WO2016092157A2

WO2016092157A2 - Method and arrangement for detecting ionizing radiation

Info

Publication number: WO2016092157A2
Application number: PCT/FI2015/050877
Authority: WO
Inventors: Heikki JUNNINEN
Original assignee: University Of Helsinki
Priority date: 2014-12-12
Filing date: 2015-12-14
Publication date: 2016-06-16
Also published as: FI20146094A; WO2016092157A3

Abstract

An arrangement (100) for detecting different ionizing radiations, such as radioactivity and galactic cosmic rays, comprises by a mass spectrometer (101) and an ionization tube (102) with a carrier gas, which is adapted to enable ionizing reactions (104) between the ionizing radiations (105) and the carrier gas and thereby enabling formation of ion particles. The arrangement comprises also a flow tube (107) for providing the carrier gas flow through the ionization tube (102). The arrangement is configured to introduce the carrier gas with said produced ion particles to the mass spectrometer (101) for detection (109) existence of said ionizing radiations (105) or other particles (106) to be detected.

Description

METHOD AND ARRANGEMENT FOR DETECTING IONIZING RADIATION

TECHNICAL FIELD OF THE INVENTION The invention relates to a method and arrangement for detecting ionizing radiation, such as radioactivity and galactic cosmic rays and especially the method and arrangement to study fast ion reaction caused by ionization radiation and to measure and determine atmospheric ion bursts and their dynamics.

BACKGROUND OF THE INVENTION

Earth and earth creatures are exposed to lots of different ionizing radiation continuously, such as high energy cosmic rays mainly originating outside the Solar System, and radioactive radiation originated e.g. from radon. The ionizing radiation has numbers of effect e.g. to health, namely it causes both good but also low grade mutations. The mutations are essential event for evolution, but many of low grade mutations might also causes cancer. Earth creatures have anyhow adapted for living under ionizing radiation, but excessive ionizing radiation, however, is detrimental to health. In atmosphere the ionizing radiation produces ions, which catalyses chemical reactions and causes particle and cloud formations, especially in the upper atmosphere.

Air ions are produced by galactic cosmic rays (GCRs), as well as by radon decay and gamma radiation originating from the soil. Terrestrial sources are usually more important than GCRs for ion production taking place within a continental planetary boundary layer, whereas in the free troposphere GCRs play the dominant role. After formation in atmosphere, the ions are rapidly converted from the primary ions (N₂ ⁺, 0₂ ⁺ and e^") to secondary molecular ions, and then either neutralized by ion-ion recombination or attached to aerosol particles and removed from the relevant mobility regime. The final composition of ambient ions is defined by the composition of neutral gases and their proton and electron affinities. Cosmic rays are divided into two groups: primary and secondary particles. The primary particles have travelled through space and, by colliding with matter in interstellar space and in Earth's atmosphere, form secondary particles called air showers. The energy range of cosmic rays is 109-1021 eV. When entering the Earth atmosphere, cosmic rays do not only form secondary cosmic rays but also ionize matter. In this way, the cosmic ray leaves a track of ions until it collides with a nucleus of molecule and gets terminated, as a result of which new secondary ionizing particles will be formed. Cosmic rays are thought to produce one ion-electron pair per 35 eV of deposited energy.

Numbers of different methods are known for determining ionizing radiation, such as semiconductor detectors having e.g. silicon sensors are commonly used, where the ionizing radiation produces charges, which can be measured electrically. Also cloud chambers are known for detecting ionizing radiation, where the ionizing radiation causes a trace into the cloud, which can be measured e.g. optically.

Typically detection methods for ionizing radiation provide the frequency of ionization events and event the energy deposited. However, there are no methods that could determine chemical composition of ions produced by one single ionization event. For example detailed investigation of the dynamics and chemistry of atmospheric ionization has been very difficult because of instrumental limitations.

SUMMARY OF THE INVENTION An object of the invention is to provide a new detection method and arrangement, which is able to measure and determine atmospheric ion bursts and their dynamics and chemistry in more details to count total radiation, which is as a sum of all radiation including among other the radioactivity radiation, galactic cosmic rays and additionally study chemical composition of single produced ion plume (result of ionization event). An additional object of the invention is to additionally detect also different non- radiating particles or molecules, when they are ionized before the detection.

