channel, then it makes sense to use the vetology unit 40 by means of a control signal HS / St from the microprocessor 50 to disable or bridge so that it no longer suppresses pulses, and to use in this mode in the calculation of the count rates, the compensation factors according to the prior art. With this switching technique, one can define a so-called high sensitivity mode with very small detection limits and a standard mode with slightly higher detection limits and thus expand the dynamic range of the measuring system.

For some reasons, not every natural event produces pseudo-coincidence, but according to the invention, the total alpha and total beta counts of the natural radiation emitted by the microprocessor 50 registered or counted, compared to the prior art significantly reduced, while the pseudo coincidence rate remains the same. This reduces the compensation factors according to the above formula, which leads to a lower detection limit for the alpha and beta channels.

Claims (9)

Method for measuring airborne alpha and beta radiation of artificial origin by means of a detector ( 10 ), in which radiation contributions of natural origin are at least partially compensated by means of an alpha-beta pseudo-coincidence difference method, with the steps: generating alpha pulses when an alpha particle is applied to the detector ( 10 ), - generating beta pulses when a beta particle hits the detector ( 10 ), generating pseudo-coincidence pulses using the alpha-beta pseudo-coincidence difference method, suppressing such alpha pulses and beta pulses that are associated with a pseudo-coincidence pulse, counting the pseudo-coincidence pulses, and counting only non-suppressed alpha pulses and beta pulses, characterized in that - the alpha activity Aα and the beta activity Aβ are calculated according to the following formulas: Aα = Kα (Rα-Roα-a-Rps) and Aβ = Kβ · (Rβ-Roβ-b · Rps), where -Kα denotes a calibration factor for an alpha channel, Kβ denotes a beta channel calibration factor, Rα denotes a count rate of un-suppressed alpha pulses, Rβ a non-repressed beta pulse count rate Denote Roα and Roβ zero-fill count rates, a and b denote compensation factors, and Rps denotes a pseudo-coincidence rate, where a is selected to be equal to about 1 and b is equal to about 3 gewä is being held. A method according to claim 1, characterized in that - in the alpha-beta pseudo-coincidence method it is checked whether a beta particle and an alpha particle are sequentially timed to the detector within a predetermined period of time ( 10 ), in which case the alpha particle and the beta particle are attributed to the natural radiation and an associated pseudocoincidence pulse is generated. A method according to claim 2, characterized in that the predetermined period of time in a range of 100 microseconds to 400 microseconds, preferably 200 microseconds to 300 microseconds, is located. Method according to one of the preceding claims, characterized in that - the radiation contributions of natural origin are radon and thoron radiation contributions. Device for measuring airborne alpha and beta radiation of artificial origin, designed to carry out the method according to one of the preceding claims, comprising: - a detector ( 10 ) for alpha and beta radiation, - an amplifier ( 20 ) which is adapted to generate alpha pulses when an alpha particle is applied to the detector ( 10 ) and generate beta pulses when a beta particle is applied to the detector ( 10 ), - a pseudo-coincidence unit ( 30 ) designed to generate pseudo coincidence pulses, - a vetology unit ( 40 ) which is adapted to suppress alpha pulses and beta pulses to which a pseudo-coincidence pulse belongs, and - a counting unit ( 50 ), which is designed only by the vetology unit ( 40 ) non-suppressed alpha pulses Rα and beta pulses Rβ, characterized in that - the counting unit ( 50 ) is designed to calculate the alpha activity Aα and the beta activity Aβ according to the following formulas: Aα = Kα * (Rα-Roα-a-Rps) and Aβ = Kβ * (Rβ-Roβ-b * Rps Kα denotes a calibration factor for the alpha channel, Kβ denotes a beta channel calibration factor, Roα and Roβ denote zero effective count rates, a and b denote compensation factors, and Rps denotes a pseudo coincidence rate, where a is approximately equal to 1 and b is approximately equal to 3 becomes. Device according to claim 5, characterized in that - the vetology unit ( 40 ) a first logic gate ( 41 ) and a second logic gate ( 42 ), each having a first and a second input, wherein the first input of the logic gates ( 41 . 42 ) with an output of the pseudo-coincidence unit ( 30 ) at which the pseudo-coincidence pulses are output. Device according to claim 5 or 6, characterized in that - the amplifier ( 20 ): - an alpha channel ( 21 ) to output the alpha pulses and - a beta channel ( 22 ) for the output of the beta pulses, and - the pseudo-coincidence unit ( 30 ): - a mono-flop ( 31 ) connected to the alpha channel ( 21 ), - a first delay element ( 32 ) with the beta channel ( 22 ), and - a logic gate ( 33 ), wherein a first input of the logic gate ( 33 ) with an output of the delay element ( 32 ) and a second input of the logic gate ( 33 ) with an output of the mono-flop ( 31 ), wherein at an output of the logic gate ( 33 ) the pseudo-coincidence pulses are output. Device according to claim 7, characterized by - a pseudo-coincidence circuit ( 60 ), comprising: - the pseudo-coincidence unit ( 30 ), - the vetology unit ( 40 ), and - a second delay element ( 61 ) connected to the alpha channel ( 21 ), wherein - an output of the second delay element ( 61 ) to the second input of the second logic gate ( 42 ) of the vetology unit ( 40 ) and - an output of the first delay element ( 32 ) of the pseudo-coincidence unit ( 30 ) to the second input of the first logic gate ( 41 ) of the vetology unit ( 40 ) connected is. Device according to one of claims 5 to 8, characterized in that - the vetology unit ( 40 ) has a first mode of operation in which it is adapted to suppress alpha pulses and beta pulses, to which a pseudo coincidence pulse belongs, and a second mode of operation in which it is adapted, alpha pulses and beta pulses not to suppress, to whom a pseudo coincidence pulse belongs.

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