The object of the invention can be achieved by the features of independent claims. The invention relates to an arrangement for detecting ionizing radiations, such as radioactivity and galactic cosmic rays, by a mass spectrometer, according to claim 1. In addition the invention relates to a corresponding method for detecting different ionizing radiations by a mass spectrometer according to claim 14.

According to an embodiment of the invention an arrangement to measure and determine atmospheric ion bursts and their dynamics and chemistry and additionally for detecting different ionizing radiations comprises at least an ionization tube, which is filled with a carrier gas and using a mass spectrometer. The carrier gas is advantageously chosen based on the needs and applications, e.g. with atmospheric ambient gas one can study ion-induced reactions occurring in atmosphere.

The ionization tube with the suitable carrier gas is used for enabling ionizing reactions between the ionizing radiations to be detected and the carrier gas. The ionization occurs in the ionization tube so between the radiation and the gas molecule of the carrier gas, when the high energy particle or radiation, such as the galactic cosmic ray, radioactive particle or e.g. alpha particle originating from radon, collides with the gas molecule of the carrier gas inside the ionization tube and excited the molecule into a higher energy state. This starts the reaction of collisions, where large portion of the ions are discharged and neutralized, but still a small portion of the molecules stay as an ion cluster, which can then be determined by the mass spectrometer. The chemical composition of the ion cluster is based on chemical composition of the carrier gas, as well as possible also on the end products of the reactions induced by the ions inside the ionization tube.

The ionization tube is advantageously a continuous flow ionization chamber for capturing ion bursts and coupled to atmospheric pressure interface time- of-flight (APi-TOF) mass spectrometer for detecting the frequency and composition of the resulting ion plumes. The APi-TOF sampling rate is, according to an embodiment, advantageously on the order of about 10- 100 Hz in order to detect individual ion bursts from ionization events and fast reactions, the sample rate depending on the application and needs of use.

Besides counting the individual ionization events, the arrangement is able to follow the rapidly changing chemical composition of ions during ion burst cascade. The embodiments of the invention can give insights into the primary ionization mechanisms and their importance in atmospheric ion and aerosol dynamics. The arrangement advantageously comprises two main parts: an APi-interface that transfers ions from atmospheric pressure to vacuum and a mass spectrometer that determines the mass-to-charge ratios of ions.

As an example the APi may have three differentially pumped chambers, with two quadrupoles and ion lenses to focus and transport the ions. Once the ions arrive at the time-of-flight chamber of the mass spectrometer, they are accelerated orthogonally to their initial flight path. This acceleration process is called an extraction, and can be repeated for example at the frequency of around 10-12 kHz depending on the desired mass range (however not limiting the scope of the invention only to this exemplary frequency). Ions are separated based on their flight time, which depends on the mass-to- charge ratio, and detected using a multi-channel plate (MCP) with intervals of 40-80 ps, for example. The mass spectrometer can have a mass resolving power of 3000-6000 Th/Th and mass accuracy better than 20 ppm (0.002%).

According to an embodiment, at a certain measurement the carrier gas is free of ambient air and thus also free of radon. In this case the mass spectrometer is used for measuring ionizing radiation originating galactic cosmic rays or other high energy particle sources, and any phenomena originating possible from ambient air or radon, are eliminated. By this arrangement the amount of ionized particles formed in the ionization tube can be determined. It is previously known that the ionizing radiation originated from galactic cosmic rays is very highly energetic, whereupon it has potential to ionize enormous amount of molecules in the ionization tube and thereby form lots of ionized molecules, which can be detected as a very high peak in the mass spectrometer. On the other hand it is known that energy radioactive sources are not so energetic, whereupon they have potential to ionize smaller amount of molecules in the ionization tube and thereby form smaller number of ionized molecules than the cosmic rays, which can be detected as a lower peak in the mass spectrometer. The peak size corresponds with the age of the plume, so the result of ionization event. Thus, when the correlation of the peak size with the age of the plume is known beforehand, the age of the plume can be assumed or estimated by comparing the size of the peak to the predetermined peak size the age of which are known.

According to an embodiment also ambient air can be provided into the ionization tube. In this embodiment the mass spectrometer measures the total ionizing radiation comprising radiation originating from galactic cosmic rays or other radioactive sources, also from ambient radon.

The portion of radon can be detected by sequentially and separately measuring the peaks of galactic cosmic rays and other radioactive sources, and on the other hand the total ionizing radiation comprising all ionizing radiation, and then by subtraction the portion of galactic cosmic rays and other radioactive sources from the total ionizing radiation, whereupon the portion induced by the radon is achieved.

In addition, according to an embodiment also an ionizing generator can be used for ionizing particles of a sample gas flow to be introduced to the mass spectrometer via said ionization tube. The sample gas then comprises particles to be detected. The particles are typically electrically neutral, so they need to be ionized before they can be detected by the mass spectrometer. The ionizing is advantageously done by the ionizing generator, which is advantageously an X-ray radiation source. Often the energy of the used X-ray photons are so called soft X-ray, which is in a range of 1 -10 keV, most advantageously about 1 -5 keV. In addition the X- ray radiation source is configured to be switched on in an operation mode and off mode both for secure reasons, but also for using the arrangement for different measurement purposes. The present invention offers advantages over the known prior art, such as the possibility to measure radiation such as the galactic cosmic rays and radioactivity radiation originating e.g. from alpha particle originating from radon, but also radioactivity radiation originating from explosives, as well as determine chemical composition of ions produced by one single ionization event. Especially the embodiments of the invention are effective for directly measuring atmospheric ion bursts produced by cosmic rays, radioactive decay or gamma radiation. The method of the invention makes it possible to monitor individual ionization events during ion bursts and to measure the rapidly changing chemical composition of the ions associated with such bursts. The resulting information will be valuable for atmospheric scientists in exploring the effect of ions on microphysical atmospheric processes and consequent effects on climate in the Earth system. The chemical composition of ions can be analyzed within short bursts that are generated by high energy particles originating e.g. from cosmic rays and radioactive decay. In addition the present invention can be used for detecting different non-radiating particles or molecules by the same method and arrangement, such as particles of drugs, different chemical substances, such as ammonia, amines, sulphuric acid and oxidized organics, in ambient air, but also explosives when they release molecules into the ambient air, which can be detected.

Moreover, by operating the mass spectrometer (APiTOF) at the order of 10- 100 Hz sampling frequency, the system of the invention is able to detect ion plumes, to separate them from the background signal, to categorize them according to the ion density, and to measure their chemical composition. Also a clear change in the chemical composition of ion plumes can be observed as they age. Distinct chemical compositions can also be observed for young and old ion plumes for both negative and positive ions. Compared with negative ion plumes, the positive ion plumes tended to be spread to a larger volume while being lower in concentration. Still in addition the embodiments of the invention provide direct information on individual ion bursts in an ambient air, including their "age" and the chemical composition of ions within such bursts. In general, the embodiment of the invention will provide new insight into the dynamics of ions and their chemistry. Such information is essential for detailed investigations on atmospheric oxidation, cluster dynamics, new particle formation and gas-to-particle conversion

BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 illustrates a principle of an exemplary arrangement for detecting different ionizing radiations by a mass spectrometer according to an advantageous embodiment of the invention, Figure 2 illustrates an exemplary of total ion counts measured at two different sampling rates, and

Figure 3 illustrates a principle of an exemplary evolution of equilibrium in measurements according to the embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 illustrates a principle of an exemplary arrangement 100 for detecting different ionizing radiations by a mass spectrometer 101 according to an advantageous embodiment of the invention. The arrangement comprises an ionization tube 102, which is filled with a carrier gas 103.

The ionizing reactions 104 are happened between the ionizing radiations to be detected and the carrier gas 103. The ionizing radiations 105 can be originated outside the ionization tube 102, which penetrates the mechanical structure of the ionization tube 102 and thereby causes the ionization. This kind of radiation is e.g. galactic cosmic rays or other radiation, such as gamma or X-ray photons the source of which is somewhere outside the ionization tube 102, but having so energetic radiation that it is able to penetrate into the ionization tube 102. Alternative, or in addition to, the arrangement may also detect particles 106, which must first be introduced into the ionization tube 102, where they then are able to cause the ionization, such as for example radon gas or other sample gas 106, which is transferred via a flow tube 107, which advantageously also provides said carrier gas flow through the ionization tube.

The arrangement comprises a controlling device 108 for controlling the access of the ambient air or sample gas flow via the flow tube 107 into the ionization tube 102. Thus, by controlling with the controlling device 108 e.g. only inert carrier gas can be fed into the flow tube 107 into and again to the ionization tube 102, whereupon only ionizations induced by ionizing radiation originating galactic cosmic rays or other high energy radioactive sources are detected, because the carrier gas is free of ambient air and thus also free of radon and other possible samples. Correspondingly, the by controlling with the controlling device 108 also the ambient air with radon, or alternative a special sample gas 106 with sample particles to be detected can be fed into the ionization tube. The arrangement 100 may be configured to determine portion of the radon or other sample measuring by sequentially and separately the peaks of the ions ionized only by the galactic cosmic rays and/or other radioactive sources, and on the other hand the total ionizing radiation comprising all ionizing radiation. The arrangement 100 is also configured to determine the portion of the radon or other sample by subtraction the portion of galactic cosmic rays and other radioactive sources from the total ionizing radiation, whereupon the portion induced by the radon or other sample is achieved. For this the arrangement 100 comprises or is coupled with a data analysing unit 109.

The arrangement may also comprise an ionizing generator 1 10, such as an X-ray radiation source, which can be used for ionizing particles of a sample gas flow 106 to be introduced to the mass spectrometer via said ionization tube. Additionally the arrangement may also comprise a filter device 1 1 1 for filtering unwanted particles. The filter device 1 1 1 may be for example a HEPA filter (High Efficiency Particulate Air filter).

Figure 2 illustrates an exemplary of total ion counts measured at two different sampling rates, namely 100 Hz (201 (black line), according to the embodiments of the invention) and 1 Hz (202 (white line), representing the prior art methods). According to the invention the sampling rate is increased to 100 Hz, whereupon a continuous-looking time series of air ions became spiky, with periods in between the spikes having few or no counts per spectrum (Figure 2). Actually, each spike in the data was associated with one ionization event caused by a single ionizing interaction. If a low sampling rate is used (let's say 0.001 -1 Hz, which is very typical rate in prior art methods), an average ion concentration over the acquisition period is recorded, whereupon the time series obtained from this type of a measurement gives an impression that the ions are relatively homogeneously distributed within the sampled air, which is not the case in reality.

The spikes in the time series of the total ion count (TIC) signal (Fig. 2) are formed when a cosmic ray or ionizing particle forms a plume of ions during the flight through the ionization chamber. We assume that large (multiple ions detected) ion plumes are results of ionization events that are detected sooner after the event compared with the small plumes. Other factors that influence the size of the plume are the angle of incidence and radial position of the ray as well as the type and energy of ionizing radiation in question. On average, however, the size of a plume is approximately related to its age. After the initial formation of the ion plume, ion-ion recombination starts to neutralize ions, which lowers their total concentration. The carrier gas in the ionization chamber transports these ions to the mass spectrometer. Higher flow rates correspond to shorter average residence times and, consequently, longer residence times tend to provide more time for the ions to be neutralized by the ion-ion recombination. Besides the ion-ion recombination, the ion plume is affected by diffusion broadening that makes the observed spikes wider. Again, with longer residence times the observed spikes get wider.

Figure 3 illustrates a principle of an exemplary evolution of equilibrium of ions in measurements according to the embodiment of the invention, where cumulative mass spectrum of observed positive ion bursts is separated by the bursts intensity. Figure 3 A) background spectrum, Figure 3 B) Sum of bursts that had maximum intensity of 20-50 counts. Assumed time from initial ionization is increasing from b) to a). Value "Total counts" indicates the total number of counts summed up for each cumulative spectrum. The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims. Especially it is to be noted that the arrangement of the invention can be used for detecting and separating lots of different originating radiations or particles in a very easy, effective and fast way due to used mass spectrometer. In particularly the invention can be utilized in detection of CBRNE weapons, for example (CBRNE is an acronym for Chemical, Biological, Radiological, Nuclear, and high yield Explosive Explosives.

Claims

1. An arrangement for detecting dynamics or chemistry of atmospheric ionization caused by different ionizing radiations, such as radioactivity and galactic cosmic rays, by a mass spectrometer,

wherein the arrangement comprises:

- an ionization tube with a carrier gas, which is adapted to enable ionizing reactions between said ionizing radiations or ionized products ionized by said ionizing radiations and said carrier gas and thereby enabling formation of ion particles, and

- a flow tube for providing said carrier gas flow through said ionization tube,

wherein

- said arrangement is configured to introduce the carrier gas with said produced ion particles into the mass spectrometer for detection of dynamics or chemistry of atmospheric ionization and existence of said ionizing radiations.

2. An arrangement of claim 1 , wherein the mass spectrometer is an atmospheric pressure interface time of flight mass spectrometer.

3. An arrangement of any of previous claims, wherein said carrier gas is free of ambient air and thus also free of radon, whereupon the mass spectrometer is used for measuring ionizing radiation originating galactic cosmic rays or other high energy radioactive sources.

4. An arrangement of claim 3, wherein the arrangement is configured to determine separate gaseous sources like radon, particles or cosmic rays by changing the flow configuration of the flow tube and thereby determining different ionizing radiations to be as originated from radon, certain particles or cosmic rays.

5. An arrangement of any of previous claims 1 -2, wherein an ambient air is provided into the ionization tube, whereupon the mass spectrometer is used for measuring total ionizing radiation comprising also radiation originating from ambient radon.

6. An arrangement of any of previous claims, wherein the arrangement is configured to separate ionization radiation originated only from radon by sequentially measuring at the first time moment the carrier gas free of ambient air and at the second time moment the carrier gas comprising also ambient air and thus also radon, whereupon the separating said ionization radiations are implemented by subtracting the results measured at the second time moment from the results measured at the first time moment.

7. An arrangement of any of previous claims, wherein the arrangement further comprises a ionizing generator for ionizing particles of a sample gas flow to be introduced to the mass spectrometer via said ionization tube.

8. An arrangement of claim 7, wherein the ionizing generator is an X-ray radiation source.

9. An arrangement of claim 8, wherein the energy of the used soft X-ray photons is in a range of 1 -10 keV, most advantageously about 1 -5 keV, and wherein said X-ray radiation source is configured to be switched in operation mode and off mode.

10. An arrangement of any of previous claims, wherein the arrangement is configured to provide laminarized flow through said ionization tube.

1 1. An arrangement of any of previous claims, wherein the ion particles forms a plume, whereupon the arrangement is configured to estimate age of the plume by comparing the size of the peak of the plume determined by the mass spectrometer to predetermined peak sizes, where a correlation of the peak sizes with ages of the plume are known beforehand.

12. An arrangement of any of previous claims, wherein the ion particles forms a plume, whereupon the arrangement is configured to detect ion plumes, to separate them from a background signal, to categorize them according to the ion density, and to measure their chemical composition.

13. An arrangement of any of previous claims, wherein said mass spectrometer is used on sampling frequency of 10-100 Hz.

14. A method for detecting dynamics or chemistry of atmospheric ionization caused by different ionizing radiations, such as radioactivity and galactic cosmic rays, by a mass spectrometer,

wherein the method comprises steps of:

- providing an ionization tube with a carrier gas, and enabling ionizing reactions between said ionizing radiations or ionized products ionized by said ionizing radiations and said carrier gas and thereby enabling formation of ion particles, and

- providing said carrier gas by a flow tube so to flow through said ionization tube for,

wherein

- introducing the carrier gas with said produced ion particles into the mass spectrometer and detecting of dynamics or chemistry of atmospheric ionization and the existence of said ionizing radiations by the mass spectrometer.

15. A method of claim 14, wherein said carrier gas is free of ambient air and thus also free of radon, whereupon the mass spectrometer is used for measuring ionizing radiation originating galactic cosmic rays or other high energy radioactive sources.

16. A method of claim 15, wherein a determination, whether the ionizing radiation is originated from radon, certain particles or cosmic rays is implemented by changing the flow configuration of the flow tube for separate gaseous sources.

17. A method of claim 14, wherein an ambient air is provided into the ionization tube, whereupon the mass spectrometer is used for measuring total ionizing radiation comprising also radiation originating from ambient radon.

18. A method of any of claims 14-17, wherein the ionization radiation is detected to be originated only from radon by sequentially measuring at the first time moment the carrier gas free of ambient air and at the second time moment the carrier gas comprising also ambient air and thus also radon, whereupon the separating said ionization radiations are implemented by subtracting the results measured at the second time moment from the results measured at the first time moment.

19. A method of any of claims 14-18, wherein particles of a sample gas flow is ionized by an ionizing generator and the ionized particles are then introduced to the mass spectrometer via said ionization tube, where said ionizing generator is X-ray radiation source.

20. A method of any of previous claims 14-19, wherein the ion particles forms a plume, and wherein age of the plume is estimated by comparing the size of the peak of the plume determined by the mass spectrometer to predetermined peak sizes, where a correlation of the peak sizes with ages of the plume are known beforehand.

21. A method of any of previous claims 14-20, wherein the ion particles forms a plume, whereupon ion plumes are detected and separated from a background signal, and categorized them according to the ion density, and measured their chemical composition.